Influence Of Thermo Hydrogen Treatment On Microstructural Evolution And High Temperature Deformation Behavior Of Titanium Alloys

The manufacturing cost of titanium alloys is high for its poor workability,limiting the application in this way.Thermo hydrogen treatment(THT) of titanium alloys,or the use of hydrogen as a temporary alloying element due to the reversible reaction of hydrogen with titanium,can improve the processing properties of titanium alloys.THT for titanium alloys can solve the oxidative problem at high temperature and improve the hot workability of the alloys,consequently,lowering the manufacturing cost.The use of THT will give an impulse to the application of titanium alloys,especially high-temperature titanium alloys to some extent.The research findings on THT theory put forward by former researchers are introduced in this paper,especially,the theoretical and experimental development in aspect of influence of THT on microstructures and mechanical properties of titanium alloys.On this basis, Ti6Al4V and Ti600 alloys are employed in this paper to study the microstructural evolution and high temperature deformation behavior of them after THT,systematically,including analysis of thermodynamic process of formation of titanium hydride and simulation of dynamic recrystallization(DRX) process of titanium alloy using a cellular automaton approach.The main conclusions to be drawn are as follows:(1) Influence of THT on microstructure,microdefect and microhardness of Ti6Al4V alloy is researched.The microstructural evolution of Ti6Al4V alloy after THT is investigated by microstructure observation and phase analysis.The influence of hydrogen content on the amount and type of microdefect is analyzed by positron annihilation technique(PAT).The influence of hydrogenation on alloying diffusion and microhardness is studied by electron probe microanalysis(EPMA) and microhardness testing,respectively.The results indicate that titaniumδhydride(fcc structure) can precipitate from both a andβphases,and the orientation relationship betweenβandδfollows[011]_δ//[012]_β,(02(?))_δ//(200)_βwhenδprecipitates fromβphase.As the hydrogen content increases,the type of microdefect is at first "vacancy+dislocation" and then it is mainly dislocation.The amount of microdefect shows a dramatic decrease after 0.1%H is charged,and then increases with increasing of hydrogen.After hydrogenation,the alloying elements Al and V redistribute in Ti6Al4V alloy. The hardness values ofαandβphases of Ti6Al4V alloy increase synchronously with increasing of hydrogen,and the hardness ofβis higher than that of a at the same hydrogen content.After vacuum dehydrogenation,αgrain boundary of hydrogenated Ti6Al4V alloy disappears,andβphase is broken up into very fine microstructure.The rolled microstructure of hydrogenated Ti6Al4V alloy is refined after dehydrogenation,which is a result of a combined action of two operating mechanisms of phase transformation and recrystallization.(2) Influence of THT on microstructural evolution and macrohardness of Ti600 alloy is researched.The microstructure of hydrogenated Ti600 alloy is investigated by microstructure observation and phase analysis,and the influence of hydrogen content on precipitation of silicide is analyzed.The influence of hydrogenation on macrohardness of Ti600 is studied by macrohardness testing.The results show thatδhydrides(fcc structure) exist in the specimens with 0.35%and 0.5%hydrogen,andδtends to be refined with increasing of hydrogen.There are two types of silicide precipitate in the Ti600 alloy after THT,one is S₃(0.35%H),and the other is S₁(0.5%H).The hardness of Ti600 alloy increases with increasing of hydrogen,and it is considered that hydride,silicide,lattice defects and martensiteα' are the major factors.(3) Thermodynamic calculation of formation of titanium hydride is operated.A modified Miedema model is employed to calculate the standard enthalpy of formation of titanium hydride TiH_x(1≤x≤2).The standard entropy of formation of titanium hydride TiH₂ is calculated by statistic thermodynamics method,and the thermodynamic process of formation of TiH₂ is analyzed.The calculated results show that the values of standard enthalpy of formation of TiH_x decrease linearly with increasing of x.The calculated results of standard enthalpy,entropy and Gibbs free energy of formation of TiH₂ at 298K are -137.46kJ·mol^-1, -143.0J·mol^-1·K^-1 and -94.85kJ·mol^-1,respectively.The reaction of Ti(s)+H₂(g)→TiH₂(s) inclines to occur spontaneously when temperature is lower than 925K,and the reaction tends to take place in the opposite direction when temperature is higher than 925K.As temperature increases,the equilibrium hydrogen pressure of formation of TiH₂ increases,and the stability of TiH₂ falls,accordingly.(4) High temperature deformation behavior of Ti600 alloy after THT by hot simulation experiments is researched,and the microstructural evolution after deformation is investigated by microstructure observation,also,the hydrogen modified high temperature deformation mechanism is analyzed.The results reveal that the flow stress,strain-hardening rate and strain energy density decrease synchronously with increasing of hydrogen when hydrogen content is less than 0.3%.The addition of 0.3%hydrogen in Ti600 alloy can decrease the hot deformation temperature by 80℃or increase the deformation strain rate by two orders of magnitude in flow stress terms.The activation energy of deformation of Ti600 alloy decreases with increasing of strain at a given hydrogen content level.At true strain 0.6,the calculated values of activation energy of deformation of Ti600 alloy without and with 0.1%,0.3%and 0.5%hydrogen are 648.4,459.0,324.3 and 420.0KJ/mol,respectively,and the value of activation energy of deformation decreases gradually with increasing of hydrogen contents from 0 to 0.3%.(5) The constitutive relationship of hydrogenated Ti600 alloy during high temperature deformation is established.For eliminating the influence of multi-correlation,the "optimum" independent variable subsets influencing the flow stress of Ti600 alloy are determined by diagnosis of colinearity and selection of variables.Then,constitutive relationship is obtained using partial least squares regression method for high temperature deformation of Ti600 alloy with 0,0.3%and 0.5%hydrogen contents,respectively.(6) A cellular automaton(CA) model is employed to simulate the dynamic recrystallization(DRX) inβphase field of Ti6Al4V alloy,and the kinetics during DRX process has been analyzed.The simulation results show that the DRX volume fraction increases remarkably with increasing of strain at a given strain rate.If an adequate strain is given,the DRX grain size of Ti6Al4V alloy decreases with increasing of strain rate,and also the inadequate microstructure is induced.The results of kinetics analysis of DRX reveal that constant nucleation rate nucleation as well as site saturated nucleation behavior occurs during DRX.The Avrami exponent obtained in the present work is a variable ranging from 2.4 to 2.9, which increases with increasing of strain rate.