MESO APPROACHES FOR THE CREEP DAMAGE BEHAVIOR OF NICKEL-BASE DIRECTIONALLY SOLIDIFIED SUPERALLOYS

A self-consistent creep damage constitutive model and a finite element model have been developed for nickel-base directionally solidified superalloys. Grain degradation and grain boundary voiding are considered. The model parameters are determined from the creep test data of a single crystal and a directionally solidified superalloy with a special crystallographic orientation. The numerical analysis shows that the modeled creep damage behaviors of nickel-base directionally solidified super-alloys with different crystallographic orientations are in good agreement with the experimental data.

An Elasticity Model Considering Grain Boundaries and Tensile Orientations for Directionally Solidified Superalloys

In order to investigate the elastic properties of directionally solidified(DS)superalloys,an elasticity model called boundaries elastic model(GBE model),considering grain boundaries and tensile orientations,is proposed in this paper.Two assumptions are adopted in the GBE model:(1)The displacement of grains,which moves along the perpendicular direction,is restricted by the grain boundaries;(2)Grain boundaries influence region(GBIR)is formed around the grain boundaries.Based on the single crystal(SC)calculation method of elastic properties,the GBE model can well predict macroscopic equivalent elastic modulus(Young’s modulus)of DS superalloys under different tensile orientations effectively.To demonstrate the correctness of the GBE model,3D finite element simulation is adopted and tensile experiments on a Ni3Al?base DS superalloy(IC10)along five tensile orientations are carried out.Meanwhile,the grain boundaries are observed by light microscopy and transmission electron microscope(TEM).Therefore,the GBE model is proved to be feasible by comparing the simulated results with the experiments.

Microstructure and wear behavior of IC10 directionally solidified superalloy repaired by directed energy deposition

Directed energy deposition has been used to repair superalloy components in aero engines and gas turbines.However,the microstructure and properties are generally inhomogeneous in components because of the different processing histories.Here,the microstructures and wear behavior of different zones(substrate,HAZ,and deposit)are investigated for the IC10 directionally solidified superalloy repaired by the directed energy deposition process.It is found that the microstructure of the deposited layers is strongly textured with a<001>-fiber texture in the building direction,and the texture intensity is continuously increased along the building direction.Two kinds ofγ’phase(primary and secondaryγ’phase)can be found in the heat-affected zone(HAZ),and the average size of primaryγ’phase is smaller than that in the substrate due to liquation.In the deposit layers,the size ofγ’phase is much smaller than those in the substrate and the primaryγ’phase of HAZ;both size and the fraction of theγ’phase decreases with the increase of building height.The wear rate of the substrate is the smallest,indicating the best wear resistance;while the wear rate of HAZ is the largest,indicating the worst wear resistance in the repaired sample.The wear rates in the deposit layers increase from the bottom to the top zones,showing a decreasing wear resistance.Abrasive wear is found to be the dominant wear mechanism of the repaired alloy,and the resistance to which is closely related to the fraction ofγ’phase in the microstructure.The understanding of the influence of microstructure on wear resistance allows for a more informed application of inhomogeneous superalloy components repaired by directed energy deposition in industry.

Fatigue behavior of uncoated and MCrAlY-coated DS nickelbased superalloys pre-exposed in hot corrosion condition

The effects of a MCrAlY coating on low-cycle fatigue(LCF) behavior of directionally solidified(DS)nickle-based superalloy DZ125 were investigated. Before the fatigue testings, the specimens were pre-exposed in high-temperature hot corrosion(HTHC) environment generating by a burner rig at 850 ℃. The results show that the coating in hot corrosion condition has beneficial effects on the fatigue resistance of superalloy. Under corrosion condition, the MCrAlY-coated specimens tested have higher fatigue lives than the uncoated specimens at the same stress level. The coating failure results from fatigue process and numerous fatigue cracks were nucleated at the specimen surface, only one main crack propagates inward and the secondary cracks away from the fracture surface are perpendicular to the loading orientation.

Substrate Effects on the High-Temperature Oxidation Behavior of Thermal Barrier Coatings

The high-temperature oxidation behaviors of the NiCrAIYSi/P-YSZ thermal barrier coatings （TBCs） produced by electron beam-physical vapor deposition （EB-PVD） on directionally solidified （DS） and single crystalline （SC） Ni-based superalloy substrates were investigated. The cross-sectional microstructure investigation, isothermal and cyclic oxidation tests were conducted for the comparison of oxidation behaviors of TBCs on different substrates. Although TBC on DS substrate has a relatively higher oxidation rate, it has a longer thermal cycling lifetime than that on SC substrate. The primary factor for TBC spallation is the mismatch of thermal expansion coefficient （TEC） of the bond coat and substrate. The morphological feature of thermally grown oxide （TGO） has a strong influence on the TBC performance. By optimizing the elemental interdiffusion between bond coat and substrate, a high quality TGO layer is formed on the DS substrate, and therefore the TBC oxidation behavior is improved.