Hot Corrosion Behaviour And High Temperature Tensile Property Of A Second Generation Ni-based Single Crystal Superalloy

Single crystal superalloys are widely used in advanced aerospace engines and gas turbines.Due to the combustion medium contains corrosive substances such as sulfur during the service of gas turbine,an excellent hot corrosion resistance is required for the material of the hot end components.Although the requirements of high temperature strength for hot end components of gas turbines is lower than those of aerospace engines,maintaining high temperature strength while possessing excellent heat and corrosion resistance is clearly beneficial in expanding the application of the alloy.Therefore,this paper aims to study the hot corrosion behavior and high temperature strengthening mechanism of a second-generation single crystal superalloy DD419,expand the data of DD419 alloy,ensure the stable service of DD419 alloy,and provide reference for the selection of single crystal superalloys.In this paper,characterization methods such as X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and electron probe microanalysis（EPMA）were used to systematically study the thermal corrosion behavior of nickel-based single crystal superalloy DD419 at different temperatures and second orientations（pure Na₂SO₄）.The influence ofγ′phase structure on the high temperature tensile properties of DD419 alloy was also studied.In studying the effect of different temperatures on the hot corrosion behavior of DD419 alloy,a comparative study was conducted with DD405 alloy.In 900℃hot corrosion experiment,DD419 alloy exhibited significantly better resistance to pure Na₂SO₄ hot corrosion than DD405 alloy.The continuous oxide layer formed in DD419 and the enrichment of Al,Cr,and Ti in the external oxidation layer effectively impeding the diffusion of O from entering the alloy interior.The analysis suggests that DD419 alloy forms a continuous oxide layer rich in Al,Cr,and Ti in the external oxidation layer,and a relatively continuous Al₂O₃ oxide layer in the internal oxidation layer,effectively delaying O from entering the alloy interior.The network-like Al₂O₃ structure distribution in DD419 alloy is less,and the internal oxidation phenomenon is not significant.The solid sulfur effect of Cr and Ti in DD419 alloy is significant,thus slowing down the hot corrosion rate.In contrast,the continuity of Al and Cr oxide layers in DD405 alloy is inferior to that in DD419 alloy,and the internal oxidation is severe,leading to inferior hot corrosion resistance of DD405 alloy.During the 950℃hot corrosion process,the protective oxide film formed by the two alloys is continuously consumed,resulting in an accelerated hot corrosion rate,shortened incubation period of the alloy,and accelerated occurrence of bulges and oxide film shedding on the alloy surface.The shedding area of surface regrows Ni O,which is difficult to resist molten salt corrosion,leading to a decrease in the hot corrosion resistance of both alloys.However,DD419 alloy has a continuous mixed oxide layer rich Al,Cr,and Ti,and the solid sulfur effect of Cr and Ti in the alloy is significant.There is no Ti element in DD405 alloy,therefore,when O and S from the external environment enter the alloy interior,the continuity of Al₂O₃ in the alloy and its solid sulfur capability are weaker than those of DD419 alloy,causing the network-like Al₂O₃ in the alloy to extend into the substrate.Therefore,the hot corrosion resistance of DD419 alloy is slightly better than DD405 alloy.The study on the effect of the secondary orientation on the thermal corrosion performance of the alloy selected[100],[210],and[110]oriented alloys for experiments.The samples with different secondary orientations have structural differences in theγ′andγphase interfaces,causing differences in the diffusion rate of metal cations during hot corrosion.Generally,the hot corrosion resistance of the oriented samples with shorter diffusion channel paths is better than that of the oriented samples with longer diffusion channel paths.During the early stage of hot corrosion in 900℃,the effect of different secondary orientations on the thermal corrosion performance of the alloy was not significant.In the later stage of thermal corrosion,the consumption of internal alloy elements and the differences in the cross-sectional structure of the alloy samples have an impact on the diffusion rates of the elements.The longer the channel path of the alloy sample,the poorer the repair ability of the protective oxide film（rich in Al,Ti,Ta,Cr,and Co）.Therefore,the hot corrosion resistance of[100]and[210]oriented samples is slightly better than[110]oriented samples.When the experimental temperature is 950℃,the increase in temperature accelerates the diffusion rates of elements in theγ′andγphases,and the difference in channel paths has a greater impact on the diffusion rates of alloy elements.The internal oxidation and internal sulfidation phenomena of[210]and[110]oriented samples are more severe than those of[100]oriented samples,and the number of internal Al₂O₃ particles and sulfides in[210]and[110]oriented samples is greater than that in[100]oriented samples.Therefore,the hot corrosion resistance of[100]oriented samples is better than that of[210]and[110]oriented samples.The influence of microstructure on high-temperature tensile properties was explored by adjustment ofγ′morphology.The study showed that as the primary aging temperature of the alloy increased,the size of theγ′phase in the alloy increased gradually.With the increase of primary aging temperature,the yield strength of the alloy increased slightly,and the effect was not significant on the tensile strength.The plastic deformation capacity of the alloy with a primary aging temperature of 1140℃was better than other primary aging temperatures.