A Dilatometric Study of the Order-Disorder Transformation in Cu-Au Alloys

Abstract

The order-disorder transformation in single crystals of Cu-Au alloys, containing 22, 25, and 30 atomic percent Au, has been studied by dilatometric means. The plots of true coefficient of thermal expansion versus temperature for previously well-ordered alloys display a slow rise from -190°C to about +50°C, followed by a flat plateau to about 250°C. From the flat plateau they rise rapidly to a peak at the critical ordering temperature $T_c$, followed by a fall to a second flat plateau which extends to at least 450°C above $T_c$. The lower portion, that portion below the flat plateau, can be well represented by a Grueneisen equation if one assumes additivity for the constants taken from the equation for the pure metals Cu and Au. The presence of the flat plateau above $T_c$ is in disagreement with Bethe-Peierls' theory of the vanishing of short range order. Plots of true coefficient of expansion versus temperature, for specimens previously quenched from above $T_c$, display valleys similar to those in the comparable specific heat curves of Sykes and Jones.