| Ti Bw/Ti60 composites with a three dimensional network reinforcement distribution were successfully fabricated by low energy milling and reactive hot pressing technique based on the system of Ti60-Ti B2 in the present work. The size of Ti60 powders is 120~220μm, and fine Ti B2 powders with that of 1~8μm are employed. The volume fraction of Ti Bw reinforcement is 1.7%, 3.4%, 5.1% and 6.8%, respectively. The feasibility of the preparation of titanium matrix composites was analyzed based on the Ti60-Ti B2 system by thermodymics analysis. The hot-pressed sintering parameters of Ti60-Ti B2 were designed. The microstructure of in-situ titanium matrix composites, the morphology and distribution of reinforcement, the interface between reinforcement and matrix were investigated using optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffractometer(XRD) and transmission electronic microscope(TEM). The mechanical properties at room and high temperatures of the as-sintered and the as-extruded composites were studied. The influence of extrusion deformation on the microstructure and properties of the composites was analyzed. The fracture mechanisms of the composites were analyzed combining with the fractographs under different states. Finally, the oxidation behavior of the three dimensional network structured Ti Bw/Ti60 composites was studied at high temperatures. Moreover, the oxidation mechanism of the network structured Ti Bw/Ti60 composites was discussed.The thermodynamic analysis shows that the Ti Bw/Ti60 composites can be fabricated based on the exothermic reaction between Ti60 and Ti B2. The volume fraction of reinforcement is determined by controlling the Ti B2 addition. The reaction based on the Ti60-Ti B2 system is completed, and the microstructure of the composites fabricated at 1300℃ under a pressure of 20 MPa for 1h is dense.The microstructure analysis shows that the presence of Ti Bw refines primary β grain and α lath. The synthesized Ti B whiskers are distributed around the Ti60 particles in the form of a three dimensional equiaxed network. The morphologies of Ti Bw mainly appears rod-like, needle-like, plate-like. Also, some Ti Bw with claw-like, dentritic and T type structure are observed. The interface bonding between reinforcement and matrix is better. The local volume fraction of Ti Bw in the network boundary increases with increasing the overall volume fraction of Ti Bw in the composites. The Ti Bw shows good alignment along the extrusion direction.The strength at room temperature of the as-sintered Ti Bw/Ti60 composites with network structure has a marginal increasement but a remarkable reduction in elongation when compared to those of Ti60 alloy. However, the strengthening effect of the composites increases obviously at high temperatures. When compared to the Ti60 alloy, the ultimate tensile strengths of the 5.1vol. %Ti Bw/Ti60 composites at 600℃, 650℃, 700℃ and 750℃ are increased by 42.5%, 39.9%, 35% and 13.2%, respectively. It is attributed to the different deformation ability of the matrix alloy at room and high temperatures.Hot extrusion has a significant effect on the menchanical properties of the as-sintered composites at room and high temperatures. The yield strength and ultimate strength of the as-sintered 5.1vol. % Ti Bw/Ti60 composites are increased from 1051 and 1095 MPa to 1271 and 1398 MPa at room temperature after extrusion deformation, respectively, and the elongation is improved from 1.2% to 3.7%. When compared to the high temperature strength of the as-sintered composites, the increasement of the high temperature strength of the as-extruded composites gradually decreases with increasing the experimental temperatures in the temperature range of 600~700℃. However, the high temperature strength of the as-extruded composites is less than that of the as-sintered composites at 750℃. In addition, the content of reinforcement within the composites has a great effect on the mechanical properties of the composites under different states. The ultimate strength of the composites with different states increases initially, succeeded by reduction with increasing the volume fractions of reinforcement in the temperature range. Also, the elongation gradually decreases.The analysis of fractographs shows that the main crack propagation of the as-sintered composites follows the boundary along the network structure at room and high temperatures. The fracture mechanism of the as-sintered composites is brittle fracture at room temperature, while the fracture mechanism transfers from quasi-cleavage fracture to ductile fracture with increasing the experimental temperatures. The fracture mechanisms of the as-extruded 1.7%, 3.4% and 5.1% composites exhibit a quasi-cleavage fracture, while the fracture mechanism of the as-extruded 6.8% composites is brittle fracture. The failure of the as-extruded composites is mainly caused by Ti Bw fracture in the temperature range of 25~700℃, and the main failure mechanism of the composites exhibits interfacial debonding at 750℃.The research on the oxidation resistance of the network structured Ti Bw/Ti60 composites indicates that the oxidation kenitics of the Ti Bw/Ti60 composites changes with increasing the experimental temperatures. Layered oxide scales were composed of more Ti O2 and less Al2O3. The Al2O3 is easily riched at the outer layer due to the rapid diffusion of the Al element. A Sn-rich layer is always presented between the oxide scale and substrate. Also, there are no spallations at interfaces in the oxidation temperature range of 600~700 ℃. However, the spallation of the Sn-rich layer with the oxide scale and substrate is found in the oxidation temperature range of 800~900 ℃. The oxidation mechanism of the Ti Bw/Ti60 composites with higher strength does not change below 700℃ when compared to that of the monolithic Ti60 alloy. The oxidation rate is controlled by the diffusion rate. Moreover, the existence of the network microstructure can contribute to improve the oxidation resistance of the composites. However, the oxidation rate of the composites is higher than that of the monolithic Ti60 alloy due to the more interfaces within the composites. |
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