The stochastics of strain localization in metallic-glass microwires

Abstract

Tensile tests on Cu/Zr-based metallic-glass (MG) micro-wires show observable first yield point, followed by shear fracture on further straining. Microscopy examination reveals discrete shear bands decorating the free surfaces of yielded and fractured MG micro-wires. Both the first-yield and the fracture stress scatter statistically as expected, but surprisingly, they do not exhibit any significant dependence on the wire length. Fundamentally, while it has been widely accepted that glass plasticity takes place via shear transformation zones (STZs), the knowledge gap between such atomic-sized STZs and the above-mentioned micro/macroscopic plasticity phenomena remains huge. In this work, molecular dynamics (MD) simulations were carried out to delineate the detailed process by which shear bands form from discrete STZs. The results show that the STZs have an increasing tendency to emerge and operate close to one another in a correlated manner along the strain path. This process leads to shear localization in the form of shear bands. An analytical model is then proposed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, as prior shear events triggers the emergence of new STZs, successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization. Finally, the MD simulations also indicate that the first observable yield point cannot be due to the emergence and operation of one single STZ. Instead, yield or fracture is controlled by the average or extreme behavior of many STZs or shear bands operated in different locations in the wire, which explains length independence of the MG wire strength – a fact also observed in other glass wires.