The Effect of Initial Oxidation on Long-Term Oxidation of NiCoCrAlY Alloy

The initial oxidation behavior of Ni-6.5Co-17.8Cr-3.7Al-0.5Y alloy is investigated at 800°C-1000°C. X-ray diffraction results show that the dominant Cr2O3 phase and secondary α-Al2O3 and NiO phases are observed on the surface of samples at all initial stages (oxidized for 16 hours). YAlO3 and θ-Al2O3 can only be detected at low temperature (800°C) while the spinel NiCr2O4 is only observed at 900°C and 1000°C. Though the growth rates of α-Al2O3 and Cr2O3 are comparable at 900°C, the former becomes much lower than the latter when the temperature changes to 1000°C. Scanning electron microscopy (SEM) images show that the α-Al2O3 grows from some irregular ditches in the chromia scale at 900°C. However, cracking and spalling are more serious at 1000°C without α-Al2O3-grown-ditches, which is in accordance with the growth rates of these oxides at different temperatures. The cracking can be explained by the results of Raman determination which indicate that the stress on the surface of specimen oxidized at 1000°C is higher than that at 900°C. Owing to this condition, a preoxidation treatment on the NiCoCrAlY alloy for 16 hours is prepared at 900°C, and then thermal cycling oxidation test is conducted at 1000°C for 200 hours. The result indicates that the initial preoxidation treatment at 900°C improves the oxidation resistance of alloy at 1000°C.

Ultralow oxygen ion diffusivity in pyrochlore-type La_(2)(Zr_(0.7)Ce_(0.3))_(2)O_(7)

Thermally grown oxides(TGOs)at the ceramic top-coat/metallic bond-coat interface are a pressing chal-lenge in advanced thermal barrier coating(TBC)systems as they can affect the performance and ser-vice lifetime of TBCs.Thus,developing novel TBC materials with ultralow oxygen ion diffusivity is very urgent.In this study,we reported the diffusive properties of oxygen ions in a novel pyrochlore-type La_(2)(Zr_(0.7)Ce_(0.3))_(2)O_(7)(LZ7C3)material.The measured ionic conductivity and atomistic simulation revealed that the oxygen ion diffusivity in LZ7C3 grains is two orders of magnitude lower than that in conventional 8 wt.%yttria-stabilized zirconia(8YSZ)grains.This is due to the relatively high energy barrier for oxygen hopping in LZ7C3.In addition,it was found that enhancing the order distribution of cations is a strategy to reduce the intrinsic oxygen diffusion of pyrochlore-type oxides.On the other hand,we observed that La^(3+) cations segregate at the grain boundaries(GBs)of LZ7C3,which results in the electrostatic poten-tial at GBs being comparable to that in the bulk.Furthermore,we found that the oxygen ion diffusion is facilitated at the GBs of LZ7C3 due to the stretched O-Zr/Ce bond and the low coordination at GBs.How-ever,the segregations of Y^(3+)cations and the increase in the number of oxygen vacancies resulted in the formation of an electrostatic layer at the GBs of 8YSZ,which shielded the oxygen ion diffusion.Despite this,the oxygen ion diffusivity in LZ7C3 was still considerably less than that in conventional 8YSZ.This study offers a stepping stone toward utilizing pyrochlore-type LZ7C3 materials as advanced TBCs at high temperatures.