Microstructure And Mechanical Properties Of Ti-22Al-25Nb Alloy Prepared By Pwoder Metallurgy Route

As a kind of new structural material developed from traditional Ti₃ Al alloy, Ti₂ Al Nb based alloy which has a good comprehensive properties such as high strength-to-weight ratio, low density and better creep resistance, has been noticed by researchers in aerospace industry. The conventional preparation methods of Ti₂ Al Nb based alloy were casting and ingot metallurgy, which had the defects such as coarse microstructures and composition segregation. The following processes and forming quality were greatly influenced by these shortages. Thus, powder metallurgy method which can obtain Ti₂ Al Nb based alloy with the uniform microstructure and good properties has became a new and prospecting technology. In the present work, gas atomized Ti-22Al-25 Nb pre-alloy powders were studied as raw materials. Finite element simulation about hot pressing sintering process was firstly applied to analyze the influence of process parameters. Based on the results of simulation, vacuum hot-pressing sintering was adopted to fabricated Ti-22Al-25 Nb alloy in the temperature range of 1000 o C-1150 o C,40 MPa pressure and 1h holding time. The impact of temperature on microstructure evolution, phase composition and mechanical property were revealed. The grain growth kinetics under various conditions were established according to the experimental data to investigate the B2 grain thermal stability. The accuracy of kinetics were verified by contrast predicted value with experimental data. The results of this paper ensured a optimum technics route of preparing Ti-22Al-25 Nb alloy and also laid a foundation for the further process.A finite element model was established to simulate the process of powders hot pressing sintering. The flow rules of powder were studied combined with the friction factors and stress distribution. The simulation was set under different pressures, holding times and height-diameter ratios and the results illustrated that the parameters had great influence on the density of specimen. The simulation provided theory reference for the parameters selection of followed experiment.Combined the result of simulation with phase diagram, the experiment was taken at the temperatures in α2+β/B2 two-phase region and β/B2 single-phase region. Researchshowed that all the hot sintered Ti-22Al-25 Nb alloys were all composed of equiaxed B2 phase, lath O phase and α2 phase distributed along the grain boundary. With the increase of temperature, the density of samples increased generally as well as the size and volume fraction of phases were changed. The results of mechanical tests indicated that alloy fabricated in 1100 o C, 40 MPa and 1h had better comprehensive properties in both room temperature and 650 o C.In order to investigate the B2 grain thermal stability, the alloy obtained in 1100 o C was heated under different heat treatment conditions. Results showed that the grain size increased with the increasing of temperature and holding time, and meanwhile the grain size uniformity were also increased. Based on the experimental data, the grain growth kinetics are established by regression analysis and moreover, the growth time exponents（n） and activation energies（Q） have been determined for all conditions. The accuracy of kinetics were being evaluated.