Unravelling the jerky glide of dislocations in body-centred cubic crystals.

In situ straining tests in high purity α-Fe thin foils at low temperatures1 have demonstrated that crystal line defects, called dislocations, have a jerky type of motion made of intermittent long jumps of several nanometres. This observation conflicts with the standard Peierls mechanism for plastic deformation in body-centred cubic crystals, where the screw dislocation jumps are limited by inter-reticular distances, that is, distances of a few angstroms. Employing atomic-scale simulations, we show that although the short jumps are initially more favourable, their realization requires the propagation of a kinked profile along the dislocation line, which yields coherent atomic vibrations acting as travelling thermal spikes. Such local heat bursts favour the thermally assisted nucleation of new kinks in the wake of primary ones. The accumulation of new kinks leads to long dislocation jumps like those observed experimentally. Our study constitutes an important step towards predictive atomic-scale theory for materials deformation.

Modeling thermoreflectance in Au and Ni from molecular dynamics.

Experimental thermoreflectance measurements using femto-second laser irradiation (Hopkinset al2011J. Heat Transfer133044505) can be used to shed light on the electron-phonon coupling in metals through a selective excitation of electrons. In these experiments the energy transfer occurs at a time scale of pico-seconds which corresponds to the typical time scale of molecular dynamics (MD) simulations. However since the electron-phonon coupling is, generally, not taken into account in MD simulations, it is in principle not possible to model thermoreflectance as well as other properties related to electron-phonon coupling such as electric conductivity and thermal transport. Here we show that it is however possible to extend MD using a method proposed by Finnis, Agnew and Foreman (FAF) (Finniset al1991Phys. Rev.B44567-74), originally implemented in order to account for electronic stopping power in particle irradiation. Although the FAF method was devoted to model high energy atomic displacements yielding local melt of the crystal, we have been able to reproduce pulsed-laser irradiation experiments at room temperature. Our computations were realized in both Au and Ni to exemplify the transferability of our results. The agreement between the calculations and the experimental results allowed us to discuss different theories for computing the amplitude of electron-phonon coupling and to select the more appropriate according to FAF. Our work paves the way to re-introduce the phenomenology of electric conductivity in MD simulations for metals.

Quantum de-trapping and transport of heavy defects in tungsten.

The diffusion of defects in crystalline materials1 controls macroscopic behaviour of a wide range of processes, including alloying, precipitation, phase transformation and creep2. In real materials, intrinsic defects are unavoidably bound to static trapping centres such as impurity atoms, meaning that their diffusion is dominated by de-trapping processes. It is generally believed that de-trapping occurs only by thermal activation. Here, we report the direct observation of the quantum de-trapping of defects below around one-third of the Debye temperature. We successfully monitored the de-trapping and migration of self-interstitial atom clusters, strongly trapped by impurity atoms in tungsten, by triggering de-trapping out of equilibrium at cryogenic temperatures, using high-energy electron irradiation and in situ transmission electron microscopy. The quantum-assisted de-trapping leads to low-temperature diffusion rates orders of magnitude higher than a naive classical estimate suggests. Our analysis shows that this phenomenon is generic to any crystalline material.

Quantum effect on thermally activated glide of dislocations.

Crystal plasticity involves the motion of dislocations under stress. So far, atomistic simulations of this process have predicted Peierls stresses, the stress needed to overcome the crystal resistance in the absence of thermal fluctuations, of more than twice the experimental values, a discrepancy best-known in body-centred cubic crystals. Here we show that a large contribution arises from the crystal zero-point vibrations, which ease dislocation motion below typically half the Debye temperature. Using Wigner's quantum transition state theory in atomistic models of crystals, we found a large decrease of the kink-pair formation enthalpy due to the quantization of the crystal vibrational modes. Consequently, the flow stress predicted by Orowan's law is strongly reduced when compared with its classical approximation and in much closer agreement with experiments. This work advocates that quantum mechanics should be accounted for in simulations of materials and not only at very low temperatures or in light-atom systems.

Ordering of atomic monolayers on a (001) cubic crystal surface.

The self-organization of a chemisorbed monolayer is studied as a two-dimensional ordering process in the presence of surface stress. As proved previously for a single phase separation, a steady surface state is yielded from the competition between the domain boundary energy and the surface stress elastic energy. In the present Letter, the resulting patterns are shown to depend on the interplay between the symmetries of both the internal layer order and the underlying crystal. For experimental relevance, our study is focused on a (001) copper surface.