Variation in mechanical properties with crystallinity of a La-based bulk metallic glass

Abstract

The influence of crystallinity on the mechanical properties of a bulk La-based metallic glass was investigated. The degree of crystallization in the metallic glass was systematically altered by varying the annealing time above the glass transition temperature, Tg. The elastic modulus, E, of the nanocrystalline-amorphous matrix composite was observed to increase abruptly at ∼40% crystallinity, Χ. The hardness, H, however increases only linearly with the increase in Χ. Instrumented impact test results show a significant decrease, by ∼90%, in impact toughness, γ, 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. Microscopic mechanisms responsible for the observed transitions in mechanical properties are discussed.