Generation Nickel-based Single Crystal Superalloy Research Articles

γ′-Rafting mechanisms under complex mechanical stress state in Ni-based single crystalline superalloys

The morphological evolution of the γ′ precipitates has been studied in a second generation nickel-based single crystal superalloy during isothermal creep straining at 1093°C under multiaxial stress state. For this purpose, a specific V-shaped specimen has been designed to induce an evolving multiaxial stress state along its thickness. These stress conditions are quantified by the stress triaxiality obtained using viscoplastic finite element simulations. γ channel width and γ′ rafting orientation have been analyzed along the thickness for several triaxiality values, using Rose diagrams and specifically developed image processing tools. The γ′ rafting process is shown to be highly sensitive to the sign and magnitude of the stress triaxiality. Finally, it is shown that the γ/γ′ topological inversion development is faster under compression.

Influences of MCrAlY Coatings and TBCs on Oxidation Behavior of a Ni-Based Single Crystal Superalloy

DD98M alloy is a second generation nickel-based single crystal superalloy without Re addition, which is developed by Institute of Metal Research, Chinese Academy of Sciences. It is a combination of attractively strong points including high strength and low cost. In this work, Ni-based single crystal superalloy DD98M was adopted as the substrate material. Two types MCrAlY (M denote metal) coatings were deposited on DD98M specimen by arc ion plating and YSZ topcoat (TC) were deposited on the substrate by electron beam physical vapor deposition (EB-PVD) on the NiCrAlY bond coat (BC). Experimental results showed that the application of the NiCrAlY and NiCoCrAlYHfSi coatings improved the oxidation resistance of DD98M obviously at 1000 °C. The adhesion of oxide scale of NiCoCrAlYHfSi coating was much better than that of NiCrAlY coating. TBCs application greatly enhanced the operating temperature and significantly improved the durability of the substrate. The thermal growth oxidation (TGO) between the bond coat and topcoat play an important role in adhesion of the whole coating system.

Effect of Secondary Orientation on Thermal Fatigue Behavior of a Nickel-base Single Crystal Superalloy DD33

The effect of secondary orientation on thermal fatigue behavior of a third generation nickelbased single crystal superalloy DD33 was investigated. Samples with different secondary orientations were machined along(100) and(110) plane, respectively, and thermal fatigue test was performed cyclicly between room temperature and 1100 oC. It was found that different initiation sites and propagation orientations of the thermal fatigue cracks were observed in samples with different secondary orientations. In samples with secondary orientation of [100], thermal cracks initiated at the edge of the holes along a direction with 45 oincline to the directional solidification(DS) direction and propagated along also the derection with 45 oincline to the DS direction. While in samples with secondary orientation of [110], thermal cracks initiated at the edge of the holes vertical to the DS direction and propagated along the DS direction. In general, samples with secondary orientation of [100] exhibited better thermal fatigue properties than that of [110] samples.

Temperature dependence of lattice mismatchand volume fraction of a fourth-generation monocrystalline nickel-based superalloy

Abstract The strain distribution in a new generation nickel-based single crystal superalloy has been determined in the temperature range 293K–1598K. Measurements have been performed using diffraction of high energy synchrotron radiation (150keV, λ = 0.008 nm) to determine the variations with temperature of the γ/γ′ lattice mismatch and of the γ′ volume fraction. The chemical segregation at the dendrite scale and the internal stress induce a large range of values for the lattice mismatch. The measurements of the γ′ volume fraction determined by X-ray diffraction are in agreement with those obtained by atom probe tomography, image analysis or computation.