Peritectic Transformation Mechanism of Cu-75Sn Alloy

Bridgman type directional solidification and isothermal peritectic transformation experiments were carried out for Cu-75Sn alloy in which the peritectic phase has trace solid solubility. Microstructure and composition analysis were characterized by scanning electron microscopy(SEM), energy dispersive spectrum analysis(EDS)and electron probe micro-analyser(EPMA). The results reveal that the directional solidification process of Cu-75Sn hyperperitectic alloy presents a typical non-equilibrium solidification and there exist a large number of primaryε phase in microstructures. EDS and EPMA analysis of the solute profile shows that the tin concentration in theε and η phases is uniform closing to the equilibrium values in the phase diagram, whereas solute concentration in the ε/η and L/η phase interfaces is changed abruptly during the peritectic transformation. The phase transition will occur when the solute concentration reaches a certain value. Using peritectic transformation mechanism, the primary phase can be eliminated effectively by static heating preservation. With increasing in holding time and decreasing in initial lath structure size, disappear velocity of primary phase is increased with the high peritectic transformation degree. In this case, the relationship between the volumetric fraction of the peritectic phase and the time t fits φ(η)=0. 597×t^0.492 in accord with square root law of solidification. Hence, the peritectic transformation process of Cu-75Sn is controlled by solute diffusion mechanism.