Effect of solution-treatment conditions on aging response in Ti-15V-3Cr-3Sn-3Al

In a beta-type titanium alloy, Ti-15V-3Cr-3Sn-3Al, solution-treatment conditions have a significant influence on the age-hardening and the aged microstructures. In the specimens which are believed to contain a high density of quenched-in vacancies, such as the specimens quenched from high temperatures in the beta region or the ones having a low density of the vacancy sinks (grain boundaries, dislocation, etc.), the aging response is greatly accelerated and the precipitation is relatively homogeneous. In contrast, in the specimens which are believed to content a low density of excess vacancies, such as the specimens quenched from the temperature just above the beta transus, the aging response is delayed and the precipitation is inhomogeneous. The acceleration of the precipitation is probably due to the promotion of the nucleation sites formation by excess vacancies. The phenomenon is also seen in the real products having thicker sections such as forgings, thick plates, etc., especially in the larger beta grains. The cooling rates also affect the concentration of quenched-in vacancies, resulting in the morphological difference between the surface area and the inside in the thick products. The phenomenon can be applied to the surface hardening method, in which the surface layer of the specimen having the balanced mechanical properties(HV350) can be hardened to HV470