Direct observations on the evolution of shear bands into cracks in metallic glass

The evolution of shear bands (SBs) into cracks was observed by using a high-resolution scanning electron microscope in Zr59Cu20Al10Ni8Ti3 metallic glassy samples after a small punch test with different strain rates. As shear strain increased along a radial SB, three distinctive regions of morphologies were found (I) bonded SB, (II) microcrack plus bonded SB, and (III) full crack. In region II with moderate shear strain, some glassy “extrusions” were also observed. Once shear offset increases to a critical value, the SB becomes a full crack. For two different SBs in one specimen, the critical shear offsets maintain approximately the same value, which sheds light on the critical shear failure condition of metallic glass. The critical shear offset was also found to be sensitive to the strain rate and a higher strain rate led to less critical shear offset. It is suggested that the structure evolution and heat evolution within a shearing SB should be responsible for the previous results.