On the Mechanism of Anomalous Slip in BCC Metals

Abstract

Computer simulations and empirical studies of the core structure of single dislocation in bcc metals over the last few decades have made enormous contributions to interpret many abnormal mechanical behaviors of bcc metals: tension/compression stress asymmetry, high Peierls (friction) stress for the motion of screw dislocations, and strong strain-rate and temperature dependence of yield and flow stresses at low temperatures [1]. However, the single-dislocation core model remains inconclusive to elucidate a peculiar anomalous slip behavior of bcc metals, which occurs on planes for which the Schmid factors are fifth and sixth in the order of largest Schmid factors for the {110} <111> slip systems, and for which the resolved shear stress is less than half that on the (101) [111] primary system. Note that the anomalous slip behavior is also known as the violation of Schmidï‚¢s law, which states that plastic deformation of a single-crystal metals would begin on a slip system (a combination of the slip plane and the slip direction) when the resolved shear stress on the slip plane and in the slip direction reached a critical value (i.e., critical resolved shear stress). The resolved shear stress (ï´) is given by ï´ = ï³ cos ï¦ cos ï¬, where ï³ is applied stress, ï¦ is angle between the stress axis and the normal to the slip plane, and ï¬ is angle between the stress axis and the slip direction. The factor cos ï¦ cos ï¬ is usually called the Schmid factor (m). Schmidï‚¢s law in general is well obeyed by close-packed face-centered cubic (fcc) and hexagonal closed-packed (hcp) metals, which deform by slip in close-packed directions on planes that are close-packed planes. Bodycentered cubic (bcc) metal is however not a close-packed structure, which deforms by slip in the most closely packed direction: