準安定β型チタン合金における(α+β)微細二相組織の形成と機械的性質

抄録

The effects of cold working (0-80%) of the β single phase on the precipitation behavior of the α phase during aging in the (α+β) two-phase region and the mechanical properties have been studied for Ti-15%Mo-5%Zr-3%Al and Ti-12%Mo-6%Zr-5%Sn metastable β alloys. The β single phase is prepared by heating the alloys above the β transition temperature followed by quenching into water. The resulting (α+β) structures are considerablly affected by cold working of the β phase. Without cold working, the α phase preferentially precipitates at β grain boundaries, and then plate-like α comes out in the β grain. When the amount of cold working is less than 50%, the precipitation of the α phase preferentially occurs not only at the β grain boundaries but at the dislocations introduced into the β grain by the cold working. When the amount of cold working becomes as large as 80% , the β subgrain structure is rapidly formed by recovering during heating up to the aging temperature or in the early stages of aging. In such a case, the α particles precipitate at these β subgrain boundaries. Therefore, the (α+β) microduplex structure, which consists of very fine β subgrains and α particles, is obtained. In the case of the Ti-15%Mo-5%Zr-3%Al alloy, the precipitation of the α phase occurs in the earlier stages of aging and the amount of the α precipitate is much larger than that in the case of the Ti-12%Mo-6%Zr-5%Sn aIloy; thus a much finer (α+β) microduplex structure is formed in the former alloy. The mechanical properties such as tensile strength and elongation are improved by forming the (α+β) microduplex structure in both Ti-15%Mo-5%Zr-3%Al and Ti-12%Mo-6%Zr-5%Sn alloy.