Fracture, fatigue and environmentally-assisted failure of a Zr-based bulk amorphous metal

Bulk amorphous alloys, such as the Zr41.2Ti13.8Cu12.5Ni10Be22.5 (at%) alloy, have received much interest of late. This study seeks to investigate mechanical properties of this amorphous metal, including fracture and fatigue behavior in the presence of air and sodium chloride solution. For the fully amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 (at%) alloy, plane-strain fracture toughness (KIc) values of 18 to 68 MPa√m have been measured, the significant sample-to-sample variability being associated with the presence of residual stresses following processing and a dependence on loading rate. Upon partial or complete crystallization, the alloy becomes severely embrittled, with KIc dropping to ∼1 MPa√m. Moreover, in NaCl solution, static load testing reveals a threshold value for cracking, KISCC, as low as ∼0.8 MPa√m. Results from fatigue testing in air and sodium chloride, as well as fatigue testing under potential control, suggest that quite distinct mechanisms of fatigue-crack propagation are active in air and NaCl solution. Specifically, in air, fatigue-crack propagation behavior is similar to that observed in polycrystalline metals and is associated with alternating blunting and re-sharpening of the crack tip, as evidenced by the presence of fatigue striations. In NaCl solution, conversely, the high crack-growth rates likely depend on an anodic process, which results in a brittle mode of failure.