Studies On The Hot Corrosion Behavior Of Three Ni-base Single Crystal Superalloys

In this graduate dissertation,we systematically investigated the effect of key alloying elements(including Re,Cr,Al and Ti)and secondary orientation on nickel-based single crystal superalloys coated with Na2SO4 salt at 900?.Furthermore,the hot corrosion behaviors of a secondary generation nickel-based single crystal superalloy with pre-coated mixtures of Na2SO4(90 wt.%)+NaCl(10 wt.%)and pure Na2SO4 salt at 750? in air without a SOx atmosphere were also investigated.X-ray diffraction(XRD)and scanning electron microscopy(SEM)equipped with an energy dispersive spectroscopy(EDS)were used to identify the corrosion products and characterize the microstructure of the corroded specimens.The high-temperature hot corrosion mechanism of nickel-based single crystal alloys with different compositions was revealed by using the first-principles simulation calculations,and the influence mechanism of secondary orientation on the hot corrosion resistance of single crystal alloys and a new low-temperature hot corrosion mechanism were explored.The Re-free alloy with 12 wt.%Cr and higher Al/Ti(Al/Ti>1)had a slower mass gain at the initiation stage of hot corrosion,but a NiO-dominant layer formed due to the insufficient Ti content along with the hot corrosion,which was less protective compared with the NiTiO3-dominant layer and might cause a synergistic dissolution between basic and acid oxides and accelerate the hot corrosion damage.In comparison,the Re-free alloy with 12 wt.%Cr and lower Al/Ti(Al/Ti<1)exhibited an excellent hot corrosion resistance due to sufficient Ti content and formation of a compact unbroken NiTiO3 layer.The experimental alloy with moderate Cr content(8.5 wt.%)and Al/Ti>1 in the present research exhibited excellent resistance to hot corrosion by adding Re element.The mechanism was proposed that adding Re may facilitate the diffusion of Cr in y phase and diffusion of Ti in ?' phase and promote the formation of Cr2O3 and TiO2,which inhibited the formation of NiO-dominant layer at the late stage of hot corrosion.The result by first-principles calculation was consistent with our hypothesis that the diffusion of Cr can be facilitated in ? phase by adding Re.Whether it is analyzed from the mass gain of the hot corrosion or the thickness of the corrosion layer of the cross section to characterize the high temperature hot corrosion performance of the alloys,the different second orientations samples of the Re-free alloys showed anisotropic hot corrosion resistance(although the difference is very small).Compared to the[210]and[100]samples,the[110]samples have the worst thermal corrosion resistance([100],[210]and[110]are the secondary orientations of the specimens).This may be due to the the different structure of ?/?' interface as rapid diffusion paths for cations diffusion during hot corrosion in the alloys.However,the hot corrosion resistance of samples with different secondary orientations for the alloy with Re did not show obvious anisotropy,which may be due to the addition of Re to promote elements diffusion and weaken the influence of the different structure of ?/?'interface.By studying the low-temperature hot corrosion behavior of the second-generation single crystal superalloy coated with salt of different compositions in air environment at 750?,it was found that the alloy suffered type-I hot corrosion(liquid phase corrosion)damage in the molten salt.Compared with the solid salt environment,the alloy had suffered more severe hot corrosion in the molten salt.In solid Na2SO4 salt,the main corrosion products of the experimental alloy after 100 h corrosion were the same with that in mixed salt.But the thickness of corrosion layer was much thinner and the size of sulfides was significantly bigger than that in mixed salt.A solid-phase reaction between alloy elements and solid Na2SO4 salt was confirmed based on thermodynamic analysis and microscopic observation.