Primary Phase Selection And Its Orientation Variation In Directionallysolidified Ti-46Al-2Cr-2Nb Alloy

With good high temperature performance, oxidation resistance and low density, TiAl based alloys is one promising high-temperature structural material in the aviation and aerospace industries. GE alloys with compositions of Ti-(46~48)Al-2Cr-2Nb are widely applied because of their good strength and ductility. Ti-46Al-2Cr-2Nb alloy is chosen in this paper, and directional solidification (DS) experiments are carried out with Bridgman technology in different solidification conditions with growth rate v=5μm/s~120μm/s, temperature gradient G=6K/mm~26K/mm, specimen diameter φ=3mm~10mm and growth length z=5mm~30mm, in order to study the effects and mechanisms of solidification conditions on the morphology and growth direction of primary phase. The experiment results show that the primary phases of directionally solidified Ti-46Al-2Cr-2Nb alloy are β and α phase respectively, when temperature gradient is G=26K/mm and G=6K/mm, not varying with the growth rate. When the temperature gradient is G=18K/mm, the primary phase transforms from β to α phase with the increase of v, where the critical transforming rate (vtr) lies in the range5μm/s <vtr <10μm/s. Moreover, when the diameter of specimen increases to6mm and10mm, the vtr increases to10μm/s <vtr <20μm/s and40μm/s <vtr <50μm/s, respectively. With the increase of temperature gradient, the primary phase transforms from β and α phase in constant growth rate. The critical growth length for the transformation of primary phase (ztr) decreases with the increase of growth rate and temperature gradient. The morphologies of the primary phases of the directionally solidified Ti-46Al-2Cr-2Nb alloy are also affected by the variation of solidification conditions, as follows: with the increase of growth rate and temperature gradient, both the primary arm dendrite spacing (λ1) and the secondary arm spacing (λ2) are decreasing. The λ1-v relationships are little affected by the transformation of primary phases, while the λ2-v relationships are greatly affected. With the increase of specimen diameter, the absolute exponent value in the λ1-v relationships decreases. Whereas, the λ1and λ2are not affected by growth length.Based on the highest interface temperature criterion, the primary phase selection of directionally solidified Ti-46Al-2Cr-2Nb alloy is investigated, which shows solidification condition influences the primary phase selection in the process of DS by affecting the interface temperature of β or α phase. The primary phase selection model based on diffusion condition can only qualitatively describe the change of the primary phase, and the calculated theoretical value of vtr is of great difference with the experimental value. When the convection in the melt is consisdered, the primary phase selection model can explain the effect of specimen diameter on the primary phase selection, but can not explain the phenomenon that the primary phase varies with the growth length. When the influence of initial transient region is introduced, the primary phase selection model can describe the variation of primary phase with the growth length, but the calculated theoretical value of ztr is of great difference with the experimental value. When the modification based on the solute enrichment caused by the growth of the specimen with a finite length is taken into account on the primary phase selection model, the primary phase transformation with growth length can be precisely predicted. The complex primary phase transformation in solidification procedure is caused by the convection and the solute enrichment. The solute enrichment caused by the growth of specimen is the dominate reason to facilitate the transformation of primary phase.When the primary phase of directionally solidified Ti-46Al-2Cr-2Nb alloy is β phase, the possible growth direction of β phase varies from <211>β,<311>β,<321>β or <322>β to the preferential growth direction of β (<100>β) with the increasing value of G/v. When the primary phase of the directionally solidified Ti-46Al-2Cr-2Nb transforms into α phase, the angle between the growth direction of α primary phase and the preferential growth direction of α (<0001>α) is about45°with G=18K/mm, not varying with the growth rate. When the temperature gradient incerases to26K/mm, the angle between the growth direction and the preferential growth direction of α phase changes from approximate90°(low growth rates) to approximate45°(high growth rates). EBSD results show that, during the procedure of primary phase transformation, the β phase and the α phase follow the Burgers relationship. The growth direction of the α phase in the final stage of DS is determined by the growth direction of the α phase in the initial stage of DS and the primary phase transformation in the process of DS, which leads to the growth direction of the α phase after the primary phase transformation deviating from its preferential growth direction.