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

研究了铜-铂合金的有序-无序过程，以阐明其一级相变的机制。取得了以下研究成果：1.建立了模拟Cu3Pt合金从短程有序态到L1_2或到L1&lt；-S&gt；有序化过程的动力学方程。在该公式中，自由能密度用局域有序程度为S_r的朗道展开来表示，表面能用S_r、S_r^2和|S_r|来表示，这样就可以考虑较低的反相能。假定由反相关联引起的能量降低的幅度与傅里叶变换的S r的幅度成正比，比例常数可以通过具有挫折性的一维两两相互作用的傅里叶变换得到。能量的降低稳定了SRO和L1&lt；2-S的状态。由上述项加上热涨落组成的动力学方程的数值计算可以得到SRO的状态和有序化过程。用X射线衍射仪和电子显微镜研究了Cu-40at%Pt合金中L11相分解为(A1+L11)相的过程。结果表明，当退火温度T_a高于某一T_m时，合金几乎完全无序到Al，随后析出L1_1。当T_a&lt；T_m时，相分解以Al在L1_1中析出进行。通过假定T_m对应于合金发生A1-L1相变的临界温度，从热力学上解释了退火过程的温度依赖性。讨论了从A1到(A1+L11)的过程。

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.