Microstructure evolution and phase transformation of the mushy zone in a quenched β-solidifying TiAl alloy

Abstract

This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique. The results indicate that the mushy zone consists of unmelted β dendrites and interdendritic liquid, whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent. The β dendrite is composed of numerous subgrains with various orientations. During quenching, the β dendrite transforms into Widmanstätten α via a precipitation reaction, owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid. Additionally, after quenching, the interdendritic liquid is transformed into γ plates. Within the single β phase field and the lower part of the mushy zone, a massive transformation of β to γ occurs . Conversely, in the β+α phase field, both β and α phases are retained to ambient temperature. During the heating process, the transformation of α → β gives rise to the formation of β variants, which affects the orientation of β dendrites in the mushy zone. The growth kinematics of the α → β transformation was elucidated, revealing the preferential growth directions of 〈111〉 and 〈112〉 for β variants. Furthermore, this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.