Hardness and plastic deformation in a bulk metallic glass

An experimental investigation into the Vickers hardness and associated plastic deformation in as-cast and annealed $Pd_{42}Ni_{40}P_{18}$ bulk metallic glass was conducted. In addition to the bulk indentation behavior, the deformation morphology underneath the indenter and its variation with annealing time was examined by employing the bonded interface technique. For both the bulk and the interface indentations, the trends in the shear band induced plastic deformation zone sizes with the indentation load agree well with those predicted from the expanding cavity model. However, the yield strength extracted from the indentation data is higher than that measured in uniaxial compression, indicating pressure sensitive plasticity. Results show that the as-cast as well as the partially crystallized alloys deform appreciably through the shear band mechanism, with semi-circular and radial shear band morphologies. The latter gets increasingly prominent with increasing annealing time. Atomic force microscopy of the deformation region reveals increasing shear band heights with load, consistent with nanoindentation results. Whereas the spacing between semicircular shear bands was found to be independent of their distance from the tip of the indenter for the as-cast alloy, it was found to increase linearly, after a small zone of constant spacing, in the annealed alloys. Implications of this study in understanding the mechanical behavior of metallic glasses and their derivatives are discussed