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Massive parallel calculations for phase transition driven by cooperative mechanisms with different time constants

不同时间常数协作机制驱动的相变大规模并行计算

基本信息

批准号：
08650761
负责人：
MOHRI Tetsuo
金额：
$ 1.34万
依托单位：
HOKKAIDO UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1996
资助国家：
日本
起止时间：
1996 至 1997
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-08650761/
关键词：
time constant relaxation time order-disorder transition Path Probability Method correlation function degree of order 規則・不規則変態経路確率法

项目摘要

In the present study, the main focus is placed on order-disorder transition.As a preliminary calculation, we performed short range order diffuse intensity calculation in the disordered region of fcc system by explicitly distinguishing the time constant of phonon vibration process and atomic diffusion process. It is revealed that kinetic effects appear in the temperature dependency of diffuse intensity as a deviation from the equilibrium locus.A purely mathematical analysis was followed and it was found that the appearance of kinetic effects corresponds to transformation and inverse transformation of a function.A pseudo-critical slowing down phenomenon has been observed by electrical resistivity measurements for Ni3Pt intermetallic compound which undergoes first order transition. In order to reveal the mechanism, we attempted numerical calculation within the frameworks of the Tetrahedron Approximation of the Cluster Variation and Path Probability Methods. In the conventional Path Probability Method, an explicit temperature dependency is not considered in the atomic diffusion term in the path probability function. In the present study, on the other hand, a thermal activation process is introduced. The resultant relaxation time reproduces the normal temperature dependency for an ordinary temperature region except near transition temperature. Then, a slight temperature dependency is further introduced in the activation energy, and it was confirmed that the experimental result of abrupt increase of relaxation time in the vicinity of transition temperature is satisfactorily reproduced. When one considers the physical meanings of activation processes involved in the path probability function, one realizes that this additional activation process corresponds to the two distinguished relaxation processes resulting in the different time constants.

本研究的重点是有序-无序转变，作为初步计算，我们通过明确区分声子振动过程和原子扩散过程的时间常数，计算了面心立方体系无序区的短程有序扩散强度。结果表明，扩散强度随温度的变化存在动力学效应，偏离了平衡轨道;纯数学分析发现，动力学效应的出现对应于函数的变换和逆变换;电阻率测量发现，经历一级相变的Ni 3 Pt金属间化合物存在伪临界慢化现象.为了揭示这一机制，我们尝试在四面体近似的簇变分和路径概率方法的框架内进行数值计算。在传统的路径概率方法中，在路径概率函数中的原子扩散项中没有考虑显式的温度依赖性。在本研究中，另一方面，热活化过程中引入。得到的弛豫时间再现正常的温度依赖性为一个普通的温度区域，除了附近的转变温度。然后，在活化能中进一步引入了轻微的温度依赖性，并且证实了在转变温度附近弛豫时间突然增加的实验结果是令人满意地再现的。当考虑路径概率函数中所涉及的激活过程的物理意义时，人们认识到，这个附加的激活过程对应于导致不同时间常数的两个不同的弛豫过程。