Research On The Titanium Matrix Composites With A Quasi-Continuous Network Reinforcement Distribution

The aim of the present work is to increase the room temperature ductility and high temperature strength of titanium matrxi composites (TMCs). TiB whiskers (TiBw) reinforced titanium matrix (TiBw/Ti) composites with a novel quasi-continuous network reinforcement distribution were designed and successfully fabricated by in situ technique, powder metallurgy (PM) method and mechanical blending and hot-press sintering process based on the system of large and spherical Ti powders and fine TiB2 powders. The prepared composites exhibit a superior combination of mechanical properties: high strength at room and high temperatures due to the strenghening effect of reinforcement and high tougheness by tailoring the novel network reinforcement distribution. The characteristic of the reinforcement quasi-continuous network structures, morphology and distribution, the matrix microstructure and the crack propagation behavior are studied by a scanning electron microscopy (SEM). To evaluate the strength, ductility, elastic modulus, possion's ratio and high temperature properties of TMCs with different treated conditions, tensile and compressive testing, elastic modulus testing at room temperature and tensile testing at high temperatures are carried out. And then, the strengthening mechanism of the network distributed reinforcement is analyzed. The effects of subsequent hot-extrusion, hot-rolling and heat treatment on the microstructure and mechanical properties of TMCs with a network reinforcement distribution are also studied in the present work.Using the same raw materials and sintering parameters but different ball milling parameters, TiBw/Ti composites with different reinforcement distributions (homogeneous and quasi-continuous network) have been successfully fabricated. The results of tensile testing and SEM microstructure analysis show that TiBw/Ti composites with a quasi-continuous network reinforcement distribution exhibit much superior strength and ductility over those with a homogeneous reinforcement distribution. For the 8.5vol.%TiBw/Ti composite with a network microstructure, the elongation is 11.5%, and the ultimate tensile strength increases by 71% compared with that of pure Ti fabricated by the identical process.A range of TiBw/TC4(200μm) composites with different volume fractions of reinforcement have been fabricated by the same process and parameters, and the network parameters are optimized by comparison. The network structure parameters mean the network size and the local reinforcement fraction. The network size is related to the matrix particle size of raw materials while the local reinforcement fraction to the matrix particle size and the average reinforcement fraction of the composite. The results of tensile testing and SEM microstructure analysis show that the optimal size of matrix particle and volume fraction of reinforcement must be considered due to the special quasi-continuous network microstructure. The optimal combination of properties of the composite with a network microstructure can be obtained by coordinating 5vol.% of the average reinforcement fraction and 200μm of the matrix particle size. TiB whiskers with multi-branching, self-joining, mechanical locking and claw-like shapes are observed besides the rod-like whisker. These unexpected pretty structures of whiskers are believed to be beneficial to the mechanical properties of TiBw/TC4 composites with a network microstructure by increasing the strengthening effect of reinforcement and the blocking effect to crack propagation. In the matrix of the prepared composite with a network structure, equiaxedαmicrostructure instead of the widmanst?tten microstructure is formed because the equiaxed network structure effectively constrains the shrinkage of matrix during the cooling process of the sintered composites, which improves the ductility of matrix.The tensile properties are obviously affected by the matrix particle size and the average reinforcement fraction. For the same matrix particle size of different composites with a network microstructure, the strength of the composites increases but the ductility decreases with increasing the average reinforcement fraction. For the same average reinforcement fraction, the strength of the composites increases but the ductility decreases with increasing the matrix particle size. When 110μm of the matrix particle size coordinates with 8.5vol.% of the average reinforcement fraction, the ultimate strength and elongation of the as-sintered TiBw/TC4 composite with a network microstructure reach to 1288MPa and 2.6%, respectively. When 200μm of the matrix particle size coordinates with 3.5vol.% of the average reinforcement fraction, the ultimate strength and elongation of the composite reach to 1035MPa and 6.5%, respectively. The main reasons for the superior strength of the composites with a quasi-continuous network microstructure are that the network structure can effectively bear load and the the interface bonding strength can be effectively increased by the gradient distribution of TiB whiskers in the network structure. The main reasons for the superior ductility are that the ductility of matrix can be effectively exploited since crack propagation is effectively blocked by the TiB whiskers in the network structure.The analysis of fractographs and crack propagation path show that partial TiB whisker breaks in the network structure, which leads to the generation of micro-crack before the overall fracture of the composite. The toughness of the composite is improved due to the increasing fracture energy by crack branching generated by the effective blocking effect of TiB whiskers in the network structure.The superior ability of the network structure to bear load due to the much higher local reinforcement fraction in the network structure, and the superior interface bonding strength due to the gradient distribution of TiB whiskers can achieve the"intercrystalline strengthening effect"at high temperatures, which makes TiBw/TC4 composites with a network microstructure exhibit superior mechanical properties at high temperatures. The tensile tests at high temperature show that on the basis of the judgment criteria of the same tensile strength, the service temperature can be increased by an increment of 150~200oC compared with that of the monolithic TC4 alloy.The mechanical properties of TiBw/TC4 composites with a quasi-continuous network microstructure can be further improved by subsequent hot deformation and heat treament. The strength and the ductility of the composite can be effectively increased by hot extrusion which leads to working hardening and heat treatment strengthening effect of matrix, orientation arrangement of TiB whisker along the extrusion direction and decrease of reinforcement local fraction. The strength of the composite can be effectively increased but the ductility is decreased by heat treatment which mainly leads to heat treatment strengthening effect of matrix. The ultimate tensile strength of the as-sintered TiBw/TC4 composite can be increased to 1423MPa by heat treatment.