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

In order to help understand the wide scatter in the reported fatigue properties reported for bulk metallic glasses, the roles of free volume and residual stress in affecting the fracture and fatigue behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass were examined. Different residual stress and free volume states were achieved by annealing below the glass transition temperature. The free volume of a Zr-based bulk metallic glass was varied via structural relaxation, and four different material conditions were investigated: as-received, stress-relieved, 1t relaxed, and 10t relaxed, where t is the relaxation time at 610K. Differential scanning calorimetry was used to measure enthalpy differences between the relaxed and unrelaxed glasses and quantify the free volume differences. Removal of the residual thermal tempering stresses by short time annealing at 573K showed a marked decrease in the fracture toughness and fatigue crack growth thresholds. While structural relaxation also showed a pronounced effect in reducing the fracture toughness, an increase in the fatigue strength was also observed. Mechanistically, the latter effect was attributed to the crack initiation portion of the lifetime as the fatigue crack-growth behavior was found to be insensitive to bulk free volume differences. Based on these results, a new mechanism is proposed for fatigue crack growth in metallic glasses whereby 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.