Fracture Toughness Study on Zr-based Bulk Metallic Glasses

Abstract

The fracture toughness of Zr-based bulk metallic glasses of various compositions was studied in the as-cast and annealed condition. Properties were characterized using x-ray and differential scanning calorimetry (DSC) and fracture surfaces were examined using electron microscopy (SEM). Quaternary Zr-Ti-Cu-Be alloys consistently had linear elastic fracture toughness values greater than 80 MPa·m^1/2, while Vitreloy 1, a Zr-Ti-Cu-Ni-Be alloy, had an average fracture toughness of 48.5 MPa·m^1/2 with a large amount of scatter. The addition of iron to Vitreloy 1 reduced the fracture toughness to 25 MPa·m^1/2. Fracture surfaces were carefully analyzed using electron microscopy. Some samples had highly jagged patterns at the beginning stage of crack propagation, and the roughness of this jagged pattern correlated well with the measured fracture toughness values. These jagged patterns, the main source of energy dissipation in the sample, were attributed to the formation of shear bands inside the sample. The Zr-Ti-Cu-Be alloy, having K_Q=85 MPa·m^1/2 as cast, was annealed at various time/temperature combinations. When the alloy was annealed 50°C below T_g, the fracture toughness dropped to 6 MPa·m^1/2, while DSC and X-ray showed the alloy to still be amorphous. The roughness of the fracture surfaces on relaxed samples also compared well with the relative fracture toughness.