Additive manufacturing of fine-grain fully lamellar titanium aluminide alloys

Additive manufacturing (AM), or 3D printing, has attracted increased attention in producing metallic parts with complex geometries, but it has proved difficult to prepare equiaxed fine-grain parts because the high thermal gradient in solidification commonly conduces the formation of coarse columnar grains. This work shows a solution to fine-grain titanium aluminide (TiAl) alloys by designing a high-frequency thermal cycling to control the solid-state phase transformations in layer-by-layer AM. After solidification, the specially-designed high-frequency thermal cycling can significantly refine the microstructure by repeatedly inducing the nucleation of new grains and suppressing the growth of these newborn fine grains. Therefore, even if solidification leads to coarse columnar grains, equiaxed fine-grain microstructure can still be obtained by solid-state phase transformations. The resulting TiAl alloys have fine heteromorphic grains (∼50 μm) and a fully lamellar microstructure. These fine grains contribute to good strength-ductility balance at room temperature, and their irregular shape and fully lamellar microstructure restrict the flow and distortion of grains at high temperatures, which stands a chance to significantly increase the operating temperature of TiAl parts.