Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

The fracture and fatigue properties of a newly developed bulk metallic glas s alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at, pct), have been examined. Exper imental measurements using conventional fatigue precracked compact-tension C(T) specimens (similar to 7-mm thick) indicated that the fully amorphous a lloy has a plane-strain fracture toughness comparable to polycrystalline al uminum alloys. However, significant variability was observed and possible s ources are identified. The fracture surfaces exhibited a vein morphology ty pical of metallic glasses, and, in some cases, evidence for local melting w as observed. Attempts were made to rationalize the fracture toughness in te rms of a previously developed micromechanical model based on the Taylor ins tability, as well as on the observation of extensive crack branching and de flection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to similar to 1 MPa root m. Commensurate with this drop in toughness was a marginal increase in hardnes s and a reduction in ductility (as measured via depth-sensing indentation e xperiments). Under cyclic loading, crack-propagation behavior in the amorph ous structure was similar to that observed in polycrystalline steel and alu minum alloys. Moreover, the crack-advance mechanism was associated with alt ernating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress /life (S/N) properties were markedly different. Crack initiation and subseq uent growth occurred quite readily, due to the lack of microstructural barr iers that would normally provide local crack-arrest points. This resulted i n a low fatigue limit of similar to 4 pet of ultimate tensile strength.