Materials Special Seminar
Department of Materials Science & Engineering
A. L. Greer
University of Cambridge, Department of Materials Science & Metallurgy
Pembroke Street, Cambridge CB2 3QZ, UK
Metallic Glasses — Plasticity and Brittleness
Metallic glasses, compared with conventional metallic structural materials, show high yield stresses, low elastic moduli, and uniquely high capacities for storage of elastic energy. Their availability in bulk form, and their formability by viscous flow at elevated temperature make them attractive materials. On the other hand, their fracture toughness varies widely, and they always have a small plastic zone size (<< 1 mm), implying sensitivity to flaws. Thus, even setting aside considerations such as cost, the structural properties of metallic glasses can be best exploited in small-scale devices (such as in MEMS). Recent work shows that metallic glasses with a ratio of shear to bulk modulus m/B > 0.41-0.43 are intrinsically brittle. A brief review will be given of the relationship between the modulus ratio and choice of constituent elements, permitting the selection of compositions less likely to be brittle and less likely to embrittle on annealing. Even metallic glasses (notably those based on noble metals) which show local plasticity, tend to appear macroscopically brittle, because the plastic flow is severely localized into shear bands. Research is now very active in many laboratories on the mechanisms of shear-banding and on ways to make plastic deformation more diffuse. Recent advances will be reviewed from both fundamental and practical points of view.