Effect Of Surface Quality And Thermal Exposure On Fatigue Performance Of ?-TiAl Alloys With Varied Yield Strength

Electropolishing and electropolishing+shot peening are studied as potential methods for improving the fatigue strength of four y-TiAl alloys(low strength alloy Ti-46Al-2.5Nb(alloy 2.5Nb),intermedium strength alloy Ti-45Al-2Mn-2Nb-1B(alloy 2Mn-2Nb);high strength alloyTi-44Al-5Nb-0.85W-0.85B(alloy 511)and alloy Ti-44Al-4Nb-4Zr-0.2Si-1B(alloy 4Nb-4Zr))before and after 10000h 700? exposure.The results show that electropolishing+shot peening outperforms electropolishing for the four alloys before exposureowing to compressive stresses being introduced to the enhanced surface layer.After exposure,however,the electropolishing outperforms the electropolishing+shot peening for the four alloys.This is because the compressive stresses introduced are released due to long-term thermal exposure and the stressed uneven surface region is more vulnerable to oxidation.The present work has found that the long-term exposure causes two kinds of effect on the inner microstructure of these alloys.Firstly,the relaxation of inner stress concentration,alleviation of segregation and passivation of inner defects and microcracks lead toan annealing-like positive effect.Secondly,the decomposition of coarse ?2 laths and the precipitation and coarsening of ? phase lead to a thermal exposure-induced embrittlement negative effect.After 10000h thermal exposure,the fatigue strength of electropolished surface is increased for low strength alloy 2.5Nb,high strength alloy 511 and 4Nb-4Zr.This is explained by considering that the positive effect of thermal exposure-induced annealing outweighs the harmful effect of thermal exposure-induced embrittlement.Moreover,a higher degree increment is obtained in the coarse-grained low strength alloy because the alloy shows lower degree of microstructure decomposition.But the fatigue strength of electropolished surface is slightly lower(only 8.3%lower)for medium strength of 2Mn-2Nb alloy.This is probably because the positive effect of thermal exposure-induced annealing slightly less than the harmful effect from exposure-induced embrittlement.It has been found that a very smooth surface without defects and stress concentration produced simply by electropolishing is more capable of resisting the deterioration of the fatigue properties than electropolishing +shot peening during long-term thermal exposure.