Research On Plastic Deformation Mechanism And Current-assisted Deformation Behavior Of Ti-22Al-25Nb Alloy Fabricated By Elemental Powder Metallurgy

Since the orthorhombic phase（Ti₂AlNb）was discovered by Banerjee,Ti₂AlNb alloy was regarded as one of the most potential aerospace engine materials used at 650⁸00℃,due to its excellent physical and mechanical properties,such as high strength,excellent oxidation and creep resistance,etc.However,the difficult cast process,strong sensitivity of microstructure and the working conditions of the subsequent deformation process limited its extensive application in aerospace.So,it is meaningful and important to study a new fabrication process,deformation mechanism and optimal microstructure.In this study,the elemental powder metallurgy process was used to fabricate Ti₂AlNb alloy.In addition,the fabrication process,plastic deformation process,high temperature deformation mechanism,microstructure and texture evolution,and mechanical properties were studied in detailed.In order to extend the application of Ti₂AlNb alloy,the current-assisted bonding technology was also introduced and the three-layer hollow structures were successfully fabricated.Firstly,the element powders of Ti,Al and Nb were selected as the started materials and the low-energy milling and step-sintering process was used to fabricate the Ti-22Al-25Nb alloy.Moreover,the influence of sintering parameters on the microstructure evolution and mechanical properties of Ti-22Al-25Nb alloys were also investigated.The low-energy milling process was the ball-to-powder weight ratio of 5:1 at 190 rpm for 4 h under the argon protection.The step-sintered process was 630°C/20 MPa/1 h and 1250°C/35MPa/2 h.Secondly,the as-sintered Ti-22Al-25Nb alloy billets was subjected to hot extrusion process to refine grain,close holes and improve the mechanical properties.Results showed that dynamic recrystallization（DRX）occurred prior near grain boundary and further refined the B2 grains during the hot extrusion process.Besides,the extruded microstructure was consisted of both DRX and elongated grains.In addition,the extruded Ti-22Al-25Nb alloy exhibited strong（110）_B2//ED fiber texture.Meanwhile,{001}<111>and{111}<13 1?>fiber textures were also induced due to DRX.The good combination of tensile strength（1122.7 MPa）and elongation（7.9%）have been obtained at room temperature.Besides,the extruded alloy also exhibited super-high tensile strength（closed to916.5 MPa at 650°C and 613.1 MPa at 800°C）.The enhanced properties can be attributed to the comprehensive effects including deformation textures,grain refinement and dissolution of bulksα₂ phase.Lastly,the hot-deformation behavior of the extruded Ti-22Al-25Nb alloy was investigated by compression testing in the 1213¹333 K temperature range,under a 0.001-1.0 s^-1 strain-rate range.The activation energy,Q,at a deformation strain of 0.3,was calculated to be 574.80 kJ·mol^-1.A strain-compensated Arrhenius-type constitutive model was established for this alloy,with an average absolute relative error（AARE）and correlation coefficient（R）of 7.64%and0.994,respectively.Based on the dynamic materials model（DMM）,processing maps were developed by combining the power dissipation and instability maps.An instability region was identified in the 1213¹270 K temperature range and0.022¹.0 s^-1 strain-rate range.The microstructure of specimens deformed under different conditions were analyzed to characterize the corresponding deformation mechanisms.The main softening mechanisms in the stability regions were DRX and dynamic globularization.The grain size of the specimen deformed at 1313K/0.001 s^-1 was approximately 15.2μm.In addition,the current-assisted technology was used to bond the extruded Ti-22Al-25Nb alloy.The effects of bonding temperature（920⁹80℃）and bonding time（10³0 min）on the microstructure evolution and shear strength of this alloy were investigated systematically.Results showed that the temperature was distributed non-uniformly and the highest temperature could be obtained at the bonding interface.As the bonding temperature and bonding time increased,the voids were increasingly shrunk and until vanished.A completely metallurgical bonding interface could be produced at 960℃/20 min/10 MPa.In addition,the shear strength of the bonded specimen depended on its interfacial microstructure.The highest shear strength of 269.3 MPa could be obtained at 960℃for 20 min.Moreover,combining the current assisted bending process,the three-layer hollow structure of Ti₂AlNb alloy was successful fabricated.