Thermal Study of Three In—Tl Alloys in the Neighborhood of the fct—fcc Transformation; Heat Capacities from 5–300°K

Abstract

The heat capacities of three In—Tl alloys, having compositions of 34.6-, 28.9-, and 18.8-at.% Tl, have been measured from 5–300°K. The thermodynamic functions have been tabulated. In this temperature range, the 28.9-at.% Tl alloy exhibited a martensitic transformation from fct to fcc. The 34.6-at.% Tl alloy remained fcc and the 18.8-at.% Tl alloy remained fct. The entropies at 298°K were found to be 61.6±1.6, 61.4±2.0, and 61.1±1.6 J ( gaw·deg)−1 for the three alloys, respectively. The alloy that transformed from fct to fcc exhibited deviations from smooth behavior in the heat capacity in the region of the transformation. While no alloy exhibited anomalous behavior below 195°K, all alloys exhibited anomalous 5%–8% rises in heat capacity above 195°K. This behavior was exhibited over a 25° to 35° range, depending on the alloy composition. This rise in heat capacity was irreversible in the 28.9- and 34.6-at.% Tl alloys, but reversible, to within experimental error, in the 18.8-at.% Tl alloy. The behavior correlated with anomalous thermal expansion coefficients of fcc and fct alloys reported by previous investigators.