Effects of free volume changes and residual stresses on the fatigue and fracture behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass

The roles of free volume and residual stress in affecting the fracture and fatigue behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass are examined. Different residual stress and free volume states were achieved by annealing below the glass transition temperature. When residual stresses from casting were relieved by annealing, there was an associated decrease in both the fracture toughness and fatigue threshold. Longer and higher temperature annealing resulted in free volume reductions due to structural relaxation that were quantified by enthalpy recovery measurements. Structural relaxation shows a pronounced effect in reducing the fracture toughness and improving the fatigue limit by affecting fatigue crack initiation. However, free volume reduction did not show any influence on the fatigue crack-growth rates or thresholds. This latter effect is attributed to the fact that the large strains at the fatigue crack tip cause a local increase in free volume that appears to dominate the local flow properties, making the initial free volume state irrelevant. The increased free volume associated with this fatigue transformation zone was verified by depth-profiled positron annihilation spectroscopy conducted on the fatigue fracture surfaces. Finally, controlling free volume and residual stresses appears to be a viable way to tailor the fracture and fatigue properties of bulk metallic glasses for given applications.