Kinetics of Phase Transformations

虽然在理解平衡相图方面取得了很大的进展，但转化动力学的计算或定量精确模拟更加困难。当存在诸如长周期超晶格相和应变与有序之间的耦合等复杂因素时尤其如此。通过将同步加速器的时间分辨x射线散射能力与最近开发的结合能计算和超级计算机技术相结合，可以取得重大进展。蒙特卡罗模拟中最先进的能量计算用于理解和预测时间分辨x射线衍射观察到的复杂有序动力学。该计算允许原子在理想晶格位置的局部弛豫以及全局晶格弛豫和对称性变化。重点是经典的CuAu合金，它具有有序和调制的相，以及有序时晶格对称性的变化。其他需要研究的重要问题包括调制相和有序相的独立有序旋轴的存在，从调制相形成有序相的动力学，以及应变和有序相之间的耦合。除了CuAu合金外，还将研究其他表现出长周期超晶格相的体系，特别是表现出调制相魔鬼阶梯的Ti-Al合金。由于长程超晶格相的存在和/或晶格常数和对称性的变化，许多合金中的有序动力学变得复杂。了解这些系统的变化是材料科学家面临的主要挑战。＊＊＊

9633596 Ludwig While much progress has been made in understanding equilibrium phase diagrams, transformation kinetics has been more difficult to calculate or simulate with quantitative accuracy. This is particularly true when there are complicating factors such as long-period superlattice phases and coupling between strain and ordering. By combining synchrotron-based-time-resolved x-ray scattering capabilities with recently-developed binding energy calculations and supercomputer technology significant advances can be made. State-of-the-art energy calculations in Monte-Carlo simulations are used to understand, and predict, complex ordering kinetics as observed by time- resolved x-ray diffraction. The calculations allow for local relaxation of atoms off their ideal lattice sites as well as global lattice relaxation and symmetry change. The focus is on the classic CuAu alloy, which possesses ordered and modulated phases as well as a lattice symmetry change upon ordering. Other important issues to be investigated include the presence of separate ordering spinodals for modulated and ordered phases, formation kinetics of the ordered phase from the modulated phase, and the coupling between strain and order. In addition to CuAu alloys, other systems which exhibit long-period superlattice phases will be investigated, in particular Ti-Al alloys which exhibit a devil's staircase of modulated phases. %%% Ordering kinetics in many alloys is complicated by the presence of long-range superlattice phases and/or changes in lattice constant and symmetry. Understanding the changes in such systems is a major challenge to materials scientists. ***