EFFECTS OF BETA-GRAIN SIZE AND MORPHOLOGY OF ALPHA-PRECIPITATE ON THE MECHANICAL-PROPERTIES OF TI-15V-3CR-3SN-3AL ALLOY

The effect of (1) β grain size and (2) α precipitate morphology on the mechanical properties (i.e., tensile strength, elongation) of a meta-stable β titanium alloy (Ti-15V-3Cr-3Sn-3Al) has been investigated. β single phase specimens of varying grain size (i.e., approximately 15 to 200 μm) were obtained by solution heat treatment above the β transus temperatures for various periods of time, followed by water quenching. The specimens were prepared for mechanical testing by (1) isothermal aging, and (2) two-stage aging, followed by water quenching. In the case of solution treated specimens, both the tensile strength and the elongation increase with decreasing β grain size. After aging in the (α + β) two-phase region, regardless of the aging method, the specimens having the same β grain size show increasing tensile strength and decreasing elongation with α precipitate refinement. However, when the morphology of α precipitates (i.e., shape, size and distribution) is approximately the same, the tensile strength is found to be independent of the β grain size, whereas the elongation increases with decreasing β grain size. Improvements in the ductility are likely due to suppression of brittle rupture at or near the grain boundaries through the refinement of β grain size.