Plasticity

Professor Jose E. Andrade from Northwestern University is the
recipient of the 2006 Zienkiewicz medal awarded biennially by the
Institution of Civil Engineers from London. The award goes to Andrade
for his contribution entitled ' Capturing strain localization in dense
sands with random density' in IJNME 2006; 67:1531-1564 DOI:
10.1002/nme.1673 (link: http://www3.interscience.wiley.com/cgi-bin/fulltext/112519037/PDFSTART)

Our sincerest congratulations to Prof. Andrade!

Our paper on the Mechanical threshold stress (MTS) model for 6061-T6 aluminum has been accepted by JoMMS.  There are several things of interest in the paper:

1) The use of a phonon drag model to predict the sharp increase in flow stress at strain rates above 10,000 /s.  This behavior is seen in a  number of materials and is hard to fit using standard power law plasticity models.  Our model does a good job in this regard.

I have attached a copy of my McMat07 talk below.  The talk is about a modified mechanical threshold stress (Follansbee-Kocks) model that can be used for strain rates > 1000 /s and high homologous temperatures.  In the model, the pressure dependence of the flow stress arises from the pressure dependence of the shear modulus.  Even after using the most accurate models for the shear modulus (and the melting temperature) we find that the pressure sensitivity of the flow stress is underestimated by our model (by quite a bit).

Recently, there have been many strain gradient theories that are used for the interpretation of size effect at the micron and submicron length scales. The basic idea of these theories is the introduction of a first, or second (or both) gradients of strain or any internal state variable in the governing equations of classical theories.

Huang et al., PRL 98, 185501 (2007)

Watch movies at: http://netserver.aip.org/cgi-bin/epaps?ID=E-PRLTAO-98-002719

We report exceptional ductile behavior in individual double-walled and triple-walled carbon nanotubes at temperatures above 2000 C, with tensile elongation of 190% and diameter reduction of 90%, during in situ tensile-loading experiments conducted inside a high-resolution transmission electron microscope. Concurrent atomic-scale microstructure observations reveal that the superelongation is attributed to a high temperature creep deformation mechanism mediated by atom or vacancy diffusion, dislocation climb, and kink motion at high temperatures. The superelongation in double-walled and triple-walled carbon nanotubes, the creep deformation mechanism, and dislocation climb in carbon nanotubes are reported here for the first time.

Recently Henry talked about software that could be used to simulate explosions and introduced CartaBlanca. Luming asked whether anyone had used the software, how good it was, and whether one needed Java to implement models into CartaBlanca.

A couple of years ago a colleague who wanted to simulate high-speed machining asked me: " Which is the best phenomenological flow stress model for metals?" I wasn't able to give an answer right away and decided to look in the literature.

What I found was, every ten years or so, a new model appears in the literature that tries to solve some of the problems of older models. However, a clear ranking of models has not been established yet.

The SES 2007 Conference, Oct. 21-24, 2007, Texas A&M University campus in College Station, Texas, home to the George Bush Presidential Library and Museum.

 Symposium: Plasticity and Damage Size Effects at the Micron and Nano Length Scales

I would like to share some of our more recent findings on nano-pillar compression, namely the role of the surface treatment in plastic deformation at the nano-scale. Recent advances in the 2-beam focused ion beams technology (FIB) have enabled researchers to not only perform high-precision nanolithography and micro-machining, but also to apply these novel fabrication techniques to investigating a broad range of materials' properties at the sub-micron and nano-scales. In our work, the FIB is utilized in manufacturing of sub-micron cylinders, or nano-pillars, as well as of TEM cross-sections to directly investigate plasticity of metals at these small length scales. Single crystal nano-pillars, ranging in diameter between 300 nm and 870 nm, were fabricated in the FIB from epitaxial gold films on MgO substrates and subsequently compressed using a Nanoindenter fitted with a custom-fabricated diamond flat punch. We show convincingly that flow stresses strongly depend on the sample size, as some of our smaller specimens were found to plastically deform in uniaxial compression at stresses as high as 600 MPa, a value ~25 times higher than for bulk gold. We believe that these high strengths are hardened by dislocation starvation. In this mechanism, once the sample is small enough, the mobile dislocations have a higher probability of annihilating at a nearby free surface than of multiplying and being pinned by other dislocations. Contrary to this, if the dislocations are trapped inside the specimen by a coating, the strengthening mechanism is expected to be different. Here we present for the first time the comparison of plastic deformation of passivated and unpassivated single crystal specimens at the sub-micron scale. The role of free surfaces is investigated by comparing stress results of both as-FIB'd, annealed, and alumina-passivated pillars. Preliminary results show that ALD-coated pillars exhibit much higher flow stresses at equivalent sizes and strains compared with the uncoated samples. We also found that while FIB damage during pillar fabrication might account for a small portion of the strength increase, it is not the major contributor.