Metastable microstructure evolution and grain refinement in a Low-Ta containing γ-TiAl alloy through heat treatment

The paper comprehensively describes the evolution of metastable microstructures in a cast low-Ta-containing γ-TiAl alloy during cooling-tempering-lamellarization heat treatments. The behavior and mechanism of microstructure refinement are discussed. The rapid cooling generates metastable microstructures, including Widmanstätten, feathery, and massive structures, with many γ variants in small sizes and different orientations. It significantly refines the coarsened colonies and randomizes the texture. The tempering can effectively stabilize the metastable microstructures. During tempering, the γ→α phase transformation is more active at higher temperatures while coarsening and recrystallization dominate at lower temperatures. After tempering, a refined microstructure with spherical grains and convoluted structures forms. Subsequent lamellarization treatment optimizes the microstructure and obtains a fine-grained nearly-lamellar microstructure with small colonies (∼70 μm) and dispersed fine γ grains (∼10 vol%), which yielded a 1.0 % elongation with a 487 MPa yield strength (0.2%) and a 589 MPa ultimate tensile strength at room temperature. Finally, the unique promotion effects and mechanism of a small amount of Ta addition on the nucleation, growth, evolution, and stabilization of metastable microstructures are concluded and discussed, which is crucial for the refinement of cast γ-TiAl alloy by heat treatment alone.