Microstructure evolution through the α→γ phase transformation in a Ti-48 At. pct Al alloy

Abstract

The {alpha} {r_arrow} {gamma} phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed {gamma}-grain structure to a mixed microstructure of the massively transformed {gamma} grains and the untransformed (meaning massively untransformed) fine {alpha}{sub 2}/{gamma} lamellae with an increase in the cooling rate from the high-temperature {alpha} phase field. Fine {gamma} grains are generated from these fine {alpha}{sub 2}/{gamma} lamellae by subsequent aging at 1,323 K. The fine {gamma} grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive {gamma} grains. This result suggests that the formation mechanism of the fine {gamma} grains during aging is similar to that of the massive {gamma} grains. When the fine {gamma}/{gamma} lamellar sample, which is formed by preliminary aging at a lower temperature (1,173 K), is aged at a higher temperature (1,323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine {gamma}-grain formation is closely related to the {alpha}{sub 2} {r_arrow} {gamma} phase transformation in the fine {alpha}{sub 2}/{gamma} lamellae.