Ti-15V-3Cr-3Sn-3Alの時効組織におよぼす焼入れ凍結空孔の影響

Abstract

Effect of the 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 concentration of excessive vacancies.In the material which is considered to have high concentration of quenched-in vacancies, ω (≤250°C) and α (≤300°C) phases precipitate relatively homogeneously in β matrix regardless of dislocations in the early stage of aging, which indicates that the precipitation sites such as complex clusters of solute atoms and vacancies are formed in β matrix in the early stage of aging. Meanwhile, in the material which is considered to have little amount of excessive vacancies, α phase nucleates on the vacancy sinks such as 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 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.