Invited Lecture
Non-Schmid effects in BCC metals from first principles

Body-centered cubic (BCC) metals are known for their atypical plasticity at low temperatures. Here, we focus on their plastic anisotropy and dependence on non-glide stresses, which contradict Schmid’s law. Plasticity of BCC metals is controlled by the glide of ½ screw dislocations. These dislocations display strong core effects at the atomic scale that are responsible for the atypical low temperature plasticity. Here, we use ab initio Density Functional Theory calculations to investigate the link between the core properties of the screw dislocations and deviations from Schmid’s law in BCC metals. We find that the dislocation trajectory systematically deviates from the average glide plane, leading to the well-known twinning/antitwinning asymmetry [1]. Furthermore, we show that the dislocation core deformation modeled with eigenstrains is directly linked to the effect of non-glide stresses. In particular, core eigenstrains measured in absence of applied stress enable to predict the hardening and softening of the Peierls barrier when the glide plane is either under compression or tension. These results are used in a modified version of Schmid’s law in order to predict the variations of the critical resolved shear stress (RSS) as a function of crystal orientation and non-glide stresses [2]. We evidence a strong twinning/antitwinning and tension/compression asymmetry of the critical RSS, directly linked to the dislocation trajectory and eigenstrain variations at the atomic scale. These results are validated by comparison with direct DFT calculations performed under shear and non-glide stresses.

[1] Dezerald et al, Nature Communications 7, 11695 (2016)

[2] Kraych et al, submitted to NPG Computational Materials