Study On The Hydrogen Enhanced Plasticity Mechanism And Deformation Behaviors Of Titanium Alloys At High Temperatures

Titanium alloys are widely used in aviation, aerospace and marine industries due to their specific strength, good heat resistance and good corrosion resistance. But at room temperature the plasticity of titanium alloys is very low and crack occurs during deformation. At high temperature flow stress and deformation temperature are both very high which causes many problems to equipment and process. Thermohydrogen process of titanium alloys can improve the hot workability of the alloys due to hydrogen enhanced-plasticity, hydrogen-induced phase transformations and reversible reaction of hydrogen with titanium by using hydrogen as a temporary alloying element. However, the influence of hydrogen on the deformation behaviors and the hydrogen-enhanced plasticity mechanism of titanium alloys at high temperatures are lack of systematic study. This paper systematically studied about the two aspects.Microstructure of titanium alloys at room temperature has much influence on their deformation behaviors at high temperatures, thus optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to study the influence of hydrogen on the microstructures and substructures of titanium alloys. The precipitation mechanism of titanium hydride in hydrogenated alloys during cooling process was disclosed. The model of microstructural evolution during cooling process was established. Heating metallographical analysis was used to analyze the effect of hydrogen on theβtransus. High temperature metallographical microscope, high temperature X-ray diffraction analysis (XRD), DSC and TG was employed to analyze the effect of hydrogen on the phase transformationsα→β,β→α+δandβ→α'. The phase transformation subquence of hydrogenated alloys was expressed and the model ofα→βphase transformation was established. The effect of hydrogen on phase transformation and microstructural evolution provided a theoretical base for thermohydrogen process of titanium alloys.The isothermal hot compression was used to study the influence of hydrogen and deformation parameters on the flow stress behavior and microstructural evolution of Ti6Al4V-xH alloys. Experimental results showed that in the two phase region, addition of hydrogen could decrease the flow stress and increase the plasticity of titanium alloys. However, when the temperature reaches theα+β/βtransus temperature, the alloy free of hydrogen has the minimum flow stress, and the flow stress increases continuously with the increase of hydrogen content. The hydrogen enhanced plasticity mechanism of titanium alloys at high temperatures was disclosed according to the influence of hydrogen on the microstructural evolution.Hydrogen was removed by vacuum annealing after deformation. The results showed that the hydrogen content was in secure range and the hydrogenated alloy with the optimal hydrogen content had the finer microstructure. Based on the microstructure at room temperature, flow stress behavior, microstructural evolution and microstructure after dehydrogenation, the optimal hydrogen content range was 0.2~0.3wt.% and the optimal deformation parameters were 800℃~850℃, which was lower about 100~150℃than the optimal deformation temperature of the unhydrogenated alloy. This offers guidance to the development of the thermohydrogen process of titanium alloys.