The effect of delayed quenching on the strength of Ti-6%A1-4%V alloy was studied, and the mechanism of strength deterioration with delay time of quenching was investigated based on the quantitative analysis of the microstructural changes. The marked reduction of strength with delay time was a phenomenon peculiar to heat treatment in a+fl two phase region, and its susceptibility depended on the chemical composition of the alloy. The microstructural changes taken place with the extension of delay

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... time of quenching were coarsening of primary a phase, formation of acicular a phase in 13 phase matrix, and lateral coarsening and the increase of the number of acicular a phase, in turn. These microstructural changes were very similar to variations in the quenched microstructure due to the mass effect of quenching in this alloy. The strength after aging in the delayed quenched plates could be analyzed with an excellent accuracy by the mixed law of two phase structures which was taken account of grain size in primary a phase. The susceptibility of delayed quenching was controlled by stability of /3 phase at the reheating temperature of quenching, that is, by the increase of the content of /I-stabilizing elements such as V.