Effect of iron content on β → α phase transformation behavior of Ti-5Al-xFe (x = 1, 2.5, 4) alloys during continuous cooling

Abstract

We investigated the effect of Fe content on the β → α phase transformation behavior of Ti-5Al-xFe (x = 1, 2.5, 4) alloys during continuous cooling. The presence of Fe delayed long-range diffusional phase transformation, thus facilitating diffusionless transformation even at low cooling rates. This effect was explained by the extremely low solubility of Fe atoms in the α phase, so that they must be significantly redistributed before the precipitation of the equilibrium α phase. Fe also reduced the phase-transformation start temperature by lowering the β-transus temperature. Differences in the β-transus temperature of the alloys accounted for the differences in the phase-transformation start temperature at a low cooling rate. However, as the cooling rate increased, the difference in the phase-transformation start temperature greatly increased, which could no longer be explained by the difference in the β-transus temperature. It was reasoned that Fe significantly stabilized the β phase; hence, the effective β-transus temperature decreased as the cooling rate increased. This reduction was intensified by an increase in the Fe content. The strong dependence of the β → α phase transformation behavior on the Fe content led to the development of different microstructures in the alloys. Hardness was strongly dependent on the resulting microstructure, which differed with the Fe content and cooling rate. Our results provide a clear understanding of the β → α phase transformation behavior of these alloys and would be useful for designing appropriate solution heat treatment. Keywords: Microstructure, Dilatometry, Continuous cooling, Phase transformation, Titanium alloy, Hardness