Metallic glasses, alloys with a disordered or ‘amorphous’ atomic structure, are new materials with a range of potential engineering applications due to their high strength and spring. However, as they are prone to brittle fracture, their development for practical use has remained limited. Researchers from Tohoku University in Japan1 have now uncovered an important theoretical relationship between stress and temperature in these materials that provides fresh insights into the deformation and failure of glassy states under various physical conditions.

Pengfei Guan, Mingwei Chen and Takeshi Egami performed detailed molecular dynamics simulations for a metallic glass composed of zirconium, copper and aluminum under realistic conditions. In their simulation, a melt at 1700 °C was cooled rapidly to produce a glassy alloy with a dense amorphous atomic packing, as seen in real samples. It is the disruption of this tight packing under shear stress that can lead to mechanical failure in these materials, so determining the exact mechanism of failure under shear could help solve the problem of brittleness in metallic glasses.

Fig. 1: Diagram showing the change in viscosity of a metallic glass with stress and temperature.Adapted from Ref. 1. Reproduced with permission. © 2010 APS

The researchers simulated changes in metallic glass viscosity by applying different rates of shear to their model under conditions of constant volume and temperature. According to Chen and Guan, each data point in these simulations actually corresponds to the average result of many molecular dynamics runs, representing a daunting amount of computation. Through these simulations, the team found that the atoms entered into a state of steady flow before reaching the breaking point. From results obtained over a range of temperatures, they also revealed a surprisingly simple mathematical relationship between stress and temperature in these systems, which allowed them to define the temperature–stress boundaries of stable glassy phases (Fig. 1).

These findings suggest that stress has a similar effect to temperature in promoting and controlling atomic motion and mechanical flow in metallic glasses, even in the absence of heating or expansion — an outcome with far-reaching consequences, according to Egami. “This result directly challenges many conventional models, and demands serious re-thinking of the atomistic mechanism of deformation in metallic glasses,” he says.