Superalloy Surface Research Articles

Quantitative Evaluation of Alitize Coating on ŽS6K Ni-Base Superalloy

The aerospace industry is one of the biggest consumers of advanced materials because of its unique combination of mechanical and physical properties and chemical stability. Highly alloyed stainless steel, titanium alloys and nickel based superalloys are mostly used for aerospace applications. The aim of the work is to evaluate protective Al Si coating applied by diffusion annealing on substrate, Ni base superalloy ZS6K. This superalloy is used for turbine blade production in aero jet engine DV 2. Using of protective alitize coating provides an increasing of heat resistance of superalloy surface and increases working temperature up to 800°C. However, overcrossing of working temperature range (for ZS6K turbine blades it is from 705°C to 750°C) sometimes happen and that is the reason for detailed study of protective coating degradation. The alitize coating were evaluated in starting stage and after various time of regular loading in real aero jet engines DV 2. Coating and its degradation was evaluated with help of quantitative metallography methods (metallography software NIS Elements) and colour contrast as well.

Simulation of oxidation-nitridation-induced microstructural degradation in a cracked Ni-based superalloy at high temperature

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.

A Study of Pack Aluminizing Process for NiCrAlY Coatings Using Response Surface Methodology

Aluminizing process is widely used to provide additional Al deposition onto superalloy surface for enhanced oxidation and corrosion resistance. In this research, an aluminizing process—pack cementation process, is used to deposit Al onto the surface of NiCrAlY coatings for increasing environmental protection. The experiment is designed using Box-Behnken approach, in which three parameters, the Al content, Ni content of the pack powder, and the temperature of the process, are selected as factors; and the thickness and Al/Ni ratio of the coatings are selected as responses. The effects of the factors on the responses are analyzed and modeled empirically. It is found that these empirical models correlate well with the results from additional sets of experiment. These models can be used to produce aluminized NiCrAlY coatings with specific thicknesses and Al/Ni ratios.

Damage of the machined surface and subsurface in orthogonal milling of FGH95 superalloy

FGH95 is one kind of powder metallurgy (PM) superalloy which has excellent mechanical proper- ties at high temperature. It has been developed for turbine disc applications to improve aeroengine efficien- cy under higher operating temperatures. However, this kind of superalloy is very difficult to machine because of its poor thermal diffusivity and work-hardening prop- erties during the machining process. The machining process can lead to damage of the machined surface and subsurface. Thus, the purpose of this paper was to investigate damage of the machined surface and subsur- face in hard machining of FGH95 PM superalloy. Or- thogonal milling experiments using coated carbide inserts were carried out on a CNC machining center. The machined surface was observed and recorded using an optical microscope, white light interferometer, and scanning electron microscope. Machined surface defects were recorded and analyzed. The effects of cutting speed on machined surface roughness, white layer thick- ness, plastic deformation, and microhardness were in- vestigated. The research results show that better surface roughness can be generated at higher cutting speeds, while several defects appeared on the FGH95 machined surface. White layer thickness and machined surface microhardness increase with the cutting speed. Also, plastic shear strain in the machined surface layer in- creases with cutting speed. The depth of plastic defor- mation decreases with the increase of cutting speed. These investigation results are essential for the evalua- tion of PM superalloy surface integrity and are signifi- cant for the prediction of PM superalloy service life.

Characterization of Engineered Nickel-Base Alloy Surface Layers Produced by Additive Friction Stir Processing

Additive friction stir processing (AFSP) was used as a novel method to create consolidated γ′-strengthened superalloy surface layers on non-age-hardenable Ni-alloy substrates. A Ni–Cr–Mo superalloy (Haynes Alloy 282) was deposited onto Ni–Cr–Fe alloy (INCONEL™ Alloy 600) plate using a cold-wire gas tungsten arc process and subsequently friction stir processed. Alloy 282 was incorporated into the surface and led to increases in near-surface hardness to levels over 275 HV0.3 as-processed. Heat treatment response and precipitate distribution also improved as a result of AFSP. The application of AFSP led to a 2-fold increase in hardness after a direct age heat treatment relative to the as-deposited GTAW weld overlay without AFSP. Enhancement of precipitate formation kinetics was attributed to stored energy from mechanical stirring of the additive Alloy 282 at high temperature. Electron backscatter diffraction and transmission electron microscopy of AFSP regions showed increased dislocation density for low AFSP heat input processing parameters.

Microstructure and Dry Sliding Wear Behavior of Laser Cladding (TiC+TiN)-NiCrWC Powder Composite Coating on DZ125 Superalloy

Ni-based TiN-TiC composite coating was fabricated on DZ125 superalloy surface by laser cladding. The phase constitution and microstructures were investigated by means of X-ray diffraction (XRD), optical microscope (OM) and scanning electron microscope (SEM). Microhardness measurements and wear experiments without lubrication were also accomplished. The experimental results showed that a pore- and crack-free coating with metallurgical bonding to the substrate was obtained. Solidification morphologies along the section of the coating varied from directional dendrite in the interface to random dendrite in the surface. The coating was mainly composed of γ-Ni, M23C6, TiN, TiC particles and a small amount of NiTi, respectively. The average microhardness of 705HK for the coating was 2.3 times higher than that of the substrate. Wear tests indicated that wear resistance of the coating was significantly improved compared with that of the substrate. The improvement in hardness and wear resistance was attributed to TiN and TiC phase and chromium carbide uniformly dispersed in the matrix of the Ni-based TiN-TiC composite coating.

Laser treatment of A286 superalloy: corrosion resistance of the treated surface

Laser controlled melting of metal surface provides a local treatment with improved surface properties such as corrosion resistance. In the present study, laser surface treatment of iron base superalloy (A286) is carried out. The corrosion resistance of the laser‐treated surface is examined through potentiodynamic tests using 0.5 N NaCl solution. The microstructural and morphological changes in the laser‐treated layer are investigated incorporating scanning electron microscopy (SEM), X‐ray diffraction (XRD) and energy dispersive spectroscopy. The residual stress formed at the laser‐treated surface is measured using the XRD technique. It is found that laser treatment enhances corrosion resistance of A286 superalloy surface, which is attributed to the formation of fine grains and dense layer at the treated surface. Although locally scattered few corrosion induced microcracks are observed at the treated surface, they are not extended to form large cracks or crack network at the surface. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of picosecond laser irradiation on nickel-based superalloy surface microstructure

The investigation presented in this paper was carried out on the nickel-based superalloy, Nimonic 263, a material widely used at elevated temperatures and pressures. The samples were exposed to a Nd3+:YAG pulsed laser, at a wavelength of 1064 nm and a pulse duration of 170 ps, and the estimated threshold damage was 2 mJ. Different pulse energies and numbers of pulses were applied in air and helium-enriched atmospheres. Spots obtained by the laser interaction with the material were observed by optical and scanning electron microscopes and analyzed by energy-dispersive spectroscopy. Vickers microhardness tests were performed. In this paper, the changes that had occurred at the sample surface microstructure as a result of laser–material interaction are discussed with the aim of contributing to the determination of optimal laser parameters in the laser surface treatment process.

Characterization of the alumina scale formed on coated and uncoated doped superalloys

To investigate the mechanisms by which Y and La dopants affect the oxidation behavior of Ni-base single‐crystal superalloys, the oxide scales formed on two variants of a commercial X4 alloy, each with and without a MCrAlYHfSi coating were characterized. The alloy systems were oxidized for 100h at 1100°C and then examined using analytical transmission electron microscopy. Without a coating, a duplex scale was formed on the superalloy surface comprised of an outer Ni‐rich spinel‐type layer and an inner columnar α‐Al2O3 layer. In this case, Hf and Ti were found segregated to the alumina grain boundaries in the outer part of the scale on both alloys but only Hf was detected near the metal–alumina interface. There was no evidence of Ta, Y or La segregation to the alumina scale grain boundaries after this exposure. The scale formed on the alloys with the thermally sprayed coating was primarily alumina, and Y and Hf segregated to the alumina grain boundaries for both alloys. There was evidence of Ti-rich oxides in the outer part of the scale indicating that Ti had diffused through the coating into the thermally grown oxide but La was not found.

Thermal spraying of silicon nitride coatings using highly accelerated precursor powder particles

Highly accelerated powder particles obtained by low-frequency pulsed detonation spraying (LFPDS), high-frequency pulsed detonation spraying (HFPDS), and high-current pulsed plasma spraying (HCPPS) were applied with the aim to deposit dense protective silicon nitride coatings that adhere well to steel and nickel superalloy surfaces. The silicon nitride-based feedstock powders comprise α-silicon nitride with sintering additives such as alumina, yttria and minor amounts of aluminum nitride and/or silica and magnesia. Coatings were characterized by testing their critical wear path length, tensile adhesion strength, microindentation hardness, salt-spray corrosion and thermoshock resistances, and friction coefficient (pin-on-disk).

Nickel-based super-alloy Inconel 600 morphological modifications by high repetition rate femtosecond Ti:sapphire laser

AbstractThe interaction of Ti:sapphire laser, operating at high repetition rate of 75 MHz, with nickel-based super-alloy Inconel 600 was studied. The laser was emitting at 800 nm and ultrashort pulse duration was 160 fs. Nickel-based super-alloy surface modification was studied in a low laser energy/fluence regime of maximum 20 nJ–15 mJ/cm2, for short (10 s) and long irradiation times (range of minutes). Surface damage threshold of this material was estimated to be 1.46 nJ, i.e., 0.001 J/cm2 in air. The radiation absorbed from Ti:sapphire laser beam under these conditions generates at the surface a series of effects, such as direct material vaporization, plasma creation, formation of nano-structures and their larger aggregates, damage accumulation, etc. Laser induced surface morphological changes observed on Inconel 600 were: (1) intensive removal of surface material with crater like features; (2) material deposition at near and farther periphery and creation of nano-aggregates/nano-structures; (3) sporadic micro-cracking of the inner and outer damage area. Generally, features created on nickel-based super-alloy surface by high repetition rate femtosecond pulses are characterized by low inner/outer damage diameter of less than 11 µm/30 µm and relatively large depth on the order of 150 µm, in both low (10 s) and high (minutes) irradiation time regimes.

Fine-Scale Structure Investigation of Nimonic 263 Superalloy Surface Damaged by Femtosecond Laser Beam

Due to the specific conditions of manufacturing, processing and utilization of nickel-base superalloys, the implementation of contemporary precision techniques is necessary in the research of the alloys features. Multi-component nickel-base superalloys, with about of 50% of nickel content, are commonly exploited in the conditions of high temperatures and pressures as well as in various aggressive operating environments. For successful quality control, which includes the monitoring of the changes in the alloy microstructure, fine-scale structure investigations are necessary. In this work, the samples of nickel-base superalloys have been exposed to 800 nm femtosecond laser in various operating regimes. Surface damages and dents caused by femtosecond laser pulses have been observed by optical and scanning microscopy.

Evolution of wear, roughness, and friction of Alloy 600 superalloy surfaces in water-submersed sliding

Self-mated wear and friction of Alloy 600 superalloy was studied in a water-submersed ring-on-rod configuration, loading the side of a 6.35 mm diameter rod across the flat surface of a rotating annular ring of 100 mm outer diameter and 70 mm inner diameter producing two sliding contacts along the ring. Tests were conducted at sliding speeds of 0.178 and 0.330 m/s for sliding distances of 100 m. Normal loads of 51 and 204 N were applied, and initial R a surface roughnesses of the rings along the sliding direction were either smooth (∼0.2 μm) or rough (∼7.5 μm). Increased initial ring roughness caused a ∼20-fold increase in rod wear at the lighter load, whereas at the heavier load increased initial roughness only caused a ∼4-fold increase in wear. At lower initial ring roughness the 4-fold decrease in normal load caused a large (one order-of-magnitude) decrease in rod wear, whereas for rings of higher initial roughness the 4-fold decrease in normal load caused only minor (2-fold or less) decreases in rod wear. Wear during this 100 m sliding distance only experienced a minor effect from the 1.8-fold change in sliding speed, as did friction. In all cases friction coefficient rapidly settled into the range 0.6–0.7, except in the cases of lower load on rings of lower initial roughness where friction coefficient remained above 1 for most of this sliding duration. At this lower load the initial ∼0.2 μm rod roughnesses increased to nearly 0.8 μm by the 100 m sliding distance, whereas at the higher load this same sliding distance resulted in roughnesses returning near to the initial 0.2 μm. It was hypothesized more highly loaded cases also went through initial roughening prior to smoothening back to 0.2 μm roughness within the 100 m sliding distance, and given additional sliding the more lightly loaded cases would also experience subsequent smoothening. Increasing sliding distance to 400 m, roughnesses indicated a smoothening back to 0.2 μm level during those lightly loaded tests, with friction coefficient correspondingly dropping from 1 into the 0.6–0.7 range observed in all other cases. Extended sliding to 400 m at light loading against rings of lower initial roughness also allowed a rod wear rate which increased with increased sliding distance to be observed, approaching the same rate observed against initially rough rings within the 100 m sliding distance.

Experimental Study on Residual Stress of K24 Superalloy in Laser Cladding Zone by Laser Shock Processing

The surface of K24 superalloy was processed with laser cladding & LSP (laser shock processing). Residual stress in the laser cladding zone by LSP was measured with X-ray stress tester X-350A, and the variational rule of residual stress in the cladding zone by tempering treatment of 8 hours and 16 hours was measured, respectively. The experimental results show that compressive residual stress of K24 superalloy surface by laser cladding & laser shock processing is above -600MPa, which exceeds residual stress by mechanical peening treatment; and there is no clear effect on residual stress by tempering treatment at 600°C for 8 hours and 16 hours, respectively, which can improve fatigue life of K24 superalloy.

Oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition at 800 °C

Isothermal oxidation of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet fabricated by Electron Beam Physical Vapor Deposition (EB-PVD) at 800 °C in air was investigated. The oxidation kinetics of microcrystalline Ni-11.5Cr-4.5Co-0.5Al superalloy sheet followed a parabolic power law at initial oxidation stage, and cubic power law for long oxidation terms. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubic power law was derived. The excellent results in agreement with the experimental data were obtained.

Oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet fabricated by electron beam physical vapor deposition at 900 °C

Isothermal oxidation of microcrystalline Ni–11.5Cr–4.5Co–0.5Al (wt%) superalloy sheet fabricated by electron beam physical vapor deposition (EB -PVD) at 900 °C in air was investigated. Mass gain curves were obtained on testing the alloy in dry air. And characterization of the surface morphology and cross-sections was performed on samples isothermally oxidized for different exposure times. Surfaces and cross-sections of the oxidized specimens were studied by scanning electron microscopy (SEM). Phase identification of the oxide scale was performed by glancing angle X-ray diffraction (GAXRD) and energy dispersive X-ray microanalysis (EDX). The experiment results indicated that the oxidation kinetics of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet follows a cubical power law at initial oxidation stage and fourth power law for long oxidation terms. The oxide scales consisted mainly of a mixed oxide at initial oxidation stage and a double-layer scale with an outer NiO oxide layer, and inner mixed oxides exposed for long terms at 900 °C. The fine oxide grains could be formed on microcrystalline superalloy surface. Taking the short circuit diffusion through grain boundaries into account, an oxidation kinetic with cubical power law in mixed oxide and fourth power law in single-phase oxide were derived. And the excellent fitting of the present model to the experimental data was obtained. The results suggested that the growing oxide layer of microcrystalline Ni–11.5Cr–4.5Co–0.5Al superalloy sheet, which controlled the ionic transportation, contained mainly mixed oxide at initial oxidation stage and the scale/substrate interface single-phase oxide for long terms.

Conductivity profile determination by eddy current for shot-peened superalloy surfaces toward residual stress assessment

This paper demonstrates a swept high frequency eddy current (SHFEC) methodology that can determine near-surface conductivity deviation profiles of shot-peened superalloy surfaces, from which residual stress state can be assessed nondestructively. Our methodology is built around a laboratory-grade SHFEC hardware and a model-based SHFEC data inversion software, both described in this article. For the demonstration, a series of shot-peened Inconel 718 block specimens is prepared and examined by the proposed SHFEC inversion technique. The conductivity depth profiles of the samples under various shot peening intensities have been obtained by the inversion. Several sensitivity and consistency test results are given to support the reliability of the inverted conductivity profiles. The extreme near-surface regions (10–20μm) of the shot-peened surfaces are also examined by various microstructural characterization methods such as scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction, to examine our inversion results microscopically.

Pre-oxidation of Inconel Alloys for Inhibition of Carbon Deposition from Heated Jet Fuel

Exposure of superalloy surfaces to jet fuel at elevated temperatures leads to the formation of carbonaceous deposits and metal sulfides. The formation of stable oxide layers on alloy surfaces can reduce the activity of the constituent transition metals that catalyze the dehydrogenation of hydrocarbons and the subsequent carbon deposit growth. The metals Ni, Cr, Fe, Mn, Al, Ti and Nb + Ta form thermodynamically stable oxide layers after oxidation above 800°C under O2 flow. In this study, we investigated the formation of oxides and spinels on three different Ni-base superalloys (Inconel 600, Inconel 718, and Inconel X750) and their activity towards carbon and sulfur deposit formation from jet fuel (JP-8) thermal stressing at 600°C and 34 atm (500 psig) for 5 hr. Metal oxide formation during pre-oxidation and the behavior of pre-oxidized samples in thermal stressing were found to depend strongly on the minor element composition of these superalloys.

The preparation and hot corrosion resistance of gradient NiCoCrAlYSiB coatings

An Al and Cr gradient NiCoCrAlYSiB coating was prepared by a two-step deposition: (i) deposition of a NiCoCrAlYSiB layer onto Ni-based superalloy surface by arc ion plating (AIP); and (ii) co-deposition of NiCoCrAlYSiB and Cr–Al by AIP and magnetron sputtering (MS), respectively. The results of EPMA show that Al and Cr have a gradient distribution in the surface layer of the gradient coatings. After annealing, the NiCoCrAlYSiB coatings consist of γ/γ′ and β-NiAl, while the gradient coatings consist of σ-(Cr, Co), AlCr 2, and Al 8Cr 5, besides γ/γ′ and β-NiAl. The phases of σ-(Cr, Co), AlCr 2, and Al 8Cr 5 can act as reservoir phases of Cr and Al to improve the hot corrosion resistance and lifetime of the coatings. In order to investigate the effect of Al and Cr on the hot corrosion of coatings, NiCoCrAlYSiB coatings and the gradient coatings were coated with 0.5 mg (Na 2SO 4+K 2SO 4 20 wt.% salt mixture)/cm 2 and oxidized at 900 °C for 20 h. XRD and EDS results show that there are only the alumina formed on the NiCoCrAlYSiB coatings, so the basic fluxing of Al 2O 3 occurred, which caused an accelerated hot corrosion after 10 h. For the gradient coatings, there formed Al 2O 3 and Cr 2O 3 at the primary stage of hot corrosion. Cr 2O 3 is easier to react with Na 2SO 4 than Al 2O 3, which reduces the oxygen ion activity of Na 2SO 4, so Al 2O 3 was protected. The formation of the continual and protective layer of Al 2O 3 and Cr 2O 3 leads to the hot corrosion kinetics of gradient coatings similar to oxidation kinetics.

Ni-based superalloy surface alloying by double-glow plasma surface alloying technique

The double-glow surface alloying technique, also called the Xu-Tec/Xu-Loy process, is a novel technique in the field of surface alloying. This technique allows alloy layers with unique physical, chemical and mechanical properties, such as nickel-based alloy layers, stainless-steel layers and age-hardened surface high-speed steel layers to be formed at the surface of treated metallic materials. In this paper, recent research of the application of the double-glow plasma surface alloying technique in the formation of corrosion resistance alloy layers is briefly reviewed. The results of a study of Ni–Cr–Mo–Nb and Ni–Cr–Mo–Cu corrosion-resistant alloying layers as well as composite alloying layers with an electric brush plating Ni interlayer are reported.