| Due to their superior properties, such as low density, excellent specific strength and modulus, good elevated temperature creep resistance and anti-oxidation, Ti Al based alloys are promising candidates for high-temperature structural materials, and have high potential applications in automobile and aerospace industry. High Nb containing Ti Al based alloys are typical of high-temperature and high-strength Ti Al alloys, which are the most widely researched Ti Al alloys. However, the difficulties in ingot preparation and deformation of high Nb containing Ti Al alloy hinder their applications. In this work, elemental powder metallurgy technique was used to synthesize high Nb containing Ti Al based alloy, with a nominal composition of Ti-45Al-10Nb(at.%). The process of mechanical ball milling, vacuum hot-pressed sintering, isothermal forging, heat treatment, and the microstructure and mechanical properties were systematically investigated.Fine-grained Ti-45Al-10 Nb composite powder was prepared by mechanical ball milling. The evolutions of surface morphology, average particle size and distribution of elements in the powder with milling time were investigated. Meanwhile, the crystal defect of 10h-milled powder was revealed. And the effect of milling time on the phase composition, grain size and microhardness of the powder were investigated. It was found that, with increased milling time, average particle size of Ti-45Al-10 Nb composite powder firstly increased. And the shape of the powder gradually became equiaxed, while the dimensions of particle size and grain size gradually decreased. However, no Ti-Al intermetallic compound was found in the whole process. The 8h-milled powder had the highest hardness, reaching 870 HV. According to thermal analysis curves of 10h-milled powders, two exothermic peaks and two endothermic peaks were respectively found in temperature range of 463~553 ℃ and 1200~1350 ℃, which was related to melting of Al and the formation and transformation of Ti-Al intermetallic compound.Ti-45Al-10 Nb alloys were fabricated by vacuum hot-pressed sintering, using 10h-milled powder. Microstructure and mechanical properties in sintered products under different parameters were studied. All as-sintered alloys consisted of three phases γ-Ti Al, α2-Ti3 Al and Ti2 Al C. With the increase of sintering temperature and dwelling time, Nb gradually dissolved into the Ti Al matrix, microstructure of the alloy became more uniformed, and the grain size of Ti-Al matrix also increased. When sintering temperature was 1350℃, coarse lamellar grains were produced in the matrix. Ti2 Al C ceramic phases were benefical for Ti-45Al-10 Nb alloy refinement and improving the sintering strength. Due to solid solution of Nb and the formation Ti2 Al C, the as-sintered alloys display excellent strength and low plasticity. Hot deformation constitutive equations for as-sintered Ti-45Al-10 Nb alloy were established, and provided guidance for the subsequent development of plastic processing parameters.High-quality Ti-45Al-10 Nb alloy pancakes were successfully produced by isothermal forging of the as-sintered alloys. The microstructure of forged alloy consisted of fine dynamic recrystallization(DRX) grain with average grain size of 10μm and several residual lamellas. It was found that recrystallization degrees differed in different regions, the complete recrystallization occured in midsection. Along the diameter direction, the crystal volume fractiongradually decreased, the remaining content of lamellar colonies gradually increased, which related to the amount of deformation and stress states in different regions of forged pancak e. Two-step forge with a 30-min dwelling at temperature had helped to increase the percentage of recrystallized microstructure. The forged alloy was found to exhibit superior mechanical properties comparable to as-sintered alloy.At room temperature, the elongation was 0.7% and tensile strength was 929 MPa. At 900 ℃, the elongation was up to 30.87%,which was 2.5 times more than the sintered state. Stretching under 1000℃ and the strain rate of 1×10-5s-1, the forged Ti-45Al-10 Nb alloy exhibited superplasticity, the engineering strain was up to 173%. It was found thatcavity generation, growth and lateral connections led to the fracture of tensile specimens.Heat treatment process of as-forged alloys was investigated, heat treatment rules for various microstructures were established, and the relationship between microstructure and mechanical properties was also studied. Fully lamellar(FL) microstructure could be obtained by treatment in single α phase field(above 1320℃). The temperature of heat treatment brought a significant impact on grain size of lamellar, while heat-treatment time comparatively less affected. Duplex(DP) microstructure was difficult to obtain by simply heat treatment in α+γ two-phase field. Even using 1280℃/8h in the field, the microstructure still consist of equiaxed γ grains. Only being treated using two-step treatment in α and α+γ two phase field, can the DP microstructure be produced. The strong microstrucstural stability of forged Ti-45Al-10 Nb alloy was due to the Nb’s solid solution improved the high temperature stability of γ phase.Compared with the forged Ti-45Al-10 Nb alloy, FLand DP microstructures alloy exhibited higher strength, but the plasticity of all three didn’t change significantly with temperature. Excellent ductility was displayed in equiaxed microstructure, which reached 42.5% at 950℃.The contents of lamellar colonies had greatest impact on fracture toughness of Ti-45Al-10 Nb alloy. In all tissue types, FL microstructure owed highest fracture toughness values for 14.6MPa·m1/2.Oxidation behavior of the as forged Ti-45Al-10 Nb alloy was investigated through isothermal oxidation tests at 850℃, 900℃, 950℃ up to 192 h. The alloy oxide layer surface morphology, phase composition, morphology and the oxide layer profile oxidation kinetics were mainly examined. The formation of continuous dense Al2O3 layer close to the matrix directly affects oxidation resistance. The formation of continuous Nb-rich oxide slown down the diffusion of Ti, and promote the formation of Al2O3 layer, which was the main reason for excellent high-temperature oxidation resistance. The as-sintered alloy displayed further high temperature oxidation resistance for finer microstructure. |
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