The Study On Microstructure And Mechanical Properties Of Ti-Al-Sn-Zr-Mo-Nb-W-Si Short-term Using High Temperature Titanium Alloy

With increasing the flight speed of short-term high speed aircraft,it puts forward high requirements on room temperature plasticity,high temperature strength and endurance performance under large stress of titanium alloy.The Al equivalent of common high temperature titanium alloy has reached the critical value,the contents of alpha stabilizer element and neutral element have no room to adjust.When designing the composition of short-term high temperature titanium alloy used at 600?650 ?,it should be focused on adjusting the contents of beta stabilizer elements(Mo,Nb,W).However,the study about the effects of beta stabilizer element on microstructure and mechanical properties of short-term high temperature titanium alloy is still not perfect.Therefore,novel Ti-6.5Al-2Sn-4Zr-(Mo-Nb-W)x-0.2Si short-term high temperature titanium alloys with different contents of Mo,Nb,W have been designed and prepared.The effects of each beta stabilizer element on microstructure and mechanical properties of the alloys were systematically studied.The relationship of heat treatment,microstructure and properties of the alloys was investigated.In addition,the deformation behavior and fracture behavior of the alloys during high temperature tensile process were analysed.The main results were drawn as follows:The microstructures and mechanical properties of the investigated materials heat-treated under the same condition were compared.After simple annealing,with increaing the contents of Mo and W,650? ultimate tensile strength of the alloys increased gradually.And the increase of W content can effectively improve the endurance performance under large stress at 650?.However,adding more Mo and W could increase the beta phase content and refine the microstructure,which would reduce the yield strength of the alloy at 650 ?.Nb had less influence on microstructure,it could effectively solid solution strengthen alpha phase and beta phase,which would improve the yield strength of the alloys.When the Kp value was between 0.2?0.3 and the contents of Mo,Nb,W were controlled in 1?2wt%,1?2wt%,2wt%respectively,the alloys could get good combination of plasticity at room temperature,tensile strength and large stress endurance performance at high temperature.The room temperature elongation and area reduction rate,650? ultimate tensile strength and yield strength of the alloys is 14%,35%,700MPa,520MPa,respectively.The creep rupture life under 650?/425MPa and 650?/350MPa could reach 1h and 2.5h.The microstructures and mechanical properties of the investigated materials with various heat treatments were studied.It's founded that with the single annealing temperature increasing,the volume fraction of primary alpha(ap)phase decreased,and the lamellar thickness of secondary alpha(as)phase increased.Meanwhile,the content of alpha stabilizer element in ap phase and beta transformed(?t)structure increased,while the content of beta stabilizer element in each phase decreased.In addition,with elevating the annealing temperature,the plasticity decreased at room temperature,while tensile strength gradually increased at 650 ?.When annealing at(Tp-20 ?)/2h,AC,the alloys got bi-modal microstructure with?30%of ap phase in the matrix of lamellar ?t structure,which could acquire good combination of plasticity at room temperature and tensile strength at high temperature.After double annealing at 600?8300C,ordered ?2 phase precipitated in a phase.With increasing the secondary annealing temperature or annealing time,the size and particle spacing of ?2 phase increased.When the size of ?2 phase was less than 6nm,with the particle size increasing,650? yield strength decreased.The interaction between moving dislocations and coherent particles was cutting mechanism.With further increasing the size of a2 phase,the interaction between moving dislocations and coherent particles was by-pass mechanism,and the yield strength decreased at 650?.The high temperature tensile deformation behaviors of the alloys were investigated.During 650? tensile process,the deformation behavior of ?p phase were mainly composed of dislocation glide,and the ap phase particle was elongated after deformed at 650 ?.However,dynamic recrystallization happened in lamellar as phase during tensile process at 650 ?.In-situ scanning electron microscopy of the alloy was performed during high temperature tensile process.When tensiled at 600?,5M0N1W alloy presented transgranular fracture characteristic.However,1M3N0.5W alloy presented intergranular fracture characteristic.The results indicated that Mo and W could improve the interface strength of the alloys,which would enhance the temperature that interface strength equal to grain strength.When tensiled at 650 ?,both 5M0N1W and 1M3N0.5W alloy presented microvoid coalescence intergranular fracture characteristic.With increasing the contents of Mo and W,the ? phase content increased,and the the average phase size decreased,which had excellent accommodative deformation capability and would inhibit the microvoid nucleation along grain or phase boundaries.In addition,adding more Mo and W could strengthen the interface,and hinder the microcrack propagating along the interface,which would improve the plasticity and ultimate tensile strength of the alloys at 650?.