Research On Three Typical Microstructures And Mechanical Properties Of Ti-22Al-25Nb Alloy

Ti₂Al Nb alloy as a new kind of Ti-Al-Nb ternary alloy has been considered as a promising candidate for high temperature structural applications in the aeronautical and aerospace industries due to its comprehensive performance at room temperature and high temperature including high specific strength, low density, high fracture toughness and excellent creep and oxidation resistance. In terms of the intrinsic brittleness of Ti₂ Al Nb alloy as an intermetallic compound, it has to go through a series of thermal processing from ingots to parts, such as isothermal forging, hot-extrusion, hot-rolling, hot forming, heat treatment, and so on. As is well known, the mechanical property is decided by the microstructure, and therefore reasonably optimize the microstructure and achieve the precise control of microstructure is the premise and basis to understand the microstructure-property relationship and improve the mechanical properties. However, the previous studies focus on adding or changing the alloying elements to adjust the mechanical properties. No systematic study has been reported so far on the correlation between the controlling microstructure parameters and mechanical properties. Thus, three typical microstructures including equiaxial, duplex, and lamellar microstructures of Ti-22Al-25 Nb alloy bars were obtained by different thermomechanical processing and adjusted by heat treatment. This paper comprehensively and systematically studies the microstructure evolution and the microstructure-property relationship via isothermal forging the Ti₂ Al Nb based alloy in different phase regions and then heat treating the alloy by different heat treatments. The main results are obtained as follows:The microstructure evolutions under different hot processing treatments are studied. The equiaxial microstructures were obtained by isothermally forging in α2+B2+O region and then heat treatment in B2+O region. The increasing of the solution temperature is helpful for decreasing the number of the equiaxed particles. The lamellar O phase can be controlled by the aging treatment. The coarsening of the lamella is caused by the increasing of the aging temperature. The duplex microstructures were obtained by isothermally forging in α2+B2 region and then heat treatment in B2+O region. The number and quantity of the equiaxed particles were determined by isothermally forging temperature, the primary coarse lamellar O phases were controlled by solution treatment temperature, and the secondary fine lamellar O phases were adjusted by age treatment temperature. The lamellar microstructures were obtained by isothermally forging in B2 region and then heat treatment in B2+O region.When the alloy is isothermally forged in α2+B2+O region, the rim-O can be obtained via peritectoid reaction of B2 and α2 phase and it can improve the glide deformation in the particle and enhance the hot workability. After the alloy is isothermally forged in α2+B2 region, the lath O precipitates within the α2 particles when heated at high temperature in O+B2 region which is related to the Nb diffusion. The α2 phase containing Nb separates into Nb lean and Nb rich regions and Nb rich regions whose composition is closer to Ti₂ Al Nb transform to the O phase. The lath O prefers to nucleate near α2/B2 interface and the retained α2 phase dissolved into B2 matrix.The deformation and failure mechanism of different microstructures are investigated. At same heat treatment systems, lamellar microstructures have the biggest strength but the poor ductility; however, equiaxed microstructures have the biggest ductility but the poor strength. In deformation behavior, the slip bands of B2 phase in equiaxial microstructures becoming waves; it means that more slip system and cross-slip in B2 phase are occurred. Slip lines can transmit between lamellar O-phase and B2-phase without stress concentration at the phase boundaries. Primary α2/O equiaxial particles and secondary fine lamellar O phase in duplex microstructures inhibited the dislocation motion. A good match of strength and ductility of the alloy can be realized through controlling the size and content of lamellar O-phase.The creep behaviors of different microstructures are also investigated. The creep rate of the primary creep stage for equiaxial microstructures is higher than that of duplex, and lamellar microstructures. Lamellar microstructures has the best creep resistance at 650℃/150 MPa compared with the other samples. The creep behaviors of the duplex, and lamellar microstructures are very similar at 650℃. This behavior relates mainly to the microstructures, and their microstructures are nearly identical except for the content of equixed particles and the width of the lath O phase. The fine-lath microstructure possesses a lower creep resistance than that of the coarse-lath microstructure, therefore, the lamellar microstructure has a higher creep resistance than duplex microstructure. As age treatment temperature increased, the trends of the creep behaviors of lamellar microstructure and duplex microstructure are almost identical, namely, the creep resistance of alloys decreased as age treatment temperature increased, while the trends of the creep behaviors of equiaxial microstructure are the opposite.The creep experiment was taken and the experiment data was analysed. The stress sensitive parameters of different microstructures at 650℃ are: lamellar microstructure: n=3.84; duplex microstructure: n=4.28; equiaxial microstructure: n= 5.14. The activity energy of different microstructures at σ=150MPa : lamellar microstructure: Q=230.85KJ/mol; duplex microstructure: Q=318.69 KJ/mol; equiaxial microstructure: Q=329.72KJ/mol. Dislocation slip and dislocation climb holds a leading post in the creep of alloys. The effects of B2 grain size on creep properties of Ti-22Al-25 Nb were also investigated. The results indicate that the lamellar microstructure with major B2 grain size has best creep resistance, while the duplex microstructure with smaller B2 grain size has worse creep resistance. It also found that creep induced phase transformation in Ti-22Al-25 Nb alloy. The formation of the fine plate-like O-phases precipitated within α2-phases is the result of the α2 phase decomposition in creep.