Microstructure Development of an Electroplating Coated Nickel-Base SC Superalloy in Oxidation Processes—Simulation Results

In this paper; a diffusion kinetic model was applied to simulate the microstructure development in a MCrAlY-superalloy system at high temperatures. Both simulation and experimental results showed that γ+γ’ microstructure was obtained in the coatings due to Al depletion after oxidation. With the help of the modelling; the mechanism of the formation of the diffusion zones in the single crystal (SC) superalloy can be also analyzed. The results revealed that the inward diffusion of Al from coating affected the depth of secondary reaction zone (SRZ) with the precipitation of TCP phases while the depth of inter-diffusion zone (IDZ) was decided by the inward diffusion of Cr.

CMSX-4® Plus (SLS): An Improved 3rd Generation Single Crystal Alloy

Ongoing demand for advanced aero gas turbine engines with lower fuel burn and commensurate reduced CO2 emissions require single crystal (SX) superalloys capable of operating at higher gas and metal temperatures beyond the capability of 2nd generation, 3% rhenium (Re)-containing SX alloys, currently used extensively in commercial and military flight engines. These complex cooled turbine blades and vane castings must have an excellent balance of high temperature mechanical properties, producibility, oxidation/hot corrosion resistance, coating compatibility including TBC performance and phase stability. The highest strength nickel-base SX superalloys currently in production (3rd generation CMSX-10K® and CMSX-10N® alloys) contain 6–7% Re. These highly alloyed, specialty alloys have some application drawbacks including some secondary reaction zone (SRZ) phase instability in the base alloy adjacent to the coatings, low temperature internal oxidation/hot corrosion attack requiring sophisticated dual role internal and external coatings and difficulty in production solution heat treatment. In addition, current 3rd generation SX alloys have relatively higher density which is a disadvantage in terms of weight and inertia in rotating part applications, and high cost due to the elevated Re content. An improved, lower Re content (4.8%), 3rd generation SX superalloy, CMSX-4® Plus (SLS) has been developed with improved properties and performance over current 3rd generation SX alloys, while lacking the drawbacks. Coatings have been successfully developed which are compatible with the base alloy and suitable bond coats for TBC application. This paper presents the characterization of CMSX-4 Plus (SLS) alloy, including composition, mechanical and physical properties, oxidation properties and phase stability, along with production status.

Interdiffusion Behaviour of NiCoCrAlY Coating and N5 Single Crystal Superalloy

This paper aims at investigating the microstructure and phases evolution of single crystal superalloy/high temperature protective coating during high temperature static oxidation, and exploring the influence of element interdiffusion behaviour on microstructure and phase evolution of the single crystal superalloy substrate. A NiCoCrAlY high-temperature protective coating was deposited on the Ni-based single-crystal superalloy by low-pressure plasma spraying technology. The coated samples were subjected to static oxidation for 200 h at a constant temperature of 1100 °C. Scanning electron microscope, energy dispersive spectrometer and X-ray diffraction were used to characterise the microstructure and phase after interdiffusion between the coating and the substrate at high temperature. The results showed that a dense thermally grown oxide layer was formed on the surface of the NiCoCrAlY coating after oxidation for over 100 h. The only interdiffusion zone was formed after oxidation for 50 h, while both interdiffusion zone and secondary reaction zone could be observed after oxidation for over 100 h. The thickness of interdiffusion zone and secondary reaction zone is increased with the extension of oxidation time, and the grain growth of topological close-packed phase in the secondary reaction zone is found. Al, Cr and Co in the coating diffuse from the coating to the substrate, while Ni and refractory materials like Ta, Mo, Re and W diffuse from the coating to the substrate. The interdiffusion of coating and substrate leads to the instability of γ/γ′ phase in the substrate, which finally results in the formation of W, Re and Cr-rich needle-like topological close-packed phase in the substrate.

Molecular mechanisms for the gas-phase conversion of intermediates during cellulose gasification under nitrogen and oxygen/nitrogen

Gas-phase conversions of volatile intermediates from cellulose (AvicelPH-101) were studied using a two-stage experimental setup and compared with those of levoglucosan (1,6-anhydro-b-D-glucopyranose). Under N2or 7% O2/N2flow, vapors produced from the pyrolysis zone (500?C) degraded in the secondary reaction zone at 400,500, 600 or 900?C (residence time:0.8-1.4 s). The 69.3% (C-based) of levoglucosan was obtained at 400?C under N2flow along with 1,6-anhydro-b-D-glucofuranose (8.3 %, C-based), indicating that these anhydrosugars are the major volatile intermediates from cellulose pyrolysis. Levoglucosan and other volatiles started to fragment at 600?C, and cellulose was completely gasified at 900?C. Most gas/tar formations are explained by gas-phase reactions of levoglucosan reported previously, except for some minor reactions originating from the molten-phase pyrolysis, which produced benzene, furans and 1,6-anhydro-b-D-glucofuranose. Synergetic effects of O2and volatiles accelerated fragmentation and cellulose gasification was completed at 600?C, which reduced benzene and hydrocarbon gas productions. The molecular mechanisms including the action of O2as a biradical are discussed. These lines of information provide insights into the development of tar-free clean gasification that maintains high efficiency.

The Formation of TBCs Using LPPS, CVD and PS-PVD Methods on CMSX-4 Single-Crystal Nickel Superalloy

In the paper new type of thermal barrier coatings characterized by good oxidation and hot corrosion resistance were presented. Bond coats were formed by overaluminizing of an MeCrAlY type coating deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttria stabilized zirconia (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). Rene 80 nickel superalloy was used as base material. The research showed that double-layer bond coat was formed with external NiAl phase layer and inner MeCrAlY layer. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process. The presence of secondary reaction zone (SRZ) was noted.

Secondary Reaction Zone Formations in Pt-Aluminised Fourth Generation Ni-Base Single Crystal Superalloys

Fourth generation superalloys are characterised by the addition of Ru which contributes to improved creep resistance whilst improving the microstructural stability. However, Ru additions have a negative effect on coated Ni-base superalloys, promoting Secondary Reaction Zone (SRZ) formation. Formation of a layer of SRZ beneath an aluminised or Pt-aluminised coating has the potential to reduce the effective cross section of a blade by in excess of 100 μm or 10% of the wall thickness. In this paper the effects of alloy composition on the formation of the SRZ in Pt-Aluminised fourth generation alloys were investigated systematically. A series of experimental fourth generation alloys was used having two distinct compositions of Co, Mo, W and Ru and conforming to a four factorial 'Design of Experiments' model. These alloys showed significant and consistent changes in the SRZ depending on alloy composition. These were in distinct contrast to the effects of these elements on stability in the bulk. Mo was demonstrated to be by far the most effective element suppressing SRZ formation, followed by Co. In contrast, both W and Ru enhance SRZ formation.