An experimental investigation on the notch toughness of Cu-Zr-based bulk metallic glasses with in-situ crystallization

The notch toughness of wear-resistant Cu43Zr43Al7Be7 bulk metallic glasses (BMGs) with in-situ crystallization is investigated. Using different cooling rates during copper mold casting and a post-cast anneal, the amount of crystallization is controlled and varied. The notch toughness correlates most closely with the composition of the remaining amorphous matrix, as measured by energy-dispersive X-ray spectroscopy (EDS). As the matrix composition shifts further away from the designed high-glass forming composition, the toughness decreases. The sample with a matrix composition that closely matches Cu43Zr43Al7Be7 is shown to be the toughest, with a notch toughness of 54 MPa-m(1/2). X-ray diffraction (XRD), EDS and electron backscatter diffraction (EBSD) were used to identify the metastable crystalline phase as Cu3Zr5Al2(+Be) with a cubic Fm (3) over barm structure. Furthermore, the crystalline phases are shown to be harder than the glassy matrix, and therefore likely contribute to the high wear-resistance of the studied BMG. Under standard XRD operating parameters, as used in the literature, all of the samples examined in this study appear to be fully amorphous, yet many are shown to contain a significant number of crystalline regions. In order to identify in-situ crystallization using XRD with Cu K alpha radiation, extremely long dwell times and high X-ray fluxes are required. This demonstrates the importance of reporting XRD operating parameters when trying to evaluate the amorphous nature of BMGs.