The “ tension transformation zone” model of amorphous alloys

Abstract

An amorphous alloy is a glassy solid that is formed through the supercooling of a melt. As the melt cools via the glass transition, its atoms freeze into a long-range disordered structure. Amorphous alloys represent a relatively young class of materials, having been first reported in 1960 when Duwez and co-workers produced Au-Si alloys by developing the rapid-quenching technology. The advent of amorphous alloys, especially the bulk samples with their characteristic size in excess of 1 mm, has aroused much interests in the basic science of glass transition, glass structure, and their intriguing properties. For crystalline metals, their structure can be well described by the period lattices and lattice defects including dislocations, twins, stacking faults, grain doundaries, etc. However, these traditional structural defects are not defined in amorphous alloys. Therefore, this type of atomic-disordered alloys manifest a series of excellent mechanical properties, including extraordinary strength, high hardness, large elastic limit and relatively high fracture toughness, making them attractive candidates for many potential applications as structural materials. At temperatures far below the glass transition temperature, the failure of amorphous alloys is generally induced by 10 nm thick shear banding with the single-dominated or multiple mode. It is well known that the shear banding is an instability mode of plastic flow from homogeneous to localized feature. Although the precise mechanism for amorphous plasticity is not well discovered, it is widely accepted that the shear-banding-mediated plasticity originates from a cascade of inelastic shear rearrangements of local atomic groups, called shear transformation zones (STZs). The STZs are thermally activated events with the transient nature, driven by shear stress and giving rise spatially to Eshelby fields. However, many recent works have shown that the failure of amorphous alloys is not always dominated by the shear banding; instead, a brittle failure will take place with a tension mode. The latter is usually accompanied with a new type of fracture surface morphology: fine dimples and/or nanoscale periodic corrugations. In order to understand such a dissipation process of fracture energy, we proposed the “tension transformation zone (TTZ)” model of amorphous alloys in 2008. The TTZ describes the brittle nucleation-controlled cavitation of local atomic groups that can be activated by shear-induced dilatation or direct hydrostatic tension. Here, we review how the TTZ model was developed, including its inherent nature, activation conditions, atomistic simulations and relevant experiments. The difference and relationship between the proposed TTZ and the classical cavitation are extensively discussed. Therefore, the energy dissipation in fracture of amorphous alloys is determined by two competing elementary processes, via. STZs and TTZs ahead of the crack tip. Based on this STZ vs. TTZ picture, the ductile-to-brittle transition of amorphous alloys can be understood as the change in the nature of transformation zones from shear-dominated STZs to dilatation-dominated TTZs. This review ends with the key aspects that deserve further study regarding the TTZ model. These aspects, at least, include (1) the experimental capture of TTZs, (2) the dynamics properties, (3) the spatio-temporal evolution, and (4) the theoretical construction from TTZs to brittle failure in amorphous alloys.