phase evolves to single α phase or α+γ phases, depending on the heating temperature. Using the γm as a starting structure, every γ grain would be divided to four new α grains in the process of heating to the single α region. This is because γ phase in TiAl alloys has four habit planes for the precipitation of α phase. In order to divide the γ grain completely and have full transformation to α phase, the specimen should be heated to above Tα. In the subsequent cooling process, γ plates would precipitate with the same orientation in every α grain. This results in the formation of a fully lamellar structure.