MICROSTRUCTURAL INVESTIGATION ON THE CRYSTALLISATION AND AGEING PROCESSES OF ADVANCED ALLOYS

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The crystallisation, phase transformation and ageing processes of various advanced materials have been investigated by means of high resolution transmission electron microscopy, differential scanning thermal analysis, X-ray diffection and vibrating sample magnetometer and the following results were obtained.(1) The crystellisation processes and magnetic properties of Pr-Fe-B and Nd-Fe-Cr-B amorphous ribbonsThe decomposition of coarse Nd_2Fe_<23>B_3 particles into coarse Nd_2Fe_<14>B_2 particles involving very fine α-Fe particles has been found in the later stage of annealing. It has also been found that this structure exhibits excellent hard magnetic properties arising from the exchange coupling interaction between coarse Nd_2Fe_<14>B_2 particles and fine α-Fe particles within them.(2) The ageing process of Al-Mg-Si alloysIn this study, it has been confimed that the structure of β" particle which contributes precipitation hardening is in good keeping with the model proposed by Andersen … More . It was also shown that the reaction following the β" precipitation is not the β' rod formation as has been believed so far but the precipitation of cuboid β particles. The existence of three different orientation relationships betwen β particles and the matrix has been recognised and one of them has been found in this study. Such relationships can be explained by considering the morphology, volume elastic energy and the interfacial energy.(3) Crystallographic study on Fe-Ni-C bainiteAlthough bainite transformation occurs via a diffusional lattice change with atomic-site correspondence and the lattice invariant shear, it has been confimed that me broad fece of bainite migrates subsequently in diffusional mode. Thus, the dislocation structures on the interface cannot be the evidence of the ledge mechanism proposed by Aaronson et al.(4) The transformation processes of near β Ti alloys and Ti-Al alloysIn Ti alloys, the effects of ω phase particles on the α" martensitic transformation has been examined. It has been shown that the initial stage of ω phase precipitation accelerates the β→α" martensitic transformation but the later stage stabilises the martensitic transformation significantly. In Ti-Al alloys, the models of massive γ structure and the anti-phase domain formation have been proposed. Less