Effects of annealing and changes in stress state on fracture toughness of bulk metallic glass

The effects of annealing and changes in stress state on the toughness of bo th 4 mm thick and 7mm thick plates of a Zr-Ti-Ni-Cu-Be alloy have been dete rmined. In the amorphous state, both notched and fatigue precracked specime ns have been tested. The effects of changing the notch root radius from a f atigue precrack to that of a blunt notch on the fracture toughness are dram atic. The toughness increases from approximately 17.9 +/- 1.8 MPa rootm in the fatigue precracked specimens to in excess of 130 MPa rootm in the notch ed specimens. These results an compared to similar tests on a range of stru ctural materials, including aluminum alloys, steels, Ti alloys, and metal m atrix 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 tre atments on the toughness are presented and rationalized via both clack path and fracture surface observations. Annealing of this bulk metallic glass a t temperatures below T-g produces increases in strength/hardness, rapid dec reases 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.