Fracture and Fatigue in a Zr-Based Bulk Metallic Glass

The fracture and fatigue properties of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (at.%) bulk metallic glass alloy have been examined. The plane-strain fracture toughness of the fully amorphous alloy was found to exceed 50 MPa√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 approximately 1 MPa√m and the hardness increasing by approximately 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 approximately 4% the ultimate tensile strength.