Cu-75Sn合金包晶转变机制研究

Peritectic Transformation Mechanism of Cu-75Sn Alloy

对包晶相为极低固溶度的Cu-75Sn合金进行Bridgman定向凝固和等温包晶转变研究,借助扫描电镜(SEM)、能谱分析(EDS)和电子探针(EPMA)等手段分析其显微组织。结果表明,Cu-75Sn过包晶合金定向凝固为典型的非平衡凝固过程,凝固组织中存在大量初生相;包晶转变前后初生相和包晶相内的溶质浓度变化不大,接近其平衡成分,但在ε/η以及L/η相界面处溶质浓度发生突变,即溶质浓度扩散达到一定值时发生相变;利用包晶转变机制,通过静置保温可以有效消除初生相。原始板条尺度越细小,保温时间越长,初生相消失越快,包晶转变程度越高;包晶相的体积分数与保温时间t满足φ(η)=0.597×t^(0.492),符合凝固平方根定律,表明包晶转变过程为溶质扩散机制。

Bridgman type directional solidification and isothermal peritectic transformation experiments were carried out for Cu-75 Sn 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-75 Sn hyperperitectic alloy presents a typical non-equilibrium solidification and there exist a large number of primaryεphase in microstructures.EDS and EPMA analysis of the solute profile shows that the tin concentration in theεandηphases is uniform closing to the equilibrium values in the phase diagram,whereas solute concentration in theε/ηand L/ηphase 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 fitsφ（η）=0.597×t-（0.492） in accord with square root law of solidification.Hence,the peritectic transformation process of Cu-75 Sn is controlled by solute diffusion mechanism.