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

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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.