A study of order-disorder phase transformation in Ag50Pd50 alloy

Abstract

In this study, the structure of Ag50Pd50 alloy was investigated with temperature dependent in-situ X-ray diffraction, residual resistometry, and differential scanning calorimetry (DSC) as well as differential thermal analysis (DTA) techniques. Isochronal annealing experiment was performed to determine the impact of ordering on the residual electrical resistivity. The residual resistivity curve shows a sharp minimum near 250 °C indicating the on-set of ordering. The in-situ X-ray diffraction data taken for a well annealed sample shows no super lattice reflections over the investigated temperature range but the lattice parameter determined from this data shows an abrupt decrease in its value at 250 °C showing an order-disorder phase transformation (ODPT). The diffraction pattern of a sample annealed at 230 °C shows splitting of the high angle 511 fundamental reflection due to the tetragonal distortion. The peak intensity and c/a ratios are found to be 1:1.94 and 1.0028, respectively. A similar phase change was also observed during a DSC experiment around 250 °C on this alloy. The existence of these anomalies may be attributed to L1o - FCC type order-disorder phase transformation at this temperature.