Research On The Thermohydrogen Processing And Room-temperature Deformation Behavior Of TC21 Alloy

TC21 alloy is a new kind of high strength, high toughness and high damage tolerance property titanium alloy developed by Chinese. The comprehensive mechanics performance of TC21 alloy is well matched, and the alloy is suitable for large aircraft components. Similar to most titanium alloys, TC21 alloy is hard to process induced by their high hot-working temperature, low plasticity during cot deformation and high deformation resistance. The use of hydrogen as a temporary alloying element in titanium alloys, so-called thermohydrogen processing (THP), is a special kind of technology for titanium alloys which can improve the plasticity and machinability, and induce phase evolution of original materials. However, the technology using THP to improve the capacity of cold plastic processing for titanium alloys is still hard to be applied actually due to the lack of theoretical research in hydrogen induced plasticity at room temperature. This paper studied the problem systematically.This paper intensive studied the effect of hydrogen as a temporary element on hydrogen absorption characteristics, microstructural evolution and room-temperature compressive properties of TC21 alloy by modern material analysis test methods based on THP experiments and room-temperature mechanical tests. The formation mechanism of hydride was discussed. The plasticization mechanism was discussed, and results can help confirming appropriate cold forming technology and deformation parameters for the hydrogenated TC21 alloy. Moreover, the hydrogenated TC21 alloys were dehydrogenated, and the best dehydrogenation process was determined. Results show that different hydrogenation temperatures have distinct effects on hydrogen absorption characteristics and microstructural evolution of TC21 alloy. The initial hydrogen absorption rate and the ultima hydrogen content achieve the maximum value at 600 ℃. With the addition of 0.68 wt.%H in the alloy, β transus temperature drops below 750 ℃ but still higher than 650 ℃. Acicular δ hydride and fine α2 precipitate exist in the alloy hydrogenated at 550 ℃, large numbers of lamellar δ hydrides precipitate in the alloy hydrogenated at 650℃, while β phase becomes the main phase in the alloy hydrogenated at 750 ℃. After hydrogenation, a’martensite and 8 hydride appear in the alloy hydrogenated at 750 ℃, the amount of a phase and a’martensite decrease while the amount of β phase and δ hydride increase with the increase of hydrogen content.δ hydrides distribute first along phase/grain boundaries, then form in β phase. The room-temperature compressive properties of the TC21-χH alloys are strongly dependent on the scales of different phases and the existing types of hydrogen. When hydrogen content in TC21 alloy is in the range of 0.6 wt.%to 0.9 wt.%, the amounts of primary a phase and acicular a’martensite are small, while the amount of ductile β phase becomes the main phase, large numbers of hydrogen atoms solubilize in β phase, and few 8 hydrides distribute along phase/grain boundaries discontinuously, thus the plasticity of TC21 alloy increases obviously. Compressive velocity has a remarkable effect on the room-temperature compressive properties of hydrogenated TC21 alloy. The ultimate compression of TC21-0.9H increases by about 30%under low compressive velocity, but the ultimate compressive deformation rate of TC21-0.9H increases by more than 100%under rapid compressive velocity, because mechanical energy transforms into internal energy during rapid compression and δ hydrides distributed along phase/grain boundaries dissolved due to inverse eutectoid reaction. Appropriate hydrogen content not only improves the plasticity but also decreases the flow stress of TC21 alloy during plastic deformation. The flow stresses of TC21-0.9H alloy decreases about 150-240 MPa under compressive test. The results of thermal analysis show that no obvious phase transformation is found in the hydrogenated TC21 alloy when heating temperature is lower than 500 ℃, metastable phases containing hydrogen begin to decompose when heating temperature is high than 500 ℃, and the mass loss rate of the alloy reaches maximum when heating temperature is in the range of 500 ℃ to 700 ℃. Hydrogen in the hydrogenated TC21 alloy can be removed after the alloy is dehydrogenated at 750 ℃ for 4 h or at 800 ℃ for 2 h, but the best treatment is at 750 ℃ for better microstructural refinement. After dehydrogenated at 750 ℃, the yield strength, compressive strength and microhardness of the alloy increase, while the ultimate compression decreases. The best hydrogenation process and forming process condition are determined according to the above results. Hydrogenation temperature of TC21 alloy is at least 750 ℃, the optimum hydrogen content is 0.9 wt.%, and the capacity of cold plastic processing for hydrogenated TC21 alloy can be further improved during rapid deformation. The dehydrogenation process was determined. Dehydrogenation temperature is 750 ℃, and the holding time is 4 h.