Simulations, Modeling, and Designs of Bulk Metallic Glasses

Abstract

Bulk metallic glasses usually have very high yield strength, at least double that of ordinary commercially used crystalline materials, and high elastic strain limit, roughly 2% in tension or compression, due to their disordered atomic structure. Unfortunately, the Achilles heel of metallic glasses is their rather limited ductility and low resistance to the propagation of a crack especially in tension. Many research efforts have been devoted to understanding the deformation and fracture behavior of bulk metallic glasses. One interesting observation is that the properties of metallic glasses are well correlated with each other. The challenge is to understand these correlations, and to utilize such understanding to design novel glasses with good glass forming ability and mechanical toughness. Following the Cooperative Shear Model, we investigated the temperature, volume, and configurational dependence of elastic properties by constructing an effective tight-binding force field for a Cu-Zr binary alloy system, and carrying out molecular dynamics simulations. We determined the isothermal Equation of State in a wide range of temperatures and pressures. Pressure-induced cavitation was observed and negative pressure is critical for triggering cavitation. The cavitation barrier height was estimated from the classical nucleation theory. The intrinsic origin of cavitation and its connection to Poisson’s ratio or the ratio of G/B are investigated. The relationship to the deformation and fracture behavior of glasses is discussed. We designed several novel bulk metallic-glass-forming systems using the link between fragility, elastic properties, and glass forming ability as a guiding tool. The compositional dependence of thermal and elastic properties of Cu-Zr-Be ternary bulk-metallic-glass forming alloys was systematically studied. Lightweight Ti-based bulk amorphous structural metals with more than double the specific strength of conventional titanium alloys have been discovered. We report a novel class of bulk amorphous alloys with benchmark thermoplastic processability, having good glass forming ability, exceptional thermal stability, unexpectedly large Angell fragility number, and good mechanical toughness. Starting from the two binary bulk glass formers in the Cu-Zr system, we systematically investigated the compositional dependence of glass formation, thermal, elastic, and mechanical properties in the Cu-Zr-Ag ternary alloys.