Effect of Quenched-in Vacancies on Aged Microstructures in Ti–15V–3Cr–3Sn–3Al

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Abstract
An effect of quenched-in vacancies on the microstructures during aging in a β titanium alloy, Ti–15V–3Cr–3Sn–3Al, is examined using two materials which are considered to have extremely different concentrations of excessive vacancies. In the material which is considered to have high concentration of quenched-in vacancies, the ω (≤250°C) and the α (≥300°C) phases precipitate relatively homogeneously in the β matrix regardless of dislocations in an early stage of aging, which indicates that the precipitation sites such as complex clusters of solute atoms and vacancies are formed in the β matrix in an early stage of aging. Meanwhile, in the material which is considered to have a small amount of excessive vacancies, the α phase nucleates on the vacancy sinks such asthe β grain boundaries, dislocations, etc. The material which is considered to have low concentration of quenched-in vacancies shows a single maximum peak of the age hardening around 400∼500°C. On the other hand, the material which is considered to have high concentration of quenched-in vacancies shows another peak around 300∼350°C in addition to the peak around 400∼500°C. The temperature range of these two peaks agrees to the temperatures at which two types of α morphologies are formed; one is the equiaxed aggregates morphology composed of the small particles formed at 300∼350°C, and the other is the lenticular α morphology formed at 400∼500°C, in which some monolithic α plates are included.

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