| The Ti2 Al Nb-based alloy with ordered orthorhombic O phase, ordered hexagonal intermetallic α2 phase, and ordered B2 phase has the advantages of low density, high strength-to-weight ratio and better creep resistance, which has been used in advanced aerospace applications more and more widely. In recent years, as potential lightweight high-temperature structural materials used in 600-800℃, Ti2 Al Nb-based alloys have been of great interest. The Ti2 Al Nb-based intermetallic compound has poor ductility and toughness at room temperature due to long-range ordered atomic structure, coexistence of metallic bond and covalent bond and limited number of operative slip systems, which restrictes the industrial application. Reasonable thermal deformation is an effective processing method to improve the plastic deformation properties of Ti2 Al Nb-based alloys. However, hot deformation behavior, dynamic softening mechanism, dynamic recrystallization kinetics, dynamic globularization kinetics and process design and optimization of hot working for the Ti2 Al Nb-based alloy are lack of systematic study. A systematically study was conducted in this paper.In this paper, gas atomized Ti2 Al Nb-based alloy powders were used as raw materials. Hot pressing sintering was adopted to fabricate Ti-22Al-25Nb(at.%) alloy with high relative density and uniform composition. Analysis of the effect of sintering temperature on the density, phase composition, microstructure evolution and tensile properties was conducted. Hot deformation behavior of the hot pressed(HP’ed) Ti-22Al-25 Nb alloy was studied. The high temperature constitutive model and hot processing maps were constructed. The hot deformation mechanisms of stable domains and instable domains were revealed, respectively. The optimal hot deformation condition was obtained. The dynamic softening mechanism types of HP’ed Ti-22Al-25 Nb alloy were determined. The dynamic recrystallization kinetics model and the dynamic globularization kinetics model were established, respectively. The deformation mechanisms of dynamic recrystallization and dynamic globularization were clarified. The effect of heat treatment conditions on the microstructure and phase composition was discussed.Based on the hot pressing sintering characteristics of the Ti-22Al-25Nb(at.%) pre-alloyed powders at the temperature range of 950-1200 ℃, all the HP’ed Ti-22Al-25 Nb alloys are composed of O and B2 two phases lamellar microstructure. The HP’ed Ti-22Al-25 Nb alloys more prone to rapid densification, and the B2 phase grain size grows significantly. The results of mechanical properties tests show that the HP’ed Ti-22Al-25 Nb alloy fabricated at a temperature of 1050 ℃ under 35 MPa pressure for 1h followed by furnace cooling has the optimal mechanical properties at room temperature and elevated temperature, respectively.The hot deformation behavior of HP’ed Ti-22Al-25 Nb alloy was researched by isothermal hot compression tests. The regression analysis of hyperbolic sine model and dynamic materials model(DDM) were adopted to obtain the high temperature constitutive model and hot processing maps. The results show that hot deformation behavior is significantly affected by the deformation temperature and strain rate. The main deformation mechanism for the plastic deformation stable domain is dynamic recrystallization or dynamic globularization. The main deformation mechanism for the instable domain is adiabatic shear band. In addition, the optimal deformation domain for this alloy is the stable domain Ⅰ(T=1010-1070℃, e& =0.001-0.3s-1). The Avrami equation and the modified Avrami equation were adopted to describe the evolution of the dynamic recrystallization volume fraction and the dynamic globularization volume fraction, respectively. And the dynamic recrystallization kinetics model and the dynamic globularization kinetics model were obtained. The results show that the dynamic recrystallization volume fraction and the dynamic globularization volume fraction exhibit a ‘S’ shape ‘slow-fast-slow’ growth trend.In addition, the isothermal heat treatment experiment was conducted to study the heat treatment characteristics of the HP’ed Ti-22Al-25 Nb alloy at the temperature range of 1010-1110℃. The results show that the phase composition and microstructure are significantly influenced by the heat treatment temperature and the holding time. The evolution law of the average size of B2 grain with holding time and heattreatment temperature were accord with Beck equation and Hillert equation. |
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