| In recent years, Ti2AlNb based alloys have been of great interest for most potential advanced structural materials used in600-750℃, which are widely applied in aircraft engine as rotating components due to their attractive properties such as high strength-to-density, excellent fracture toughness and good creep resistance. A compositionally modified Ti-25Al-14Nb-2Mo-lFe alloy was created by adding the quaternary β-phase stabilizing elements Mo and Fe in substitution for a portion of Nb in the baseline Ti-22Al-27Nb alloy. Earlier studies have indicated that the Mo and Fe modified alloy exhibits higher0.2%yield strength and better creep properties. However, the deformation processing "window" for Ti2AlNb based alloys is quite narrow. The alloy is rather difficult to deform into a complex shape because the hot deformed microstructures are very sensitive to the processing parameters, such as deformation temperature, strain rate and strain. Therefore, it is necessary to study the high temperature deformation behavior and deformation mechanisms of the modified alloy so as to provide theoretical basis for optimizing the hot working parameters.Isothermal compression testing of Ti-25Al-14Nb-2Mo-lFe alloy was carried out at deformation temperatures between950and1100℃with strain rates between0.001and1s-1. The hot deformation behavior of the modified alloy was characterized based on the analysis of the stress-strain behavior and the processing map for obtaining optimum processing windows. The constitutive equation of the alloy was established by using multiple regression analysis. Based on dynamic materials model and the Prasad’s instability criterion, the processing map for the modified alloy was constructed. The main results are summarized as follows.(1) The flow stress of the modified alloy is sensitive to the variation of deformation temperature and strain rate. The stress-strain curves exhibit a peak flow stress, flow softening and steady state flow behavior. Moreover, at lower strain rates, the stress-strain curves exhibit steady state flow. The flow softening tendency is great at lower temperatures and higher strain rates, which has been attributed to local temperature rise, dynamic recrystallization or flow instability.(2) A hyperbolic-sine Arrhenius-type equation is used to describe the relationship between strain rate, flow stress and temperature at high temperature deformation for the modified alloy. The apparent activation energy of deformation and the other material constants are calculated and constitutive equation that describes the flow stress as a function of the strain rate and deformation temperature is proposed for high temperature deformation of this alloy.(3) Based on dynamic materials model and the Prasad’s instability criterion, the processing map for the modified alloy in the deformation temperature range of950-1100℃and the strain rate range of0.001-1s-1at the strain of0.7is constructed. The hot deformation mechanisms of the alloy are analyzed by using the processing map combined with microstructure observation, which can identify the range of hot processing parameters for the "safe" domain and "unsafe" domain.(4) Based on the processing map, the characterization of hot deformation behavior for the stability domain is investigated. The results indicate that the processing map exhibits two peaks in power dissipation, one is43%occurring at950℃/0.001s-1that is associated with lamellar globularization, the other is58%occurring at1050-C/0.001s1, which displays the feature of discontinuous dynamic recrystallization.(5) Based on the processing map, the characterization of hot deformation behavior for the instability domain is investigated. The results indicate that the material undergoes flow instabilities at strain rates higher than Is-1and large height reduction. This instability domain exhibits adiabatic shear bands, flow localization and cracking which should be avoided during hot processing. |
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