Pressure dependence of the free volume in amorphous Pd 40Ni 40P 20 and its implications for the diffusion process

Abstract

Pd 40Ni 40P 20, a metallic glass that has been shown to reach a temperature-dependent metastable equilibrium state before crystallisation, has recently been the subject of an extensive study on its structural relaxation and atomic mobility. A consistent picture of the properties of the glass in terms of the free-volume model has been obtained. In the present paper we present a study on the pressure dependence of the free volume in this glass. After bringing the glass in equilibrium at 563 K at ambient pressure, 12 h anneals at the same temperature are performed at hydrostatic pressures up to 0.75 GPa. The structural state of the glass after these anneals is monitored by means of Differential Scanning Calorimetry, revealing the glass-transition peak to be distinctly dependent on the pressure applied during the anneals. Interpreting the glass-transition peak according to the free-volume model yields a linear pressure dependence of the reduced free volume, quantified by dx/dp= −0.0025 GPa −1. Using this result, the activation volume for diffusion ΔV= (11 ±4) A 3, which was previously obtained from the pressure dependence of the diffusivity of Au in the same glass, could be separated into a formation volume ΔV F= (5 ±1) A 3, which reflects the pressure dependence of the concentration of diffusion carriers and a migration volume ΔV M= (5 ±4) A 3, which reflects the influence of the pressure on the diffusion event itself. These values lend support to a cooperative mechanism for Au diffusion in Pd 40Ni 40P 20, contrary to the conclusion based on the activation volume only.