Fracture and fatigue in a Zr-based bulk metallic glass

The fracture and fatigue properties of the Zr4.12Ti13.8Cu12.5Ni10Be22.5 (at.%) bulk metallic glass alloy have been examined. The plane-strain fracture toughness of the fully amorphous alloy was found to exceed 50 MPa root m, although results were sensitive to strain rate, showed significant variability and were influenced by the presence of residual stresses following processing. Fracture surfaces exhibited a characteristic vein morphology, consistent with micromechanical models for meniscus instabilities. Local melting was evident, consistent with the emission of light during rupture and very high local temperatures (>1000 K) measured during fracture. Upon partial or complete crystallization, the alloy was severely embrittled, with toughnesses dropping to similar to 1 MPa root m and the hardness increasing by similar to 10%. Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline metals; the crack-advance mechanism was associated with alternating crack-tip blunting and resharpening, as evidenced by presence of fatigue striations. Conversely, the (unnotched) stress-life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a very Low fatigue limit of similar to 4% the ultimate tensile strength.