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Post-Doc/PhD in mechanics of grain boundaries at Max-Planck-Institut für Eisenforschung, Düsseldorf

MPI für Eisenforschung, Düsseldorf, Department for Microstructure Physics and Alloy Design
Post-Doc or PhD Position: Characterizing and modeling the mechanics of crystalline interfaces

Requirements
A solid background in physical metallurgy, polycrystal mechanics,
dislocation plasticity, continuum mechanics and proficiency in English
are required. Prior experience in microstructural characterization such
as EBSD or AFM is beneficial. Programming skills (Fortran, Python,
Matlab) and experience in numerical simulation and finite element
modeling are strongly appreciated.

Background
The strengthening effect of grain boundaries is one of the key components in the development of modern structural materials.  Despite intense research, the precise nature of the often beneficial effects of grain boundaries have not been understood to a level which would allow for theory guided optimization of microstructures and accelerated alloy development. The current project aims to combine, improve, and apply recently developed approaches to investigate and quantify the micromechanical behavior of grain boundaries. Linking the topographic information around indents with state of the art simulation methods, a sound understanding of the interplay between grain boundaries and heterogeneous plasticity in titanium polycrystals is sought.
The goals of this research are: (1) Carry out indentation within the interiors of large grains of alpha-titanium to effectively collect single crystal data coupled with extensive (three-dimensional) characterization of the resulting plastic defect fields surrounding the indents.  By correlating with models of the indentation, a precise constitutive description of the anisotropic plasticity of single-crystalline titanium shall be developed.  (2) Extension of this methodology to indentations close to grain boundaries, i.e. quasi bi-crystal deformation. (3) Comparison of the measured characteristics of indentations at grain boundaries to simulated indentations as predicted by a constitutive model calibrated using the single crystal indentations. (4) Based on this qualitative understanding, a grain boundary transmissivity description will be developed, implemented into a non-local crystal plasticity formulation, and validated against the collected indent characteristics. 

Job Opening
Funding for this position is immediately available through the DFG-NSF
Materials World Network for a period of three years (2012-2015). The
work will be carried out in close cooperation with our partners at
Michigan State University. We are an equal opportunity employer.
Applications will be reviewed in the order that they are received until
the position is filled.

Please send your application as a single pdf file or by mail to
Max-Planck-Institut für Eisenforschung
Dr.-Ing. Claudio Zambaldi   c.zambaldi (at) mpie.de
Abt. Mikrostrukturphysik und Legierungsdesign
Max-Planck-Str. 1
40237 Düsseldorf

References
C. Zambaldi, Y. Yang, T.R. Bieler, D. Raabe (2012) JMR, 27(1), 356-367, doi: 10.1557/jmr.2011.334
Y. Yang, L. Wang, C. Zambaldi, P. Eisenlohr, R. Barabash, W. Liu, M.R. Stoudt, M.A. Crimp, T.R. Bieler (2011) JOM, 63(9), 66-73, doi: 10.1007/s11837-011-0161-8

Comments

A three-year Ph.D. scholarship is available at the Department of Mechanical Engineering, Solid Mechanics Section, from July 1, 2012.

Recent lattice rotation measurements performed on non-uniformly deformed single crystals of Aluminium, Nickel and Tantalum have given detailed information about the development of dislocation structures as a consequence of deformation. Geometrically necessary dislocations are known to give rise to size-effects in materials, according to the general rule that smaller is stronger. This Ph.D. study aims at developing material models that describe both the complex microstructure and the size-effect in single crystals upon deformation. Critical experiments, based on electron backscatter diffraction, are to be carried out in collaboration with a strong research group at Columbia University in New York, USA. Hence, a period of about 6 months must be expected for a research visit abroad.

The Solid Mechanics Section at the Department of Mechanical Engineering, has a long tradition of front edge research within the area of materials mechanics, and we have close collaborations with other strong groups both in Denmark and abroad.

We offer a stimulating environment, focused on research at an international level. The Ph.D. Student will become part of a research group at DTU funded by the Danish Council for Independent Research under the project 'Higher Order Theories in Solid Mechanics'.

Qualifications
The successful candidate must have (or soon obtain) a master's degree in mechanical engineering, with a strong background in solid mechanics and material mechanics.

Approval and Enrolment
The scholarships for the PhD degree are subject to academic approval, and the candidates will be enrolled in one of the general degree programmes of DTU. For information about the general requirements for enrolment and the general planning of the scholarship studies, please see the DTU PhD Guide.

Salary and appointment terms
The salary and appointment terms are consistent with the current rules for PhD degree students. The period of employment is 3 years.

Further information
Further information is available at www.mek.dtu.dk, or by contacting Associate Professor Christian Niordson, cn@mek.dtu.dk, +45 4525 4287.

Application
We must have your online application by 13 April 2012. Please open the link "apply for this job online", fill in the application form and attach the following documents;

  • A letter motivating the application (cover letter)
  • Curriculum vitae
  • Grade transcripts and BSc/MSc diploma
  • Excel sheet with translation of grades to the Danish grading system (see guidelines and excel spreadsheet here)

Candidates may apply prior to ob­tai­ning their master's degree, but cannot begin before having received it.

All interested candidates irrespective of age, gender, race, disability, religion or ethnic background are encouraged to apply.

DTU Mechanical Engineering covers the fundamental engineering disciplines within Naval Architecture and Offshore Engineering, Energetics, Solid Mechanics, Fluid Mechanics, Thermodynamics, Control and Engineering Design, Hydrodynamics, Structural Engineering and Materials. It has a scientific staff of about 100 persons, 85 PhD students and a technical/administrative support staff of 75 persons.

 

Apply here

Sujet de thèse proposé dans le cadre du projet ANR Fluti
Équipe Comportement et Calcul des Structures, Centre des Matériaux, Mines ParisTech,
Experimental and numerical analysis of sustained load cracking in Titanium
· Partnership : LMS Ecole Polytechnique, ICMPE
· Duration : Dec. 2012 to Dec. 2015
· Key-words : Titanium, toughness, sustained load cracking, hydrogen and oxygen influence,
dynamic strain aging, numerical crack propagation
· Funding : ANR (French National Research Agency)
Abstract :
The Centre des Matériaux located in Evry (35km south of Paris) is a laboratory associated with the
CNRS, employing around 181 people including 38 researchers, 38 technicians, 82 PhD students and
10 Post-Doctoral researchers. Research concerns materials processing and surface modification, the
microstructural characterization and experimental study of the behaviour of materials. These studies
are carried out in close contractual collaboration with industrial partners.
The influence of H and O on toughness and sustained load cracking (SLC) will be characterized. The
influence of these impurities on the visco-plastic behaviour (yield stress, type and degree of
hardening, strain rate sensitivity, activation volume) will also be investigated. Constitutive equations
describing dynamic strain aging in relation with H and O contents and taking anisotropy into account
will be formulated and identified. They will be used in finite element simulations of SLC taking into
account H concentration at the crack tip and the impact of H on the local behaviour and damage
process.
Main Objective:
Titanium alloys are widely used for aircraft or rocket engine manufacturing due to their high strengthto-
weight ratio. They are also appreciated for steam turbines, naval and offshore applications as well
as geothermal brine wells thanks to their resistance to corrosion. In the future, they could also be used
for the first wall, blanket and magnetic coil structures of fusion reactors, for their good resistance to
radiation damage and fast rate of residual radioactivity decay.
However, these alloys are prone to room-temperature creep and several associated phenomena
which can be detrimental to structural integrity, such as creep-fatigue synergy (the “cold dwell effect”
responsible for several in-service failures of aircraft engines during the past decades), sustained load
cracking (mainly due to creep at the crack tip at low H content and to titanium hydrides precipitation
and fracture at high H content) and absence of threshold for fatigue crack growth at high Kmax (the
“Marci effect”, also associated with creep at the crack tip).
Titanium alloys always contain some residual solute impurities -among which oxygen- known to
increase their yield stress through a reduction in the mobility of screw dislocations and hydrogen,
which, in some circumstances, has an opposite effect (it triggers dislocations glide and softens the
material) while in some other loading conditions, it also hardens the alloy. In the present practice, the
H content is kept below an upper limit (around 150 weight ppm for alloys used in aeronautics) so as to
avoid titanium hydrides precipitation, but no attempt to adjust the H content for optimum mechanical
properties was made.
However, the amount of retained hydrogen in the range 0 to 150ppm has a large – but yet
unexplained- influence on toughness, room-temperature creep (either enhanced or inhibited by H,
depending on the alloy) as well as on the resistance to creep-fatigue, fatigue cracking at high Kmax and
sustained load cracking. When solute H enhances cold creep, sustained load cracking is triggered,
while in the conditions for which it slows down creep, it also improves the resistance to SLC. There is
an evident but yet unexplained correlation between the hydrogen content, the viscoplastic behaviour
and the resistance to SLC. The aim of this project is to make the underlying mechanisms clear, so as
to determine the optimum hydrogen content (as a function of the O content) to minimize cold creep,
sustained load cracking and their consequences.
Task 1 – Experiments:
Tension and torsion tests at various loading rates or with loading rate jumps, as well as creep, loadingunloading-
reloading and relaxation tests will be performed at LMS on each of the material batches, so
as to investigate the anisotropy in the mechanical behaviour.
Toughness and sustained load cracking tests followed by fractographic observations will be performed
at CdM on CT specimens so as to determine the evolution in KIC and the threshold stress intensity
factor for SLC with the impurity content and detect possible modifications in fracture mechanisms
(ductile voids, interfacial fracture or facetted fracture surfaces for example).
Task 2 – Models:
Macroscopic constitutive visco-plastic equations including the influence of H and O on DSA will be
identified at CdM from experimental stress-strain curves for α Ti. A macroscopic model accounting for
dynamic strain aging (DSA) will be improved including H and/or O concentrations. The constitutive
equations include ageing terms in the yield function and in isotropic and kinematic hardenings. The
creep arrest and the relaxation response of materials should then be accurately reproduced.
Task 3 – Simulations:
Finite element simulations of SLC in α titanium for various impurity content will be developed at CdM ,
assuming that the H distribution around the crack tip (in the interstitial lattice sites and trap sites) is in
equilibrium with the local tress and plastic strain distributions (which seems reasonable for sustained
load, considering the high diffusivity of hydrogen in titanium). The constitutive viscoplastic equations
depending on the impurity level determined previously will be used in these simulations.
Firstly, a crack in a material showing DSA will be simulated in order to observe the strain localisation
phenomena around the crack tip. In a second time, the diffusion of H in the calculated stress gradient
will be estimated from a coupled mechanics-diffusion simulation. The H distribution will then be
evaluated for each crack length. Finally, simulations of SLC will be performed including the obtained
field of H concentration and adding cohesive zones in order to simulate the crack propagation in such
conditions.
References:
[1] D.N. Williams, Effects of hydrogen in Ti alloy on subcritical crack growth under sustained load, Mat.
Sci. Eng. 24, 1976, 53-63
[2] F. Mignot, V. Doquet, C. Sarrazin-Baudoux, Micromechanical investigation of the abnormal cracking
of Ti6246 at high mean stress, , Journal of Mechanical Behaviour of Materials, 16 n°3, 2005, pp195-
210
[3] S. Graff, S. Forest, JL. Strudel, C. Prioul, P. Pilvin, JL. Béchade, Strain localization phenomena
associated with static and dynamic strain aging in notched specimens : experiments and finite element
simulations, Mat. Sci. & Eng. A 387-389 (2004) 181-185
[4] S. Graff, S. Forest, JL. Strudel, C. Prioul, P. Pilvin, JL. Béchade, Finite element simulations of
dynamic strain aging effects at V-notches and crack tips, Scripta Mat. 52 (2005) 1181-1186
To apply:
The candidate would ideally own a master related to Material Science or Numerical Simulation. A good
background in Finite Elements theory and practice is required. The candidate must be proficient in
English, shall talk C++ and preferably be used at working with a UNIX environment.
Applicants should supply the following :
· a detailed Cv
· a covering letter explaining the applicant’s motivation for the position
· detailed exam results
· references
which must to arrive at Centre des Matériaux de l’Ecole des Mines de Paris, B.P. 87 – 91003 EVRY
CEDEX, for the attention of recruitment department, and/or by e-mail : recrutement@mat.ensmp.fr

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