Effect of crystallinity on the impact toughness of a la-based bulk metallic glass

The influence of crystallinity on the ductile-brittle transition in a bulk lanthanum-based metallic glass was investigated. The degree of crystallization in the metallic glass, which was processed through the arc-melting and water-quenching route, was systematically altered by varying the annealing time above the glass transition temperature. The resulting amorphous/crystalline microstructures were characterized by XRD, DSC, and TEM techniques. Instrumented impact test results show a significant decrease, by approx. 90%, in impact toughness with the introduction of a small percentage of crystallinity. This decrease in toughness was also associated with a distinct change in fracture morphology, from a ductile vein-type fracture in the bulk glass to intergranular cleavage fracture in the crystalline material. The observed fracture transition was rationalized by recourse to the mechanism of stress relaxation due to viscous flow. For this purpose, variations in elastic modulus and dynamic viscosity with respect to the crystallinity were measured using a dynamic mechanical analyzer. The characteristic relaxation times were computed from the viscosity data and were used to explain the ductile-brittle transition. Microscopic mechanisms responsible for the fracture transition are also discussed.