In the present work the effect of surface roughness on oxidation behavior during the early stages of high temperature exposure of Ni-base superalloy IN 625 is described. The surface roughness was described using standard contact profilometer as well as novel method, fractal analysis. It was found that the different surface preparation resulted in a difference in roughness with a parameter increase of at least one order of magnitude for the ground sample as compared with the polished sample. The oxidation test was performed in a horizontal tube furnace. Post exposure analyses including glow discharge optical emission spectrometry (GD OES) and scanning electron microscopy (SEM), which revealed that grinding lowers the oxidation kinetics of IN 625. It was found that surface preparation influences the oxide scale composition and morphology. The hypothesis explaining the mechanism responsible for the changes in oxidation behavior is proposed as well.
Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.
In the present work, it is shown that the surface preparation method used on two Ni-based superalloys prior to aluminizing chemical vapor deposition (CVD) is one of the most important factors determining the oxidation resistance of aluminized Ni-based superalloys. It was found that grit blasting the substrate surface negatively affects the oxidation resistance of the aluminized coatings. For grit blasted and aluminized IN 625, a thicker outer NiAl coating was formed compared to that of IN 738. In contrast, no effect on NiAl coating thickness was found for grit blasted and aluminized IN 738. Ho…
Alumina forming, oxidation and thermal shock resistant MAX phases are of a high interest for high temperature applications. Herein we report, on bonding and resulting interactions between a Ni-based superalloy, NSA, and two alumina forming MAX phases. The diffusion couples Cr${_2}$AlC/Inconel-718/Ti${_2}$AlC were assembled and heated to 1000 or 1100°C in a vacuum hot press under loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting interfaces were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Good bonding between Cr$…
The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1wt% Mn and one containing 0.5wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protect…
A new computational tool has been developed to model, discover, and optimize new alloys that simultaneously satisfy up to eleven physical criteria. An artificial neural network is trained from pre-existing materials data that enables the prediction of individual material properties both as a function of composition and heat treatment routine, which allows it to optimize the material properties to search for the material with properties most likely to exceed a target criteria. We design a new polycrystalline nickel-base superalloy with the optimal combination of cost, density, gamma' phase cont…
The effects of surface preparations on oxidation kinetics and oxide scale morphology for the commercially available Ni-based superalloy IN 713C have been investigated. The ground and polished samples were exposed in air at 800-1100C. The ground specimens were found to demonstrate lower oxidation kinetics compared to those after polishing. The grinding also affected the oxide scale morphology, resulting in a protective alumina scale, while the polished samples developed Ni-/Cr-rich mixed oxides on the surface. Better oxidation resistance of the ground surfaces is related to a higher concentrati…
A new class of crack-resistant nickel-based superalloy containing high $γ^\prime$ fraction is studied for the laser-powder bed fusion (L-PBF) process. The effects of the (Nb+Ta)/Al ratio is emphasised, a strategy that is shown to confer excellent low-temperature strength whilst maintaining oxidation resistance at high temperatures via stable alumina scale formation. The processability of the new alloys is characterised with respect to defect assessment by micro-focus x-ray computed tomography; use is made of a prototype turbine blade geometry and the heritage alloy CM247LC as a benchmark. In a…
Atomic force microscopy (AFM) is a well-known tool for studying surface roughness and to collect depth information about features on the top atomic layer of samples. By combining secondary ion mass spectroscopy (SIMS) with focused ion beam (FIB) milling in a scanning electron microscope (SEM), chemical information of sputtered structures can be visualized and located with high lateral and depth resolution. In this paper, a high vacuum (HV) compatible AFM has been installed in a TESCAN FIB-SEM instrument that was equipped with a time-of-flight secondary ion mass spectroscopy (ToF-SIMS) detector…
In the present paper, the analysis of polishing, grinding and sand-blasting processes on oxidation behavior of nickel, copper and nickel-copper alloy has been investigated. Surface roughness has been measured using two methods: by contact profilometer and by fractal analysis. It has been proved that sand-blasting process influences the surface roughness the most but also contaminates the surface with alumina particles. The oxidation tests were performed in thermogravimetrical furnace. The obtained mass gain plots, SEM microphotographs of cross-sections and SEM EDS analysis of the samples prove…
We study the formation of nanometer thick oxide layers around nanoparticles (plateaus) in a wet nitrogen environment over 32 days. We determine both the plateau height and the lateral force needed to remove the particles with an atomic force microscope (AFM). The height is well described by the Deal-Grove model for a thermally activated diffusion limited oxidation process. Furthermore, we observe that height and force appear to be correlated suggesting that the rise in adhesion forces is mainly governed by an oxidation process. Since the plateaus are the result of a change in the chemical stru…
We report the results of an x-ray scattering study that reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of \~5.0*10-6 Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidatio…
Fe based alloys are commonly used in almost every sector of human life. For different reasons, the surfaces of the real parts are prepared using different methods, e.g., mirror-like polishing, grit-blasting, etc. The purpose of the present work is to answer the question how the surface preparation influences the oxidation behavior of Fe-based alloys. To answer this question, a high purity model alloy, Fe 5 wt Al, was isothermally oxidized in a thermogravimetrical furnace. The post-exposure analysis included SEM/EDS (WDS) and XRD. The surface roughness was determined by a contact and laser prof…
A study of wetting was carried out on porous silicon films with pore diameters spanning three orders of magnitude. Water contact angle measurements on adjoining porous and nonporous regions yielded Wenzel roughness ratios that were either unphysical (less than unity) or unrealistically low when compared to those expected from specific surface area considerations. Moreover, results obtained from a sample consisting of a microporous film on a macroporous layer gave contact angles that were very similar to those found on films consisting of only a single microporous layer, contrary to what would …
Electrochemical oxidation of hematite (α-Fe2O3) nano-particulate films at 600 mV vs. Ag+/AgCl reference in KOH electrolyte forms a species at the hematite surface which causes a new transition in the upper Hubbard band between the Fe(3d)-O(2p) state region and the Fe(4sp)-O(2p) region, as evidenced by oxygen near edge x-ray absorption fine structure (NEXAFS) spectra. The electrochemical origin of this transition suggests that it is related with a surface state. This transition, not known for pristine α-Fe2O3 is at about the same x-ray energy, where pristine 1% Si doped Si:Fe2O3 has such transi…
The Ni-based superalloy Alloy 718 is used in aircraft engines as high-pressure turbine discs and must endure challenging demands on high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale $γ^{\prime}$- and $γ^{\prime \prime}$-precipitates commonly found in duplet and triplet co-precipitate morphologies provide high-temperature strength under these harsh operating conditions. Direct ageing of Alloy 718 is an attractive alternative manufacturing route known to increase the yield strength at 650 $^°$C by at least +10 $\%$, by both retaining high dislocation densities and chan…
