Fatigue crack propagation behavior and fracture toughness in a Ni-free ZrCuFeAlAg bulk metallic glass

Abstract

The fatigue crack propagation and fracture toughness of a Ni-free Zr60.14Cu22.31Fe4.85Al9.7Ag3 bulk metallic glass (BMG) have been studied by using single edge notched beam (SE(B)) samples prepared from the as-cast BMG plates. It is shown that the metallic glass follows similar fatigue crack propagation behavior to traditional ductile polycrystalline metals, i.e., exhibiting “S” shape of da/dN∼ΔK curve with a fatigue threshold (ΔKth) of 3.71MPa√m and a Paris law exponent of 3.52 in Paris regime. The fatigue-fractured surfaces show universal fatigue striations at different stress intensity levels. The striation spacing (d) fits well with the fatigue crack propagation rate (da/dN) in high stress intensity regime, but deviates significantly from crack propagation rate in low stress intensity regime, i.e., d is larger than da/dN, reflecting that the crack propagation in low stress intensity range was driven by the accumulation of damage from a number of cyclic loadings. A shear slip pair model based on STZ theory and linear-elastic-fracture mechanics (LEFM) analysis are established to interpret the mechanism of fatigue striation formation and crack propagation of the BMG. In addition, the fracture toughness of the BMG was also studied by three-point bending test of pre-cracked samples, which yields a high KJ value of 116.7MPa√m. The high fracture toughness is believed to be related to the high Poisson’s ratio (ν=0.368) of the BMG, which helps to the formation of abundant shear bands in front of crack tip and to the retardation of crack propagation by crack path deflection.