Rolling parameters, microstructure control, and mechanical properties of powder metallurgy Ti-44Al-3Nb-(Mo, V, Y) alloy: The impact of rolling temperatures

Flow softening behavior, microstructure evolution and hot working parameters of powder metallurgy Ti-44Al-3Nb-(Mo, V, Y) alloy were systematically studied by isothermal hot compression. Based on the analysis of processing maps and the corresponding microstructure, power dissipation efficiency peak occurred at two domains, which were associated with dynamic recrystallization (DRX) and super-plasticity. Pile-up of high density dislocation and a large number of twins were the primary deformation mechanisms at low temperature and high strain rate. As the temperature increases, fine DRX grains promote the movement and rotation of grain boundary, the softening of beta phase and the transformation of beta ->alpha/alpha(2)+gamma above 1200 degrees C improve the deformation capacity of the alloy significantly. Crack-free TiAl sheets were successfully obtained by direct hot rolling process at 1080 degrees C and 1220 degrees C. The corresponding microstructure and mechanical properties were analyzed. The sheets rolled at 1080 degrees C was characterized by near gamma (NG) +beta(0) microstructure with a streamline along the rolling direction, while duplex (DP) +beta(0) microstructure appeared to be the main microstructure at rolling temperature of 1220 degrees C. The as-rolled sheet exhibits excellent mechanical properties at 800 degrees C. The sheet rolled at 1080 degrees C exhibits 320 MPa yield strength and 468 MPa ultimate tensile strength with 142% elongation. The yield strength, ultimate tensile strength, and elongation of the as-rolled sheet are 446 MPa, 586 MPa, and 3.0%, respectively, when rolled at 1220 degrees C. The excellent ductility of the sample rolled at 1080 degrees C is attributed to formation of fine DRX grains due to consumption of twins and substructure under tensile stress, which promote grain boundary slip. Lamellar colonies with fine interlamellar spacing and alpha(2) laths formed at 1220 degrees C can effectively prevent dislocation motion, resulting in a significant increase in the strength of the alloy.