ICMCTF2016 Session AP: Symposium A Poster Session

The investigation results of surface layer modification influence on hydrogen interaction behavior with zirconium alloy Zr1Nb have presented in this work. The modification of surface layer was performed using pulsed electron beam irradiation with energy density equal from 10 to 25 J/cm². Hydrogen sorption and diffusion processes were investigated at temperatures equal from 350 to 550 °С. Hydrogen sorption rates and activation energies of hydrogen diffusion at zirconium alloy before and after different modes of pulsed electron beam irradiation were obtained from hydrogen sorption curves. Analysis of hydrogen desorption curves showed significant effect of pulsed electron beam irradiation on hydrogen desorption behavior. There is increasing in temperature of maximum hydrogen desorption intensity and activation energy of hydrogen desorption. The results of sorption and desorption investigation were supported by XRD, scanning and transmission electron microscopy.

The aluminum slurry coatings are required for improvement of oxidation resistance of energy equipment such as inner surfaces of boiler tubes for new highly efficient coal power plants operating under ultra-supercritical parameters (over 300 bar / 600-620ºC).

Fe aluminide coatings on ferritic steel P92 produced by Al slurry application have shown to be very protective for steam oxidation at 650º C. These coatings are generated by heat treatment of the applied slurry at 700º C for 10 h and are composed mostly of Fe2Al5 on top of a thin FeAl layer.

Modeling is helpful for estimation of coatings lifetime, which is the duration of maintenance of their protective properties. Coating failure typically happens when the surface concentration of the protective element, usually Al, falls below a critical value or due to coating cracking caused by thermo-mechanical fatigue and spallation.

The lifetime of such coаtings is determined by diffusion and oxidation processes and usually is about 60000 h. (~6-8 years) and more.

Experimental approach for diffusion process investigation in general is practically impossible due to its long duration. Calculation approach is impossible practically too due to models coefficients absence (Al diffusion coefficients for coаting-base alloy system for instance). Due to this fact, the models are not adequate to real physical processes and they can’t calculate the coating life time with desired accuracy.

Calculational and experimental approach was developed for solving this problem. The models adequacy to physical processes is provided by model parameters estimation with short-time experiment data for coating – base alloy systems.

The model is based on the numerical solution of second Fick’s law with a mass flux dependent on the chemical potential gradient (Nernst-Plank formulation) that makes possible to consider phase transformations which are present in the studied coating. Simulations shown in the present work were performed considering phase transformations and results are more realistic than the ones without them.

Diffusion coefficients available in the literature are given for conditions that differ from those needed for slurry aluminide coatings. Therefore, for the present work they were determined by a calculation and experimental approach, which comprises model parameters identification by short-time experimental data for coating–substrate systems.

Multi-component, high-entropy alloys (HEAs) are an interesting class of materials that have been observed to exhibit high ductility and strength, microstructural stability at high temperatures, and oxidation resistance. These reported properties make them ideal for use in high-temperature oxidizing environments. More recently, a compositionally optimized Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ HEA that forms a dominant γ / γ’ microstructure has emerged. Mechanically, this novel HEA tends to outperform other FCC solid-solution forming commercial alloys like IN617 and Alloy 800H. However, the oxidation behavior at elevated temperatures has not been addressed. This work investigates the 1000°C oxidation behaviors of bulk Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ in comparison to direct current (DC) magnetron sputtered Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ HEA coatings onto CMSX-8 superalloy substrates. The resulting oxidation mechanisms will be discussed relative to conventional M-Cr-Al oxide formation models and thermodynamic predictions based on the CALPHAD method.

Nickel-aluminides display excellent oxidation resistance in high temperature environments. This makes them ideal candidates for use as protective alumina-forming coatings to preserve more expensive superalloys that are commonly used in the aerospace industry. With regard to binary Ni-Al, enhanced oxidation behaviors have been reported with small alloying additions of reactive elements such as Hf, and through grain refinement to the nanocrystalline scale. This study investigates the 1000°C oxidation resistance of direct current (DC) magnetron sputtered NiAl-0.1Hf (at.%) nanocrystalline coatings on CMSX-8 superalloy substrates. For comparison, the baseline oxidation behavior of the bulk, uncoated CMSX-8 superalloy will also be examined. The results will serve to evaluate the oxidation performance of CMSX-8 and to fundamentally address alumina formation with regard to reactive element doping and grain refinement. The experimental results will be compared to thermodynamic predictions based on the CALPHAD method.

Grain-refinement to the nanocrystalline regime has been shown to enhance the oxidation behaviors of a number of metals and alloys. This work investigates the 1000°C oxidation performance of direct current (DC) magnetron sputtered nanocrystalline Ni-10Cr-10Al-0.3Y (wt. %) on CSMX-8 superalloy substrates. To serve as a baseline, the oxidation behavior of the uncoated CMSX-8 was also evaluated. The microstructural changes that occur during oxidation have been elucidated. The results will be compared to thermodynamic model predictions using the CALPHAD method and analogous coating substrate systems.

Corrosion of Zirconium alloys (Zircaloys) on fuel rod cladding can lead to embrittlement and mechanical failure that reduces the service life. In the absence of coolant water, corrosion of fuel rods in nuclear reactors can have disastrous consequences, such as those seen in the 2011 Fukushima disaster. Coatings for cladding materials have thus been identified as a strategy worthy of investigation to increase corrosion resistance and mitigate the chemical reactions that lead to hydrogen production and its subsequent absorption in Zircaloys.

Plasma nitriding is a thermo-chemical diffusion technique used widely to improve the wear and corrosion resistance of engineering tooling and load-bearing machine parts, extending their service life. The aim of this work was to test the hypothesis that this widely used treatment technique could improve the corrosion resistance of Zircaloys via the zirconium nitride compound layer formed by Triode Plasma Nitriding (TPN) of the alloy surface.

Optical micrographs of low temperature autoclave corrosion testing at 230^oC for 8h on TPN-treated pure Zr substrates showed some darker grains relative to others and SEM micrographs revealed local spallation on the surface of some grains, due to preferential oxidation in grains of certain crystallographic orientations – although no evidence of any significant grain growth nor large scale corrosion was observed.

High temperature corrosion testing in water at 360^oC for 3 days was performed to test coating stability under near-supercritical conditions. The findings revealed that the TPN layer did not prevent oxidation, but rather enhanced it. Glancing-Angle XRD revealed the formation of predominantly monoclinic ZrO₂.

A uniformly distributed ZrO₂ layer, approximately 10 microns thick, was determined from cross-sectional SEM micrographs and interestingly, showed the formation of microcracks, the most extensive of which was observed on the outer edge of the oxide layer. This observation suggests that compressive stresses are highest at the substrate/oxide interface and diminish with distance away from this region. These results are consistent with the extensive microcracks observed on the planar surface. The EDX results indicate complete coverage of the nitride film by ZrO₂.

Scratch testing revealed that a low critical load leads to failure which is attributed to the oxide scale formed, rather than the TPN layer itself. By comparison, a clear critical load is not observed for the TPN layer up to an indenter normal force of 50 N. Nanoindentation results showed that the hardness of the oxide layer was much lower than that of the nitride compound layer.

Next generation gas turbine engines will require new materials that are durable for tens of thousands of hours at temperatures in excess of 2700°F. Although Si-based ceramic matrix composites (CMCs) offer the high temperature capability with reduced weight and cooling requirements, environmental barrier coatings (EBCs) are needed to protect these materials in combustion environments. In addition to an environmentally stable top coat, advanced bond coats are also required for adhesion and durability. Historically Si metal has been used for EBC bond coats, but this material has an upper use temperature of ~2400°F. Advanced composite bond coats of Si-HfO₂ developed at NASA show promise for 2700°F capability. Si-HfO₂ bond coats were deposited with a Yb₂Si₂O₇ EBC topcoat on SiC using Plasma Spray-Physical Vapor Deposition (PS-PVD) facility at NASA Glenn Research Center. PS-PVD is a hybrid technique that combines conventional thermal spray and vapor phase methods to tailor microstructures and compositions with processing conditions. Durability up to 2700°F was tested by cycling samples in gradient and isothermal heating environments. Thermal conductivity in the multilayers were measured and composition, crystal structure and microstructure were evaluated for as-deposited and cycled samples.

Ceramic material candidates for thermal-barrier coatings (TBC) are required to show low thermal conductivity and long thermal stability at high temperature. The compounds from the Al₂O₃-Y₂O₃ binary phase system has the potential for such application. Yttrium aluminum garnet (YAG) with Y:Al ratio 3:5 is a well known material with high thermal stability and a good chemical resistance. Less examined phase in this system is yttrium aluminum perovskite (YAP) with Y:Al ratio 1:1. YAP shows similar properties as YAG but synthesis of YAP is more complicated due to the side phases contamination, especially when a traditional solid state reaction method is used. Both phases are able to make solid solution with lanthanides which significantly changes their thermal properties.

This work shows results of the microstructure analysis and thermal properties of M_xY_1-xAlO₃ (M=Ce, Eu, Pr, Lu). All samples were prepared by sol-gel combustion method and/or the solid state reaction method with a high-energy milling step) and then heat-treated at the temperature of 1650^oC in the form of powder or pellets. Morphology and phase composition of specimens were characterized by the scanning electron microscopy (SEM) method accompanied by the energy dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction (XRD). Thermal properties of specimens like thermal stability, conductivity and expansion were examined using differential thermal analysis (DTA), thermal diffusivity measurement (LFA) and dilatometry (DIL), respectively. Results show influence of modification of YAP solid solution by lanthanides on thermal properties of received compounds.

The financial support by NCN under the project No: UMO-2013/09/D/ST8/03976 is gratefully acknowledged.