Research On The Microstructure Evolution And Mechanical Property Of The Isothermally Forged Ti-22Al-25Nb Alloys

With the rapid development of the high technology for the aerospace industry, theaviation component should have a good comprehensive properties with highstrength-to-density. Ti2AlNb based alloys have been of interest for most potentialhigh-temperature structural materials used in650to800℃which widely applied in theaerospace field due to attractive properties such as low density, high strength-to-density,good oxidation resistance and nonmagnetic property. However, because of complexrelationships between phases, intrinsic brittleness, microstructure and properties varyingsensitively with thermal mechanical processing, it is especially important to recognizecomprehensively and systematically the evolution mechanism of microstructure andcorresponding performance variation of the alloys under various heat treatments, whichis also urgent to be solved to accelerate their application in aerospace field. As is wellknown, the mechanical property is decided by the microstructure, and thereforereasonably optimize the microstructure and achieve the precise control of microstructureis the premise and basis to understand the microstructure-property relationship andimprove the mechanical properties. However, the previous studies focus on adding orchanging the alloying elements to adjust the mechanical properties. Thus, this papercomprehensively and systematically studies the microstructure evolution and themicrostructure-property relationship via isothermal forging the Ti2AlNb based alloy indifferent phase regions and then heat treating the alloy by different heat treatments. Themain results are obtained as follows:In order to control the B2grain growth during hot deformation, the B2graingrowth behavior of the alloy has been deeply studied. The results indicate that the B2grain size increases as the heating temperature and time increases, and the kinetic of theB2grain growth is established. The activation energy is increased with extending theholding time which can be explained by taking into account the grain size increasewhich produces a decrease in the grain boundary area per unit volume. This might meanthat the grain boundary interfacial energy per unit volume decreases and therefore, thedriving force for grain growth is lower. In addition, Zener model is a reliable model toexplain the pining effect of the equiaxed particles on B2grain boundary migration.The microstructure evolutions under different hot processing treatments are studied.Generally, as the isothermal forging temperature increases, the B2grain size increases and the number of the equiaxed particles decreases. The dynamic globularization occurswhen the alloy is isothermally forged in O+B2region which is induced by the shearingfracture, resulting in the softing during hot working. When the alloy is isothermallyforged in α2+B2+O region, the rim-O can be obtained via peritectoid reaction of B2andα2phase and it can improve the glide deformation in the particle and enhance the hotworkability..The microstructure evolutions under different heat treatments are quantitativelystudied and the mechanism of the evolution is discussed. Generally, the equiaxedparticles can be controlled by the solution treatment. The increasing of the solutiontemperature is helpful for decreasing the number of the equiaxed particles. The lamellacan be controlled by the aging treatment. The coarsening of the lamella is caused by theincreasing of the aging temperature. The Al element increases and the Nb elementdecreases in the B2phase as the solution temperature increases.The alloy with bimodal lath size distribution can be obtained when forged in α2+B2region. The coarse lath O could be obtained when solution treated at O+B2region andthe fine lath O was emerged during the aging process. The static globularization occurswhen the alloy is isothermally forged in B2region and then solution treated in α2+B2+Oregion. The static globularization could be divided into two stages. The first is the grainboundary separation which is related to the embling of the B2phase, and the second isthe coarsening of grain occurs during prolonged cooling.The source of the equiaxed particles and its phase transformation during heattreatment is investigated. The larger equiaxed particles in the solution treatedmicrostructure are retained by the forging microstructure and the smaller equiaxedparticles is obtained by the static globularization. After the alloy is isothermally forgedin α2+B2region, the lath O precipitates within the α2particles when heated at hightemperature in O+B2region which is related to the Nb diffusion. The α2phasecontaining Nb separates into Nb lean and Nb rich regions and Nb rich regions whosecomposition is closer to Ti2AlNb transform to the O phase. The lath O prefers tonucleate near α2/B2interface and the retained α2phase dissolved into B2matrix.The mechanical property of the Ti-22Al-25Nb is investigated and the effects of theforging, solution and aging temperature on the tensile properties are introduced. As theisothermal forging temperature increases, the strength and ductility of the alloy has afirst increases and then decreases. When the alloy is forged in B2single phase region,the dislocation density in the particle and lamella is limited and the main B2matrixmakes main contributions to the deformation. When the alloy is forged below B2single phase region, the dislocation density in the particle and lamella increases, and thedislocation tangle pile up can also be seen which are given great efforts to enhance theductility. When solution treated in O+B2region, as the solution temperature increases,the B2volume increase and particle decrease are the main reason that the alloy ownshigh ductility. A key principle to improve the tensile property of the alloy is to beisothermally forged nearby the B2-transus in order to control the amount of the particleand the B2grain size and after that aged at low temperature to obtain fine lamella.