Kinetics of the eutectoid transformation in the Cu-In system

Abstract

The present study concerns a detailed investigation of the kinetics of the eutectoid transformation in the Cu–In system based on both the isothermal growth rate of the eutectoid colony (monitored by microstructural change) and enthalpy changes during non-isothermal heating (determined by differential scanning calorimetry) of solution-treated and quenched samples. The maximum growth distance of the eutectoid cells and the equilibrium interlamellar spacing have been determined by optical and scanning electron microscopy in the temperature range 600–825 K. The reaction front velocity was observed to increase with the isothermal ageing temperature in the temperature range studied. A detailed analysis of the isothermal growth kinetics through the models available in the literature has yielded an activation energy of 125–127 kJ mol-1 for the operating diffusion process, which is comparable with that for discontinuous precipitation in Cu–In or for grain boundary tracer diffusion of 115In in Cu, but significantly lower than that for volume diffusion of In in the β Cu–In alloy. A subsequent differential scanning calorimetric study has indicated a similar activation energy of 133 kJ mol-1 for the concerned eutectoid transformation. It is thus concluded that the eutectoid transformation in the Cu–In system is a boundary-diffusion-controlled process.