Research On The Microstructure And Properties Of High Temperature Titanium Alloy Matrix Composites Enhanced By TiC And TiB

Melting-casting process in situ autogenous technique was adopted to prepare the TMCs with the matrix of Ti-5.5Al-4Sn-4Zr-0.3Mo-1Nb-0.5Si and reinforcement of TiC+TiB. The contents of reinforcement in the three titanium matrix composites were 9vol%TiC+1vol%TiB, 7vol%TiC+3vol%TiB and 5vol%TiC+5vol%TiB respectively . In addition, different heat treatment processes for each material were conducted. Then the microstructure and properties of as-cast materials as well as the materials after heat treatment were analyzed and tested.It is shown that the two reinforcements distribute along the grain boundary. Most of the TiC is granular or equiaxial, but the shape of TiB are needle or fibrous. The two reinforcements which are added simultaneously incline to grow mutually and to suppress the growing of each other. In the final titanium matrix composites, the size of reinforcements is relatively small. The introduction of TiC+TiB also plays a role of refinement on the matrix in a certain extent.After the heat treatment, no change takes place in the microstructure of TiB almost, and the microstructure is stable in high temperature. However, the microstructure of TiC has changed obviously. Meanwhile, these changes have the same tendency in composites with different contents of reinforcements: strip microstructures of TiC are molten and granulated, and TiC particles are gradually dissolved, whereas this tendency for TiC particles near to TiB is less a lot. In the whole process of heat treatment, TiB plays a certain restraining role to the solution of TiC.The compressive strength of casting Ti-based composites is obviously higher than the matrix of titanium alloy. The composite with reinforcements whose content is 5vol%TiC+5vol%TiB has the maximum increasing degree, and its compressive strength increases to 1699.26MPa, but the plasticity descends substantially. After the heat treatment, this phenomenon is obviously developed. The plasticity increases, at the same time, the strength of Ti-based composites becomes higher than that of casting Ti-based composites to some extent. Fracture textures after compression experiment analyzed, it is found that the interfaces between reinforcements and matrix combine well.The properties of Ti-based composites after the heat treatment enhance obviously, because the reinforcements after the heat treatment distribute more uniformly and dispersedly; the TiC grains are refined; the reinforcements become smooth and non-strident; usually, stress concentration occurs near edges and corners of the reinforcements, the phenomenon is developed well after the heat treatment. In addition, the compressive strength and compressibility of composites increase meanwhile. The final analysis data shows that the properties of Ti-based composites with reinforcements whose content is 9vol%TiC+1vol%TiB are optimal after the heat treatment at 1050℃in four hours; its compressive strength increases to 1969.67MPa from 1602.34MPa of casting Ti-based composites, and the compressibility increases to 20.76 percent from 15.19 percent.The final analysis and test indicate that the titanium matrix composites with the content of reinforced phase of 9vol%TiC+1vol%TiB after holding at 1050℃for 4h can obtain the optimal properties. The compressive strength in as-cast are improved from 1602.34MPa to 1969.67MPa and the compression ratio are ameliorated from 15.19% to 20.76%.