Densification and Strain Hardening of a Metallic Glass under Tension at Room Temperature

The deformation of metallic glasses involves two competing processes: a disordering process involving dilatation, free volume accumulation, and softening, and a relaxation process involving diffusional ordering and densification. For metallic glasses at room temperature and under uniaxial loading, disordering usually dominates, and the glass can fail catastrophically as the softening process runs away in a localized mode. Here we demonstrate conditions where the opposite, unexpected, situation occurs: the densifying process dominates, resulting in stable plastic deformation and work hardening at room temperature. We report densification and hardening during deformation in a Zr-based glass under multiaxial loading, in a notched tensile geometry. The effect is driven by stress-enhanced diffusional relaxation, and is attended by a reduction in exothermic heat and hardening signatures similar to those observed in the classical thermal relaxation of glasses. The result is significant, stable, plastic, extensional flow in metallic glasses, which suggest a possibility of designing tough glasses based on their flow properties.

Figure 1

The effect of stress state on the rates of free volume generation and annihilation in a metallic glass under tension. Under a uniaxial tensile stress state (a), the rate of generation is always higher than that of annihilation over a wide range of stress (b), resulting in a net increase in free volume and softening. Under a triaxial tensile stress state in a notched sample (d), the rates of generation and annihilation crossover at a critical stress (f) above which the free volume decreases resulting in densification and hardening (e).

Figure 2

Optical and SEM micrographs of a deeply notched metallic glass tensile specimen.

Figure 3

Tensile test data for notched and unnotched bars. Here the $y$ axis of the raw load-displacement curves is normalized using the minimum load bearing area of the sample (i.e., the full cross section for an unnotched sample, and the cross section in the notch region for the notched sample), to permit comparison between the two different geometries. The notched specimen exhibits work hardening characteristics and large, stable extensional flow, whereas the unnotched sample fails catastrophically at the elastic limit. The inset shows the permanent extension in the notched region before and after tensile testing.

Figure 4

Investigation of property changes after tensile deformation. Hardening behavior of the notched region after tensile deformation (a), was revealed by microhardness traces. The DSC curves of the deformed specimen also show a signature reduction in endothermic heat, similar to those specimens after annealing (b), reflective of ordering or densification.