Effect of relaxation on pressure sensitivity index in a Zr-based metallic glass

Shear-banding features of as-cast and annealed Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass were investigated through Rockwell indentation tests. Isothermal annealing of the as-cast samples was conducted at temperatures below its glass transition temperature, T-g. The exothermal enthalpy during continuous heating below T-g decreases with increasing annealing temperature, indicating the gradual reduction of free-volume upon annealing. The observation on the morphology of shear-banding pattern around the indents implies a reduced shear bands activity in the annealed samples. The included angles (20) between two families of shear bands emanating from the edge of Rockwell indent decrease from 88 degrees for the as-cast sample to 79 degrees for the sample annealed at 633 K for 1 h, indicating a pressure sensitive plasticity. By Mohr-Coulomb criterion, the pressure sensitive index can be obtained on the basis of the measured 20, which increases with increasing annealing temperature, indicating an increase of atomistic friction due to the reduction of the free volume upon annealing. (C) 2007 Elsevier B.V. All rights reserved.