抄録
Formability of the quenched Ti-2∼8% Al-2∼8% Co alloys was studied by the tensile tests and the cold-rolling test. The values of δmax⁄σ(δ=δmax) were calculated as “formability index” from the tensile test data, where δ is the true strain, δmax is the limit value of the uniform elongation and σ is the true stress. High formability indices were obtained with the specimens transforming from β to martensite by stress. Particularly, the formability index of Ti-4% Al-4% Co alloy which was quenched from 900°C corresponded to commercial pure titanium. Low formability indices were obtained with the specimens having a stable β structure. The specimens having α+Ti2Co, α+β+Ti2Co and α+β structures had moderate formability indices. Results of the cold-rolling test indicated that the specimens transforming remarkably to martensite by stress was low in reduction ability, and that the specimens having a stable β phase was high in reduction ability. Particularly, Ti-4% Al-8% Co and Ti-6% Al-6% Co quenched from β phase excelled in reduction ability. This difference may be attributed to the reduction ability during rolling which is not determined by the extent of uniform elongation but by the occurrence of cracks during the deformation beyond the limit of uniform elongation. In other words, owing to the low rate of strain hardening, Ti-4% Al-8% Co alloy quenched from β phase has a small uniform elongation, but it can be deformed heavily beyond the limit of uniform elongation. On the other hand, the specimens forming stress-induced martensite with a high rate of strain hardening have a large uniform elongation, but they show cracking when deformed beyond the limit of uniform elongation. It can be concluded that the specimens having a relatively stable β phase are suitable for the forming which does not require a large uniform elongation, and that the specimens readily forming stress-induced martensite are suitable for the forming in which a large uniform elongation is required.
