Effect Of Surface Quality And Thermal Exposure On Fatigue Strength Of ?-TiAl Based Alloys

There different strength ?-TiAl alloys:Ti-48Al-2Cr-2Nb-1B(2Cr-2Nb)?Ti-46Al-8Ta(8Ta)and Ti-44Al-4Nb-4Hf-0.2Si-1B(4Nb-4Hf)with electropolishing and electropolishing+shot peening were researched on fatigue property under three exposure conditions of no exposure,block exposure(without surface oxidation)and individual exposure-oxidation at 700? for 10000h.This paper systematically studied the change of surface defects,surface oxidation,stress concentration and the microstructure before and after thermal exposure.And then analyzed the influence mechanism on fatigue performance.It has been found that electropolished+shot peened surface of three alloys is superior to the single electropolished surface on fatigue property without thermal exposure because of the introduction of the surface compressive layer.The increased quantity has a positive relationship with the strength of the alloys.If the strength of the alloys is higher,the fatigue property will increase more.During the long-term block thermal exposure without oxidation,two effects happened in the alloy influenced the fatigue property of ?-TiAl alloys.The first effect is long-term block thermal exposure lead to a beneficial effect similar to the annealing.It can cause the relaxation of inner stress concentration,alleviation of segregation and the passivation of inner defects which will increase the resistant to fatigue crack initiation.This is an enhanced effect.The second effect is the decomposition of coarse of ?2 and the phase change of?2??2P(B2+?).The embrittlement of the releasing oxygen caused by microstructure decomposition and the embrittlement of phase change caused by the formation the phase B2+? have a harmful effect on the resistant to fatigue crack initiation.Both surfaces of alloy 2Cr-2Nb and alloy 4Nb-4Hf show an improved fatigue strength in relation to the non-exposed,giving an exposure-induced fatigue strengthening phenomenon because of enhanced effect is stronger than the embrittlement effect.The increase of fatigue of low-strength alloy 2Cr-2Nb with less decomposition and low embrittlement is more obvious.During the individual thermal exposure+oxidation process,the above two opposing effects are still working.But surface oxidation one unfavorable factor to fatigue resistance is introduced.Research shows that the low-strength alloy 2Cr-2Nb and the high-strength alloy 4Nb-4Hf which are treated by single electrolytic polishing still performed the increase of fatigue strength after thermal exposure.This indicates that the beneficial effects of thermal exposure of these two alloys are strong enough to counteract the brittleness effects,whereas the medium-strength alloy 8Ta performed the exposure embrittlement and the brittle effect plays a dominant role.Oxidation happened on the two kinds of surface,but the electropolished surface is more effective in retaining oxidation resistance than electropolished+shot peened surface.Compared with electropolished surface,electropolished+shot peened surface has an extra harmful factor,that the compressive stresses and deformation hardening are released.This has a negative effect on fatigue resistance of the alloys.Research shows three different strength alloys after individual thermal exposure+oxidation process,the fatigue strength of electropolished surface increased and the fatigue strength of electropolished+shot peened surface weakened.But both surfaces of alloy 8Ta are weakened in relation to non-exposed.