Theoretical Analysis of the Diffusion Processes Determining the Oxidation Rate of Alloys

Wagner, Carl

doi:10.1149/1.2779605

Cited by 805 publications

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“…It is indicated in Eq. (2) that the Al flux is very large at the beginning of oxidation, which agrees well with the classic oxidation modelling where the initial flux at the boundary is nearly infinite [60]. Though the assumed β depletion zone is similar to that formed after the initial heat treatment, the resultant Al flux after an initial oxide has been applied may still be too large.…”

Section: Comparison Between Modelling and Experimental Resultssupporting

confidence: 82%

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

Chen

Jackson

Voisey

et al. 2016

Surface and Coatings Technology

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This paper investigates the β-phase depletion behaviour during oxidation of free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) bond coats prepared by high velocity oxy-fuel (HVOF) thermal spraying. The microstructure of the coatings was characterised using scanning electron microscopy with energy dispersive X-ray (EDX) analysis and electron backscatter diffraction (EBSD). It comprises a two phase structure of fcc-Ni and bcc -NiAl, with grain sizes varying largely from 0.5 to 2 µm for both phases.Isothermal oxidation tests of the free-standing coatings were carried out at 1100 C for times up to 250 h. The phase depletion behaviour at the surface was measured and was also simulated using Thermo-Calc and DICTRA software. An Al flux function derived from an oxide growth model was employed as the boundary condition in the diffusion model. The diffusion calculations were performed using the TTNi7 thermodynamic database together with the MOB2 mobility database. Reasonable agreement was achieved between the measured and the predicted element concentration and phase fraction profiles after various 2 time periods. Grain boundary diffusion is likely to be important to element diffusion in this HVOF sprayed CoNiCrAlY coating due to the sub-micron grains. Keywords IntroductionThermal barrier coatings (TBCs) are widely used to protect high temperature components in turbine engines from harsh operating environments [1,2]. TBC systems consist of a ceramic top coat, a metallic bond coat and a superalloy substrate [3][4][5]. Additionally, a thermally grown oxide (TGO) forms at the interface between the top coat and the bond coat during service at elevated temperature due to the fact that oxygen permeates through the ceramic top coat and oxidises the bond coat. The durability of the overall TBC system is largely determined by the microstructural, chemical and mechanical characteristics of the bond coat due to interdiffusion of element between the MCrAlY and the superalloy substrate. This is not considered in the present paper but has been studied by others, e.g. [29][30][31][32][33][34][35][36][37][38][39][40][41]. In these research works, the emphasis has been on substrate/bond coat interdiffusion and specific comparisons between measured and predicted β-phase depletion at the oxide/bond coat interface have not been considered. On the other hand, several analytical models concerned with the oxidation of 4 two phase systems in which the secondary phase dissolves during oxidation have been reported, e.g. [42][43][44][45][46][47][48]. In these models, the secondary phase depletion behaviour during oxidation can generally be represented by the parabolic diffusion law, Eq. (1), ~√( 1) where is the width of the second phase depletion zone and is the oxidation time.Considering the significance of β depletion on the degradation of TBCs, therefore, the aims of the work reported in this paper were to investigate specifically the kinetics of β depletion at the oxide surface in free-standing MCrAlY coatings ...

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Section: Comparison Between Modelling and Experimental Resultssupporting

confidence: 82%

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

Chen

Jackson

Voisey

et al. 2016

Surface and Coatings Technology

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“…29) Similar phenomena have been observed in other alloys with a noble component. 31) In the iron-copper-nickel system at the temperatures studied, the iron-depleted phase that forms is liquid and transport of iron through the liquid to the oxide should not be an issue. The solid enriched material behind the liquid layer should consist of approximately 80 wt% iron 30) and transport of iron through this material should also not affect the oxidation rate.…”

Section: Discussionmentioning

confidence: 97%

The Effect of Silicon on the High Temperature Oxidation Behavior of Low-carbon Steels Containing the Residual Elements Copper and Nickel

Webler

Sridhar

2007

ISIJ Int.

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“…1) The advanced Ni-based superalloys are characterized by higher Re content; second generation single crystal (SC) superalloys contain about 3 mass% (about 1 mol%) Re [2][3][4] and third generation SC superalloys contain 5 to 6 mass% (about 1.7 to 2 mol%) Re. [4][5][6] So, influence of the Re addition on the oxidation resistance of superalloys raises great concern.…”

Section: Introductionmentioning

confidence: 99%

Degradation of High-temperature Oxidation Resistance for Ni-based Alloys by Re Addition and the Optimization of Re/Al Content.

et al. 2003

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High-temperature oxidation tests were conducted with two groups of Ni-based superalloys in order to elucidate the Re effect on oxidation resistance. One group of alloys was the Ni-8Cr-2Ti-10Al-Re system, characterized by 10 mol% Al content. The other group of alloys was the Ni-8Cr-2Ti-15Al-Re system, characterized by 15 mol% Al content. The alloy compositions were chosen with the aid of the d-electrons concept, so that any undesirable phases would not appear in the microstructure. Both cyclic and isothermal oxidation tests were carried out at 1 373 K for a total time of 720 ks. The oxidation resistance decreased clearly with increasing Re content in the 10-Al series alloys, but did not in the 15-Al series alloys. The alloys having a Re/Al ratio (mol%) up to 0.1 exhibited very small mass change by oxidation, so the Re/Al ratio was indeed a good indication for the design of superalloys having a good oxidation resistance.KEY WORDS: Ni-based superalloy; high-temperature oxidation resistance; Re effect. Alloy PreparationButton ingots of the experimental alloys were prepared by arc-melting under the purified Ar gas atmosphere. Each button ingot was cut into pieces and remelted several times in order to get a compositional homogeneity. The ingots then underwent a solution heat treatment followed by aging heat treatments. The heat treatment conditions were determined on the basis of the experimental data obtained from the differential thermal analysis (DTA). As shown in Table 2, two-steps aging processes were adopted in this experiment. Microstructural ObservationThe heat-treated ingots were cut into small specimens. Those specimens were mounted with a conductive filled phenolic mounting compound and the specimen surfaces were polished with abrasive papers and then with alumina powders down to 0.05 mm mesh. Polished surfaces were etched in the 1 : 1 volume% HNO 3 and HCl solution for about 30 sec. The microstructural observation of the etched specimens of all the alloys was made using a scanning electron microscopy (SEM). The volume fraction of the gЈ phase in each alloy was measured by analyzing the SEM image in a conventional way. Oxidation TestsThe specimens for oxidation tests were prepared by cutting each alloy ingot into 10ϫ5ϫ1 mm size, and then the surfaces were polished mechanically using emery papers down to # 600 grits. Two kinds of oxidation tests were employed in this study. One was the cyclic oxidation test, in which the specimen was kept in a furnace at 1 373 K for 72 ks followed by air cooling in each cycle. This thermal cycle was repeated for 10 times, so the total exposure time at 1 373 K was 720 ks. The other was the isothermal oxidation test, in which the specimen was set in a furnace for the exposure to static air at 1 373 K for 720 ks followed by air cooling. This test temperature is much lower than the solution-heat-treatment temperature and is equal to the firststep-aging temperature (see Table 2), and therefore it is considered that the volume fraction and shape of the gЈ phase scarcely vary.The...

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Theoretical Analysis of the Diffusion Processes Determining the Oxidation Rate of Alloys

Cited by 805 publications

References 0 publications

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

The Effect of Silicon on the High Temperature Oxidation Behavior of Low-carbon Steels Containing the Residual Elements Copper and Nickel

Degradation of High-temperature Oxidation Resistance for Ni-based Alloys by Re Addition and the Optimization of Re/Al Content.

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