Modelling microstructural evolution in binary alloys
Modelling microstructural evolution in binary alloys
复制标题
模拟二元合金的微观结构演化
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
T. Rautiainen
中科院分区：
文献类型：
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作者：
T. Rautiainen
在这篇论文中，当使用确定性平均场和随机蒙特卡罗方法对相分离和随后的微观结构粗化进行建模时，系统地研究了形态、粗化机制和动力学。对于平均场方法，采用了Khachaturyan的微观扩散方程，以及它与环境相关的迁移率的变体。Monte Carlo模拟进行了与空缺和川崎动力学，并在空缺的情况下应用的停留时间算法。在平均场模型中，微观结构的演化是由自由能泛函的直接最小化引起的，原子扩散的机制并没有明确地出现。在Monte Carlo模型中，占位位的变化受相邻原子的直接交换（川崎动力学）或空位运动的影响。本文研究了平均场模型和蒙特卡罗模型在描述二元合金相变中的对应关系。给出了确定性和随机模型之间的这些差异影响相变的情况下的几个例子，并分析了潜在的差异。还研究了蒙特卡罗模型中扩散机制的选择如何影响微观结构的演变。大多数蒙特卡罗研究都是用川崎动力学进行的，尽管在真实的金属中这种直接交换不太可能发生。它将显示如何空位扩散机制产生各种粗化机制在一定的温度范围内，其中的川崎动力学未能捕获。因此，动力学和所得的形态，特别是在低温下，受到影响。最后，在平均场和蒙特卡罗模型的时间尺度的物理性的问题进行了讨论。通常假设蒙特卡罗时间和真实的物理时间之间存在线性关系，尽管对此没有严格的证明。在平均场模型中，时间是由原子迁移率定义的，通过考察相变过程中实际扩散机制的影响，对微观平均场模型和具有川崎动力学的Monte Carlo模型中的时间尺度进行了讨论.
In this thesis morphologies, coarsening mechanisms and kinetics are examined in a systematic way, when phase separation and subsequent microstructural coarsening is modelled using deterministic mean field and stochastic Monte Carlo methods. For the mean field approach a microscopic diffusion equation due to Khachaturyan is employed, and a variation of it with an environment dependent mobility. Monte Carlo simulations are carried out with vacancy and Kawasaki dynamics, and a residence time algorithm is applied in the vacancy case. In mean field models microstructural evolution results from a direct minimization of a free energy functional, and the mechanism of atomic diffusion does not appear explicitly. In Monte Carlo models, changes in site occupancies are effected by direct exchanges of neighbouring atoms (Kawasaki dynamics), or through vacancy motion. In this thesis the correspondence between mean field and Monte Carlo models in describing phase transformations in binary alloys is examined. Several examples of cases in which these differences between deterministic and stochastic models affect the phase transformation are given, and the underlying differences are analyzed. It is also investigated how the choice of diffusion mechanism in the Monte Carlo model affects the microstructural evolution. Most Monte Carlo studies have been carried out with Kawasaki dynamics, although in real metals such direct exchanges are very unlikely to occur. It will be shown how the vacancy diffusion mechanism produces a variety of coarsening mechanisms over a range of temperatures, which the Kawasaki dynamics fails to capture. Consequently, kinetics and resulting morphologies, especially at low temperatures, are affected. Finally, the question of physicality of time scales in mean field and Monte Carlo models is addressed. Often a linear dependence between Monte Carlo time and real physical time is assumed, although there is no rigorous justifcation for this. In mean field models, time is defined through the atomic mobility.By examining the effect of a realistic diffusion mechanism in systems undergoing phase transformation, a critical discussion of time scales in microscopic mean field models and a Monte Carlo model with Kawasaki dynamics is presented.