Recrystallization in Ti-15Mo Base Beta Titanium Alloys

Abstract

The precipitation site of alpha, which is related to the mechanical properties of the aged Ti-Mo base beta alloys is, as previously reported, affected by the preceding solution-treatment. It seems that this phenomenon is related to the recrystallization behavior of the alloys. The effects of the preceding working and the solution-treatment on the precipitation site of alpha during aging were studied in Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al alloys.
The beta transus was determined using an optical microscope. By observing the structure after heat treatment at temperatures above or below the estimated beta transus, the precipitating or vanishing point was determined. Detection of a small quantity of alpha on the grain boundaries was difficult. Consequently the specimens in which alpha was uniformly precipitated in the grains by pre-aging, were examined. The beta transi of Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al obtained in the present study were 730 and 785°C respectively.
Two steps of recrystallization were observed in the alloys. Alpha precipitated non-uniformly during aging even in the specimens, in which most of the deformed grains were changed to equiaxed ones in the initial stage of recrystallization. The fact that alpha precipitated in some grains and not in others is due to the incompleteness of the recrystallization. Alpha precipitated preferentially on the grain boundaries after the final stage of recrystallization.
The relation between the temperature and the time of the two stages of recrystallization is summarized in a diagram. The final stage recrystallization temperature was affected by the preceding deformation conditions.