Formation mechanism of ordering waves and ordering process in Cu-Pt alloys

研究了Cu-Pt合金中的有序-无序过程，以阐明一级相变的机制。得到以下结果.建立了Cu_3Pt合金从短程有序态（SRO）到L1_2或L1_2有序化过程的动力学方程<-s>。在该公式中，自由能密度用局部阶数S_r的朗道展开式表示。表面能用S_r、S_r^2和|S_r|从而可以考虑低反相能量。反相关联引起的能量降低的幅度与经傅里叶变换的S_r的幅度成正比。通过对具有阻挫的一维两两相互作用进行傅立叶变换，可以得到比例常数。能量的降低使SRO和L1_2态稳定<2-s>。SRO的状态和有序化过程可以通过由上述项加上热涨落组成的动力学方程的数值计算来得到.用X射线衍射和电子显微镜研究了Cu-40at%Pt合金中L1_1相分解为（A1+L1_1）相的过程。结果表明，当退火温度T_a高于一定的T_m时，合金几乎完全无序化为A_1，然后析出L_1_1。当T_a&lt;T_m时，L1_1中A1的析出是相分解的主要原因。通过假定T_m对应于合金发生A_1-L_1_1相变的临界温度，从理论上解释了退火过程的温度依赖性。并对A_1 →（A_1 + L_1_1）的过程进行了讨论。

Order-disorder processes in Cu-Pt alloys were studied in order to clarify the mechanism of the first order transformation. The following results were obtained.1. The kinetic equation was established for simulation of ordering process from short range ordered state (SRO) to L1_2 or to L1_<-s> in a Cu_3Pt alloy. In the formulation, the density of free energy is expressed by a Landau expansion with the local degree of order S_r. Surface energy is represented in terms of S_r, S_r^2 and |S_r| so that a low antiphase energy may be taken into account. The magnitude of energy reduction due to antiphase correlation is assumed to be proportional to the amplitude of Fourier transformed S_r. The proportionality constant can be obtained by Fourier transformation of one-dimensional pairwise interactions which have frustration. The energy reduction stabilizes the states of SRO and L1_<2-s>. The state of SRO and the ordering processes can be produced by numerical calculation of the kinetic equation which consists of the above terms plus thermal fluctuation.2. The process of phase decomposition from L1_1 into (A1+L1_1) in Cu-40 at%Pt alloy was investigated by X-ray diffraction and electron microscopy. It was found that, if the annealing temperature T_a is higher than a certain one T_m, the alloy is disordered almost completely to A1, followed by precipitation of L1_1. When T_a<T_m, the phase decomposition proceeds by precipitation of A1 in L1_1. The temperature dependence of annealing process is explained thermodynamically by assuming that T_m corresponds to the critical temperature of the alloy for A1-L1_1 transformation. The process from A1 to (A1+L1_1) was also discussed.