| γ-TiAl based alloys have excellent high-temperature strength, creep resistance, oxidation resistance, flame retardant, low density and high elastic modulus, which is promising to be applied to aerospace and automotive industries. However, there are some disadvantages for forming of TiAl alloy, such as the coarse structure after the casting and high flow stress during the forging. The thermohydrogen treatment (THT) is a technique in which hydrogen is used as a temporary alloying element to be introduced into the alloys. Grain refinement and improvement of hot deformation performance can be obtained by the positive effect of hydrogen.Because the diffusion of hydrogen in solid alloy is relatively slow, the conventional THT is usually applied to small-scale or thin-wall samples. In order to overcome the disadvantage of low efficiency of the conventional THT, a novel hydrogenation technique was put forward by our research group, which is melt hydrogenation technique (MH). In the MH processing, the alloys are melted in argon/hydrogen mixture. The hydrogen decomposes and diffuses into alloy melt and retains in the alloys during the cooling process. The MH is applied to TiAl alloy in this study, and its effect on solidification structure, mechanical property and removal of impurities are studied.The control of hydrogen content and duration during MH process was studied first, including solubility and diffusion coefficient of hydrogen in TiAl alloy melt. A formula of hydrogen solubility in TiAl alloy melt was obtained based on thermodynamic theory and experimental data. The relationship of hydrogen solubility in TiAl binary alloy melt between composition, partial pressure of hydrogen and melt temperature was established. The experiment value of hydrogen solubility is consistent with the calculated values. The equivalent diffusion coefficient of hydrogen in TiAl alloy melt was estimated by assuming one-dimention conduction, together with Fick's second law and error function method, which is 5.5×10-3 cm2/s.The removal effect of MH on the impurities O, C and N in TiAl alloy melt was investigated. The method showed good deoxidation, decarburization and weak denitrification effect on TiAl alloy melt, as evidence by the decrease of impurities when the partial pressure of hydrogen and the melting duration were increased. The deoxidation, decarburization and denitrification effect of hydrogen was analyzed via change of Gibbs free energy together with the hydrogen action in alloy melt, which is due to the reactions of hydrogen with the impurities near certain regions of the melt surface.The MH was performed on Ti-44Al, Ti-47Al, Ti-49Al alloys and its effect on solidification structures was studied. After MH, the coarse columnar grains of TiAl alloys become much refiner columnar plus equiaxed grains, and some grains with the six-fold symmetry appear except the four-fold symmetrical grains. The hydrogen-induced refinement is due to hydrogen-decreased critical nucleation energy and hydrogen-enhanced constitutional supercooling ahead of the solid-liquid interface. In addition, the lamellar spacing of Ti-47Al alloy was decreased by MH.The effect of hydrogen on hot deformation of Ti-47Al alloy was studied using a Gleeble hot simulator. At the appropriate temperature and strain rate, the flow stress and peak stress of Ti-47Al alloy are reduced with increasing hydrogen addition. The peak stress can be reduced by about 33 % with 1 at.% hydrogen addition at 1150°C, 1×10-2 s-1. There are less dislocation tangles and more twinning and stacking faults in hydrogenated alloys. The decrease of flow stress and peak stress by melt hydrogenation is attributed to hydrogen-induced dislocation mobility and hydrogen-promoted dynamic recrystallization and twinning.
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