Investigation of microscopic mechanisms of diffusional phase transformations

本课课组开展扩散相变研究，为材料的微观组织控制奠定实践基础，主要针对合金中有序-无序转变和相分离的微观过程——原子扩散和异相边界的原子结构这两个方面进行研究，这两个方面控制着母相与转变产物之间边界的迁移率。研究结果可以总结如下。(1)对L1_2结构镍基金属间化合物（Ni_3Al， Ni_3Ga和Ni_3Ge）中的原子扩散进行了实验研究，并对点缺陷的热力学进行了理论分析。α-亚晶格中的空位机制是这类材料的主要扩散机制。还精确地测定了某些金属玻璃和准晶体中各组分的扩散系数。(2)在FePd的畴结构中，在序上施加应力和磁场是非常有效的。这一现象的起源已经从理论上得到了解释。(3)采用高分辨率透射电镜等技术研究了Ni-Mo和Cu-Pt合金的近程有序。系统地叙述了其微观结构和排序过程。(4)通过小角散射实验和计算机模拟研究了某些铝基合金相变的初始阶段。在建立的沉淀硬化合金设计指导原则的基础上，设计并制备了一种抗拉强度在900 MPa以上的新型铝合金。(5)用x射线异常散射技术研究了金属玻璃中的原子结构（排列）。澄清了这些材料热稳定性的来源。(6)通过直接观察和浓度剖面测量研究了异相界面的原子结构和组成变化，为讨论异相界面的稳定性奠定了基础。(7)通过研究钢中非均匀形核析出相与母相界面的原子结构及其晶体学关系，建立了高强高韧性钢的微观组织控制方法。少

This group carried out research on diffusional phase transformations to establish practical basis for microstructure-control of materials, aiming particularly at the two aspects :-the microscopic processes of order-disorder transformation and phase separation in alloys-atomic diffusion and atomic structure of heterophase boundaries, both of which control the mobility of the boundaries between the parent phase and the transformation products.The results of the research can be summarized as follows.(1) Atomic diffusion in nickel-based intermetallic compounds of the L1_2 structure (Ni_3Al, Ni_3Ga and Ni_3Ge) has been investigated experimentally, in parallel with theoretical analyses of the thermodynamics of point-defects. The vacancy mechanism in the α-sublattice has shown to be the dominant mechanism of diffusion in this class of materials. The diffusion coefficients of constituents in some metallic glasses and quasicrystals have also been determined precisely.(2) Application of stress o … More r magnetic field on ordering has been shown to be very effective in producing strong variations in domain structures in FePd. The origin of the phenomenon has been explained theoretically.(3) The short-range order in Ni-Mo and Cu-Pt alloys have been investigated by high-resolution transmission electron microscopy and some other techniques. A systematic account of the microscopic structure and the ordering processes has been given.(4) The initial stage of the phase transformation in some aluminium-based alloys has been studied by small-angle scattering experiments and computer simulation. On the basis of the guiding principle for the design of precipitation-hardened alloys established, a new aluminium alloy has been designed and prepared, which exhibits tensile strength above 900 MPa.(5) The atomic structure (arrangement) in metallic glasses has been studied by the X-ray anomalous scattering technique. The origin of the thermal stability of those materials have been clarified.(6) The atomic structure and the variations in composition of heterophase interfaces have been investigated by direct observations and concentration profile measurements to establish the basis for discussing the stability of those interfaces.(7) By studying the atomic structure of the interfaces between inhomogeneously-nucleated precipitates and the parent phase in steels and their crystallographic relationships, the method of microstructure control for high-strength, high-toughness steels has been estabilished. Less