MCrAlY Alloy Research Articles

Effects of High Al Content on the Phase Constituents and Thermal Properties in NiCoCrAlY Alloys.

MCrAlY (M = Ni and/or Co) metallic coatings are essential for the protection of hot-end components against thermal and corrosion damage. Increasing the Al content is considered a feasible solution to improve the high-temperature performance of MCrAlY coatings. In this paper, the effects of high Al contents (12-20 wt.%) on the phase constituents and cast microstructures in MCrAlY alloys were studied by high-energy X-ray diffraction and electron microscopy techniques combined with phase equilibria calculations. High Al content improved the stability of β, σ, and α phases. Meanwhile, an evolution of the cast microstructure morphology from a dendrite structure to an equiaxed grain structure was observed. The thermal properties were analyzed, which were closely related to the phase constituents and solid-to-solid phase transitions at evaluated temperatures. This work is instructive for developing high-Al-content MCrAlY coatings for next-generation thermal barrier applications.

Effect of Hf-Doped MCrAlY Alloy on the Structure and Properties of Thermally Grown Oxide Layer

Effect of Hf-Doped MCrAlY Alloy on the Structure and Properties of Thermally Grown Oxide Layer

Hf and Ta co-doping MCrAlY alloy to improve the lifetime of coatings

Thermal barrier coating (TBC) is one of the key components of an aircraft turbine or stationary gas turbine. NiCoCrAlY is the most commonly used bond coat (BC) and its composition and impurities have an impact on the lifetime of the TBC. However, the performance of NiCoCrAlY is very unstable due to the influence of impurities. Reactive elements (RE) Hf and Ta are co-doped into the bond coat to improve the adhesion of thermally grown oxide (TGO) layers to extend the lifetime of TBC. Yttrium stabilized zirconia (YSZ) is sprayed on free-standing alloys of NiCoCrAlY and NiCoCrAlYHfTa, respectively. The samples are cyclically oxidized at 1050 °C until the peeling off of coatings. The lifetime of NiCoCrAlY coatings is only 240 h, while the lifetime of NiCoCrAlYHfTa coatings reaches 2664 h. The morphology and structure of the oxide scale are characterized to study the mechanisms of lifetime extension.

Advanced MCrAlY alloys with doubled TBC lifetime

Thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades. However, the most commonly used MCrAlY bond coats which largely determine air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. APS TBC has been found predominate failure within top coat near interface/or at interface with thermally grown oxides (TGO). TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in the top coat or both. By focusing on the reduction of tensile stress in the top coats, three new bond coat alloys have been designed and developed. More than doubled TBC lifetime has been achieved by using the new alloys as bond coat. Extended thermal cycle lifetime is attributed to the unique properties of the new alloys, such as remarkably lower coefficient of thermal expansion (CTE), lower weight fraction of β phase, lower aluminum migration rate and the absence of mixed/spinel TGO oxides.

Effect of Water Vapor on the Microstructure of Al2O3 on the Free-Standing MCrAlY Alloy at 1100 °C

The oxidation resistance of the MCrAlY binding coat is due to the formation of protective Al2O3 oxide scale at high temperature. The oxidation behavior of NiCrAlYHf alloy in 1100 °C air and air-water vapor atmosphere was studied. The effect of water vapor on the microstructure and distribution of reactive elements was discussed. The results showed that the oxide scale in air has a double layer structure composed of columnar and equiaxed crystals, while the oxide scale in water vapor contains fine alumina grains, which provides more channels for the diffusion of reactive elements. In addition, The Cr element in the oxide scale is mainly concentrated in the outer equiaxed crystal zone, and the Hf oxide is mainly concentrated in the columnar crystal boundary. In air-water vapor atmosphere, the Cr element is uniformly distributed in the oxide scale.

Design of novel NiSiAlY alloys in marine salt-spray environment: Part I. Al-Si-Y and Ni-Si-Y subsystems

MCrAlY (M=Ni, Co, NiCo) alloys have been widely used as a bond layer for thermal barrier coatings (TBCs). However, oxides form discontinuously on MCrAlY surface in the marine environment since Cr2O3 can react with NaCl and H2O at temperatures above 500 ℃. Replacing Cr with other alloying elements (such as Si) might prevent severe salt-spray corrosion effectively. To save time and efforts in materials design of the Ni-Si-Al-Y system, we obtained the thermodynamic descriptions of the ternary compounds by coupling the CALPHAD (CALculation of PHAse Diagrams) approach and first-principles calculations. Seven and eighteen ternary compounds were evaluated and calculated in the Al-Si-Y and Ni-Si-Y systems, respectively. Phase diagrams of the two systems were depicted after a series of experimental Al/Ni-Si-RE (RE denotes rare earth elements, including La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, and Er) systems being analyzed. It is found that the RE elements with the same reference states tend to form ternary compounds with the same crystal structures. The model-based descriptions are the basis for the research and development of novel NiSiAlY alloys in resisting high-temperature corrosion.

The Isothermal Oxidation of MCrAlY Protective Coatings

The thermal barrier coatings (TBC) are commonly used for protection of jet engine parts. In presented article the influence of chemical composition of TBCs bond coats on isothermal oxidation resistance was analysed. The bond coat was plasma sprayed (APS) using different MCrAlY alloys produced by Oerlikon-Metco: Metco 4451, AMDRY 997, AMDRY 962, AMDRY 365-1, AMDRY 995C. The conducted research showed big difference in coating thickness despite the same spraying parameters. The difference in porosity was not observed. The isothermal oxidation test was conducted during 500h at 1100°C in static laboratory air. The obtained results showed, that degradation mechanism of MCrAlY bond coats regardless of chemical composition is similar. The formation of scale contained aluminium and chromium oxides was observed. The internal oxidation effect was also observed in produced coatings.

A study on the effect of nano-CeO2 dispersion on the characteristics of thermally-grown oxide (TGO) formed on NiCoCrAlY powders and coatings during isothermal oxidation

Abstract The present investigation is focusing on the high-temperature oxidation behavior of conventional and nano-CeO2 modified MCrAlY alloys, in the form of powders and coatings. Emphasis is placed upon the influence of MCrAlY alloy form on the oxidation performance. The NiCoCrAlY- CeO2 nanocomposite powders were prepared by mechanical milling and were subsequently thermally sprayed by high velocity oxygen-fuel (HVOF) technique. Isothermal oxidation tests were carried out at 1000 °C for up to 100 h. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) experimental analyses with thermodynamic equilibrium calculations were used for analysis of the oxide scales. The results indicated that the effect of nano-CeO2 addition on the oxidation properties was more pronounced in the form of coating. Moreover, the oxidation rate of the nano-CeO2 modified coating was found to be 87% lower than that of conventional coating, which was suggested to be related to the incorporation of nano-sized CeO2 dispersion into the Al2O3 scale. The different oxidation mechanisms were discussed.

A comparative study on the microstructure evolution of conventional and nanostructured MCrAlY powders at high-temperature

Previous studies have revealed that nanostructured MCrAlY alloys with nano-oxide dispersoids show promising potential for high-temperature applications. In an effort to provide a fundamental understanding of microstructure-oxidation performance derived insights, the influence of nano-scaled microstructure and oxide dispersion on the oxidation behavior of MCrAlY powder material is investigated in this study. The microstructural evolution within the conventional and nanostructured powders was investigated during thermal exposure at 1000 °C for up to 100 h. The results indicated that a pure alumina layer was formed onto the surface of conventional powder and the thickness of Al2O3 layer increased gradually with the oxidation time. However, a duplex-structure comprising an outer spinel layer and an inner alumina layer was formed onto the surface of the nanostructured powder. The different oxidation mechanisms of the two powders were discussed.

Significantly improving the oxidation and spallation resistance of a MCrAlY alloy by controlling the distribution of yttrium

The NiCoCrAlY alloy with uniform yttrium distribution was fabricated via powder milling and spark plasma sintering. After powder milling, yttrium was uniformly distributed in the alloy in the form of Ni-Y precipitates (50–150 nm) and nano-sized Y2O3 particles (5–30 nm). It was found that the uniform yttrium distribution could: 1) lower thermally grown oxides (TGO) growth rate; 2) reduce the growth stress in the TGO; 3) suppress S segregation to the interface and the formation of interfacial imperfections (e.g. oxide intrusions, pores). Consequently, the oxidation and spallation resistance of the TGO was significantly improved.

High-Temperature Wettability and Interactions between Y-Containing Ni-Based Alloys and Various Oxide Ceramics.

To obtain appropriate crucible materials for vacuum induction melting of MCrAlY alloys, four different oxide ceramics, including MgO, Y2O3, Al2O3, and ZrO2, with various microstructures were designed and characterized. The high-temperature wettability and interactions between Ni-20Co-20Cr-10Al-1.5Y alloys and oxide ceramics were studied by sessile drop experiments under vacuum. The results showed that all the systems exhibited non-wetting behavior. The contact angles were stable during the melting process of alloys and the equilibrium contact angles were 140° (MgO), 148° (Y2O3), 154° (Al2O3), and 157° (ZrO2), respectively. The interfacial reaction between the ceramic substrates and alloys occurred at high temperature. Though the ceramics had different microstructures, similar continuous Y2O3 reaction layer with thicknesses of about 25 μm at the alloy-ceramic interface in MgO, Al2O3, and ZrO2 systems formed. The average area percentage of oxides in the alloy matrices were 0.59% (MgO), 0.11% (Al2O3), 0.09% (ZrO2), and 0.02% (Y2O3), respectively. The alloys, after reacting with MgO ceramic, had the highest inclusion content, while those with the lowest content were in the Y2O3 system. Y2O3 ceramic was the most beneficial for vacuum induction melting of high-purity Y-containing Ni-based alloys.

Thermo-Calc and DICTRA modelling of the β-phase depletion behaviour in CoNiCrAlY coating alloys at different Al contents

The β-phase depletion kinetics in MCrAlY coating alloys at different Al contents has been investigated by diffusion simulations and comparing with available experimental data. The thermodynamic software, Thermo-Calc, and the finite difference software, DICTRA, were employed to calculate phase equilibriums and to perform diffusion calculations using the thermodynamic database, TTNi7, and the mobility database, MOB2. The phase fractions and compositions in MCrAlY alloys with different Al contents were calculated by Thermo-Calc. At high temperature exposures, the β-NiAl phase in MCrAlY coating alloys depletes progressively due to the consumption of Al during oxidation and interdiffusion. The Al flux due to oxidation was approached from the oxide growth kinetics at the oxide/coating interface in the DICTRA diffusion model. Initial oxides and the associated initial β-phase depletion zones were assumed as the initial conditions. It is found that the MCrAlY with highest Al content exhibits the least β-phase depletion due to its large β-phase fraction and high Al content. In addition, it is shown that the loss of β phase to the substrate due to interdiffusion is more significant than the β-phase depletion from oxidation.

Influence of composition and microstructure on the tribological property of SPS sintered MCrAlY alloys at elevated temperatures

In order to explore the mechanical and tribological properties of high-temperature alloy materials, the present study aims to investigate the synthesis, microstructure, composition, mechanical and tribological properties of high-temperature monolithic MCrAlY (M = Ni or NiCo) alloys. The alloys were prepared by spark plasma sintering technique. The microstructure and composition were studied by Scanning Electron Microscopy and X-ray diffractometry. The tribological properties of Ni-based alloys were tested by friction tester against Si3N4 balls at elevated temperatures. The results showed that the NiCrAlY alloy was composed of γ-Ni(Cr) solid solution, β-NiAl, and γ′-Ni3Al with little CrAl. The NiCoCrAlY alloy was consisted of γ-Ni(Cr/Co) solid solution, β-NiAl and γ′-Ni3Al. The hardness of NiCrAlY alloy was higher than that of NiCoCrAlY alloy. However, its flexural strength was lower than the NiCoCrAlY alloy. The more uniform microstructure of NiCoCrAlY alloy along with the evenly dispersed fine reinforced phases and the less pores defects contributed to the higher flexural strength. The μs and WRs of NiCrAlY and NiCoCrAlY alloys increased firstly to reach the highest value at 200 and 400 °C, and then declined with the temperature increasing to 800 °C. The WRs of NiCrAlY were lower than that of NiCoCrAlY for all the temperatures. Adhesive wear dominated the wear mechanism of NiCrAlY/Si3N4 tribo-pair, while abrasive wear was the main wear mechanism for NiCoCrAlY/Si3N4 tribo-pair. At high temperature, oxidation of worn surfaces to a certain extent decreased the wear for both the tribo-pairs. The higher hardness of NiCrAlY may contributed to the lower wear than NiCoCrAlY.

On the Reaction Mechanism of MCrAlY Alloys with Oxide–Sulfate Deposits at 1100 °C

The corrosion of γ-(Ni,Co) + β-(Ni,Co)Al NiCoCrAlY alloys exposed to CaO-rich, Na2SO4-containing deposits in air and CO2–H2O–O2 at 1100 °C was studied with a focus on the mechanisms governing the accelerated attack of γ-rich compositions. Internal oxidation of Al resulted from the breakdown of an initially formed Al2O3 scale, due to its reaction with the deposit. Compared to deposit-free conditions, this scale exhibited: (i) a delayed Al2O3 structure transition from metastable θ to stable α; (ii) finer grains; (iii) a reduced adherence to the metal, to the extent that a gap developed at the metal/oxide interface; and (iv) significant permeability to S, as well as N during air exposures. The occurrence of sulfidation in S-free gases reflected the establishment of an elevated S activity under the molten Na2SO4, due to the locally reduced $$p_{{{\text{O}}_{ 2} }}$$ . The factors affecting the Al2O3-scale microstructure were examined, as well as the associated consequences on Al consumption and on the transition to internal oxidation. The roles of alloy and gas compositions in the identified mechanisms were also discussed.

Parameters Evaluation of Bond-Coat Deposited by CO<sub>2</sub> Laser Beam for Aeronautical Turbine Blades

The high temperature environments where aeronautical turbine blades are exposed makes mandatory the use of ceramic coatings. A bond coat comprised by a MCrAlY alloy (M=Co, Ni) is necessary to match the blade metallic substrate with the ceramic, also acting as a corrosion barrier. The laser treatment of metals and alloys is based in the surface local heating, followed by high cooling rates. The laser parameters such as scanning speed and laser power, plays an important role on the morphological, mechanical and chemical characteristics of the deposited material. In the present investigation, the NiCrAlY deposition was performed in stainless steel substrates with a CO2 laser beam. Different laser parameters of scanning speed and number of scanning cycles were implemented. The samples were characterized by optical microscopy and measurements of Vickers hardness. The results show that it is possible to achieve coatings with high hardness, free of pores or any pronounced defects, metallurgically bonded to the substrate.

大気圧プラズマ溶射によるMCrAlY皮膜の高温酸化特性

MCrAlY alloy thermal spray coatings are applied to protect the substrate from oxidation and corrosion under high temperature environment. Various MCrAlY alloys can be coated by different thermal spray processes; LPS, HVOF, and APS. MCrAlY coating by APS is said to be inferior in oxidation resistance, however few studies have been conducted on oxidation property of APS MCrAlY alloy coatings. In this study, CoNiCrAlY, NiCoCrAlY, and NiCrAlY alloys of two different particle sizes were prepared and their coatings were formed by APS process. These MCrAlY alloy powders and coatings were exposed to high temperature and their surface oxide formations were analyzed to determine the growth rate and the activation energy from the growth of the oxide layer thickness. Furthermore, microstructure observations and chemical composition analysis of coatings were conducted. From these results, influence of MCrAlY powder size difference and oxides contained in the coating on high temperature oxidation property of APS MCrAlY coatings was revealed.

Mechanical dispersion of platinum particles and its effect on the microstructure of MCrAlY alloy prepared by SPS

Spark plasma sintering (SPS) technique was used to densify fine Pt-particles with a MCrAlY alloy powder, at temperatures ranging between 1000 and 1050°C in order to obtain consolidated tablets. Fine dispersion of Pt-particles into a full densified matrix was achieved on the products while the overall SPS processing time did not exceed 30min. Both chemical constitution and microstructure, as well as porosity evaluation were investigated by using scanning electron microscopy, chemical analysis and X-ray diffraction. The fine Pt-particles embedded into the MCrAlY matrix have no marked influence over the final microstructure but in its sintering consolidation behavior. The effect of Pt-particles, as-deposited on the surface of MCrAlY/Pt might mean important technical advantages on the obtained products due to the refined microstructure and the solubility level attained, which can lead to attractive applications for high temperature corrosion systems.

Analysis of YSZ-Al<sub>2</sub>O<sub>3</sub>/YSZ Flame Sprayed Thermal Barrier Coating to Thermal Resistance

TBC (Thermal Barrier Coating) with YSZ-Al2O3/YSZ as top coat (TC) and MCrAlY alloy as bond coat (BC) marked as YSZ-Al2O3/YSZ double layer TBC was used for thermal barrier material on rocket nozzle wall. Several test have been conducted (thermal test and SEM-EDX). Thermal test was divided into two part, Thermal Torch and Non-isothermal Oxidation Test which is used for measure the resistance of TBC structure against oxyacethylene flame penetration as a miniature of thermal static test for rocket nozzle until the specimen was failure and used to measure the resistance against non-isothermal oxidation as well as investigate the growth of TGO (Thermally Grown Oxide) layer on interface between TC/BC for thermal torch and non-isothermal oxidation test respectively. This test was performed at 1100o C with 10°C/min heating rate. The results showed that the improvement of feed rate on topcoat powder gave the effect on the surface morphology of topcoat. Lower feed rate bring in a coarse surface morphology and tend to be porous. Al2O3/YSZ composite layer affect on the growth of TGO layer after thermal test, which indicates the lower growth of TGO especially for 15%Al2O3/8YSZ. TGA test showed that the sample began to oxidize at temperature range c.a 1000 – 1030°C, however 1st derivative analysis indicated that 15% Al2O3/8YSZ, 14 g/min was the most stable sampel. Moreover this material has a good resistance from torch destruction.

Thermo-mechanical Fatigue Failure of Thermal Barrier Coated Superalloy Specimen

Failure behavior of thermal barrier coated (TBC) Ni-based superalloy specimens were studied from the aspect of the effect of bond coat material behavior on low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) at various temperatures and under various loading conditions. Initially, monotonic tensile tests were carried out on a MCrAlY alloy bond coat material in the temperature range of 298 K to 1273 K (25 °C to 1000 °C). Special attention was paid to understand the ductile to brittle transition temperature (DBTT). Next, LCF and TMF tests were carried out on the thermal barrier coated Ni-based alloy IN738 specimen. After these tests, the specimens were sectioned to understand their failure mechanisms on the basis of DBTT of the bond coat material. Experimental results demonstrated that the LCF and TMF lives of the TBC specimen were closely related to the DBTT of the bond coat material, and also the TMF lives were different from those of LCF tests. It has also been observed that the crack density in the bond coat in the TBC specimen was significantly dependent on the test conditions. More importantly, not only the number of cracks but also the crack penetration probability into substrate were shown to be sensitive to the DBTT.

Characterisation of Fe–20Cr–6Al–Y model alloy in supercritical water

MCrAlY (M = Fe, Ni or Co) alloys belong to a class of coating materials that provide corrosion and oxidation resistance to Fe or Ni based substrate alloys by forming a dense alumina layer on the surface. In order to assess its potential as a corrosion resistant coating on components in supercritical water cooled reactor core, a Fe–22Cr–6Al–0·6Y model alloy was tested in supercritical water (SCW) (500°C and 25 MPa) for over 6000 h. The long-term corrosion behaviour of samples with various surface preparation means was evaluated by measuring weight change per unit surface area and examining surface microstructure and oxide formation using scanning electron microscopy and X-ray diffraction. The results showed that surface preparation alone led to changes in weight gain as great as seven times. Finely polished surface allowed for more oxidation to take place in SCW, hence more weight change due to Fe3O4 (and spinel) formation. Simple grinding with abrasive paper yielded the least and the most stable weight change while grit blasting had resulted in the greatest weight gain, probably due to the increased surface area and more oxygen inward diffusion. Comparing to other alloys tested under similar condition, the ground Fe–22Cr–6Al–0·6Y had the lowest weight change. Although not detected, the formation of Al2O3 or an Al/Y modified Cr2O3 superficial layer was likely the reason for such low weight change.