| Titanium aluminide alloy castings of advanced aero-engines are constantly subjected to temperature and stress changes during service,which can easily lead to high cycle fatigue,that is,the fatigue caused by 10~4～10~7 cycles or more under the cyclic stress lower than the yield strength.The as-cast microstructure of titanium aluminide alloy is coarse,which makes the fatigue performance poor,so the as-cast structure must be refined by heat treatment.However,at present,the heat treatments of titanium aluminide alloy castings are complex,take a long time,require special equipment,and will cause the castings to deform or even crack.At the same time,the surface processes before service and the thermal exposure environment during service for titanium aluminide alloy castings will lead to complex changes of the surface condition.The changes of surface roughness,hardness and residual stress will affect the fatigue resistance and make the castings suffer the risk of premature failure.Therefore,it is an urgent need to study the microstructure control using heat treatment and the high cycle fatigue behavior under practical application surface conditions of titanium aluminide alloy castings for ensuring their safe service.In order to meet the above requirements,this paper makes an in-depth study on the microstructure control using heat treatment and the high cycle fatigue behavior under three typical surface application conditions.The main work and results are as follows.(1)The microstructure evolutions of Ti-47Al-2Cr-2Nb castings after different heat treatments were studied.The one-step heat treatment of duplex(DP)microstructure and two-step heat treatment of nearly lamellar(NL)microstructure were put forward after hot isostatic pressing,and the grain refinement mechanisms of heat treatments were revealed.The results show that the lamellar colony grains are hereditary during conventional heat treatments in the(α+γ)phase region and the orientation of lamellar colony grains does not change after heat treatments,which are not conducive to the grain refinement.By optimizing the parameters of heat treatment,the heat treatments of DP and NL microstructures with grain refinement are established,which are 1185°C/6 h/furnace cooling and 1185°C/6 h/furnace cooling+1330°C/0.25 h/furnace cooling,respectively.The as-cast grain size can be refined by 75.5%and 40.2%,respectively.The grain refinement mechanism of DP microstructure heat treatment is that a large number ofγgrains precipitate inside the lamellar colony grains and break the original coarse lamellar colony grains.The nucleation ofγnuclei in the lamellar colony grains comes from Al element segregation and continuous coarsening ofγlamellae.The grain refinement mechanism of NL microstructure heat treatment is that theγ→αtransformation takes place in theαsingle phase region for a short time which destroys the original coarse lamellar colony grains,and the equiaxedγgrains can pin the growth ofαgrains,so that the lamellar colony grains formed after cooling are smaller.The yield strength of the specimens with DP microstructure is slightly higher than that of the specimens with NL microstructure,but their elongation is much higher than that of the specimens with NL microstructure.(2)The high cycle fatigue deformation behaviors of Ti-47Al-2Cr-2Nb castings with DP and NL microstructures were studied,and the fatigue deformation mechanisms of DP and NL microstructures were revealed.The results show that the fatigue deformation mechanism of DP microstructure is mainly dislocation slip in the coarseγlamellae and equiaxedγgrains,and there is also a small amount of twin deformation.The fatigue deformation mechanisms of NL microstructure are the dislocation slip and twin deformation inγlamellae.The twin deformation can easily cause high stress concentration and promote the initiations of fatigue microcracks during the fatigue deformation.(3)The surface condition evolutions and high cycle fatigue fracture behaviors of the sand blasting(SB),sand blasting plus shot peening(SBSP)and sand blasting plus mechanical grinding(SBMG)Ti-47Al-2Cr-2Nb casting specimens with DP and NL microstructures after thermal exposure were studied,and the intrinsic relationship between surface condition and fatigue resistance was revealed.The surface roughness of SB,SBSP and SBMG specimens with DP and NL microstructures decreases successively.The microstructures in the surface layer of SB,SBSP and SBMG specimens with DP and NL microstructures recrystallize and recover successively from outside to inside during thermal exposure,and the surface hardness and residual compressive stress are greatly relaxed.The surface roughness becomes the main factor affecting the fatigue resistance.The lower the surface roughness,the smaller the surface stress concentration,the greater the fatigue crack initiation resistance and the higher the fatigue limit.Therefore,the fatigue limit of SBMG specimens is obviously higher than those of SB and SBSP specimens.The fracture surfaces of the specimens with DP microstructure are smooth,and the fatigue sources are located in the micro-dents on the surfaces.The fracture modes are trans-lamellar and trans-γgrain fractures.The fracture surfaces of the specimens with NL microstructure are rough,so it is difficult to distinguish the locations of the fatigue sources.The possible fatigue sources are located in the micro-dents or oxide on the surfaces,and the fracture mode is trans-lamellar fracture.Because the fatigue limit of the SBMG specimens with DP microstructure is higher than that of the specimens with NL microstructure and the fatigue life dispersion of the specimens with DP microstructure is lower,the DP microstructure and SBMG surface process should be selected for practical application of Ti-47Al-2Cr-2Nb castings. |
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