| The so-called thermohydrogen processing (THP) is a technique in which hydrogen is used as a temporary alloying element in titanium alloys by controlling the microstructure and phase structure to improve the mechanical working properties. Usually, the titanium alloys are alwalys solid state during the hydrogenation process of THP technique. Hydrogen is absorbed by the solid state titanium alloys. However, this method needs long time to complete the hydrogenation process and can only hydrogenate the thin titanium alloys plate. This problem can be solve by Melt hydrogenation, which was proposed in this thesis, because the diffusivity of hydrogen increase quickly with the temperature. Titanium alloys are remelted in H2/Ar gaseous mixture and the hydrogen content in the alloy can be controlled by the variation of hydrogen partial pressure in the gaseous mixture. Hydrogen is absorbed by titanium alloy melt, then hydrogen exists in the alloy after the alloy cooling to room temperature. Melt hydrogenation can be used to hydrogenate the big bulk alloy specimens, just like titanium alloy ingots.Firstly, the thermodynamics problem about the hydrogen content in titanium alloy was studied in the thesis. The hydrogen content in TC4 alloy is calculated with two methods. One is that the weight variation, hydrogen weight, of the alloy specimens was measured by a balance. Another is that the mole number of hydrogen absorbed by the alloy was deduced based on the ideal gas state equation. Then the second method was verified by the experimental data and theoretical calculation. The solution heat of hydrogen in TC4 alloy is about -38.04kJ/mol which was deduced by the experimental data. Based on the famous Sieverts law, the equations of the hydrogen content in TC4 alloy, TC21 alloy ,Ti600 alloy and the hydrogen partial pressure was established. Based on these equation, the hydrogen content in titanium alloy can be controlled by hydrogen partial pressure. Hydrogenation processs is very fast in melt hydrogenation because the diffusion coefficient of hydrogen in the alloy melt, which increases with the partial pressure of hydrogen and temperature, is very large.The deoxidation of TC4 alloy which remelted in H2/Ar gaseous mixture was studied. It was found that the oxygen content of TC4 alloy decreased obviously after remelted in H2. The deoxidation mechanism of hydrogen was proposed: H2+O =H2O, then the water molecules flow into the gaseous mixture from the alloy melt surface. The influence of oxygen and deoxidation on the microstructures of TC4 alloy was studied, respectively. Oxygen make the microstructures of TC4 alloy coarse. The microstructures of TC4 alloy were refined after remelted in hydrogen. The refinement in the grain boundary is very obvious.Macrostructures and microstructures of titanium alloys were studied with stereoscope optical microscope(OM) and scanning electron microscope(SEM). It was found that both the macrostructures and microstructures of titanium alloys were refined by hydrogen and the structures get more fine with the hydrogen content increase. The grains were refined because hydrogen increases the nucleation rate and the floating crystals. The microstructures were also refined because the growth ofαmartensite restrained by the increase ofβphase. The hydrogen content increase resulted in the formation ofα′andα" martensite. Directional solidification phenomenon was found in some specimens becase of hydrogen.Transmission electron microscope(TEM) was used to study the influence of hydrogen on the substructures of titanium alloys. It was found that the hydride existed inαphase of hydrogenated titanium alloys. The single hydride and twinning hydride were both found in titanium alloy. The fraction of hydride increased with the increase of hydrogen content. The formation mechanism of hydride was presented in this thesis.The thermoplasticity of TC4 alloy tests were carried out on Gleeble-1500 thermal simulation testing machine. The influence of temperature and strain rate on the high temperature flow stress was studied, respectively. The influence of hydrogen content on the thermoplasticity of TC4 alloy is the key point of these research. It is found that the addition of hydrogen makes the flow stress of TC4 alloy increase when the hydrogen content is less than 5.31×10-2 wt.%; on the contrary, hydrogen decrease the flow stress of the alloy. The hydrogen enhanced plasticity mechanism of titanium alloys at high temperatures was achieved according to the influence of hydrogen on the microstructural evolution. It was found that the optimal hydrogen content range was about 0.1 wt.%(about 30% hydrogen in the gaseous mixyure) after melt hydrogenation.
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