Shuozhi Xu's blog
https://imechanica.org/blog/17621
enA fully-funded Ph.D. student position at the University of Oklahoma
https://imechanica.org/node/26492
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>Dear Colleagues,</span></p>
<p>My research group in the School of Aerospace and Mechanical Engineering at the University of Oklahoma is looking for a Ph.D. student to work in the general fields of computational materials science and solid mechanics. Please share the attached flyer with anyone who may be interested.</p>
<p>Thank you</p>
<p>Shuozhi Xu</p>
<p> </p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
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<tr class="odd"><td><span class="file"><img class="file-icon" alt="PDF icon" title="application/pdf" src="/modules/file/icons/application-pdf.png" /> <a href="https://imechanica.org/files/Shuozhi-Xu-OU-ad-1op-2023.pdf" type="application/pdf; length=537474">Shuozhi-Xu-OU-ad-1op-2023.pdf</a></span></td><td>524.88 KB</td> </tr>
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</div></div></div>Wed, 18 Jan 2023 00:59:15 +0000Shuozhi Xu26492 at https://imechanica.orghttps://imechanica.org/node/26492#commentshttps://imechanica.org/crss/node/26492Graduate research assistant positions at the North Carolina State University
https://imechanica.org/node/26315
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/597">mechanics of materials</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A research group led by Dr. Liming Xiong (currently at Iowa State and will be moving to NCSU) (<a href="https://www.aere.iastate.edu/lmxiong/">https://www.aere.iastate.edu/lmxiong/</a>) at North Carolina State University, Raleigh, NC, is looking for full-support ($2600/month + tuition) graduate research assistants in the general field of theoretical, applied, and computational mechanics of materials. Research topics will fall into a broad area of modeling and simulation of mechanical and transport behaviors, such as dislocation plasticity, fracture, phase transformation, thermal transport, and mass transport, in a variety of engineering materials.</p>
<p>Self-motivated individuals who have research experience in one or more of the following areas are strongly encouraged to apply:</p>
<ul><li>Atomistic simulations, Coarse-graining and Data-driven Finite Element</li>
<li>Dislocation Dynamics, Phase Field Method, XFEM, Crystal Plasticity Finite Element</li>
<li>Continuum and Micro-continuum Mechanics</li>
<li>Multiscale Materials Modeling</li>
<li>VASP, Gaussian, LAMMPS, and GULP</li>
<li>C++, OpenMPI, Python</li>
</ul><p>To apply for the position, please submit a single PDF file via email (<a href="mailto:lmxiong@iastate.edu">lmxiong@iastate.edu</a>) with (i) your most recent CV; (ii) relevant research experience or publications if there are any, as well as (iii) a list of references. NCSU is located in Raleigh, the capital of North Carolina, which forms one vertex of the world-famous Research Triangle Park (RTP). RTP is an innovative environment, both as a metropolitan area with one of the most diverse industrial bases in the world, and as a center of excellence promoting technology and science. The Research Triangle area is routinely recognized in nationwide surveys as one of the best places to live in the U.S. We enjoy outstanding public schools, affordable housing, farmer’s markets and festivals, and great weather- all in proximity to the mountains and the seashore.</p>
</div></div></div>Wed, 26 Oct 2022 02:02:49 +0000Shuozhi Xu26315 at https://imechanica.orghttps://imechanica.org/node/26315#commentshttps://imechanica.org/crss/node/26315Dislocation dynamics in heterogeneous nanostructured materials
https://imechanica.org/node/26234
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/4675">dislocation dynamics</a></div><div class="field-item odd"><a href="/taxonomy/term/466">nanostructure</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span><a href="https://doi.org/10.1016/j.jmps.2022.105031">https://doi.org/10.1016/j.jmps.2022.105031</a></span></p>
<p><span>Abstract</span></p>
<p><span>Crystalline materials can be strengthened by introducing dissimilar phases that impede dislocation glide. At the same time, the changes in microstructure and chemistry usually make the materials less ductile. One way to circumvent the strength–ductility dilemma is to take advantage of heterogeneous nanophases which simultaneously serve as dislocation barriers and sources. Owing to their superior mechanical properties, heterogeneous nanostructured materials (HNMs) have attracted a lot of attention worldwide. Nevertheless, it has been difficult to characterize dislocation dynamics in HNMs using classical continuum models, mainly due to the challenges in describing the elastic and plastic heterogeneity among the phases. In this work, we advance a phase-field dislocation dynamics (PFDD) model to treat multi-phase materials, consisting of phases differing in composition, structural order, and size in the same system. We then apply the advanced PFDD model to exploring two important but divergent materials design problems in HNMs: dislocation/obstacle interactions and dislocation/interface interactions. Results show that the interactions between a dislocation and distribution of obstacles varying in structure and composition cannot be understood by simply interpolating from their individual interactions with a dislocation. It is also found that materials containing interfaces with nanoscale thicknesses and compositional gradients have a much higher dislocation bypass stress than those with sharp interfaces, providing an explanation for the simultaneous high strength and toughness of thick interface-containing nanolaminates as observed in recent experiments.</span></p>
</div></div></div>Fri, 23 Sep 2022 16:44:44 +0000Shuozhi Xu26234 at https://imechanica.orghttps://imechanica.org/node/26234#commentshttps://imechanica.org/crss/node/26234Fully-funded Ph.D. student positions at the University of Oklahoma
https://imechanica.org/node/25941
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I will join the School of Aerospace and Mechanical Engineering at the University of Oklahoma as a tenure-track Assistant Professor. I have two fully-funded Ph.D. student positions in my group. The students will work in general fields of computational materials science and solid mechanics. Please share the attached flyer with anyone who may be interested.</p>
<p>Thank you</p>
<p>Shuozhi Xu</p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
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<tr class="odd"><td><span class="file"><img class="file-icon" alt="PDF icon" title="application/pdf" src="/modules/file/icons/application-pdf.png" /> <a href="https://imechanica.org/files/Shuozhi-Xu-OU-ad.pdf" type="application/pdf; length=538942">Shuozhi-Xu-OU-ad.pdf</a></span></td><td>526.31 KB</td> </tr>
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</div></div></div>Mon, 02 May 2022 02:37:47 +0000Shuozhi Xu25941 at https://imechanica.orghttps://imechanica.org/node/25941#commentshttps://imechanica.org/crss/node/25941Line-length-dependent dislocation glide in refractory multi-principal element alloys
https://imechanica.org/node/25781
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/499">dislocations</a></div><div class="field-item odd"><a href="/taxonomy/term/12833">multi-principal element alloys</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span><a href="http://dx.doi.org/10.1063/5.0080849">http://dx.doi.org/10.1063/5.0080849</a></span></p>
<p><span>Abstract</span></p>
<p><span>Plastic deformation of refractory multi-principal element alloys (RMPEAs) is known to differ greatly from those of refractory pure metals. The fundamental cause is the different dislocation dynamics in the two types of metals. In this Letter, we use atomistic simulations to quantify dislocation glide in two RMPEAs: MoNbTi and NbTiZr. Edge and screw dislocations on the {110} and {112} slip planes are studied. A series of dislocation line lengths, ranging from 1 nm to 50 nm, are employed to elucidate the line-length-dependence. To serve as references, the same simulations are performed on pure metals. For the RMPEAs, the dependence of critical stresses on length becomes undetectable within the statistical dispersion for dislocations longer than 25 nm, as a result of the change in dislocation behavior. This length is in good agreement with those predicted by analytical models. Compared to the pure metals, the critical stress anisotropy among different slip planes and character angles is substantially reduced, providing an explanation for the homogeneous plasticity in RMPEAs observed in prior experiments.</span></p>
</div></div></div>Tue, 15 Feb 2022 01:40:59 +0000Shuozhi Xu25781 at https://imechanica.orghttps://imechanica.org/node/25781#commentshttps://imechanica.org/crss/node/25781Phase-field modeling of the interactions between an edge dislocation and an array of obstacles
https://imechanica.org/node/25655
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/9383">phase-field</a></div><div class="field-item odd"><a href="/taxonomy/term/11120">Dislocation-Obstacle Interactions</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.cma.2021.114426">https://doi.org/10.1016/j.cma.2021.114426</a></p>
<p>Abstract</p>
<p>Obstacles, such as voids and precipitates, are prevalent in crystalline materials. They strengthen crystals by serving as barriers to dislocation glide. In this work, we develop a phase-field dislocation dynamics (PFDD) technique for investigating the interactions between dislocations and second-phase obstacles, which can be either voids or precipitates. The PFDD technique is constructed to account for elastic heterogeneity, elastic anisotropy, dissociation of the dislocation, and dislocation transmission across bicrystalline interfaces. Within the framework, we present a model for “pseudo-voids”, which are voids shearable by dislocations, in contrast to unphysical, unshearable voids in conventional phase-field dislocation formulations. We employ the PFDD technique to investigate the in-plane interactions between an edge dislocation and an array of nano-scale obstacles with different spacings. In this application, the interactions take place in glide planes of either a face-centered cubic (FCC) Cu or a body-centered cubic (BCC) Nb matrix, while the precipitates have a Cu1-<em>x</em>Nb<em>x</em> composition, with <em>x</em> varying from 0.1 to 0.9. Our atomistic simulations find that the alloy precipitates can have an FCC, an amorphous, or a BCC phase, depending on the compositional ratio between Cu and Nb, i.e., value of <em>x</em>. Among all types of obstacles, the critical stresses for dislocation bypass are the highest for unshearable amorphous precipitates, followed by shearable crystalline precipitates, and then the pseudo-voids.</p>
</div></div></div>Sun, 26 Dec 2021 05:27:56 +0000Shuozhi Xu25655 at https://imechanica.orghttps://imechanica.org/node/25655#commentshttps://imechanica.org/crss/node/25655Local slip resistances in equal-molar MoNbTi multi-principal element alloy
https://imechanica.org/node/24666
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12833">multi-principal element alloys</a></div><div class="field-item odd"><a href="/taxonomy/term/12975">local slip resistance</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.actamat.2020.10.042">https://doi.org/10.1016/j.actamat.2020.10.042</a></p>
<p>Abstract</p>
<p>In this work, we calculate the local slip resistances (LSRs) in equal-molar MoNbTi multi-principal element alloy via molecular static simulations. We consider dislocations of either screw or edge character gliding on four types of slip planes, {110}, {112}, {123}, and {134}, in either forward or backward sense of the <111> slip direction. As references, we also compute the Peierls stresses of the same dislocations in two natural reference metals, Mo and Nb, and a synthetic one, the mean-field, <em>A</em>-atom potential-based MoNbTi. Further, we compare the LSRs with the corresponding ideal shear strengths that do not account for the lattice distortions induced by dislocation cores. We show that for neither dislocation character is the LSR on the {110} plane the lowest in MoNbTi, in contrast to Mo and Nb. For edge dislocations, slip on the {134} plane is the easiest, but for the screw dislocations, it is the hardest. For screw dislocations, the {112} glide plane is the most favored, while for edge dislocations, it is the least favored. We also find that the screw-to-edge ratio in the slip resistance is reduced by one order of magnitude in MoNbTi compared to that of any pure reference metal for the same type of slip plane. These results suggest that, in contrast to pure body-centered cubic (BCC) metals, BCC MPEAs could deform by a multiplicity of slip modes due to the lower screw-to-edge ratios and the lower LSRs for edge dislocations on the three higher order planes.</p>
</div></div></div>Wed, 21 Oct 2020 02:27:37 +0000Shuozhi Xu24666 at https://imechanica.orghttps://imechanica.org/node/24666#commentshttps://imechanica.org/crss/node/24666Multiplicity of dislocation pathways in a refractory multiprincipal element alloy
https://imechanica.org/node/24630
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12967">multiprincipal element alloys</a></div><div class="field-item odd"><a href="/taxonomy/term/12968">refractory metals</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1126/science.aba3722">https://doi.org/10.1126/science.aba3722</a></p>
<p>Abstract</p>
<p>Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum.</p>
</div></div></div>Thu, 01 Oct 2020 22:19:14 +0000Shuozhi Xu24630 at https://imechanica.orghttps://imechanica.org/node/24630#commentshttps://imechanica.org/crss/node/24630The effect of local chemical ordering on Frank-Read source activation in a refractory multi-principal element alloy
https://imechanica.org/node/24562
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12833">multi-principal element alloys</a></div><div class="field-item odd"><a href="/taxonomy/term/2375">phase-field modeling</a></div><div class="field-item even"><a href="/taxonomy/term/4675">dislocation dynamics</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.ijplas.2020.102850">https://doi.org/10.1016/j.ijplas.2020.102850</a></p>
<p>Abstract</p>
<p>In this work, we investigate the operation of Frank-Read (FR) sources in a refractory multi-principal element alloy (MPEA). Simulations of discrete dislocation motion in MPEAs is enabled by the development of a phase field dislocation dynamics model that treats the atomic-scale fluctuations in lattice energies across the glide plane, present due to local ordering in the chemical composition within the nominally random MPEA atomic structure. We consider, through simulation, a range of length scales over which ordering occurs, varying from short-range lengths, a few times dislocation core width, to long-range lengths, an order of magnitude longer than the core. Characteristic of this body-centered cubic MPEA, the simulations also include screw/edge character dependence in glide resistance, as informed by atomic scale simulation. The critical stresses to activate the source for the same source size are found to be statistically distributed, as a direct consequence of the underlying variation in lattice energy. Analysis of critical state for activating edge and screw FR sources in the MPEA reveals that FR source operation occurs via a two-step mechanism, involving athermal kink-pair formation, unlike the conventional FR source operation in a material with no composition fluctuations. This mechanism lowers the average critical stress required to activate the FR source and causes the statistical dispersion in critical stress to depend on the range of composition ordering. More importantly, it leads to a more severe dependence of source strength on FR source length than predicted by line tension alone.</p>
</div></div></div>Thu, 03 Sep 2020 01:20:39 +0000Shuozhi Xu24562 at https://imechanica.orghttps://imechanica.org/node/24562#commentshttps://imechanica.org/crss/node/24562Si/Ge (111) semicoherent interfaces: Responses to an in-plane shear and interactions with lattice dislocations
https://imechanica.org/node/24539
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12942">semicoherent interfaces</a></div><div class="field-item odd"><a href="/taxonomy/term/1910">multiscale modeling</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1002/pssb.202000274">https://doi.org/10.1002/pssb.202000274</a></p>
<p>Abstract</p>
<p>Concurrent atomistic–continuum simulations are employed to study Si/Ge (111) semicoherent interfaces in terms of their responses to an in‐plane shear and interactions with lattice dislocations. Three types of Si/Ge interfaces, differing in interfacial structures and energy, are considered. Type I interface coincides with the shuffle‐set slip plane and contains a hexagonal network of edge dislocations. Type II and Type III interfaces both coincide with the glide‐set slip plane, yet they contain, respectively, a triangular and a hexagonal network of Shockley partial dislocations. The simulations show that among the three types of interfaces, 1) Type I interface is the least stable subject to an in‐plane shear and 2) Type III interface impedes the gliding of lattice dislocations the most significantly.</p>
</div></div></div>Tue, 25 Aug 2020 02:35:21 +0000Shuozhi Xu24539 at https://imechanica.orghttps://imechanica.org/node/24539#commentshttps://imechanica.org/crss/node/24539Effects of lattice distortion and chemical short-range order on the mechanisms of deformation in medium entropy alloy CoCrNi
https://imechanica.org/node/24538
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/11633">high entropy alloys</a></div><div class="field-item odd"><a href="/taxonomy/term/1188">atomistic simulation</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.actamat.2020.08.044">https://doi.org/10.1016/j.actamat.2020.08.044</a></p>
<p>Abstract</p>
<p>As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A-atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO.</p>
</div></div></div>Tue, 25 Aug 2020 02:31:34 +0000Shuozhi Xu24538 at https://imechanica.orghttps://imechanica.org/node/24538#commentshttps://imechanica.org/crss/node/24538Atomistic calculations of the generalized stacking fault energies in two refractory multi-principal element alloys
https://imechanica.org/node/24227
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12833">multi-principal element alloys</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.intermet.2020.106844">https://doi.org/10.1016/j.intermet.2020.106844</a></p>
<p>Abstract</p>
<p>In this work, we utilize atomistic simulations to calculate the generalized stacking fault energies (GSFEs), which are related to the dislocation glide process, on four types of slip planes – {110}, {112}, {123}, and {134} – in two refractory multi-principal element alloys (MPEAs): MoNbTi and NbTiZr. To serve as a reference material for MoNbTi, we develop, validate, and employ an <em>A</em>-atom interatomic potential, which is expected to represent the response of the nominal random solution. Our calculations show that, owing to the variation in local chemical composition within small finite nanometer sized planes, (i) the peak GSFE values vary significantly among parallel planes; (ii) within the same specific {110} plane, substantial differences in the GSFE curves along the two non-parallel <111> directions are observed; (iii) the {110} GSFE curves develop an asymmetry, such that the peak energy is not achieved at half the lattice periodicity length, (iv) the GSFE value after a shift equaling the lattice periodicity length is not recovered; and (v) on average, the peak GSFE values are close to the volume fraction average of the peak GSFEs of their constituents.</p>
</div></div></div>Sat, 23 May 2020 15:37:49 +0000Shuozhi Xu24227 at https://imechanica.orghttps://imechanica.org/node/24227#commentshttps://imechanica.org/crss/node/24227Frank-Read source operation in six body-centered cubic refractory metals
https://imechanica.org/node/24170
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12806">phase-field modeling; frank-read source</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.jmps.2020.104017">https://doi.org/10.1016/j.jmps.2020.104017</a></p>
<p>Abstract</p>
<p>The Frank-Read (FR) source is a well-known intragranular dislocation source that plays an important role in size-dependent dislocation multiplication in metallic crystals. In this work, we extend a phase-field dislocation dynamics (PFDD) technique to study FR source operation on the {110}, {112}, and {123} slip planes in body-centered cubic (BCC) crystals. Here, the periodic lattice potentials for shearing across these planes used in PFDD simulations are provided by density functional theory (DFT) calculations for six BCC refractory metals, Cr, Mo, Nb, Ta, V, and W. The DFT calculations show that the group 6 elements (Cr, Mo, and W) have higher generalized stacking fault energies than the group 5 elements (Nb, Ta, and V). With PFDD, we focus on the effects of the GSFE curve shape, initial character angle, slip plane crystallography, and elastic anisotropy (measured by the Zener ratio, <em>A</em>c) on the critical stresses to activate the FR source. For the same FR source of any character angle in the same metal, the critical stress on the {123} plane is lower than those on the {110} and {112} planes. It is also shown that elastic anisotropy decreases the critical stress when <em>A</em>c<span> < 1 and increases it when </span><em>A</em>c<span> > 1. We also find that in both Cr and Nb, which possess the lowest values of </span><em>A</em>c<span> among the six metals, elastic anisotropy causes the critical stress on {110} planes to achieve a local maximum for the mixed 45o oriented FR source.</span></p>
</div></div></div>Tue, 05 May 2020 20:46:08 +0000Shuozhi Xu24170 at https://imechanica.orghttps://imechanica.org/node/24170#commentshttps://imechanica.org/crss/node/24170Ph.D. positions in Dr. Xiaoyu Tang’s group in the Department of Mechanical and Industrial Engineering at Northeastern University in the area of Fluid Mechanics and Soft Matter (2020 Fall / 2021 Spring / 2021 Fall)
https://imechanica.org/node/24116
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><strong>Overview</strong></p>
<p>Dr. Xiaoyu Tang’s group in the Department of Mechanical and Industrial Engineering at Northeastern University has multiple fully funded Ph.D. positions in the area of Fluid Mechanics and Soft Matter. Research in the group focuses on multiphase flow, microfluidics, and active matter, using both experimental and numerical techniques. Selected candidates are expected to start in the Spring or Fall of 2021. Under special cases, a Fall 2020 start date may also be arranged. Visiting students/scholars are always welcome.</p>
<p><strong>Qualification</strong></p>
<p>Students with Bachelor’s or Master’s degree from all science and engineering majors, especially Mechanical Engineering, Chemical Engineering, Materials Science, Physics, are encouraged to apply. Self-motivated candidates with a strong foundation are particularly welcome. Research experience in either experiment or simulation and publications are preferred but not required.</p>
<p><strong>How to Apply</strong></p>
<p>Interested candidates should contact Dr. Tang (<a href="mailto:xiaoyu.tang.neu@gmail.com">xiaoyu.tang.neu@gmail.com</a>) with anticipated start date and their CV detailing academic and/or research achievements. Review of applications will start immediately and continue until the positions are filled.</p>
<p><strong>About Dr. Tang</strong></p>
<p>Dr. Tang obtained her Ph.D. in Mechanical and Aerospace Engineering from Princeton University and Bachelor’s degree in Thermal Engineering from Tsinghua University. She is currently a postdoctoral researcher in the Department of Chemical Engineering at University of California, Santa Barbara. She will join Northeastern University as a tenure-track Assistant Professor.</p>
<p><strong>About NEU</strong></p>
<p>Northeastern University is a private research university located in Boston, Massachusetts. According to the 2020 edition of US News and World Report rankings, Northeastern University is #31 in Best Graduate Engineering School, #40 in National Universities, and #1 in Best Co-op/Internships. The famous cooperative education program at Northeastern integrates classroom study with professional experience and contains over 3,100 partners across all seven continents. World-class research conducted in the university has attracted $178.8M external funding in 2019.</p>
</div></div></div>Sat, 18 Apr 2020 01:22:15 +0000Shuozhi Xu24116 at https://imechanica.orghttps://imechanica.org/node/24116#commentshttps://imechanica.org/crss/node/24116Fully-funded PhD student positions in the field of materials in extreme environments using small-scale computational methods
https://imechanica.org/node/24086
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6176">computational materials science</a></div><div class="field-item odd"><a href="/taxonomy/term/12807">materials in extreme environment</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://scholar.google.com/citations?user=Pz7lLpUAAAAJ&hl=en">Dr. Yanqing Su</a>'s group in the Department of Mechanical and Aerospace Engineering at the Utah State University (USU) is looking for fully funded Ph.D. students in the field of materials in extreme environments using small-scale computational methods. Self-motivated individuals who have research experience in one or more of the following areas are strongly encouraged to apply for the Ph.D. positions in our group:</p>
<ul><li>First-principle calculations (e.g., VASP)</li>
<li>Quantum mechanics</li>
<li>Atomistic Simulation (e.g., LAMMPS)</li>
<li>FORTRAN, C, C++, Python, OpenMPI</li>
</ul><p>Interested applicants please send an email to Dr. Su (<a href="mailto:yanqingsu@ucsb.edu">yanqingsu@ucsb.edu</a>) with (i) your most recent CV, including TOEFL and GRE scores, research experience, a list of publications if applicable; (ii) an unofficial transcript; and (iii) a list of references. This position is available starting Fall 2020 or Spring 2021.</p>
<p>More information on the admission requirement and applications can be found at <a href="https://engineering.usu.edu/mae/students/graduate/apply">https://engineering.usu.edu/mae/students/graduate/apply</a>. Located in Logan, UT, USU is just minutes from two mountain ranges and within a half-day's drive of six national parks, including Yellowstone and Arches. It provides big-school opportunities with a small-school feel, and all for a great value.</p>
</div></div></div>Sun, 05 Apr 2020 19:55:03 +0000Shuozhi Xu24086 at https://imechanica.orghttps://imechanica.org/node/24086#commentshttps://imechanica.org/crss/node/24086Atomistic mechanism for vacancy-enhanced grain boundary migration
https://imechanica.org/node/24051
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/7217">Grain Boundary Migration</a></div><div class="field-item odd"><a href="/taxonomy/term/9288">vacancy</a></div><div class="field-item even"><a href="/taxonomy/term/1188">atomistic simulation</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="http://dx.doi.org/10.1103/PhysRevMaterials.4.033602">http://dx.doi.org/10.1103/PhysRevMaterials.4.033602</a></p>
<p>Abstract</p>
<p>Mechanical behavior of polycrystalline materials is intimately connected to migration of grain boundaries, which in turn is dramatically impacted by the presence of defects. In this paper, we present atomistic simulations to elucidate the elementary mechanism that dictates the role of vacancies in enhancing grain boundary migration via shear-coupled normal motion. The minimum energy pathway and the associated energy barriers are calculated using the nudged elastic band method. Fully three-dimensional atomistic simulations provide excellent verification of the three-dimensional disconnection model and furnish quantitative evidence that vacancies facilitate grain boundary migration by weakening the line tension of a disconnection loop. It is also revealed that vacancies serve as energetically favorable sites for the nucleation of grain boundary disconnections, thereby inducing shear-coupled grain boundary migration.</p>
</div></div></div>Mon, 09 Mar 2020 20:35:34 +0000Shuozhi Xu24051 at https://imechanica.orghttps://imechanica.org/node/24051#commentshttps://imechanica.org/crss/node/24051Call for papers: Nanofabrication, Atomic and Close-to-atomic Scale Manufacturing
https://imechanica.org/node/23999
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12762">nanomanufacturing</a></div><div class="field-item odd"><a href="/taxonomy/term/268">nanofabrication</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A Special Issue entitled "Nanofabrication, Atomic and Close-to-atomic Scale Manufacturing" for the journal <em>Nanomanufacturing and Metrology</em> is now open for submission:</p>
<p>The submission deadline is June 15, 2020. For more information, please refer to the attached flier or this webpage:</p>
<p><a href="https://www.springer.com/journal/41871/updates/17701520">https://www.springer.com/journal/41871/updates/17701520</a></p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
<tbody>
<tr class="odd"><td><span class="file"><img class="file-icon" alt="PDF icon" title="application/pdf" src="/modules/file/icons/application-pdf.png" /> <a href="https://imechanica.org/files/NMM%20Special%20Issue.pdf" type="application/pdf; length=124859">NMM Special Issue.pdf</a></span></td><td>121.93 KB</td> </tr>
</tbody>
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</div></div></div>Fri, 14 Feb 2020 17:31:08 +0000Shuozhi Xu23999 at https://imechanica.orghttps://imechanica.org/node/23999#commentshttps://imechanica.org/crss/node/23999Comparative modeling of the disregistry and Peierls stress for dissociated edge and screw dislocations in Al
https://imechanica.org/node/23963
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/605">Dislocation</a></div><div class="field-item odd"><a href="/taxonomy/term/2375">phase-field modeling</a></div><div class="field-item even"><a href="/taxonomy/term/1910">multiscale modeling</a></div><div class="field-item odd"><a href="/taxonomy/term/12752">Peierls-Nabarrow model</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.ijplas.2020.102689">https://doi.org/10.1016/j.ijplas.2020.102689</a></p>
<p>Abstract</p>
<p>Many elementary deformation processes in metals involve the motion of dislocations. The planes of glide and specific processes dislocations prefer depend heavily on their atomic core structures. Atomistic simulations are desirable for dislocation modeling but their application to even sub-micron scale problems is in general computationally costly. Accordingly, continuum-based approaches, such as the phase-field microelasticity, phase-field dislocation dynamics (PFDD), generalized Peierls-Nabarro (GPN) models, and the concurrent atomistic–continuum (CAC) method, have attracted increasing attention in the field of dislocation modeling because they well represent both short-range cores interactions and long-range stress fields of dislocations. To better understand their similarities and differences, it is useful to compare these methods in the context of benchmark simulations and predictions. In this paper, we apply the CAC method and different PFDD variants – one of them is equivalent to a GPN model – to simulate an extended (i.e., dissociated) dislocation in Al with initially pure edge or pure screw character in terms of the disregistry. CAC and discrete forms of PFDD are also employed to calculate the Peierls stress. By conducting comprehensive convergence studies, we quantify the dependence of these measures on time/grid resolution and simulation cell size. Several important but often overlooked differences between PFDD/GPN variants are clarified. Our work sheds light on the advantages and limitations of each method, as well as the path towards enabling them to effectively model complex dislocation processes at larger length scales.</p>
</div></div></div>Mon, 03 Feb 2020 07:28:32 +0000Shuozhi Xu23963 at https://imechanica.orghttps://imechanica.org/node/23963#commentshttps://imechanica.org/crss/node/23963Open PhD positions in the general field of theoretical, applied, and computational mechanics of materials
https://imechanica.org/node/23942
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12212">funded PhD position</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A <a href="https://www.aere.iastate.edu/lmxiong/">research group</a> at Iowa State University is looking for fully funded research assistants in the general field of theoretical, applied, and computational mechanics of materials. Research topics will fall into a broad area of modeling and simulation of mechanical and transport behaviors, such as plasticity, fracture, phase transformation, thermal transport, and mass transport, in a variety of engineering materials.</p>
<p>In particular, self-motivated individuals who have research experience in one or more of the following areas are strongly encouraged to apply for the full-support Ph.D. positions in our group:</p>
<ul><li>Continuum Mechanics</li>
<li>Finite Element Modeling, XFEM</li>
<li>Atomistic Simulation (LAMMPS, DL_POLY, GULP)</li>
<li>First-principle calculations</li>
<li>Dislocation Dynamics</li>
<li>Phase Field Method</li>
<li>Crystal Plasticity</li>
<li>Multiscale Modeling of Materials</li>
<li>FORTRAN, C++, OpenMPI</li>
</ul><p>To apply for the positions, please submit a single PDF file via email (<a href="mailto:lmxiong@iastate.edu">lmxiong@iastate.edu</a>) with (i) your most recent CV; (ii) a transcript; (iii) relevant research experience or publications if there are any, as well as (iv) a list of references.</p>
</div></div></div>Tue, 28 Jan 2020 23:51:59 +0000Shuozhi Xu23942 at https://imechanica.orghttps://imechanica.org/node/23942#commentshttps://imechanica.org/crss/node/23942Modeling dislocations with arbitrary character angle in face-centered cubic transition metals using the phase-field dislocation dynamics method with full anisotropic elasticity
https://imechanica.org/node/23674
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6692">phase field</a></div><div class="field-item odd"><a href="/taxonomy/term/605">Dislocation</a></div><div class="field-item even"><a href="/taxonomy/term/2682">elastic anisotropy</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.mechmat.2019.103200">https://doi.org/10.1016/j.mechmat.2019.103200</a></p>
<p>Abstract</p>
<p>In this study, we present a phase-field dislocation dynamics (PFDD) method that includes full anisotropic elasticity. We apply it to calculate the equilibrium core structures of dislocations with arbitrary character angle in eight face-centered cubic transition metals. The calculations investigate the effects of the gradient energy density in the total energy density and the choice of the averaging scheme to determine the isotropic equivalent elastic moduli (i.e., Voigt, Reuss, and Hill). We show that the addition of the gradient energy term increases the intrinsic stacking fault (ISF) widths for the edge and screw dislocations in most of the metals studied here, but decreases the ISF widths for the edge dislocations in four metals: Ir, Ni, Pd, and Rh. The analysis indicates that among the three isotropic averaging schemes, the Voigt isotropic equivalent modulus best predicts the ISF widths of the edge dislocations and the Reuss scheme for the ISF widths of the screw dislocations, compared to the full elastic anisotropy. Finally, a critical character angle (~60o) is revealed, at which the PFDD simulations with full elastic anisotropy and those with the isotropic Hill average predict the same ISF width. Our work advances the basic understanding of the elastic anisotropic effects on the equilibrium dislocation core structures and can help guide the choice of isotropic averaged moduli.</p>
</div></div></div>Sat, 12 Oct 2019 01:05:23 +0000Shuozhi Xu23674 at https://imechanica.orghttps://imechanica.org/node/23674#commentshttps://imechanica.org/crss/node/23674Density functional theory calculations of generalized stacking fault energy surfaces for eight face-centered cubic transition metals
https://imechanica.org/node/23573
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/4465">density functional theory</a></div><div class="field-item odd"><a href="/taxonomy/term/12594">generalized stacking fault energy</a></div><div class="field-item even"><a href="/taxonomy/term/12595">face-centered cubic</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://aip.scitation.org/doi/10.1063/1.5115282">https://aip.scitation.org/doi/10.1063/1.5115282</a></p>
<p>Abstract</p>
<p>In this work, we use density functional theory to calculate the entire generalized stacking fault energy (GSFE) surface for eight transition metals with a face-centered cubic structure: Ag, Au, Cu, Ir, Ni, Pd, Pt, and Rh. Analysis of the ⟨112⟩ GSFE curves finds that the displacements corresponding to the unstable stacking fault energy are larger than the ideal value for all eight metals except Ag and Cu. Over the entire surface, Pt is found to not possess well-defined local maxima or minima, suggesting spreading in favor of dissociation of the dislocation core, unlike the other seven metals. Our calculations also reveal that at a large ⟨112⟩ displacement, where atoms on two {111} adjacent planes are aligned, an anomalous local minimum occurs for Ir and Rh. The oddity is explained by relatively large, localized atomic displacements that take place in the two metals to accommodate the alignment that do not occur in the other six metals. In addition to the fully calculated surfaces, we characterize a continuous 11-term Fourier-series function, which provides a particularly excellent representation of the GSFE surfaces for Ag, Au, Cu, Ni, and Pd.</p>
<p>Data: <a href="https://archive.materialscloud.org/2019.0041/">https://archive.materialscloud.org/2019.0041/</a></p>
</div></div></div>Fri, 13 Sep 2019 02:35:34 +0000Shuozhi Xu23573 at https://imechanica.orghttps://imechanica.org/node/23573#commentshttps://imechanica.org/crss/node/23573Uniaxial deformation of tungsten nanopillars/nanowires/nanotubes: Atomistic and coarse-grained atomistic simulations
https://imechanica.org/node/23554
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12590">tungsten</a></div><div class="field-item odd"><a href="/taxonomy/term/1143">nanotube</a></div><div class="field-item even"><a href="/taxonomy/term/1744">nanowire</a></div><div class="field-item odd"><a href="/taxonomy/term/11294">Nanopillar</a></div><div class="field-item even"><a href="/taxonomy/term/712">atomistic simulations</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleague,</p>
<p>In the last two years, we published six papers on uniaxial deformation of tungsten nanopillars/nanowires/nanotubes using atomistic and coarse-grained atomistic simulations:</p>
<ul><li>Travis Trusty, Shuozhi Xu, Irene J. Beyerlein, Atomistic simulations of tungsten nanotubes under uniform tensile loading, J. Appl. Phys. 126 (2019) 095105, <a href="http://dx.doi.org/10.1063/1.5110167">http://dx.doi.org/10.1063/1.5110167</a></li>
<li>Shuozhi Xu, Modelling plastic deformation of nano/submicron-sized tungsten pillars under compression: A coarse-grained atomistic approach, Int. J. Multiscale Comput. Eng. 16 (2018) 367--376, <a href="http://dx.doi.org/10.1615/IntJMultCompEng.2018026027">http://dx.doi.org/10.1615/IntJMultCompEng.2018026027</a></li>
<li>Shuozhi Xu, Saeed Zare Chavoshi, Yanqing Su, Deformation mechanisms in nanotwinned tungsten nanopillars: Effects of coherent twin boundary spacing, Phys. Status Solidi RRL 12 (2018) 1700399, <a href="http://dx.doi.org/10.1002/pssr.201700399">http://dx.doi.org/10.1002/pssr.201700399</a></li>
<li>Shuozhi Xu, Saeed Zare Chavoshi, Uniaxial deformation of nanotwinned nanotubes in body-centered cubic tungsten, Curr. Appl. Phys. 18 (2018) 114--121, <a href="http://dx.doi.org/10.1016/j.cap.2017.10.003">http://dx.doi.org/10.1016/j.cap.2017.10.003</a></li>
<li>Shuozhi Xu, Yanqing Su, Dengke Chen, Longlei Li, An atomistic study of the deformation behavior of tungsten nanowires, Appl. Phys. A 123 (2017) 788, <a href="http://dx.doi.org/10.1007/s00339-017-1414-3">http://dx.doi.org/10.1007/s00339-017-1414-3</a></li>
<li>Shuozhi Xu, Jacob K. Startt, Thomas G. Payne, Chaitanya S. Deo, David L. McDowell, Size-dependent plastic deformation of twinned nanopillars in body-centered cubic tungsten, J. Appl. Phys. 121 (2017) 175101, <a href="http://dx.doi.org/10.1063/1.4982754">http://dx.doi.org/10.1063/1.4982754</a></li>
</ul><p>I hope you enjoy reading them!</p>
<p>Best</p>
<p>Shuozhi</p>
</div></div></div>Tue, 03 Sep 2019 20:07:41 +0000Shuozhi Xu23554 at https://imechanica.orghttps://imechanica.org/node/23554#commentshttps://imechanica.org/crss/node/23554Ab initio-informed phase-field modeling of dislocation core structures in equal-molar CoNiRu multi-principal element alloys
https://imechanica.org/node/23513
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/11633">high entropy alloys</a></div><div class="field-item odd"><a href="/taxonomy/term/605">Dislocation</a></div><div class="field-item even"><a href="/taxonomy/term/7824">Ab initio calculations</a></div><div class="field-item odd"><a href="/taxonomy/term/10625">phase field methods</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1088/1361-651X/ab3b62">https://doi.org/10.1088/1361-651X/ab3b62</a></p>
<p>Abstract</p>
<p>In this work, selecting the equal-molar CoNiRu multi-principal element alloy (MPEA) as a model material, we study dislocation core structures starting from first principles. We begin by sifting through all possible configurations to find those corresponding to elastic stability and energetically favored face-centered cubic (fcc) phases and, then, for these configurations, employ a phase field-based model to predict the extent of dislocations lying within them. The main findings are that for the fcc phase, (i) large variations in atomic configuration for the same chemical composition can cause significant changes in the generalized stacking fault energy surface and (ii) the dispersion in defect fault energies are chiefly responsible for substantial variations in the intrinsic stacking fault (ISF) widths of screw and edge dislocations. For instance, positive the ISF energy can vary by 10 times, with the lower values correlated with entirely Ni and Ru atoms and higher values with only Co and Ru atoms across the slip plane. Variations in lattice parameter and stiffness tensor accompany local differences in atomic configuration are also taken into account but shown to play a lesser role. We find that the dislocation can experience profound variations (3–7-fold changes) in its associated ISF width along its line, with the screw dislocation experiencing a greater variation than the edge dislocation (6.02–43.22 Å for the screw dislocation, and 19.6–62.62 Å for the edge dislocation). We envision that the ab initio-informed phase-field modeling method developed here can be readily adapted to MPEAs with other chemical compositions.</p>
</div></div></div>Thu, 15 Aug 2019 22:59:32 +0000Shuozhi Xu23513 at https://imechanica.orghttps://imechanica.org/node/23513#commentshttps://imechanica.org/crss/node/23513Open PhD Position in Atomistic and Multiscale Computational Mechanics at University of Alabama
https://imechanica.org/node/23399
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A fully funded PhD position is available as early as Fall, 2019 in Dr. Ning Zhang’s group at the University of Alabama (Tuscaloosa), the department of Aerospace Engineering and Mechanics. This position is in the general area of mechanics, structural and multifunctional materials with focus on mechanical and thermal properties.</p>
<p>The research topics include:</p>
<p>(1) Hypersonic flight materials;</p>
<p>(2) 2D materials;</p>
<p>(3) Shape memory ceramics/alloys;</p>
<p>(4) High entropy alloys;</p>
<p>(5) Biomaterials.</p>
<p>We are looking for highly self-motivated students with background in mechanical engineering, materials science, engineering mechanics, solid mechanics, civil engineering or related area. PhD students in our group will be involved in first principle calculations (e.g., DFT), molecular dynamics simulation, and phase field modeling.</p>
<p>Interested applicants please send an email to Dr. Ning Zhang <a href="mailto:ningzhang@eng.ua.edu">ningzhang@eng.ua.edu</a> with your resume, cover letter, GRE and TOEFL scores if applicable. This position is available immediately, which can start from Fall 2019, or Spring 2020.</p>
</div></div></div>Fri, 28 Jun 2019 21:15:59 +0000Shuozhi Xu23399 at https://imechanica.orghttps://imechanica.org/node/23399#commentshttps://imechanica.org/crss/node/23399A comparison of different continuum approaches in modeling mixed-type dislocations in Al
https://imechanica.org/node/23393
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/2375">phase-field modeling</a></div><div class="field-item odd"><a href="/taxonomy/term/605">Dislocation</a></div><div class="field-item even"><a href="/taxonomy/term/10954">concurrent atomistic-continuum</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1088/1361-651X/ab2d16">https://doi.org/10.1088/1361-651X/ab2d16</a></p>
<p>Abstract</p>
<p>Mixed-type dislocations are prevalent in metals and play an important role in their plastic deformation. Key characteristics of mixed-type dislocations cannot simply be extrapolated from those of dislocations with pure edge or pure screw characters. However, mixed-type dislocations traditionally received disproportionately less attention in the modeling and simulation community. In this work, we explore core structures of mixed-type dislocations in Al using three continuum approaches, namely, the phase-field dislocation dynamics (PFDD) method, the atomistic phase-field microelasticity (APFM) method, and the concurrent atomistic-continuum (CAC) method. Results are benchmarked against molecular statics. We advance the PFDD and APFM methods in several aspects such that they can better describe the dislocation core structure. In particular, in these two approaches, the gradient energy coefficients for mixed-type dislocations are determined based on those for pure-type ones using a trigonometric interpolation scheme, which is shown to provide better prediction than a linear interpolation scheme. The dependence of the in-slip-plane spatial numerical resolution in PFDD and CAC is also quantified.</p>
</div></div></div>Wed, 26 Jun 2019 23:42:26 +0000Shuozhi Xu23393 at https://imechanica.orghttps://imechanica.org/node/23393#commentshttps://imechanica.org/crss/node/23393Sequential obstacle interactions with dislocations in a planar array
https://imechanica.org/node/23309
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12515">dislocation; obstacle; multiscale modeling</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="https://doi.org/10.1016/j.actamat.2019.05.030">https://doi.org/10.1016/j.actamat.2019.05.030</a></p>
<p>Abstract</p>
<p>The strengthening of metals by nano-scale obstacles is mainly attributed to the impediment to glide dislocations offered by these obstacles. It is important to understand the mechanisms for dislocation bypass of obstacles having nano-scale dimension, including the atomic-scale structure changes sustained by both obstacles and dislocations after the bypass process. Recently, atomic-scale modeling has provided much insight into obstacle interactions involving a single dislocation. However, the more naturally occurring scenarios involving a sequence of encounters with arrays of moving dislocations are not as well understood owing to prohibitively large length scale requirements for atomistic models. In this study, we utilize a novel multiscale concurrent atomistic-continuum method to simulate a sequence of interactions between glide dislocations in an array with a spherical nano-obstacle (either a void or an impenetrable precipitate) in Al. In the case of a void, the bypassing array of dislocations progressively weakens the void until it splits the originally spherical void into two hemispheres. We present an analytical model for the depinning stress for the first dislocation in the array. In the case of a large impenetrable precipitate, sequential dislocations in the array bypass via alternating mechanisms of Orowan looping and Hirsch looping. The residual dislocation loop created around the precipitate by the bypass of the first dislocation is completely removed by the passage of the subsequent dislocation. These mechanisms can benefit the design of materials that are reinforced with nanophase inhomogeneities to achieve ultra-high strength.</p>
</div></div></div>Tue, 21 May 2019 17:00:12 +0000Shuozhi Xu23309 at https://imechanica.orghttps://imechanica.org/node/23309#commentshttps://imechanica.org/crss/node/23309A Ph.D. student position in the research area of atomistic and multiscale simulation modeling at the Louisiana Tech University
https://imechanica.org/node/23211
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12212">funded PhD position</a></div><div class="field-item odd"><a href="/taxonomy/term/3762">atomistic modeling</a></div><div class="field-item even"><a href="/taxonomy/term/1910">multiscale modeling</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>The College of Engineering and Science at Louisiana Tech University is seeking self-motivated candidates for a Graduate Research Assistant (Ph.D. student) position, with the earliest starting date in Fall 2019.</p>
<p>Successful candidates will join the research group of Dr. Xiang (Shawn) Chen (<a href="https://www.latech.edu/faculty-staff/single-entry/name/xiang-shawn-chen-1/">https://www.latech.edu/faculty-staff/single-entry/name/xiang-shawn-chen-1</a><span>), who is in the research area of atomistic and multiscale simulation/modeling. A list of related publications can be found below: </span><a href="https://scholar.google.com/citations?user=qdz0cy4AAAAJ&hl=en">https://scholar.google.com/citations?user=qdz0cy4AAAAJ&hl=en</a></p>
<p>Job responsibilities: Use atomistic and multiscale simulation / modeling to study mechanical and thermal properties of functional materials. Job duties may involve methodology development (theoretical / computational) and applications.</p>
<p>Requirements: B.S. or M.S. Degree in Mechanical Engineering, Materials Science/Engineering, Physics, or other related disciplines. In particular, self-motivated individuals who have research experience in one or more of the following areas are strongly encouraged to apply:</p>
<ul><li>Finite Element Modeling, Atomistic Simulation</li>
<li>Continuum Mechanics</li>
<li>Solid State Physics</li>
<li>Crystal Plasticity, Dislocation Dynamics, Thermal Transport</li>
<li>Multiscale Modeling of Materials</li>
<li>LAMMPS, DL_POLY, GULP</li>
<li>FORTRAN, C++, OpenMPI</li>
</ul><p>To apply: Please email Dr. Xiang (Shawn) Chen (<a href="mailto:shawnxc@latech.edu">shawnxc@latech.edu</a>) with a detailed CV, including your GPA, scores of TOEFL (or IELTS) and GRE, if applicable and available.</p>
</div></div></div>Mon, 01 Apr 2019 18:37:14 +0000Shuozhi Xu23211 at https://imechanica.orghttps://imechanica.org/node/23211#commentshttps://imechanica.org/crss/node/23211Phase-field-based calculations of the disregistry fields of static extended dislocations in FCC metals
https://imechanica.org/node/23124
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/605">Dislocation</a></div><div class="field-item odd"><a href="/taxonomy/term/2375">phase-field modeling</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><a href="http://dx.doi.org/10.1080/14786435.2019.1582850">http://dx.doi.org/10.1080/14786435.2019.1582850</a></p>
<p>Abstract</p>
<p>In the continuum context, the displacements of atoms induced by a dislocation can be approximated by a continuum disregistry field. In this work, two phase-field (PF)-based approaches and their variants are employed to calculate the disregistry fields of static, extended dislocations of pure edge and pure screw character in two face-centred cubic metals: Au and Al, which have distinct stable stacking fault energy and elastic anisotropy. A new truncated Fourier series form is developed to approximate the generalised stacking fault energy (GSFE) surface, which shows significant improvement over the previously employed Fourier series form. By measuring the intrinsic stacking fault (ISF) width and partial dislocation core size in different ways, the PF-based disregistry fields are quantitatively compared against those predicted by molecular statics. In particular, two new measures for the ISF widths are proposed and shown to overcome drawbacks of the more commonly used standards in the literature. Our calculations also show that continuum formulation of the elastic energy and the GSFE for a homogeneous surface can successfully characterise the core structure. Last, our comparisons highlight the significance of including the gradient energy in the free energy formulation when an accurate description of the dislocation core structure is desired.</p>
<p> </p>
</div></div></div>Thu, 28 Feb 2019 01:01:35 +0000Shuozhi Xu23124 at https://imechanica.orghttps://imechanica.org/node/23124#commentshttps://imechanica.org/crss/node/23124Nanoindentation and nanoscratching at finite temperatures: Three reviews
https://imechanica.org/node/22707
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12232">nanoidentation</a></div><div class="field-item odd"><a href="/taxonomy/term/12233">nanoscratching</a></div><div class="field-item even"><a href="/taxonomy/term/2543">review paper</a></div><div class="field-item odd"><a href="/taxonomy/term/12234">finite temperature</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleague,</p>
<p><a href="https://scholar.google.com/citations?user=vqOdWCcAAAAJ&hl=en">Dr. Saeed Zare Chavoshi</a> and I have co-authored three review articles, concerning nanoindentation and nanoscratching at finite temperatures from the computational and experimental perspectives:</p>
<ul><li>Saeed Zare Chavoshi, Shuozhi Xu, Nanoindentation/scratching at finite temperatures: Insights from atomistic-based modelling, Prog. Mater. Sci. 100 (2019) 1--20, <a href="http://dx.doi.org/10.1016/j.pmatsci.2018.09.002">http://dx.doi.org/10.1016/j.pmatsci.2018.09.002</a></li>
<li>Saeed Zare Chavoshi, Shuozhi Xu, A review on micro- and nanoscratching<span>/tribology at high temperatures: Instrumentation and experimentation, J. Mater. Eng. Perform. 27 (2018) 3844--3858, <a href="http://dx.doi.org/10.1007/s11665-018-3493-5">http://dx.doi.org/10.1007/s11665-018-3493-5</a></span></li>
<li>Saeed Zare Chavoshi, Shuozhi Xu, Temperature-dependent nanoindentation response of materials, MRS Comm. 8 (2018) 15--28, <a href="http://dx.doi.org/10.1557/mrc.2018.19">http://dx.doi.org/10.1557/mrc.2018.19</a></li>
</ul><p>We hope you enjoy reading them. Please let us know if you have any questions.</p>
<p>Best</p>
<p>Shuozhi</p>
</div></div></div>Tue, 02 Oct 2018 15:39:48 +0000Shuozhi Xu22707 at https://imechanica.orghttps://imechanica.org/node/22707#commentshttps://imechanica.org/crss/node/22707A Ph.D. student position in the research area of atomistic and multiscale simulation modeling at the Louisiana Tech University
https://imechanica.org/node/22679
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12212">funded PhD position</a></div><div class="field-item odd"><a href="/taxonomy/term/3762">atomistic modeling</a></div><div class="field-item even"><a href="/taxonomy/term/1910">multiscale modeling</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>The College of Engineering and Science at Louisiana Tech University is seeking self-motivated candidates for a Graduate Research Assistant (Ph.D. student) position, with the earliest starting date in Winter 2018.</p>
<p>Successful candidates will join the research group of Dr. Xiang (Shawn) Chen, who is in the research area of atomistic and multiscale simulation/modeling. A list of related publications can be found below: <a href="https://scholar.google.com/citations?user=qdz0cy4AAAAJ&hl=en">https://scholar.google.com/citations?user=qdz0cy4AAAAJ&hl=en</a></p>
<p>Job responsibilities: Use atomistic and multiscale simulation / modeling to study mechanical and thermal properties of functional materials. Job duties may involve methodology development (theoretical / computational) and applications.</p>
<p>Requirements: B.S. or M.S. Degree in Mechanical Engineering, Materials Science/Engineering, Physics, or other related disciplines. In particular, self-motivated individuals who have research experience in one or more of the following areas are strongly encouraged to apply:</p>
<ul><li>Finite Element Modeling, Atomistic Simulation</li>
<li>Continuum Mechanics</li>
<li>Solid State Physics</li>
<li>Crystal Plasticity, Dislocation Dynamics, Thermal Transport</li>
<li>Multiscale Modeling of Materials</li>
<li>LAMMPS, DL_POLY, GULP</li>
<li>FORTRAN, C++, OpenMPI</li>
</ul><p>To apply: Please email Dr. Xiang (Shawn) Chen (<a href="mailto:shawnxc@latech.edu">shawnxc@latech.edu</a>) with a detailed CV, including your GPA, scores of TOEFL (or IELTS) and GRE, if applicable and available.</p>
</div></div></div>Mon, 24 Sep 2018 16:56:00 +0000Shuozhi Xu22679 at https://imechanica.orghttps://imechanica.org/node/22679#commentshttps://imechanica.org/crss/node/22679Error | iMechanica