Dislocation Driven Problems in Atomistic Modelling of Materials

Abstract

Understanding the mechanical properties of technologically advanced materials from quantum mechanical predictions based on electronic structure calculations remains one of the most challenging problems in modern computational materials science. In this paper, we illustrate this challenge from our current investigations on dislocation behaviour in bcc transition metals that are promising candidates for materials subject to fast neutron irradiations in future fusion power plants. Starting with the relationship between the brittleness and the negative Cauchy pressure of elastic constants in materials within the so-called Harris-Foulkes approximation to the density functional theory (DFT), we briefly discuss the importance of the generic form of interatomic potentials in order to reproduce a correct Cauchy pressure. The latter in turn plays an important role in predicting dislocation properties in fcc iridium and therefore allows us to explain experimental observation of the intrinsic brittleness of this material. We then investigate the behaviour of the (1/2)[111] screw dislocation that controls plastic deformation in bcc metals from atomistic simulation. Here we show the atomic phenomena associated with the non-planar core structure of dislocations in bcc iron from the Stoner tight-binding bond model. The crucial point comes from the accurate evaluation of forces implemented within the charge neutrality conditions in the treatment of the spin-polarized dependence in the electronic structure calculations. In agreement with DFT studies, the magnetic bond-order potentials predict a non-degenerate core structure for screw dislocations in Fe. Finally, a new analytic expression has been derived for the migration energy barrier for the one-dimensional (1D) motion of crowdions, which are the most stable self-interstitial atom (SIA) defects predicted by our DFT calculations. Importantly, the latter study is strongly supported by the recent observation of 1D diffusion of nanometer-sized dislocation loops, observed very recently under in situ electron microscope irradiation for bcc transition metals.

Ab initio study of AlxMoNbTiV high-entropy alloys

“…As shown in figure 2, the Al x MoNbTiV HEAs have strong a metallic character due to the large positive (c 12 − c 44 ). We also observe that increasing Al content makes the value of the Cauchy pressure decrease [34] suggesting that Al reduces the metallic character of the bonding in Al x MoNbTiV HEAs.…”

Effect of Ta addition on the structural, thermodynamic and mechanical properties of CoCrFeNi high entropy alloys

“…Positive Cauchy pressure is featured by ductile alloys, while negative Cauchy pressure is a signature of brittle alloys. 63 The fcc structure of x ¼ 0 and the bcc structures of x ¼ 0, 0.4 and 0.8 are brittle, the rest of the structures are ductile. A z is used to predict the elastic anisotropy of materials.…”

Mechanical and Thermal Properties of Low-Density Al20+xCr20-xMo20-yTi20V20+y Alloys

“…Gu et al [47] reported that bulk metallic glasses with ν > 31 are very ductile. Moreover, the positive sign value of Cauchy pressure (C 11 − C 44 ) is associated with the metallic nature of polycrystalline material [48]. All three designed RHEAs show metallic nature, as all of them have a positive value of Cauchy pressure.…”