Research On Mechanical Properties Of In-situ Synthesized 7715D Titanium Matrix Composites With Equiaxed And Lamellar Microstructures

Recently; titanium matrix composites (TMCs); especially the high temperature titanium matrix composites would become the most promising structural materials for aero-space engines because of their high strength and elastic module compared to their weight; especially their good mechanical properties at elevated temperature. Particles or fibers reinforced TMCs fabricated by in situ synthesis technology attracted more and more attention because the reinforcements were distributed uniformly and the matrix-reinforcements interfaces were clean; well bonded; and thermodynamically stable which caused excellent properties of TMCs at elevated temperature. Moreover; they were cost-efficient and easy fabricated.With the incorporation of reinforcements; the mechanical properties of TMCs were dependent on the reinforcement's properties; the reinforcement's size; the reinforcement's volume fraction; the reinforcement's distribution and the interface between matrix and reinforcements. The microstructure of matrix also played a critical role on the mechanical property of TMCs. Due to different advantages of different microstructures; the excellent comprehensive properties; such as room temperatures tensile test; creep resistance; and solid thermal stability; used to be hard to reach for titanium alloys. With the incorporation of reinforcements; the microstructure evolution would happen and influence the comprehensive properties. In order to obtain superior comprehensive mechanical properties; the kinds of reinforcements and volume fraction of them; as well as factor of thermal treatment; have been studied. Recently; the research on comparison of comprehensive properties between equiaxed structure and lamellar structure of TMCs are limited; so the study on mechanical properties of 7715D with equiaxed and lamellar structures are necessary.In this thesis; three types of TMCs reinforced by TiB,TiC andLa2O3 with different fraction of volume were synthesized by common casting and hot-forging technology. They are TMC1(0.39vol%TiB+0.11vol% La2O3),TMC2(1.42vol%TiB+0.4vol% La2O3)和TMC3(3.09vol%TiB+1.21vol%TiC+0.4vol% La2O3). Heat treatments were carried atβfield for lamellar microstructure andα+βfield for equiaxed Microstructure. After two kinds of solution-treatment; researches on the effect of microstructure of matrix on the comprehensive mechanical properties of TMCs were conducted. Moreover; how to make the strengthening effect of reinforcements be given full play were also discussed. The tensile test at ambient temperature; high-temperature tensile test with different velocity and temperature; creep deformation properties and thermal stability were tested. Micro-structures of the composites and reinforcements were investigated by X-Ray Diffraction (XRD); Optical Microscopy (OM); Scanning Electronic Microscope (SEM) and Transmission Electronic Microscope (TEM) to provide some suggestion about optimizing the microstructure of matrix and comprehensive mechanical properties. In this research; the main work was done as following:(1) Theβ-transus temperatures of TMCs have been obtained by the method of metallographic analysis. After casting; the ingots were hot-forged at 1050?C and rolled at 1000 ?C. After heat-working; two kinds of different heat treatments (HT1 and HT2) were carried out for the TMCs. The microstructures of TMCs via HT1 and HT2 were observed by optical microscope. TMCs obtained equiaxed and fully lamellar microstructures via HT1 and HT2 respectively. The reinforcements were uniformly distributed along forging direction in the TMCs. TiB whiskers were of needle-like and TiC was of granular; and sizes of La2O3 particles were in the nano-scale. They distributed in the matrix with no apparent reaction layer between the matrix and reinforcement. Increasing volume fraction of reinforcement could significantly lead to the rise ofβ-transus temperature; especially for TMC3 with the addition of element C. Consequently; the critical temperature of TMCs thermal-processing also increased;α+βtwo-phase region expanded. With the addition of C;α-phase content increased significantly and the width ofαlath in fully lamellar microstructures broadened under the same cooling rate.(2) The tensile properties of TMCs with equiaxed and lamellar structures at ambient temperature are studied. The yield strength and tensile strength improved significantly with the increase of reinforcement content due to TiB short fibers bearing and TiC; La2O3 particle dispersion strengthening. The improvement of strength by increasing TiB reinforcements did not necessarily mean the reduction of plasticity. It depended on the degree of initial micro-flaws on TiB whiskers during the process of casting and processing. The appropriate micro-flaws were beneficial to the acquisition of good plasticity. Adding the element C caused the increase ofβ-phase transition point; leading to the increase ofαphase fraction; which caused significantly improvement of the tensile strength of materials and significantly reduction of plasticity. Coarse lamellar microstructure of matrix alloys usually display poor plasticity compared with equiaxed microstructure. However; with the addition of reinforcements; the lamellar microstructures became fine which had better effect on plasticity. Lamellar microstructure can more effectively retard the crack propagation from whiskers; which is also conducive to good plasticity.(3) The high-temperature tensile properties and creep resistance have also been studied. Increasing the content of reinforcement in TMCs was conducive to the improvement of high-temperature tensile properties; but the effect of fineαlath of matrix with lamellar structure on the creep properties were much greater than that of the reinforcements. At high-temperature tensile tests; TMCs were very sensitive to strain rate at the range of 10-4 /s-10-3/s. With the decrease of strain rate; the high-temperature tensile strength decrease; elongation increase. TMC3 obtained a high elongation in 10-4 /s compared with the poor room temperature tensile test. At high temperature and low strain rate; the critical aspect ratio of TiB short fibers increased. TiB short fibers whose aspect ratio was lower than the critical aspect ratio would be easily debonding at the interface. Lamellar microstructure were much more effective in retarding the propagation of micro-voids or cracks caused by debonding and showed better high temperature mechanical properties.(4) Thermal stability test on TMCs were carried out at 550 ?C -650 ?C. The result showed that heat exposure at 550 ?C after 120 hours caused little reduction of room temperature ductility; heat exposure at 600 ?C led significantly reduction of room temperature ductility; while after heat exposure at 650 ?C the ductility at room temperature recovered. A large number of precipitates phase appeared along the boundary ofαphase at 600 ?C were the principle reason for ductility loss at room temperature; while at 650 ?C the re-solution of hard particle phase occurred. Compared with equiaxed microstructure; the lamellar microstructure were serious affected mainly due to larger area of boundary ofαlath and more precipitation of these hard phases.