Microstructure and Mechanical Properties of Powder Metallurgical TiAl-Based Alloy Made by Micron Bimodal-Sized Powders

Three powder metallurgical Ti-48Al-2Cr-2Nb compacts were prepared using spherical pre-alloyed powders, mechanically milled powders, and a mixture of the spherical pre-alloyed powders and the mechanical milled powders in a weight ratio of 1:4. Different microstructures corresponding to coarse grains, ultrafine grains, and bimodal-size grains, respectively, were obtained. The compact with a bimodal grain structure exhibits a good combination of high-yield compressive strength (similar to 1393 MPa) and improved compression ratio to fracture (similar to 13.9%) at room temperature due to the effects of back-stress and ductile gamma-TiAl single-phase layer generated near the ultrafine/coarse grain interface. At high temperatures, the compressive properties of the compact with the bimodal grain size distribution are sensitive to the temperature. A relatively high deformation resistance is achieved at 750 degrees C. At this temperature, the coarse grain region of the bimodal grain-sized microstructure undergoes more strain, and the dynamic recrystallization is promoted with increasing strain, improving the ductility. By contrast, the ultrafine grains in the bimodal grain size microstructure dominate the dynamic softening when the temperature is higher than 850 degrees C due to their accelerated dynamic recrystallization and easy grain boundary slip that are responsible for the good formability and the sharp decrease in deformation resistance of this alloy.