In an initial stage of a eutectoid reaction of a Ti-40 at.% Al alloy, hcp→D019+L10, an amorphous state was found to form around an intermediate precipitate with the B19 structure, together with a change in crystal structure of the matrix from the hcp structure to the D019 one. Characteristic features of the amorphous state are that the destruction of the crystal lattice mainly takes place in the (001) plane of the D019 structure but is very slightly extended along the  direction. The shape of the amorphous region is just like a thin film. On the basis of the present data, the origin of the two-dimensional-like character of the lattice destruction is simply discussed.
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