Investigation of microscopic mechanisms of diffusional phase transformations

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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