Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.

We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain boundary sliding are supplanted by amorphization in a highly localized nanoscale band, which allows for massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are lack of dislocation slip systems, low thermal conduction, and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting-point elements: specifically, we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.

We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation.

It is now possible to create perfect crystal nanowires of many metals. The deformation of such objects requires a good understanding of the processes involved in plasticity at the nanoscale. Isotropic compression of such nanometre scale micropillars is a good model system to understand the plasticity. Here we investigate these phenomena using Molecular Dynamics (MD) simulations of nanometre scale single crystal BCC iron pillars in compression.We find that pillars with large length to width ratio may buckle under high strain rates. The type of buckling behaviour depends sensitively on the boundary conditions used: periodic boundary conditions allow for rotation at top and bottom of the pillar, and result in an S shaped buckle, by contrast fixed boundaries enforce a C shape. Pillars with a length to width ratio closer to that used in experimental micropillar compression studies show deformation behaviour dominated by slip, in agreement with the experiments. For micropillars oriented along <100>, slip occurs on <110> planes and localized slip bands are formed. Pillars of this size experience higher stresses than bulk materials before yielding takes place. One might expect that this may be in part due to the lack of nucleation sites needed to induce slip. However, further simulations with possible dislocation sources: a shorter iron pillar containing a spherical grain boundary, and a similar pillar containing jagged edges did not show a decreased yield strength.

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