Effect of growth rate and diameter on microstructure and hardness of directionally solidified Ti-46Al-8Nb alloy

Bridgman-type directional solidification experiments were conducted for Ti-46Al-8Nb (mole fraction, %) alloy. The effects of the growth rate and the diameter on the microstructure, phase transition and hardness of the alloy were investigated. The results show that with the increase of the growth rate and the decrease of the diameter, the fully β phase solidification changes to the peritectic solidification, and the final microstructure is composed of the α2/γ lamellar structure and a multiphase microstructure (B2 phase, α2/γ lamellar structure) respectively, which can be attributed to the solute enrichment resulting from the decreasing diffusion and convection ability. The occurrence of peritectic reaction at high growth rate promotes the solute segregation heavily and the coarse lamellar spacing in Al- and Nb-rich region, which greatly decreases the hardness values and leads to the discontinuity of the hardness curves with the increase of the growth rate. Comparatively, the Ti-46Al-8Nb alloy has lower hardness values than the other applied TiAl-based alloys in previous studies.