Effect Of Minor Elements On The Microstructure And Mechanical Properties Of A Nickel-Base Single Crystal Superalloy

Minor elements, such as C, Hf and Y, have very important impacts on the microstructure and mechanical properties of nickel-base single crystal superalloy, but the effects exhibit different laws in different alloy systems and its mechanism has yet to be further explored. In this dissertation, The effect of C, Hf and Y on the microstructure and mechanical properties of a rhenium-free second-generation single crystal superalloy, DD98M, were investigated by optical microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and X-ray diffraction characterization methods.The main results are summarized as follows:Ta and Ti-rich MC-type carbides were formed in the interdendritic regions by the addition of C. The carbides morphologies were changed by the addition of Hf and Y. The Volume fraction of γ/γ’ eutectic was reduced by the formation of carbides, but increased by the addition of Hf and Y. The addition of C, Hf and Y significantly reduced the incipient melting temperature of the alloy.Partial rod-shaped carbides decomposed into granular MC carbides during solution heat treatment in C and C+Hf added alloys, but the blocky carbides in C+Y added alloy were relatively stable. After full heat treatment, all W and Mo-rich M6C-type secondary carbides and Cr-rich M23C6-type secondary carbides precipitated in the three carbon-containing alloys. M6C-type carbides exhibit octahedral morphology in the dendrite core and lamellar and granular morphology in the interdendritic region. M23C6-type carbides with small size and irregular morphology precipitated in the matrix channels.During long-term aging at 1000℃ for fully heat-treated alloy samples, MC carbides undergo degradation reactions in the form of MC+γ→M6C+γ’, and gradually transformed into M6C-type carbides. MC carbides degenerates significantly in the C and C+Hf added alloys, while MC carbides with blocky morphology degenerates slightly in the C+Y added alloy. During long-term aging process, A large number of needle-like topological close-packed (TCP) phases (μ phase) precipitates in the dendrite core in DD98M baseline alloy, While in the C, C+Hf and C+Y added alloys, no TCP phases were observed to precipitate. M6C-type carbides with lamellar morphology precipitate in the dendrite core in the C and C+Hf added alloys. Numerous granular morphology M6C-type carbides precipitate in C+Y added alloy. M6C-type carbides consume a large number of W, Mo and Cr elements which are also TCP phase forming elements, thereby inhibiting the precipitation of TCP phases.The addition of C, Hf and Y had no significant impact on tensile properties of the alloy at room temperature. At 760℃ and 1000℃, the yield strengths and ultimate tensile strength of the three carbon-containing alloys were lower than that of DD98M baseline alloy. The addition of Y slightly improved the tensile strength and ductility at 760℃, while the addition of Hf and Y improved the tensile elongation at 1000℃. At both 800℃/750MPa and 1010℃/235MPa conditions, DD98M baseline alloy exhibited the highest stress rupture lives. The addition of Hf and Y had little impact on stress rupture properties compared to only C added alloy.