Investigation Of Microstructural Degradation And Its Effect On Stress Rupture Properties Of Nickel-base Single Crystal Superalloys

In this dissertation,several Ni-base single crystal superalloys have been employed to investigate the formation of topologically close-packed(TCP)phase,secondary reaction zone(SRZ)and other microstructural degradations and their effects on stress rupture properties by scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron probe microanalysis(EPMA)and some other research methods.The effect of heat treatment on the TCP precipitations was investigated.The effect of Ta on the formation of inter-diffusion zone(IDZ),SRZ and y’ rafting of coated alloys with NiCrAlYSi coating was analyzed.At the same time,the effect of TCP on stress rupture and creep properties under different conditions was studied.The main conclusions can be drawn as follows:A significant amount of σ phases were distinguished in the interdendritic regions of the as-cast 2Re(wt.%)samples after thermal exposure at 900℃.The σ phases were preferentially precipitated at the periphery of coarse γ/γ’ eutectic,which is ascribed to the local segregation of Cr and Ti at the periphery of coarse γ/γ’ eutectic.Due to the volume fraction of γ’ phases was significantly low in the dendrite core and less supersaturation of Re and W in γ matrix was expected,no TCP phase can be observed in the dendrite of cast sample.No TCP phases were present in the interdendritic regions and dendrite core,and small quantities of μ phases were detected in the secondary dendrite arm of low temperature short-term heat treated sample.A lot of μ phases were distinguished in the dendrite of two high temperature long-term heat treated samples,and higher volume fraction of μ phase was observed in the sample with higher solution heat treatment temperature.Microstructural analysis suggested that higher γ’ volume fraction in the dendrite core led to a reduced volume fraction of γ matrix and therefore strong supersaturation of Re and W in γ channel,and this accounted for the different precipitation behavior of μ phases in the three heat treated samples.After exposure at 1000℃ for 100h,the initial γ/γ’ transformed to γ’ phases and Cr,Re-rich M23C6 carbides precipitated in the IDZ of NiCrAlYSi coated 2Ta,6Ta and 8Ta(wt.%)alloy.Moreover,SRZ with σ and μ phases was formed.The formation of IDZ and SRZ was primarily due to the inward diffusion of Al.Different behavior of SRZ formation in the alloys with various Ta content can be observed,which highlights the potency of Ta addition in promoting SRZ formation.This may be ascribed to the fact that Ta promoted the precipitation of TCP phases.After exposure at 1100℃ for 100h,the initial γ/γ’ transformed to γ layer and discontinuous coarse γ’ phases in the IDZ,and no SRZ can be found primarily due to the inward diffusion of Ni.The formation of IDZ was significantly suppressed by Ta addition,which may be related to the retardation of interdiffusion between the coating and the substrate due to the increased γ’ volume fraction.The γ’ rafts parallel to the coating/substrate interface in the substrates beneath IDZ were found in the 2Ta,6Ta and 8Ta alloys both at 1000℃ and 1100℃.It is believed that the parallel rafts can be attributed to the strain induced by grit blasting.The parallel rafts were not affect by Ta addition and thermal exposure condition.The γ’ rafts perpendicular to the coating/substrate interface in the substrates beneath IDZ were found in 6Ta and 8Ta alloy.The perpendicular rafts were probably ascribed to the high chemical gradient of Al,Re and W caused by the interdiffusion between the coating and the substrate.Ta addition and thermal exposure condition had important effects on the perpendicular rafts by changing the chemical gradient of these elements and the γ/γ’ misfit stresses.8Ta alloy was selected to investigate the effect of TCP on the stress rupture properties.Two kind of heat treatment,HT A(1290℃/6h)and HT B(1290℃/7h),were subjected to the 8Ta alloy.After exposure at 900℃,the size and shape of γ’ phase of HT A sample is similar with that of HT B sample and small quantities of μ phases(<0.7 vol%)were detected in the dendrite of HT A samples,while a significant amount of σ phases(0.72～2.6 vol%)were detected in the HT B samples.During creep at 900℃/400 MPa,the main deformation mechanisms of HT A and HT B samples were that dislocations climbed and slipped in the γ matrix channel and small quantities of antiphase boundaries(APBs)coupled dislocation pairs sheared in the γ’phases.A certain amount of TCP phases(0.72～2.6 vol%)were detrimental to the creep properties of HT A and HT B samples.When the content of TCP phase is 2.6 vol%,the creep life of HT B sample is decreased by 24.2%.The γ matrix damage mechanism of TCP phase,resulting in matrix depletion of solid solution strengtheners,mainly accounted for the detrimental effect of TCP phase.With the increasing of TCP phases content,the y matrix damage mechanism,the destruction y/y’ regular structure mechanism and the embrittlement mechanism of TCP phase occurred in turn,which is detrimental to the mechanical properties.During creep at 760℃/758 MPa,the main deformation mechanisms of HT A and HT B samples were that a significant amount of stacking faults sheared of γ’phases and a small amount of antiphase boundaries(APBs)coupled dislocation pairs sheared in the γ’ phases.The effect of embrittlement mechanism of TCP phase played a major role,when the content of TCP phase is up to 2.1 vol%.However,the cracks mainly originated and propagated in the interdendritic regions,especially around carbides.Therefore,the influence of TCP phases on the stress rupture properties was very limited.