Phase Transformation and Fine Fully Lamellar (FFL) Structure Formation in a High Nb-Containing Beta-Gamma TiAl Alloy

TiAl intermetallics with fine fully lamellar (FFL) microstructure exhibit great potential application in high-performance combustion engines. In this paper, the phase transformation sequence of a high Nb-containing beta-gamma TiAl alloy is obtained by using metallographic method. The results indicate that the single α-phase field of Ti-43Al-8Nb-0.2W-0.2B alloy is inexistent or extremely narrow, which is caused by slight W element possibly. The temperatures of γ-solvus and β-transus are in the range of 1225–1230 °C and 1400–1410 °C, respectively. Based on the phase transformation temperatures, an improved multi-step heat treatment of homogenizing in single β-phase field followed by slightly higher than γ-solvus is designed to obtain a FFL microstructure. The average lamellar colony size of the optimal microstructure is about 80 µm and the volume fraction of B2 phase is about 1%. Further analyses indicate that the elements redistribution and the secondary particles pinning effect are the mainly mechanisms of transforming to FFL microstructure.