Effects of Annealing and Changes in Stress State on Fracture Toughness of Bulk Metallic Glass

Abstract

The effects of annealing and changes in stress state on the toughness of both 4 mm thick and 7 mm thick plates of a Zr–Ti–Ni–Cu–Be alloy have been determined. In the amorphous state, both notched and fatigue precracked specimens have been tested. The effects of changing the notch root radius from a fatigue precrack to that of a blunt notch on the fracture toughness are dramatic. The toughness increases from approximately 17.9±1.8 MPa\\sqrtm in the fatigue precracked specimens to in excess of 130 MPa\\sqrtm in the notched specimens. These results are compared to similar tests on a range of structural materials, including aluminum alloys, steels, Ti alloys, and metal matrix composites. The increased toughness obtained by increasing the notch root radius in this bulk metallic glass far exceeds that typically observed in other structural materials. Possible reasons for this are presented. In addition, the effects of changes in loading rate and various annealing treatments on the toughness are presented and rationalized via both crack path and fracture surface observations. Annealing of this bulk metallic glass at temperatures below T_g produces increases in strength/hardness, rapid decreases in toughness, and a corresponding change in the fracture morphology. Changes in loading rate did not have a significant effect on the toughness for either notched or fatigue precracked specimens.