Study On High Temperature Stress Relaxation And Creep Behavior Of TC4 Alloy Sheet

Titanium alloys are applied in aerospace,automobile,shipping and other fields due to their high specific strength,high temperature performance and good corrosion resistance,et al.In the forming or service stages,forming accuracy,forming limit as well as forming quality are required and expected.At present,the thermal creep forming and thermo-hydrogen processing of titanium alloys have been widely used in industry.The combination of these methods is a potential way to further improve the plasticity of titanium alloys and solve the forming difficulty of complex components.TC4 alloy is currently the most widely used titanium alloy.In this paper,the characteristics of high temperature stress relaxation and creep behavior of titanium alloy are systematically studied.Based on classical Maxwell exponential decay function,a revised model following the time-hardening rule and corresponding method were proposed to describe the stress relaxation behavior.Compared with the original one,the revised model has the advantages of fewer parameters but higher accuracy.The effect of grain size on stress relaxation behavior of TC4alloy at elevated temperature was studied.The results showed that the stress relaxation limit SRL,creep strain rate and residual stress reaching 1.5SRL decrease with the increment of grain size.Furthermore,the SRL and average grain size are in good linear relationship.The stress exponents ranged from 1.260 to 2.144 were calculated from SRT for sheets with different grain size.These exponents indicate different SRT mechanisms though the range is somewhat narrow.The dominant SR mechanism of as-received TC4 sheet is grain rotation with grain boundary sliding accommodated.It shifts to dislocation sliding,edge dislocation climb,dynamic recovery and even continuous dynamic recrystallization with increasing grain size.The grain size decrease is advantageous to lower the SRL due to both grain rotation and dislocation movement.The effects of hydrogen on the morphology,high temperature creep and stress relaxation behavior of the alloy were discussed.By means of optical microscopy and scanning electron microscopy,et al,it was found that the?/?phase ratio increases with hydrogen content whilst??and?~??martensite precipitated in succession in?phase.The?/?or?/?phase boundaries become wider.Hydrogen promotes creep process through softening the alloy during primary and steady creep stage,but embrittling it during accelerated stage.The alloy after hydrogenation is more sensitive to initial stress and temperature.In accordance with the mechanism of creep promotion by hydrogen,the addition of hydrogen increases the stress relaxation rate and reduces the stress relaxation limit.During service,titanium alloy parts may bear the force cycling load at high temperature.The results showed that the stress relaxation rate of TC4titanium alloy increases with the increase of temperature.The stress relaxation limit decreases from 10.79 MPa to 4.36 MPa with the increase of temperature in the range of 600-690?.When the temperature is higher than690?,the stress relaxation limit decreases to near zero.The initial stress decreases approximately linearly with the increase of temperature.The trend of recycling stress relaxation curve of TC4 titanium alloy at different temperatures is similar,that is,the smaller the cycle number is,the faster the stress relaxation and the smaller the residual stress relaxation are.The corresponding stress relaxation resistance R can be obtained by using an empirical equation.The results showed that the smaller the cycle number is,the greater the cumulative stress relaxation resistance is.Within the test range,the mechanical properties of TC4 titanium alloy at room temperature have little change after reloading stress relaxation.