Investigations On The Metastable Phase Transformations And Cold Deformation Behaviors Of The Hydrogenated Titanium Alloys

Titanium alloys are widespread availability in aviation industry due to the high strength-to-weight and the resistant to corrosion. However, its low room temperature plastic, high resistance of deformation, and easily to crack during deformation limited the cold working properties. Thermohydrogen processing (THP), based on the modifying effect of hydrogen as an alloying element on phases and kinetics of phase transformation in titanium alloys, has been used as a new method to improve the mechanical properties of titanium alloys. Hydrogen induced cold plasticity is one of the mainly aspects, but the investigation on this aspects is little in national. Therefore, this work begins from the studying of hydrogen induced phase transformation, and then investigated the heat treatment for improve the cold forming and the mechanism of the cold deformation systematically.OM, XRD, TEM were utilized to investigate the effect of hydrogen on the microstructure evolution of TC4 alloys and TC16 alloys. A quartz tube device was designed and theβphase transition temperature of the hydrogenated titanium alloys was investigated using metallographic method by the device. The effect of hydrogen on the metastable phase transition was investigated and the corresponging TC4-H, TC16-H phase diagram were built, which establish the theoretical basis of the hydrogen treatment. The results showed that, as aβstabilizer, the addition of hydrogen decreased theβphase transition temperature of the TC4, TC16 alloys, promoted the formation of a" martensites and the metastableβphase. However, theβphase can not be stabilized completely to room temperature due to the limited solid solubility of hydrogen in titanium alloy. Even though, the appearance of the a" martensites and the metastableβphase still provide a basis for the improvement of cold deformation. According to the phase transition, the optimum for the improvement of cold deformation is quenching 10℃above theβphase transition temperatureThe compression test, tensile test, charpy impact test and dynamic upset test were conducted to investigate the dynamic and the static deformation behaviors of the hydrogenated TC4 alloys. OM, XRD, TEM were utilized to investigate the mechanism of the cold deformation. The results showed that the deformation limit of hydrogenated specimen increases with the increase of hydrogen concentration, and reaches maximum in specimen containing 0.6～0.9wt % H, nearly one times of the as-received. However, the results of the tensile test and the charpy impact test showed that the mechanical properties of hydrogenated alloy decreased and embrittled and shown no relationship with the treatment. According to the evolution of the microstructure after deformation, it can be concluded that the improvement of the cold deformation for TC4 alloy are owing to:①the addition of hydrogen promoted the formation ofα" martensites and the metastableβphase;②the stress induced martensites occurred during deformation;③hydrogen decreased the critical stress of the formation of dislocation and twins;④the formation of nano-structured hydrides.The compression tests, tensile tests, dynamic upset test and SHPB were conducted to investigate the dynamic and the static deformation behaviors of the hydrogenated TC16 alloys. OM, XRD, TEM were utilized to investigate the mechanism of the cold deformation. The results showed that the tensile properties decreased greatly after hydrogenation. Under the static deformation, the deformation limit decreased with the increase of the hydrogen concentration, while the alloy exhibited well deformation abilities under dynamic deformation, no crack occurred even upsetting to 70% in specimens containing 1.0wt %H. According to the evolution of the microstructure after deformation, it can be concluded that the improvement of the cold deformation for TC16 alloy are owing to:①the addition of hydrogen decreased theβtransition temperature and improved the stability of theβphase,②adiabatic temperature rise occurred during the fast deformation which supplied the conditions for hydrogen induced plasticity,③hydrogen decreased the critical stress of the formation of dislocation and twins and promoted the stress induced martensites occurred during deformation.Metallographic method was utilized to investigate the decomposition of theα" martensites and the metastableβphase, and according to the results the refining processing and the dehydrogenation heat treatment of the TC4 alloy were formulated. The results showed the decomposition of these metastable phases occurs by a nucleation and growth process controlled by atom diffusion and the shortest incubation of the decomposition is at 700℃, in which the decomposition time is shortest. The addition of hydrogen promoted the diffusion of theβstabilizer, resulting in the formation of the stableβphase, which finally retained in the alloy during the dehydrogenation. The mechanical properties of the alloy after dehydrogenation showed that specimens containing 0.45wt % H with full martensites after hydrogen treatment shows a high degree increase of strength. However, a fine grain and a comprehensive mechanical properties was obtain in specimens contains 0.8wt % H.