The influence of loading on crystallization behavior of bulk metallic glasses

Metallic glasses are amorphous metals with unique properties, such as high strength, elasticity, wear resistance, and thermoplastic processing capability. Thermoplastic forming is enabled by the existence of metastable supercooled liquid state in metallic glasses above the glass transition temperature. The thermoplastic manufacturing critically depends on the crystallization time (processing time window), temperature (viscosity), applied load, and strain-rate. Among these parameters, the effects of crystallization time and processing temperature have been extensively studied in metallic glasses. However, the effects of load and loading rate are generally ignored. Recent studies indicate that the load and loading rate can affect the structures of metallic glass supercooled liquids and hence their crystallization kinetics and thermoplastic processing ability. Here, we systematically study the effects of load on supercooled liquid state of three different metallic glass formers: Pt-based, Zr-based, and Pd-based. The results clearly suggest that load-response of metallic glass supercooled liquids is strongly alloy dependent. The onset of crystallization for the Pt-based metallic glass supercooled liquid is reduced after subjecting it to higher loads whereas the onset of crystallization for Zr-based and Pd-based metallic glasses remains largely unaffected. However, the crystallization peak time is reduced for three metallic glasses. We associate this deformation-induced crystallization to the free volume generated by plastic flow. Further studies on loading rate underway to obtain comprehensive understanding of metallic glass supercooled liquids during thermoplastic embossing.