Microstructures, micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

Abstract

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb (at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated. The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 µm∙s-1 under the temperature gradients of 15-20 K∙mm-1, and the primary phase is transformed into an α phase at relatively higher growth rates (V >20 µm∙s-1). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 µm∙s-1 under a temperature gradient of 15 K∙mm-1. The increase of temperature gradient to 20 K∙mm-1 can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as \(L\xrightarrow{{L \to \beta }}L + \beta \xrightarrow{{L + \beta \to \alpha }}\alpha + {\beta _R}\xrightarrow{{\beta \to \alpha }}\alpha \xrightarrow{{\alpha \to \gamma }}\alpha + \gamma \xrightarrow{{\alpha \to {\alpha _2} + \gamma }}{\gamma _R} + \left( {{\alpha _2} + \gamma } \right)\), in which βR and γR mean the residual β and γ