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A Study on Thermohydrogen Treatment of (TiC+TiB)/Ti-1100 Composite Synthesized Using In-situ Technology

Author: SunShuanLi
Tutor: LvWeiJie
School: Shanghai Jiaotong University
Course: Materials Science
Keywords: High-temperature titanium alloy Titanium matrix composite Thermohydrogen treatment Superplasticity Microstructure
CLC: TB331
Type: Master's thesis
Year: 2011
Downloads: 95
Quote: 3
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Abstract

With the development of aeronautics and space industry, high temperature titanium alloys which possess great ultimate strength, high module and low density have abstracted great attention. The titanium matrix composites due to higher specific strength, more excellent high-temperature resistance and creep performance illustrate a good prospect of application. However, poor plasticity and high flow stress at high temperature and high demanding for equipments result in the difficulty of working of TMCs, restricting largely their application in practice. Thermohydrogen treatment (THT) as a newly developed technology can improve the high-temperature plasticity of titanium alloys, decrease the flow stress and processing temperature during hot working, which supplies a new method for ameliorating poor hot-working character of TMCs. To overcome the hot-working difficulty of TMCs, in the work, Ti-1100 matrix composites with good comprehensive mechanical properties was prepared using in-situ technology. The microstructure, phase transformation temperature and micro hardness of hydrogenated TMCs were studied. In addition, the studying of the superplastic behavior and failure mechanism was also carried out. The main conclusions were listed as follows.Based on the reaction among titanium, B4C powder and graphite, three kinds of Ti-1100 matrix composites with different reinforcement content were prepared by consumable vacuum arc remelting (VAR). It was found from tensile test that as hydrogen content increased the ultimate tensile strength at room temperature and elevated temperature increased evidently, and the elongations were more than 13% and 23%, respectively.The microstructure, phase transformation and microhardness of hydrogenated Ti-1100 matrix composites were studied. The addition of plenty of hydrogen leaded to the precipitation of hydride. And the infiltration of hydrogen resulted in titanium lattice expansion, largening the lattice constant. The results indicated a decrease inβtransus temperature of TMCs with an increase in hydrogen content. Moreover, the microhardness also decreased after hydrogenation.In high-temperature tensile test, the maximum elongation of hydrogenated TMCs with reinforcement content of 5 vol.% was 294% while that of 10 vol.% (TiB+TiC)/Ti-1100 composite was 256%. Within the studied temperature range, hydrogen increased the elongation in tentile test.Hydrogen decreased the optimal superplastic temperature and flow stress and increased the optimum superplastic strain rate for two kinds of TMCs. The flow stress decreased with the test temperature increment, and increased with the initial strain rate. Furthermore, due to the addition of hydrogen the activation energy Q of TMCs had a decrease.According to the microstructure analysis after deformation under different condition, the lower strain rate didn’t always mean the better elongation, which was because grain recrystallized and grew at high temperature for a long time. It can be seen based on SEM analysis of hydrogenated TMCs before and after deformation that the failure in tensile test was probably ascribed to the cracks or cavities resulting from debonding interface between matrix and reinforcement.

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