Characterization of the hot deformation behavior of a Ti–22Al–25Nb alloy using processing maps based on the Murty criterion

Isothermal compression testing of Ti–22Al–25Nb alloy was carried out at deformation temperatures between 940 and 1060 °C with strain rate between 0.001 and 10 s−1, and a height reduction of 50%. The hot deformation behavior of Ti–22Al–25Nb alloy was characterized based on an analysis of the stress–strain behavior, kinetics and the processing map, for obtaining optimum processing windows and achieving desired microstructures during hot working. The constitutive equation was established, which described the flow stress as a function of the strain rate and deformation temperature. The apparent activation energies were calculated to be 788.77 kJ/mol in the α2 + β/B2 + O phase region and 436.23 kJ/mol in the α2 + B2 phase region, respectively. Based on Dynamic Material Model and the Murty instability criterion, the processing map for the Ti–22Al–25Nb alloy was constructed for strain of 0.6. The map exhibits a stable domain for the temperature range of 940–1060 °C and strain rate range of 0.001–0.1 s−1 with two peaks in power dissipation of 51 and 56%, occurring at 940 °C/0.001 s−1 and 1060 °C/0.001 s−1, respectively. One is associated with lamellar globularization, and the other displays a phenomenon of recrystallization. Therefore, the desired processing condition of the Ti–22Al–25Nb alloy is 940 °C/0.001 s−1 in the α2 + β/B2 + O phase field. Moreover, the material also undergoes flow instabilities at strain rates higher than 1 s−1. This instability domain exhibits flow localization and adiabatic shear bands which should be avoided during hot processing in order to obtain satisfactory properties.