| With the development of the aerospace industry,the requirements for lightweight,high-strength,high-temperature oxidation resistance materials are more demanding than ever before.Ti2AlNb-based alloy has attracted wide attention due to its low density,high specific strength,good elevated temperature strength and good high temperature oxidation resistance.The structural parts of Ti2AlNb-based alloy materials are mostly prepared and processed by hot forming,however,it is hard to machine due to its large high temperature deformation resistance,effective thermal window narrower,low yield and the microstructure is sensitive to the forming process,which has become the bottleneck of large-scale production of Ti2AlNb-based alloy and restricts the industrial production of Ti2AlNb-based alloy.Based on this,in order to promote the promotion and application of Ti2AlNb-based alloy in aerospace domain,this article is based on Ti-25Al-19Nb alloy,adding the element Mo of the stable phase B2 with different contents to refine its microstructure and improve its hot deformation ability,comprehensively analyzed the mechanism of Mo element in the hot deformation processing of Ti2AlNb-based alloy.The isothermal forging process was used to obtain a duplex structure consisting of primary equiaxed α2/O+ secondary lamellar α2/O and B2 matrix,and to studied microstructure controlling characterized of the Ti2AlNb-based alloy during heat treatment.In terms of alloy service,dynamic mechanical response behavior of the Ti2AlNb-based alloys with initial microstructure of lamellar and bimorphic structure were studied by dynamic loading.In addition,related basic research work has been carried out on the high temperature pack rolling process of the Ti2AlNb-based alloy.The summary is as follows:(1)Research on the influence mechanism of Mo element on Ti2AlNb-based alloy:Mo element can refine the lamellar α2/O phase of Ti2AlNb-based alloy,but it will have a certain influence on the phase transition temperature of the alloy,which easily leads to the narrowing of the O+B2+α2 three phase region of Ti2AlNb-based alloy.The thermal simulation compression test shows that Mo element will affect the compression flow stress and plasticity of the alloy.The high content of Mo element can increase the compressive peak stress of the Ti2AlNb-based alloy,but the plasticity is poor and it is easy to cause instability during the deformation process.The processing of the alloy is extremely unfavorable.The study found that the high-temperature deformation performance of the 0.5Mo alloy(Ti-25Al-19Nb-0.5Mo alloy)is significantly better than that of the 2Mo alloy(Ti-25Al-19Nb-2Mo alloy).The instability behavior of Ti2AlNb-based alloy during thermal deformation is related to the content of Mo element.In addition,during the high-temperature compression deformation process,the spherification of the lamellar in 0.5Mo alloy is mainly through the transformation of O→B2+α2,and the 2Mo alloy is mainly directly converted from the O phase to the B2 phase.(2)Microstructure controlling characterized:based on 0.5Mo alloy,the duplex micro structure obtained through the isothermal forging process is the initial micro structure,and established the CCT curve of continuous cooling transition of B2 phase of the alloy,revealed the evolution behavior of microstructure under different cooling rates.As the temperature decreases,the phase transformation process of the alloy is mainly B2→B2+α2→O+B2→O under a certain cooling rate.When the cooling rate exceeds about 1200℃/min,the O phase in the microstructure mainly comes from the direct transformation of B2 phase and which is metastable state.The microstructure precipitation method is also different under different cooling rate conditions.At a cooling rate of 5℃/min,the lamellar grain first precipitates from the grain boundary and grows into the grain.The volume fraction of the α2/O phase is higher after cooling to room temperature,and the hardness HV value is larger.At a cooling rate of 100℃/min,the lamellar grain is simultaneously precipitated from the grain boundary grains.The volume fraction of O phase and a2 phase is low,and the hardness HV value is not much different from the HV value of the structure obtained in the B2 phase region.The microstructure of the alloy can be controlled to a certain extent by adjusting the cooling rate and the cooling termination temperature.(3)Research on dynamic mechanical response behavior and deformation mechanism:Though the dynamic loading test,the dynamic plastic deformation model was established.And we found the deformation coordination of the Ti2AINb-based alloy with the lamellar structure mainly depends on the fragmentation,bending and rotation of the lamellar grains.In addition to the deformation coordination mechanism of the lamellar structure,the equiaxed grains of the duplex structure will also restrict the deformation of the lamellar bundles and the migration of dislocations,resulting in the dynamic compression strength of the duplex structure being much higher than that of the lamellar structure at room temperature/2000 s-1 rate.And based on the plastic deformation absorbed energy during the dynamic loading process,it is revealed that the alloy with duplex structure of the lamellar and the equiaxed grains has less sensitive to adiabatic shear at room temperature.(4)The hot deformation behavior and sheet rolling processing:The forged 0.5Mo alloy with duplex microstructure has good thermal processing performance.In the process of thermal deformation,the compression and deformation mechanisms of the alloy microstructure before and after spheroidization are dominated by fragmentation and bending of the lamellar α2/O phase slip and elongation of the spheroidized grains,respectively.And the softening mechanism is mainly the dynamic recrystallization of the O phase and the α2 phase,in which the B2 phase mainly plays the role of deformation coordination.Based on the analysis of the processing map,it is determined that the safe hot working area of the alloy is in the range of 865～950 ℃/0.15～0.001 s-1 and 950～1100℃/1～0.001 s-1.The tensile strength of the sheet obtained by hot rolling of 0.5Mo alloy by using high temperature pack rolling process after forging at a rolling temperature of 950℃(in the O+B2 two-phase region)and 1000℃(O+B2+α2 three-phase region),both are optimal for plasticity and plasticity,getting the thickness of 2.12 mm and 1.95 mm of alloy plate,respectively.The surface of the plate had no obvious defects,and its tensile strength and elongation were 957.90 MPa and 3.24%,903.38 MPa and 2.60%,respectively. |
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