P. M. Derlet

Atomistic simulations as guidance to experiments

Abstract Atomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modeling technique and a detailed structural characterisation of the simulated samples.

Length scale effects in the simulation of deformation properties of nanocrystalline metals

The observed inverse grain size strain rate dependency in Al columnar structures [Acta Mater 49 (2001) 2713] can be explained by a geometrical argument arising when a dislocation traverses the entire grain. The paper addresses also the differences in dislocation activity in a 2D-columnar and a full 3D-nanostructured geometry.

Atomistic simulation of dislocation emission in nanosized grain boundaries

The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB) in nanocrystalline metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free-volume migration. As in previous work, the nucleation occurs at a GBD, which, upon nucleation and propagation, is removed. In the present case, free-volume migration occurs away from the nucleation region both before and after the nucleation event.

Grown-in twin boundaries affecting deformation mechanisms in nc-metals

Molecular dynamics simulations have recently shown that the presence of grown-in twin boundaries in nc-Al promotes slip activity in the form of twin boundary migration. In this letter we investigate the effect of grown-in twin boundaries on the plastic deformation mechanism in nc-Ni and Cu, and show that (1) for these particular fcc metals twin boundary migration is not the favored deformation mechanism and (2) that the Schmid factors of the grown-in twin plane play a correspondingly important role. The results are explained in terms of the different ratios of the extrema of the generalized planar fault curves.

Radiation damage near grain boundaries

Large-scale molecular dynamics of cascade production of the primary damage state are performed in nanocrystalline nickel with an average grain diameter of 12 nm and primary knock-on atom kinetic energies ranging from 5 to 30 keV. The role of the grain boundary during the cascade production of irradiated NC Ni is discussed in terms of grain-boundary structure. It is shown that regions of misfit in the grain boundaries can absorb self-interstitials and that stacking-fault tetrahedra are formed in the neighbourhood of the grain boundary.

The role played by two parallel free surfaces in the deformation mechanism of nanocrystalline metals: a molecular dynamics simulation

Abstract We report molecular dynamics simulations which investigate the influence of two parallel free surfaces in the deformation mechanisms of model nanocrystalline fee materials. The purpose of this simulation is to study which phenomena observed in in-situ tensile experiments performed in the electron microscope can be expected to be intrinsic properties of the deformation process and which phenomena result from the thin-film geometry necessary to obtain optimal (high-resolution) transmission electron microscopy contrast and resolution. It is found that, in nanocrystalline samples with thin-film thickness comparable with that used in experiment, the extent of the surfaces influence is of the order of the grain size. For a sample with a mean grain size of 5 nm, this is evidenced by an increase in intergranular deformation (sliding) at the surface and, for a sample with a mean grain size of 12nm, in addition to increased sliding at the surface, there is a significant increase in dislocation activity that extends throughout the sample.

Following peak profiles during elastic and plastic deformation : A synchrotron-based technique

Understanding the elastic and plastic deformation properties of nanostructured metals requires the development of in situ testing methods that can follow the footprints of the deformation mechanism(s) during mechanical testing. Here we present an in situ synchrotron x-ray-diffraction technique which allows the measurement of diffraction profiles continuously during mechanical testing, providing an in situ peak profile analysis capability. The in situ approach is achieved thanks to the development of a microstrip detector allowing the instantaneous measurement of the diffraction pattern over a 2θ range of 60°. This in situ technique allows for the first time a comparison of the footprints of the plastic deformation mechanism during loading and after unloading. The measurements are performed on several types of freestanding dog bones, covering sample thicknesses down to the submicron range.

Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass

At the transition from a static to a dynamic deformation regime of a shear band in bulk metallic glasses, stress transients in terms of overshoots are observed. We interpret this phenomenon with a repeated shear-melting transition and are able to access a characteristic time for a liquidlike to solidlike transition in the shear band as a function of temperature, enabling us to understand why shear bands arrest during inhomogenous serrated flow in bulk metallic glasses.

Thermodynamic phase transitions in a frustrated magnetic metamaterial.

Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we create a thermally active artificial kagome spin ice that is made up of a large array of dipolar interacting nanomagnets and undergoes phase transitions predicted by microscopic theory. We use low energy muon spectroscopy to probe the dynamic behaviour of the interacting nanomagnets and observe peaks in the muon relaxation rate that can be identified with the critical temperatures of the predicted phase transitions. This provides experimental evidence that a frustrated magnetic metamaterial can be engineered to admit thermodynamic phases.

Temperature-dependent residual broadening of x-ray diffraction spectra in nanocrystalline plasticity

In situ x-ray diffraction peak profile analysis at room temperature has shown that peak broadening during plastic deformation is reversible upon unloading for nanocrystalline metals, demonstrating the lack of a developing permanent dislocation network. In this letter, we show that the peak broadening is not reversible when similar load-unload cycles are performed at 180 K. However, by then warming the sample to 300 K, peak broadening recovers to a great extent and all subsequent plastic deformation load∕unload cycles are characterized again by a reversible peak broadening. The temperature-dependent residual peak broadening provides explicit evidence of a thermal component in the nanocrystalline deformation mechanism.