Crystallographic study on the precipitation and the phase transformation processes in alloys

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(1) Mechanism of intermediate structure formation in steels and metastable beta phase titanium alloysThe habit plane, the orientation relationship between ferrite and austenite, the habit plane of the cementite platelets, the orientation relationship between ferrite and cementite in the upper bainite in low carbon nickel steels have been analysed by means of transmission electron microscopy. From these results, A model of upper bainite transformation mechanism, Shear-assisted Diffusional Transformation Mechanism, where the lattice change from austenite to ferrite is assumed to occur via individual atomic jumps across the phase interface with keeping the atomic-site correspondence but the elastic strain due to this phase change should be relaxed by the lattice invariant shear, has been proposed. This model has been successfully applied to the Widmanstatten alpha formation in the titanium alloy. It has also been shown that the bainite-like structure consisting acicular ferrite and carbide often observed in Fe-Cr-C alloys is definitely not a bainite but a kind of eutectoid structure.(2) Crystallographic analyses for the precipitation processed in non-ferrous alloysThe omega phase precipitation in a metastable beta phase titanium alloy and the effects of it on the beta*alpha and beta*alpha" have been examined. In addition to this, the early stage of ageing in an Al-Li based alloy was also been investigated by means of HRTEM.Using the Fourier and the inverse Fourier Transforms of the high resolution images, it has been shown that the congruent ordering processes occur in this alloy.