Estimation of high temperature phase equilibria in directionally solidified intermetallic Ti-45.9Al-8Nb alloy

摘要

High temperature phase equilibria were studied in directionally solidified (DS) intermetallic Ti-45.9Al-8Nb (at. percent) alloy by the means of quench during directional solidification (QDS) experiments, microstructure analysis and differential thermal analysis (DTA). During directional solidification at constant growth rates V ranging from 5.56 X 10~(-6) to 1.18 X 10~(-4) m s~(-1) and three measured temperature gradients in liquid at the solid-liquid interface G_L of 3.5 X 10~3, 5 X 10~3 and 8 X 10~3 Km~(-1) the primary solidification phase is identified to be beta phase (Ti based solid solution with bcc crystal structure). The microstructural analysis of quenched mushy zones excluded formation of alpha phase (Ti based solid solution with hcp crystal structure) by a peritectic reaction during directional solidification. Experimentally determined phase transformation sequences L (liquid) - > L + beta - > beta - > beta + alpha and estimated high temperature phase stability are different from those resulting from thermodynamic calculations using Thermo-Calc software and TiAl database. Due to preferred [001] crystallographic orientation of the beta dendrites, lamellar alpha_2(Ti_3Al)/gamma(TiAl) boundaries in DS ingots are observed to be inclined at the angles of 0 deg and 45 deg to the growth direction.