A Study On Thermohydrogen Treatment Of In-situ Synthesized Titanium Matrix Composite

Since titanium matrix composites (TMCs) exhibit higher specific strength and module in comparison with titanium alloys, excellent high temperature mechanical properties and creep resistant properties, they have been thought as promising materials. Near-net plastic shape forming have advantages on less processing procedures, low cost, and high utilization rate of raw materials, which is appropriate for TMCs manufacture. However, poor plasticity and high flow stress at high temperature, highly demanding for equipments, and high cost of preventing oxidation restrict the hot working of TMCs. An emerging technology thermohydrogen treatment (THT) can enhance plasticity of titanium alloys, decrease the flow stress and deformation temperature in hot working. This technology is able to reduce the cost of hot working of titanium alloys. However, the research about utilizing THT to improve hot workability of TMCs has not been reported.To improve the hot workability of TMCs, a study on thermohydrogen treatment of in-situ synthesized TMCs was carried out in this thesis. In-situ TMC with good comprehensive mechanical properties was prepared. The hydrogenation behavior, the microstructure and phase transformation of hydrogenated TMCs were studied. The laws of flow behavior and microstructural evaluation were clarified. Constitutive relation and processing map of hydrogenated TMC were constructed. Superplasticity of hydrogenated TMCs was investigated and superplastic mechanism was explored. Finally, TMCs with large size were processed using THT, and the microstructure and mechanical properties were investigated. The main conclusions were listed below. In-situ synthesized (TiB+TiC)/Ti-6Al-4V composite with 5 vol.% reinforcements was successfully prepared utilizing the reaction among titanium, B4C powder and graphite. The yield strength and ultimate tensile strength of the TMC bar at room temperature were both increased more than 10% in comparison with those of Ti-6Al-4V alloy, and the elongation was 12.6%.The hydrogenation behavior, microstructure and phase transformation of hydrogenated TMCs were studied. The microstructure grew up with increasing hydrogenation temperature. After hydrogenation at 750℃,βandδphases increased with increasing hydrogen concentration. The volume effect ofδphase resulted in plenty of dislocations inαphase and twins inδphase. Hydrogen significantly decreasedα+β/βphase transformation temperature. Phase transformation temperature decreased 200℃and reached the eutectoid point at 0.45 wt.% H and greater hydrogen concentration.The flow behaviors and microstructural evaluations of TMCs with different hydrogen concentrations were studied, and the laws of high temperature deformation were clarified. The flow stress decreased with increasing temperature and increased with increasing strain rate. Hydrogen decreased the flow stress of TMC. The flow stress first decreased and then increased with increasing hydrogen concentration. Hydrogen induced softening ofαphase, hydrogen induced hardening ofβphase and hydride formation resulted in the minimum flow stress. Hydrogen decreased the deformation temperature and increased the strain rate at the same stress level. Dynamic recrystallization (DRX) was the main softening mechanism of hydrogenated TMCs inα+βfield. Hydrogen accelerated the DRX ofαphase and decreased the DRX temperature. And hydrogen improved the accommodation of matrix and reinforcements in deformation. At high strain rate, the microstructure of TMC is relatively fine and the volume fraction of DRX ofαphase is relatively low. Optimum hydrogen concentration is determined as the one at which maximum amount ofβphase is obtained inα+βfield, at which the flow stress is minimum.According to the results of isothermal compression tests which were carried out at the deformation temperatures ranging from 700℃to 950℃and strain rate ranging from 0.001s?1 to 10s?1, the constitutive relation and processing map of as-received and hydrogenated TMCs (0.40 wt.% H) were constructed. The apparent activation energy of hydrogenated TMC deformed inα+βfield hardly changed. Low deformation temperature and reinforcements increased the value of apparent activation energy inβfield. Hydrogen decreased the temperature at which high efficiency of power dissipation was obtained and reduced instability region in the processing map of TMCs. At low temperature and high strain rate, the flow localization at adiabatic conditions and cracked interfaces between matrix and reinforcements caused the instability. At medium temperature and medium strain rate, DRX is the main reason for high efficiency of power dissipation.The superplastic property, flow behavior and microstructural evaluation of hydrogenated TMCs were studied. And the superplastic mechanism was explored. Hydrogen decreased the optimum superplastic temperature and increased optimum strain rate of TMCs. At the same elongation level, the superplastic temperature was decreased by 100℃and strain rate was increased by one order of magnitude. Hydrogen reduced the cracks and cavities from the poor accommodation between matrix and reinforcements in deformation and therefore enhanced the superplasticity of TMC. Hydrides and reinforcements stabilized the hydrogenated microstructure of TMCs with high hydrogen concentration, and this result facilitated the best superplastic property at medium and high strain rate. Grain/phase boundaries movement was the main superplastic mechanism of hydrogenated TMCs, and the dislocation movements and DRX were important accommodation mode.TMCs with large size were processed using THT, and the microstructure and mechanical properties were investigated. Forging were performed at 1000℃~880℃. TMCs with 0 wt.% H and 0.15 wt.% H obtained equiaxedα+βmicrostructures. However, TMC with 0.60 wt.% H obtainedα′′martensite plusβmicrostructure. After vacuum annealing, ultimate tensile strengths of TMC with 0.60 wt.% H at room temperature and high temperature (400℃) were both 100MPa higher than those of TMC with 0 wt.% H. Theα′′martensite andβphase in TMC with 0.60 wt.% H decomposed and formed fine and equiaxedα+βmicrostructure during vacuum annealing, which enhanced the strength.