Mechanical behavior of Fe₇₅Mo₅P₁₀C_7.5 B_2.5 bulk-metallic glass under torsional loading

Pure- and cyclic-torsional studies were conducted on a Fe₇₅Mo₅P₁₀C_7.5 B_2.5 (atomic percent, at.%) bulk-metallic glass at room temperature for an understanding of its damage and fracture mechanisms. Under pure-torsional loading, the metallic glass exhibited very little plastic strain before fracture. The fracture initiated along the maximum tensile-stress plane, which is about 45° to the axial direction. The shear-fracture strength (∼510MPa) is much lower than the compressive-fracture strength (∼3280MPa), which suggests that different deformation mechanisms be present under various loading modes. Instead of an apparent vein-type structure, the fracture morphologies revealed a crack-initiation site, a mirror region, a mist region, and a hackle region. Under cyclic-torsional loading, fatigue cracks initiated from casting defects, and propagate generally along the maximum tensile-stress plane. A slight cyclic-hardening behavior was observed in initial loading steps. The fatigue-fracture surface consists of three main regions: the fatigue crack-initiation, crack-propagation, and final-fast-fracture areas. The striations resulting from the blunting and re-sharpening of the fatigue crack tip were observed in the crack-propagation region. Based on these results, the damage and fracture mechanisms of the metallic glass induced by torsional loadings are elucidated.