Study On Hydrogen Absorption Kinetics And Microstructure Of TC4 And TC21 Titanium Alloys

Both TC4 and TC21 alloys belong to ?+? titanium alloy.Because of its excellent comprehensive mechanical properties,TC4 alloy has become the most widely used titanium alloy in the world.TC21 alloy is a new kind of high strength,high toughness and high damage tolerance property titanium alloy developed by Chinese.Like most other titanium alloys,these two alloys are prone to cracking when deforming at room temperature and hard to process due to high flow stress.Using hydrogen as a temporary alloying element in titanium alloys,so-called thermohydrogen processing(THP),is a special kind of method for titanium to refine their microstructure and improve the machinability owing to the phase evolution caused by hydrogen.In this paper,the hydrogen absorption kinetics and effects of hydrogenation on the microstructures of titanium alloys were studied.Four hydrogenation temperatures were selected(823K,923 K,1023K and 1123K)and four initial hydrogen pressures(16k Pa,31 k Pa,46 k Pa and 61 k Pa)were selected at each temperature,the change of hydrogen pressure during hydrogenation processing was recorded to investigate hydrogen absorption kinetics,the results are as follows: The hydrogen content of hydrogenated TC4 alloy increases first and then decreases with the rise of hydrogenation temperature,when the hydrogenation temperature is 923 K,the hydrogen content of hydrogenated TC4 alloy reaches the maximum value.The laws of hydrogen absorption rate of TC21 alloy is similar to that of TC4 alloy,There is no obvious induction stage in hydrogen absorption reaction of the two titanium alloys,at the beginning of the reaction,the rate of hydrogen absorption reaches its maximum,with the reaction proceeding,the rate of hydrogen absorption gradually slows down and then the hydrogen content in titanium alloy reaches the saturation under this condition and the reaction reaches equilibrium.Hydrogenation temperature has a great influence on the hydrogen absorption rate of titanium alloy,with the rise of hydrogenation temperature,the hydrogen absorption rate of titanium alloy increases gradually,specifically,the rate constants of hydrogen absorption reaction of two titanium alloys increase with the rise of hydrogenation temperature.Accordingly,the time required for hydrogen absorption reaction to reach equilibrium is gradually shortened with the rise of hydrogenation temperature.When the hydrogenation temperature is 823 K,the hydrogen absorption reaction of TC4 and TC21 alloys is not fully activated,at this time,the hydrogen absorption rate constant of the alloy is very small compared with that of the alloy at higher hydrogenation temperature.The apparent activation energy of hydrogen absorption of TC4 alloy and TC21 alloy are 79.42 k J/mol and 70.60 k J/mol,respectively,by solving Arrhenius equation.The room temperature microstructures of two titanium alloys before and after hydrogenation were studied by optical metallographic observation,X-ray diffraction and transmission electron microscopy analysis.The initial microstructure of TC4 alloy is strip-like at room temperature,when the initial hydrogen pressure is a constant,the microstructure of TC4 alloy hydrogenated at 823 K is similar to the original structure,??martensite and ? hydride are formed during hydrogenation process with further rise of hydrogenation temperature.Needle-like structure appears in the room temperature microstructure of the alloy.the number of needle-like structure increases and measurement becomes smaller gradually with the increase of hydrogenation temperature.The room temperature structure of the original TC21 alloy is a black-and-white duplex structure.When hydrogenation temperature is invariant,the relative chemical potential of the ? phase and the ? phase changes with the increase of hydrogen content in the alloy.The ? phase changes from the non-corrosive phase to the corrosive phase,while the ?phase is just the opposite,which leads to the reversal of the optical contrast of the ? phase and the ? phase after hydrogenation.The optical contrast of ? phase gradually darkens and the ? phase gradually turns to white-bright from gray-black.