Breakdown of the Schmid law in bcc molybdenum related to the effect of shear stress perpendicular to the slip direction

TL;DR

This study uses atomistic modeling to show that shear stresses perpendicular to the slip direction in bcc molybdenum significantly influence dislocation motion by altering the dislocation core, challenging the traditional Schmid law assumptions.

Contribution

It reveals how shear stresses perpendicular to the slip direction affect dislocation core structure and critical shear stress, providing new insights into slip system activation in bcc metals.

Findings

Shear stresses perpendicular to slip direction can alter the Peierls stress.

Dislocation core structure is sensitive to non-glide shear stresses.

Slip systems with lower Schmid factors may be favored due to these effects.

Abstract

The breakdown of the Schmid law in bcc metals has been known for a long time. The asymmetry of shearing in the slip direction <111> in the positive and negative sense, respectively, commonly identified with the twinning-antitwinning asymmetry, is undoubtedly one of the reasons. However, effect of stress components other than the shear stress in the slip direction may be important. In this paper we investigate by atomistic modeling the effect of shear stresses perpendicular to the Burgers vector on the glide of a/2<111> screw dislocations. We show that these shear stresses can significantly elevate or reduce the critical resolved shear stress (CRSS) in the direction of the Burgers vector needed for the dislocation motion, i.e. the Peierls stress. This occurs owing to the changes of the core induced by these stresses. This effect may be the reason why slip systems with smaller Schmid factors may be preferred over that with the largest Schmid factor.