| Ni12.9Cr7.8A16.5Co superalloy sheet with the thickness of 0.3mm was prepared by electron beam physical vapor deposition (EB-PVD). The sheet has been heat treated for 16h at 800℃with a vacuum of 3×10-3Pa.The oxidation behavior were characterised by thermogravimetry (TG), XRD, SEM and EDS. Then the oxidation mechanism of this superalloy was studied by an analysis of component distribution and diffusion. Exfoliation of oxide layer was researched which based on the flexion of sheet,moreover, a kind of model was put forward to calculate the mass change especially on the oxidation kinetics.As a result, the isothermal oxidation kinetics curve of the superalloy at 900℃followed the rule of three power parabola, and the oxidation rate was k11= 2.40×10-4mg3/cm6·s.The isothermal oxidation kinetics curve of the superalloy at 1000℃followed the rule of two power parabola, and the oxidation rate was k12=3.72×10-6mg2/cm4·s.The cycle oxidation kinetics curve of the superalloy at 1000℃followed the rule of two power parabola,and the oxidation rate was kc=1.14×10-5mg2/cm4·s.Oxide layer could be easily formed in a short time when it was the based-plane,however,it took long time to form a compact oxide layer at the deposition-plane.The intergranular diffusion of Cr, Al and O elements and the diffusion of ions or electrons were the oxidation mechanism at the based-plane, while transgranular diffusion, intergranular diffusion and the diffusion of ions or electrons were the oxidation mechanism at the deposition-plane.The main component of the oxide layer at the based-plane was Cr2O3 while it was compactθ-Al2O3 layer at the deposition-plane.There's no internal oxidation at the deposition-plane,while there's a more wide internal oxidation zone at the based-plane.So theθ-Al2O3 oxide layer could effectively inhibit O diffusion other than Cr2O3 oxide layer,and had a better antioxidation.That's all because of the loose Cr2O3 oxide layer and the intergranular diffusion at the based-plane.There's no obvious different pattern between the compression fault and tensile break at the based-plane, and the Cr2O3 layer can be quickly formed at the failure part.However,the oxide layer at the deposition-plane exhibited a plastic deformation and brittle failure when it was loaded by compressive stress. If it was loaded by tensile stress,a new and compact oxide layer would quickly form at the failure part.Thus,the best running condition was that there were tensile stress acting on the deposition-plane and compressive stress acting on the based-plane. Under this condition, the mass change of oxides when there's exfoliation working at the Ni12.9Cr7.8A16.5Co superally sheet was given as:... |
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