| In this paper,the Pt-Al coating for the second-generation single crystal(SX)superalloy were adopted as the object of study.The microstructure stability during long-term ageing process and mechanical behaviours under various thermal-mechanical test conditions of SX superalloys applied with and without Pt-Al coating was studied by using X-ray,scanning electron microscopy(SEM),transmission electron microscopy(TEM)and back electron scattering technology(EBSD).The influence of Pt-Al coating on the microstructure evolution and mechanical behavior of SX superalloy was studied and analyzed in detail.The Pt-Al coating prepared by plating Pt and depositing Al on the surface of the second-generation SX superalloy was about 60μm thickness.The γ’ rafting phase and topological closed packed(TCP)phase were appeared in the region of superalloy substrate dose to the coating(5μm-10μm)by element interdiffusion.Compared with the long-term aging at 800℃,the long-term aging at 1100℃ was more serious to the microstructure degradation of Pt-Al coating and SX substrate,simultaneously.After 100h during long-term aging at 1100℃,the oxide layer on the surface of the coating was observed obviously,and the thickness of the oxide layer gradually raised with the extension of time.Moreover,the coating fall off seriously.The thickness of the microstructre degradation after 1000 h increased to about 200 μm.where a large number of TCP phases and the β-NiAl phase were destroyed by γ’ phase.The Pt-Al coating resulted in a remarkable degradation in both ultimate tensile strength(UTS)and yield strength(YS)of the SX substrate in the range of room temperature(RT)to 1100℃.However,Pt-Al coating reduced the plasticity of SX superalloy at temperatures between RT and 1100℃,while increased the plasticity of SX superalloy at temperatures higher than 750℃.At RT to 750℃,the tip cracks of the brittle Pt-Al coating nucleated and developed toward to the SX superalloy substrate,reducing the effective bearing area of the sample and degrading the strength and plasticity of the SX superalloy.In the range of 750℃ to 1100℃:the surface cracks were effectively suppressed in the coating.The synergistic deformation ability of the tough Pt-Al coating and the SX superalloy was enhanced,which improved the toughness of the SX superalloy.The dislocation characteristics in th region of superalloy substrate near the coating was diiferent from that in the inner substrate.At RT,more dislocations cut into the γ’ phase in substrate close to the coating owing to the enhanced stress caused by tip crack penetrating into the SX superalloy.At medium temperature,the quantity of stacking faults in γ’ phases reduced and the dislocation ring and superdislocation were observed in the γ’ phase.At high temperature,the elements inter-diffusion behavior at the coat/substrate interface is preferred and the mismatch degree of γ/γ’ interface was reduced These factors led to the formation of the loose dislocation network,reducing its ability to retard the dislocation to shear into the γ’ phase.According to the creep results,the creep properties of the coated superalloy were inferior to those of the uncoated superalloy to some extent under all testing conditions.The maximum deterioration in the creep life and strain was obtained at 1100℃/112MPa and 750℃/820MPa.At low temperature and high stress,the damage of the Pt-Al coating to the SX superalloy mainly originated from the preferential emergence of cracks inside the coating and continuous expansion to the interior of the substrate,reducing the effective bearing area of the sample and damaging its creep performance.At high temperature and low stress,the performance damage mainly attributed to the microstructure degradation near the coating/substrate interface,including the increase in the thickness of the IDZ inside the coating,the formation of the bulk γ’ phase within the coating and the precipitation of the TCP phase in substrate beneath the coating.TEM was adopted to investigate that under the condition of high stress and low temperature,the stress field near the crack tip increased and the number of stacking faults in the substrate near the coating reduced.Meanwhile,the number of super-dislocations in the γ’ phase increased.At high temperature and low stress,the degree of inter-diffusion of the elements was aggrevated,so that the γ/γ’ mismatch degree in the SX substrate near the coating reduced.Therefore,the interface dislocation network after creep in this region was sparse and irregular,which was hard to hind dislocation shearing into γ’ phases.Following the creep test,deconvolution calculations were prformed,which indicated that the harmful effect of Pt-Al coating was mainly located in the third creep stage,rath er than the first and second creep stages.Furthermore,during the second creep stage,the stress of IDZ was higher than that of outer β region owing to the fact that lots of precipitates remained in IDZ.The high cycle fatigue(HCF)results revealed that Pt-Al coating was harmful to the HCF property of SX superalloy under high stresses at 700℃.However,the Pt-Al coating was found to be beneficial at 800℃ or under low stresses at 700℃.At 700℃within high stress amplitude,the brittle Pt-Al coating preferentially germinated tip cracks,and the depth of the tip cracks penetrated into the superalloy substrate,which caused some damage to its HCF performance.At low stress amplitudes,the high strength coating/substrate interface effectively hindered the expansion of coat cracks.At 800℃,a larger range of rafting γ’ inside the coating beneath the coating retarded the inward expansion of the coat cracks,prolonging the fatigue life of the SX superalloy.Moreover,EBSD results showed the all coating cracks mainly initiated at the surface of the coated specimens and started from grains with {123}<111>slip system within coating.TEM tests indicated that at 700℃ with low stress amplitude in the superalloy dose to the coating,the dislocation networks formed due to the activity of various slip systems.At 800℃ with high stress amplitude,combined with stress increase and Co,Cr atoms diffusion from substate to coating contributed to dislocation shearing into Y phases and leading to the appearance of stacking faults in the precipitates.Meanwhile,the stress concentration caused by 800℃ coating on the substrate was lower than 700℃. |
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