Development of the bifurcation theory for the microstructure changes and an application to the superalloys

当析出相颗粒受到弹性约束时，随着时效的进行，显微组织的演变过程中出现了一些有趣的现象。微观结构变化的异常行为的典型例子是：1）颗粒减速成若干个小颗粒;我们提出的形状分叉理论为这些现象提供了理论支持。本研究对这些有趣的现象进行了研究，得到了更广泛的实验数据和更精确的理论背景，得到的结果如下：（1）Ni-Al-Ti合金中观察到颗粒粗化的减速现象。当粗化速率开始减慢时，实验得到的平均粒径与基于形状分叉理论的计算值吻合得很好。(2)在Al-Zn系GP区粗化过程中，实验观察到生长速率的减慢。基于Cahn-Hilliard非线性扩散方程，通过计算机模拟研究了这种随时间变化的生长速率减慢。定量计算了含GP区的斑片状微结构中波长随时间的演化。形状分叉理论与计算机模拟动力学分析相结合，将是研究复杂微结构变化的静态和动态稳定性的一种非常有用的方法。(3)采用“成分梯度法”对Fe-Si二元合金有序/无序相变线上的微观形貌变化进行了实验研究。在A2/B2转变组分附近观察到精细的B2畴结构。

Several interesting phenomena during microstructure evolution with aging have recently been observed when precipitate particles are elastically constrained. The typical exmples of the extraordinally behavior of the microstructure changes are 1) the deceleration of particle into several small particle. These phenomena have theoretically supported by "Shape Bifurcation Theory", proposed by us. In the present research, we study these interesting phenomena to get more wide range of experimental data and also to give more accurate theoretical background.The results obtained are as follows ;(1) The deceleration of particle coarsening is observed in Ni-Al-Ti alloy. The average particle size experimentally obtained when the coarsening rate started to slow down shows quite good agreement with the calculated value based on the shape bifurcation theory.(2) In the coarsening of the GP zone in Al-Zn system, the deceleration of the growth rate is observed experimentally. This time dependent slow down of the growth rate is investigated by the computer simulation on the basis of the Cahn-Hilliard non-linear diffusion equation. The time evolution of the wave length in mottled microstructure containing GP zones is calculated quantitatively. The shape bifurcation theory combined with the kinetic analysis by means of the computer simulation will extremely be a useful method to investigate the static and dynamic stability of the complex microstructure changes.(3) The morphological microstructure changes across the order/disorder phase transition line in the Fe-Si binary alloy is experimentally studied by using the "composition gradient method". The fine B2 domain structure was observed at the vicinity of the A2/B2 transition composition.