| 1. |
- Yuan, Kang, et al.
(författare)
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Hot corrosion of MCrAlY coatings in sulphate and SO2 environment at 900°C : Is SO2 necessarily bad?
- 2015
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 261, s. 41-53
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Tidskriftsartikel (refereegranskat)abstract
- MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposed-to-be protective alumina scale in a molten sulphate environment. In this study, two MCrAlY coatings, coating A (10wt.% Al, 20wt.% Cr) and coating B (7wt.% Al, 28wt.% Cr), were tested in 48-hour cycles at 900°C in a sodium-potassium sulphate environment with and without SO2 (500ppm). The aim was to study the effect of SO2 at the typical type-I-hot-corrosion temperature-900°C. The results showed that the corrosion behavior of the MCrAlY coatings depended not only on the coating composition but also on the corrosion environment. It was found that in coating A alumina scale was more resistant in the sulphate-plus-SO2 condition than that in the sulphate-only condition. Such phenomenon indicated a beneficial effect of SO2. On Coating B, however, mixed oxide layers, consisting of alumina and other oxides and sulfides, formed after a certain number of cycles in the sulphate environments with or without SO2 gas. In this coating, the addition of SO2 in the sulphate environment promoted the formation of non-dense chromia oxides which may form from oxidation of Cr-sulfides.
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| 2. |
- Yuan, Kang, et al.
(författare)
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Analysis on Microstructural Evolution of PtAl Diffusion Coating on Ni-based Superalloy Influenced by Creep Process
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The microstructural evolution of PtAl diffusion coating on a nickel-base superalloy IN792 was studied to assess the influence of high temperature (850 °C and 950 °C) and static creep loading conditions. The chemical composition analysis by EDS reveals that local enrichment of Cr, W and Mo is responsible for the formation of σ, Cr-α and μ/R particles in the different characteristic zones in the PtAl coating. Being taken as an approach to describe the coating’s microstructure evolution, a so-called Effective Coating Thickness (ECT) is defined and was measured for different creep conditions. It is found that the value of ECT is controlled by the inward diffusion of Al and Pt, being demonstrated by the existence of a linear correlation between ECT and diffusion coefficient of Al in substrate; t: diffusion time). Simulated ECT development of the PtAl diffusion coating agrees well with the experimental results at 850 °C and 950 °C.
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| 3. |
- Yuan, Kang, et al.
(författare)
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Creep Fracture Mechanism of Polycrystalline Ni-based Superalloy with Diffusion Coatings
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superalloy components for gas turbines. The aim of this study is to investigate the effect of coatings (NiAl and PtAl) on the creep fracture mechanism of samples with a substrate of IN792. The samples have been creep tested at two temperatures (850 °C and 950 °C) and different applied tensile stresses, until failure between 205 and 21000 hrs. The observation of cross-sections by SEM shows that the microstructural evolution in the coating is dependent on the diffusion of alloying elements in the sample. Furthermore the time and temperature induced growth of the coating is found to be controlled only by inward diffusion of Al. Grain-boundary cracking is the basic fracture mode in the substrate in all samples irrespective if the crack is initiated from coating or substrate. The analysis of microstructure shows that the diffusion coatings display two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore can be considered as non-load carrying regions. After recalculating the creep stress to exclude the final effective coating thickness from the total sample thickness, the coated samples showed similar creep rupture behavior as the uncoated samples in the Larson-Miller diagram.
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| 4. |
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| 5. |
- Yuan, Kang, et al.
(författare)
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Hot Corrosion Behavior of HVOF-sprayed CoNiCrAlYSi Coatings in a Sulphate Environment
- 2015
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Ingår i: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 122:part A, s. 47-53
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Tidskriftsartikel (refereegranskat)abstract
- HVOF-sprayed CoNiCrAlYSi coatings were tested at 900 °C in a hot corrosion environment containing sodium–potassium sulphates. The HVOF spraying caused the typical splat-on-splat structure. The results after the hot corrosion testing showed that the corrosion preferentially occurred at the coating surface and the splat boundaries. The oxidation along the splat boundaries can isolate the splat from the underlying coating matrix. In those isolated splats or coating parts, internal oxidation and nitridation of Al took place, following that the Al-depleted coating fragments were then oxidized to spinels. For those coatings which had a worse splat boundary quality (i.e. with higher porosity and intersplat oxides) or had a worse coating surface quality (i.e. with more small coating fragments therefore more interfaces), heavier corrosion attack was observed on those coatings due to the corrosion of the splats or the coating fragments. The results indicated that the as-sprayed coating quality including porosity and surface morphology was important for the hot-corrosion resistance of the coatings.
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| 6. |
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| 7. |
- Yuan, Kang, et al.
(författare)
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MCrAIY Coating Design Based on Oxidation-Diffusion Modelling. Part I : Microstructural Evolution
- 2014
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 254, s. 79-96
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Tidskriftsartikel (refereegranskat)abstract
- To improve the efficiency of modern gas turbines, it is highly desired to develop durable MCrAlY alloys, to be used as protective coatings against oxidation and corrosion for superalloys which are the base materials for some hot components like turbine blades and vanes. In this paper, an oxidation-diffusion model was used to simulate the diffusion of alloying elements and the corresponding microstructural changes in different superalloy-coating systems at high temperature. Two important processes are considered in this model: oxidation of the coating and interdiffusion between the superalloy and the coating. The model showed an accurate predictability of the diffusion and microstructural evolution in real superalloy-coating diffusion couples studied at high-temperature exposure. The model was further applied to investigate the elemental effects of Ni, Co, Cr and Al on the microstructural evolution, considering the development of two important phases in superalloys and coatings, i.e. FCC-γ′ and BCC-β, at different temperatures. The results in this paper deepen the knowledge of the MCrAlY coating design for superalloy-coating systems in high-temperature applications.
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| 8. |
- Yuan, Kang, et al.
(författare)
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Modeling of the microstructural evolution and lifetime prediction of MCrAlX coatings on Nickel based superalloys in high temperature oxidation
- 2013
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Ingår i: Surface and Coatings Technology. - : Elsevier. - 0257-8972. ; 232:15, s. 204-215
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Tidskriftsartikel (refereegranskat)abstract
- At high temperature, MCrAlX coatings are deposited onto the surface of superalloy to provide oxidation and corrosion protection by forming a thermalgrowing oxide scale. In this project, the oxidation behavior of an HVOF CoNiCrAlYSi coating on IN792 was studied in both isothermal oxidation (900, 1000 and 1100 °C) and thermal cycling (1100-100 °C). The microstructural evolution in the MCrAlX coatings after oxidation was investigated. It was found that Al-rich β phase is gradually consumed due to two effects: surface oxidation and coating-substrate interdiffusion. Some voids and oxides along the coating-substrate interface, or inside the coating, were considered to play a role in blocking the diffusion of alloying elements. Based on the microstructural observation, an oxidation-diffusion model was developed by using Matlab and DICTRA software to predict the lifetime of MCrAlX coatings. Several effects influencing the microstructural evolution were included: surface oxidation, coating-substrate interdiffusion and diffusion blocking. The results showed good agreement between the experimental composition profiles and modeling ones. Furthermore, by choosing suitable diffusion blocking factors, the accuracy of coating life prediction could be improved.
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| 9. |
- Yuan, Kang, 1987-
(författare)
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Oxidation and Corrosion of New MCrAlX Coatings : Modelling and Experiments
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- MCrAlY coatings (“M” for Ni and/or Co) are widely used for the protection of superalloy components operated at high temperatures such as in the hot sections of gas turbines. The exposure to high temperature can cause coating degradation due to oxidation or hot corrosion at the coating surface. Microstructures in the coating and the coating life are affected also by the diffusion of alloying elements through the coating-superalloy interface. This PhD project, by applying thermodynamic modelling and experimental tests, investigates the oxidation and hot corrosion behavior of new MCrAlX coatings, in which X, referring to minor elements, is used to highlight the functions of such elements.In order to understand and predict the coating degradation progress during thermal exposure, an oxidation-diffusion model has been established for MCrAlX coating-superalloy systems, which integrates the oxidation of aluminum at coating surface, diffusion of alloying elements, and the diffusion-blocking effect in the materials. The predicted chemical composition profile and microstructure agreed well with experimental results in a CoNiCrAlYSiTa-Inconel 792 system. The model was further applied in several coating-superalloy systems to study the influence of coating composition, superalloy composition and temperature on the evolution of microstructure in the coating and the coating life. The results have demonstrated the potential of the model in designing new durable MCrAlX coatings. In addition to the applications in coating-superalloy systems, the model was also adapted for studying the microstructural development in a superalloy in which internal oxidation and nitridation occurred in an oxidation process.The oxidation behavior of some HVOF MCrAlX coatings was studied by thermal exposure at different temperatures (900, 1000, 1100 °C). Different spinels formed above the alumina scale, depending on the oxidation temperature. The minor alloying elements, Ru and Ir, had no direct influence on the oxidation behavior but may affect the phase stability in the coating.MCrAlX coatings were also tested in 48-hour cycles at 900 °C in different hot corrosion environments containing sulphates and/or SO2. The results showed that the coating performance was dependent on coating quality, concentration of Al and Cr in the coating, and the hot corrosion condition. It was also found that the addition of SO2 in the environment may not necessarily be bad for hot corrosion resistance of some MCrAlY coatings.
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| 10. |
- Yuan, Kang, et al.
(författare)
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Simulation of oxidation-nitridation-induced microstructural degradation in a cracked Ni-based superalloy at high temperature
- 2014
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Ingår i: MATEC Web of Conferences Vol. 14 (2014), EUROSUPERALLOYS 2014 – 2<sup>nd</sup> European Symposium on Superalloys and their Applications Giens, France, May 12–16, 2014. - Les Ulis, France : E D P Sciences. ; , s. 16004-
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Konferensbidrag (refereegranskat)abstract
- In turbine engines, high temperature components made of superalloys may crack in a creep process during service. With the inward flux of the gases, e.g. oxygen and nitrogen, along those cracks, the microstructure of the superalloy substrate nearby the cracks may degrade by internal oxidation and nitridation. The aim of this study is to investigate and simulate the oxidation-nitridation-induced microstructural degradation in superalloys by taking a variant of Ni-based superalloy IN-792 as a sample. After the creep testing of the superalloy in air, the microstructures on the cross section of the superalloy were analysed in a scanning electron microscope, equipped with energy/wavelength dispersive systems. Internal oxidation and nitridation, presenting by Al/Ti oxides and nitrides, were observed under a porous and even cracked Cr-oxide scale which was formed on the superalloy surface or along the creep cracks connecting the superalloy surface. Meanwhile, the reinforcing γ′ precipitates were depleted. Such oxidation-nitridation-induced microstructural degradation was simulated by using an oxidation-diffusion model, focusing the diffusion of the alloying elements in metallic phases of the superalloy.
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