Abstract
The aim of the present work is to obtain a detailed knowledge of aging properties of Ti-Mo base beta alloys. Effects of solution-treatment, normal aging, two-step aging and isothermal transformation on the tensile properties, in connection with the microstructures, of three beta alloys were investigated.
Though aluminum suppressed precipitation of the omega phase, the embrittlement due to the alpha phase which was hardened by aluminum was remarkable in the Al-bearing alloy, Ti-12Mo-5Zr-3Al, in which the alpha phase precipitated in abundance. Ti-15Mo-5Zr-3Al showed a high strength of 143 kg/mm2 even after aging at a rather high temperature, 550°C, because aluminum strengthened the alpha. The optimum aging temperature range for the Al-free alloy, Ti-15Mo-5Zr, was narrow, in which balanced tensile properties with a ultimate tensile strength of 140 kg/mm2 and an elongation of 11% were obtained.
The alpha which precipitated primarily at grain boundaries after isothermal transformation was not effective for strengthening. There was no fundamental differences between normal and two-step aging in this study.
It is attributed to the recrystallization of beta grains that the precipitation site of the alpha phase during aging was affected by the preceding solution treatment. The Ti-15Mo-5Zr solution-treated at the temperature just below the beta-transus, consisted of the recrystallized and non-recrystallized grains, and the alpha phase which was not hardened by aluminum precipitated preferentially on dislocations in the latter grains. In the alloy solution-treated at a temperature considerably higher the beta transus, the alpha phase precipitated at grain boundaries. Ductility of the former alloy was much higher than that of the latter.
