Effects of growth rate on primary phase and microstructures of directionally solidified Ti-46Al-2Cr-2Nb alloy

TiAl alloys with fully lamellar structure has been intensively studied for excellent fracture toughness and creep properties. Directional solidification is an effective way to control the lamellar structure. Thus it is important to investigate the effect of solidification parameters on the structure of TiAl alloys during directional solidification. In this work, microstructures of directionally solidified Ti-46Al-2Cr-2Nb (atomic fraction, %) alloy has been investigated through Bridgman-type directional solidification experiment at constant temperature gradient and wide range of growth rates (v=10-120 μm/s). The type of primary phase, the primary dendritic arm spacing (λ₁), the secondary dendritic arm spacing (λ₂) and the lamellar spacing (λ_e) are investigated. It is found that: the primary phase of the specimens transformed from β phase to λ_e phase with the increase of growth rates; the transformation trend of primary phases with growth rates can be predicted by the phase selection model based on the highest interface temperature criterion. λ₁, λ₂, and λ_e all decrease with the increasing of v. The relationship between λ₁ and v is not affected by the transformation of primary phases, which follows λ₁=700.6v^-0.24 relationship. However the relationship between λ₂ and v is associated with the type of primary phases, λ₁ and v follows λ₁=44.0v^-0.10 relationship when β is the primary phases, while follows A2=57.3v^-0.23 relationship when the primary phase has transformed to α phase. The relationship between λ_e and v can be expressed by λ_e=16.4v^-0.76. Compared with TiAl binary alloys, lamellar spacing is more effectively refined by increasing the growth rates in Ti-46Al-2Cr-2Nb alloy.