Fabrication, Microstructure And Mechanical Properties Of Multiple-reinforced Titanium Matrix Composites Synthesized By Powder Metallurgy

Titanium matrix composites (TMCs) have become a research hotspot for their advantages such as high specific strength, high specific modulus, and good mechanical properties at elevated temperature and so on.Multiple-reinforced TMCs obtain better comprehensive properties and structure-function integrated characteristics due to the coupling effect of reinforcements with different features and various sizes. But the high cost of titanium alloy products is the main restrictive factor for their industrial application. As a near net forming process, Powder Metallurgy (PM) could greatly reduce the cost of products, which makes the large-scale industrial production of titanium alloy products be possible.In this research, the reaction between titanium and graphite, B4C and LaB6 were used to in-situ synthesize multiple-reinforced TMCs by the PM technique of hot-pressing. The process parameters were optimized, TMCs with various proportions of reinforcements and rare earth elements were synthesized, their microstructure, forming mechanism, mechanical properties and fracture mechanism were studied. The main work and conclusions are as follows:1. Through comparation of the TMCs synthesized under different process parameters, the optimized hot-pressing parameters were selected as: Pressure, 50MPa; Temperature, 1200℃; Time, 3h. The in-situ reaction was completed and TMCs nearly achieved full density under this condition.2. Optical microscopy (OM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to synthesize the TMCs'phases and microstructure. The results show the reinforcements are uniformly distributed in the matrix by viewing as whole, but some gather and intercross by viewing as a part. They are mainly classified as four kinds: TiB with whisker-like shape; TiC with near-equiaxed shape; inter-mixed TiB and TiC with block structure or long strip shape, which are often irregular and large. The bond between the reinforcements and the matrix is firm. The forming mechanism of PM TMCs includes sintering process and solid state reaction, and the growth mechanism of reinforcements is solid state diffusion.3. Titanium alloys produced by PM gain high strength and better toughness than by casting. The in-situ synthesized TiB and TiC significantly improve the strength, but decrease the plasiticity of matrix alloys. At room temperature, the TMCs'strength increases as the content of reinforcements increases; the strengthening mechanism attributes to the load undertaking of reinforcements, refinement of matrix's grain size and strain regions caused by reinforcements; the failure mechanism includes the fracture of whisk-like TiB or reinforcements with large volume, and the debonding of some long strips. At elevated temperature, TMCs'strength may be lower while the content of reinforcements is more; the strengthening mechanism includes the reinforcements'load undertaking and their impediment effect on the motion of dislocation and grain boudaries; the facture is mainly due to the debonding of long strip reinforcements.4. When the rare earth element is added through LaB6, La2O3 is synthesized with the shape of granular or cluster structure and surrounded by reinforcements with large volume which are mixed with TiB and TiC. Their lattices mismatch, which cause the bonding infirm. Such reinforcements would easily crack at room temperature and debond at high temperature, which decrease the strength and plasiticity of TMCs. New adding methods of the rare earth elements should be considered to achieve their reinforcing effect. In a word, TMCs synthesized by PM technique have unifrm reinforcements, fine bonding and good mechanical properties. This work lays the foundation for industrial production of TMCs products with good properties and low cost.