Studies On Room Temperature Creep And Dwell Fatigue Properties Of Marine Engineering Titanium Alloys

Titanium alloys have become important marine engineering materials due to their high specific strengths,excellent corrosion resistance and satisfactory weldability.Both the two deep-sea manned submersibles of China,Jiaolong and Fendouzhe,use titanium alloys as their spherical shell materials.Jiaolong uses Ti64 with extra-low interstitial element content,while Fendouzhe employs Ti62A with the advantages of high strength and high toughness.Although titanium alloys have a series of distinct advantages,some titanium alloys always show a problem which can not be ignored:room temperature creep,that is,the phenomenon of plastic strain accumulation with time caused by load holding at room temperature.The room temperature creep behavior of titanium alloy will affect the dimensional accuracy of structural components,cause stress relaxation,and affect the safe and effective service of structural components.When the manned submersible cruises in the deep-sea high-pressure environment,it experiences a long time of load holding process,so their spherical shell materials' room temperature creep properties are worthy of attention.In the process of one deep-sea operation,the submersible experienced the process of submergence-cruise-ascent,which belongs to one kind of dwell fatigue wave.The dwell fatigue problem of titanium alloy has caused several major aviation accidents,so dwell fatigue properties of titanium alloys used in submersibles are also worthy of attention.Academically,the cold dwell effect of fatigue in titanium alloys is closely related to room temperature creep.In this paper,the room temperature creep and dwell fatigue properties of Ti62A and Ti64 for marine engineering are studied,which can provide basic data support for engineering application,and at the same time,some related academic problems are also analysed.Results of studies on room temperature creep properties show that the composition of the alloy plays a key role in the creep properties at room temperature.When the normalized creep stress is 0.95?y,the room temperature creep resistance increases with[Mo]eq as Ti62A-1>Ti62A-2>Ti64>Ti6242.For Ti64,the creep stress threshold is close to 0.80?y;for Ti6242,the creep stress threshold is close to 0.85?y;for Ti62A,the creep stress threshold is slightly lower than 0.75?y.Above the creep stress threshold,the room temperature creep behavior increases with the increase of creep stress level.At the same normalized creep stress 0.95?y,tensile room temperature creep is more obvious than compressive creep.The existence of texture will lead to the anisotropy of creep properties at room temperature,and the change of texture along the direction of plate thickness will also lead to the change of creep properties along the direction of plate thickness.With the increase of the peak density of<0001>in the loading direction,the work hardening exponent n increases linearly,while the creep exponent b decreases linearly,and the room temperature creep property becomes better.The decrease of the ratio of primary ? phase is conducive to the improvement of creep properties,while the coarsening of primary ? phase is not conducive to the improvement of creep properties.The larger the pre-plastic-strain is,the more obvious the inhibition on room temperature creep is.Although pre-plastic-strain can inhibit the subsequent creep of titanium alloy,it will worsen the subsequent fatigue performance of titanium alloy.The mechanism is that the greater the plastic strain is,the stronger the internal inelastic behavior of the fatigue sample is,which leads to the increase of fatigue damage rate and the decrease of fatigue life.Results of studies on cold dwell fatigue properties show that the alloy composition also plays a key role in cold dwell fatigue properties at room temperature,and the cold dwell effect of Ti62A,Ti64 and Ti6242 alloys increase in turn with the lower and lower creep resistance at room temperature.The only difference in the waveform between dwell fatigue and cyclic fatigue,load holding process,causes the difference in the plastic strain accumulation,which is the necessary condition of dwell effect.The higher the creep behavior at room temperature,the more significant difference of plastic strain accumulation will be produced during the two kinds of fatigue,which leads to the higher dwell sensitivity.The increase of holding time and stress level can increase the dwell effect of fatigue,while the pre-plastic-strain can inhibit the dwell effect.When the ratio of holding time in dwell fatigue waveform is high,the accumulation rate of plastic strain in dwell fatigue could be higher than that in normal room temperature creep,which means the existence of fatigue-creep interaction in dwell fatigue.In addition,a simple and feasible model for predicting the dwell fatigue life of titanium alloys at room temperature is established.