Effect of Cr and Si additions on the continuous-cooling-transformation kinetics of γ-based Ti-45 at.% Al alloy

The effect of Cr and Si additions on the continuous-cooling-transformation (CCT) kinetics of Ti45Al alloy has been studied at cooling rates from about 0.5 to 500 degreesC s(-1) using our recently constructed in-situ real-time resistivity-temperature measurement apparatus. The alpha --> gamma (L) lamellar reaction occurs at slow cooling rates and only the alpha --> alpha (2) ordering reaction occurs at fast cooling rates; no massive reaction occurs at intermediate cooling rates. The addition of Cr results in a simple decrease in the reaction temperatures for the alpha --> gammaL lamellar reaction without a change in the critical cooling rate for the ordering reaction only, in partial agreement with the depression of the alpha transus temperature. The ordering start temperature was measured to be slightly higher. The addition of Si, on the other hand, significantly accelerates the kinetics of alpha-->gamma (L) lamellar reaction by shifting its CCT curve towards a higher temperature and a shorter time. This is because silicides formed at high temperatures on grain boundaries raise the gamma -phase nucleation temperature and also because Si enhances its growth rate, probably by increasing the chemical diffusivity of alpha phase. The addition of Cr to a Si-containing Ti-45 at.% Al alloy significantly refines its lamellar spacing either by suppressing the Si segregation to grain boundaries or by reducing the alpha-gamma interfacial energy. The antiphase domain size in the alpha (2) phase is distinctively small in the Si-containing alloy probably owing to its interfacial segregation tendency.