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.