Features of the →a2+y precipitation reaction in a Ti-40at. %Al alloy were investigated by transmission electron microscopy. In an as-quenched sample without aging, the a phase with the hep structure already transforms into the a 2 phase with the D019 one as a matrix and a phase with the B19 one as an intermediate precipitate. When the sample is aged at 1273K, amorphous regions are nucleated at the B19 intermediate precipitates and grow in the matrix. On further aging, the amorphous region is converted to a lenticular-shape precipitate with the L10 structure of the γ phase. The α 2/γ lamellar structure is eventually produced by the lateral growth of the lenticular-shape precipitates.
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