| In order to improve the temperature capability of nickel-based single crystal superalloys to meet the need for an increased turbine entry temperature, refractory elements such as Re, W, Ta, Mo, most notably Re, are added to enhance the high-temperature properties through both the solid solution strengthing in theγmatrix and precipitation strengthening via theγ' formation. The additions of these refractory elements improve the temperature capability of materials, and in the meanwhile increase the tendency of forming topologically close-packed (TCP) phases during prolonged exposures to elevated temperatures in service. This results in destabilization of theγ/γ' microstructure and gives rise to the negative effect on the preparation and processing of the alloys. Besides, Re additions increase the cost and density of the alloys since Re is both expensive and heavy. These drawbacks limit Re addition in the nickel-based single crystal superalloys. In this paper, a Re-containing nickel-based single crystal superalloy has been studied based on thermodynamic calculation and experiments. The microstructure stability ofγ/γ' phase and elemental partitioning both betweenγandγ' phase and at its interface have been investigated. In addition, attempts have been made to the design of some low Re-containing nickel-based single crystal superalloys which bear a temperature capability of the second-generation nickel-based single crystal superalloys. The Re content in the alloys was set to 1 wt.%, and the contents of other alloying elements were slightly modified to compensate for the strengthening effect from Re.A nickel-based single crystal superalloy containing 5 wt.% Re has been prepared by optical heating floating zone method. Theγ/γ' microstructure and the elemental distribution in the microstructure have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectrometry (EDS) using specimens under different heat treatment conditions. In the studied alloy, phase-separation (γphases precipitate withinγ' precipitates) was observed, and its process, mechanism and the effects on the microstructural evolution have been systematically investigated. After the first-step aging at 1100γC,γ' phases were found to mainly precipitate in cuboidal shape and no phase-separation was observed. Re, Co, Cr and Mo were enriched in theγmatrix, while Ni and Al were concentrated in theγ' phase. W partitioned equally to theγandγ' phase. After second-step aging at 870γC, fineγ' phases were further precipitated in spherical shape and phase-separation withinγ' precipitates was observed. The elements of Re, Co, Cr, Mo, were enriched in theγmatrix, while Ni and Al were concentrated in theγ' phase. Besides, there was certain enrichment of W in the vicinity of theγ/γ' interface, and the contents of W in theγ' phase was higher than that in theγmatrix.Besides, some typical nickel-based single crystal superalloys have been studied by thermodynamic calculation, including the incipient melting temperature, density, the volume fraction ofγ', theγ/γ' misfit, the TCP contents, heat treatment window, and the temperature range of mush zone etc. All the thermodynamic properties have been investigated using JMatPro and the latest relevant database for nickel-based superalloys. According to requests for single-crystal blade design,γ/γ' microstructure stability and process-ability, design criteria of the alloys were made. Based on these, a series of alloys with different contents were designed by fixing the contents of some elements such as Re, Al, Cr, etc. and changing the relative contents of W, Mo, Co, Ta. The Re content was set to 1 wt.%. Meanwhile, the incipient melting temperature, density, the volume fraction ofγ', theγ/γ' misfit, TCP contents, heat treatment window, and the temperature range of mush zone of all the designed alloys were estimated by JMatPro. Finally, some low Re-containing alloys with reaching the temperature capability of second-generation nickel-based single crystal superalloys have been obtained.
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