两相法和单相法在集团变分法中的应用

APPLICATION OF TWO-PHASE AND ONE-PHASE CAL- CULATION IN THE CLUSTER VARIATION METHOD

本文提出了用于集团变分法中计算相平衡的两相法和计算失稳点的单相法．对于一级相变，两相法要求一种结构的对称群包含另一种结构的对称群，这样可以得到精度很高的相变点．一级相变的相变点和失稳点不同，在温度－化学势图和温度－组分图中，单相法得到的失稳点都滞后于两相法得到的相变点；随着变化步长的减小可以得到精确的失稳点．对于二级相变，相变点和失稳点相同，两种方法都可以得到相变点和失稳点．虽然在化学势变化步长很小时难以用单相法确切判断二级相变的失稳点，但在变化步长下是很小时，单相法已经可以给出足够精确的相变点．

The two-phase method used in phase transformation point calculation, and the one-phase method used in spinodal calculation, are proposed for the cluster variation method calculation. For the first order transformation, the two-phase calculation needs that the symmetry group of one phase should contain that of the other phase, the transition point and the spinodal point are different, and the spinodal point obtained by the one-phase method appears later than the transition point obtained by the two-phase method in either temperature-chemical potential curve or temparature-constituent curve; and the accurate spinodal point can be obtained with small dimensionless chemical potential changing step. For the second order phase transformation, the transition point and the spinodal point are the same, and both can be obtained by either the two- or one-phase method. The spinodal point of good accuracy can be obtained with not so small dimensionless chemical potential changing step for the second order phase transformation.