Evaluation of the mechanical properties and environmental resistance of René 125 and X-40 superalloys coated with controlled composition reaction-sintered CoNiCrAlY

The application of MCrAlY-type coatings to high temperature turbine components is primarily provided by electron beam physical vapor deposition (PVD) processing. The process of coating with controlled composition reaction-sintered (CCRS) CoNiCrAlY was developed as a cost-effective alternative using only conventional equipment and processing techniques.The CCRS method involves a two-stage process, the first step consisting of the application of the modifier (CoNiCrY powder) onto the cleaned prepared surface in an appropriate slurry bisque. This is followed by pack processing in a controlled activity aluminum pack where reaction sintering occurs to form the final CoNiCrAlY composition.The CCRS CoNiCrAlY coating was applied to both nickel- and cobalt-base superalloys, René 125 and X-40, and to a limited number of mechanically alloyed MA754 specimens. Mechanical properties evaluated at ambient and elevated temperatures include tensile strength, strain tolerance and cycle fatigue. In addition, burner rig tests were conducted at 870°C (1600°F) and 927°C (1700°F) for hot corrosion resistance and at 1149°C (2100°F) for oxidation resistance. For each of the tests, specimens coated with PVD CoCrAlY, PVD NiCrAlY and CODEP B were evaluated under similar conditions in order to compare the performance of these coatings currently widely used.Generally, the results showed that the CCRS CoNiCrAlY coated specimens were equivalent, and in some cases superior, to the PVD and CODEP B coatings. The combination of 15–17 wt.% Al in the CoNiCrAlY coating relative to the PVD coating with 8–13 wt.% Al and the reaction sintering method produced a highly protective coating characterized by multiphase microstructure, ductility and inexpensive processing.     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.     

The influence of protective coatings on thermal fatigue resistance of Udimet 710

A study was made to determine the influence of surface protective coatings on the thermal fatigue resistance of the nickel-base alloy Udimet⁷ 710 (U-710). Single- edge wedge-type specimens were thermally cycled between 80 and 1915°F in fluidized beds with an immersion time of 4 min in each bed. Thermal fatigue resistance was measured by determining the number of thermal cycles required for crack initiation. Crack propagation rates were also determined wherever possible. All the coatings employed in this study improved the thermal fatigue lifetime of U- 710. In terms of crack initiation resistance, plasma-sprayed CoCrAlY-type coatings were ranked the best and pack aluminide coatings the least effective. An inverse correlation was observed between the hardness of the coatings and the thermal fatigue lifetime. The pack aluminide coatings were not found to be beneficial from the point of view of crack propagation: although they delay crack initiation by eliminating substrate grain boundary crack initiation sites, once a crack starts it grows rapidly. In addition to oxidation resistance, the coating microstructure plays a crucial role in crack initiation and propagation. Voids aligned between columnar grains in a coating were found to be more detrimental than those uniformly distributed in the microstructure.     

Production of FeCrAlY and CoCrAlY coatings by laser surface fusion and their oxidation behavior

In this paper the results of a study aimed at coating large areas with FeCrAlY and CoCrAlY alloys by laser surface fusion are described. Metallurgical characteristics and oxidation behavior of the coatings are discussed.The results indicate that fusion of the coating materials was complete and a metallurgical bond was formed between the coatings and the type 304 stainless steel (SS) substrate as a result of laser processing. Some dilution of the coating alloys, in particular the aluminum content, was noticed due to melting of a portion of the substrate. Some small cracks, observed in the CoCrAlY coating, were formed as a result of shrinkage during solidification.Oxidation studies were performed at 1000–1185 °C in air for various times. The coatings, although diluted, showed excellent oxidation resistance and protection, while the type 304 SS substrate deteriorated rapidly. The effect of the thermal and oxidation treatment on the microstructural and chemical stabilities of the coatings are discussed.     

Pack codeposition of Al and Cr to form diffusion coatings resistant to high temperature oxidation and corrosion for γ-TiAl

Abstract

Thermochemical analyses were carried out for a series of pack powder mixtures formulated for codepositing Al with Cr to form diffusion coatings on γ-TiAl resistant to high temperature oxidation by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for codepositing Al with Cr to form diffusion coatings with an adherent and coherent coating structure. The results of the thermochemical calculations performed indicated that codeposition of Al and Cr is possible with CrCl₃.6H₂O and AlCl₃ activated pack powders containing elemental Al and Cr as depositing sources. However, experimental results obtained at 1100°C revealed that CrCl₃.6H₂O is not suitable for use as an activator for codepositing Al with Cr on γ-TiAl. It caused a significant degree of degradation indicated by weight losses instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer Cr doped TiAl₃ layer containing Al₆₇Cr₈Ti₂₅ phase and an inner layer containing TiAl₃ and TiAl₂ phases were successfully codeposited at 1100°C using pack powder mixtures activated by AlCl₃. It is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and chromium. Conditions that affect the pack codeposition process, and hence need to be carefully controlled, are discussed in relation to the mechanism of the formation of diffusion coatings with an integral structure free from microcracking on γ-TiAl.     

铝锌合金在镁合金AZ91D中的扩渗机理及腐蚀性能研究

采用包埋铝和锌的方法在镁合金AZ91D表面制备了铝-锌合金化涂层,并且利用x射线衍射、能谱分析研究了该涂层的组织和耐腐蚀性能。该涂层外层为AlMg2Zn, Mg7Zn3 和Mg17Al12 金属间化合物层;内层为Mg17Al12金属间化合物分布于α-Mg晶界。研究表明,与镁合金基体相比较在质量分数为3.5wt.% NaCl扩渗层显示出较好的耐腐蚀性能。盐雾腐蚀说明,Al-Mg-Zn合金化扩渗层对于降低腐蚀速率起到有效作用。此外,合金化扩渗层与基体的结合是冶金结合,且涂层的显微硬度显著提高。     

Focused ion beam assisted analysis of the oxidation of a NiAl coating on pure Ni

Abstract

Discs of (pure nickel 15mm diameter and 4mm thickness) have been polished to a 1 μm finish, aluminised using a standard pack CVD process to give a β-NiAl coating and then the surface of this coating polished to a 1 μm finish again. Samples have then been isothermally oxidised in a muffle furnace for 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 hours at 950°C, and subjected to surface and cross-sectional microscopy and analysis. Scales were also cross-sectioned and imaged using Focused Ion Beam Milling. The oxidation rate was determined to be 2:6×10^?13 g² cm^?4 s^?1 and for times less than 512 hours the predominant alumina phase was θ alumina. Al depletion effects were observed due to both oxidation and interdiffusion between the Ni substrate and the NiAl coating. Using these effects, it has been shown that tensile strains of up to 9% can arise and these strains readily explain the formation of intrefacial elliptical pores observed by FIB cross sectioning. FIB sectioning also indicated that different coating grains exhibit different oxide-coating interfaces which arise due to different grain orientations. The faceting of the coating-oxide interface and the development of cuboidal faceted coating structures beneath spalled oxide is thought to be due to substructure development (sub-grain development or recrystallisation) arising from the plastic strains generated by Al depletion effects.     

Interaction between the TiC(TiCN)-Ni-Mo hardmetals and chromium vapours

The interaction between TiC or TiCN-based hardmetals with a Ni-Mo binder, or cermets, and chromium vapour in a vacuum was investigated over a wide temperature range acceptable for depositing wear-resistant coatings without the formation of a liquid phase in the cermets. Computer modelling in the Ti-C-Cr system showed that a direct interaction of TiC with chromium, leading to the formation of chromium carbides, is not possible because of the high thermodynamic stability of titanium carbide. It was established experimentally that as a result of the interaction between the cermets and chromium vapours, a coating characterized by a two-layer structure was deposited on the cermet surface. The coating consists of an inner layer adjacent to the substrate, which is composed of the chromium and carbon solid solution in nickel, and an outer layer composed of a mixture of (Cr, Ni)₇C₃ and (Cr, Ni)₂₃C₆. The activation energy of the deposition process is 387 kJ mol^–1 which is close to the value of the chromium heat of evaporation. The coating deposition process is supposed to be limited by the rate of the external supply of chromium from the vapour phase. The results of the investigation of the structure, composition and morphology of the coating are presented. A mechanism responsible for the interaction of the cermets with chromium vapour leading to the formation of the two-layer coating, is proposed.     

Properties of electrodeposited nickel-cobalt (22–50%) alloys

Electrodeposited nickel-cobalt (EDNiCo) alloys ranging from 22 to 50% cobalt were produced and tested for strength as a function of thermal treatment and cobalt content. The electrolytes used were nickel and cobalt sulfamates with a total metal content of 75 g 1^?1. Cobalt content was varied from 6 to 11% in the electrolyte. Cobalt was supplied by continuously metering in cobalt sulfamate solution. All alloys with cobalt content between 22 and 50% demonstrated yield strengths greater than 60 hbar in both the as-deposited condition and after stress relief at 345°C for 1 h. Highest as-deposited strength was exhibited by the EDNi43Co at 166 hbar (245 × 10³ lb in^?2) ultimate tensile strength and 117 hbar (170 × 10³ lb in^?) yield strength. The EDNi50Co was slightly lower at 161 hbar ultimate tensile strength. Stress relief of the EDNi43Co alloy at 455°C produced a strong material with very good ductility (elongation 25%, reduction of area 50%). A similar treatment of the EDNi50Co at 455°C resulted in a strength about 50% greater than that of EDNi in a hydrogen environment.     

On the effect of Temperature Cycling on the coating of a pack aluminized low alloyed steel (13 CrMo 44)

Coated pack aluminized low alloyed steels are known for their good resistance against high temperature corrosion up to 700 °C, where diffusion stability is still sufficient. In typical high temperature applications, coating and substrate are subjected to thermal fatigue. In this study the metallurgical stability of a coated, pack aluminized, low alloyed steel is reported. The specimens were cyclically quenched from 700 °C to room temperature or liquid nitrogen temperature respectively, to assess the importance of thermal shock severity on the kinetic of degradation. In particular, crack initiation and growth were studied. The dependence of microcrack densities upon numbers of cycles, cooling rates and specimen geometry is reported.     

Characteristics of subcooled water boiling on structured surfaces

Characteristics of the process and heat transfer of subcooled water boiling on mesostructured surfaces obtained by microarc oxidation of titanium foil with formation of a TiO₂ layer and deposition of Al₂O₃ particles from boiling nanofluid have been experimentally investigated. The experiments have been carried out in the forced flow of deaerated water in a vertical rectangular channel, 21 × 5 mm in size. The ranges of regime parameters are as follows: water mass velocity is up to 650 kg/(m2 s), subcooling is 30–75°C, pressure is ~10⁵ Pa, and heat flux rate is 0.7–5.0 MW/m². It is established that the number of active nucleation sites is (70–80) × 105 1/(m² s) at the heat flux of 1.5–2.0 MW/m². Significant subcooling of the liquid and good wettability of the structured surface provide intense deactivation and lead to random spatial distribution of the nucleation sites. The characteristic size of vapor bubbles is about 200–250 μm and the bubble lifetime is 200–500 μs. Application of the coating prepared by microarc oxidation enhances heat transfer by 20–30%. At high subcoolings of liquid, the characteristics of boiling on smooth surfaces and surfaces with the coating were fairly close.     

Y_2O_3对铌硅化物基超高温合金硅化物渗层组织的影响

采用包埋共渗工艺在铌硅化物基超高温合金表面制备了Si-Y2O3共渗层,共渗温度为1050℃,共渗时间为10h。利用SEM,EDS和XRD等方法分析了渗剂中Y2O3添加量对渗层结构、组织形貌及其成分分布的影响,并与相同包埋渗温度和时间下单独渗Si渗层的组织进行了对比。结果表明:在渗剂中添加不同含量Y2O3后的渗层具有相似的结构,均具有明显分层的结构,由外至内依次为(Nb,X)Si2(X表示Ti,Hf和Cr)层,(Nb,X)5Si3过渡层和富Al扩散区。与单独渗Si渗层相比,渗剂中添加Y2O3没有改变渗层表层的相组成,但抑制了渗层中孔洞的产生,使相同包埋渗温度和时间处理后Si-Y2O3共渗层的组织较单独渗Si渗层的更为致密。EDS能谱分析结果表明,Y在渗层中的分布是不均匀的,在靠近过渡层与基体界面处的Y含量较高,并由内向外逐渐递减。随渗剂中Y2O3含量增加,渗层中的平均Y含量出现先增加后降低的规律。当渗剂中Y2O3的加入量为1%~2%(质量分数)时,Y2O3具有明显的催渗作用。     

Low temperature plastic deformation of FeCrNi steels in magnetic fields to 2.71 × 103 Am−1

The low temperature (T=4.2 K) plastic deformation of stainless FeCrNi steels (18Cr8Ni, 18Cr10Ni, 18Cr15Ni, 18Cr20Ni and 18Cr25Ni) has been studied in constant (H = 2.71 × 10³ Am^?1) and transient magnetic fields (H ≤ 2.71 × 10³ Am^?1) changing exponentially in time at different rates. It is stated that the constant magnetic field of 2.71 × 10³ Am^?1 does not influence the tensile properties and plastic deformation of steels, but the transient magnetic fields lead to changes in deforming stress. The conclusion is made that the effects observed are due to the linear magnetostriction of steels and the Joule heating by eddy currents.     

Oxidation Resistance of the Aluminide Coating Formed on Carbon Steels

Low and medium carbon steels were aluminized by the pack aluminizing technique using halideactivated pure-Al and Fe-Al packs. The effect of mixture composition, aluminizing temperatureand time and C content of the steel substrate on the structure and thickness of the aluminidelayer, and on the oxidation resistance was investigated. The optimum oxidation resistance canbe achieved with a low carbon steel substrate when the intermetallic phases Fe3Al and FeAlform the surface of the aluminide layer. In this case, the Al concentration at the surface of thealuminide coating is at least ≥15 wt pct. Formation of high Al concentration phases (FeAl3 andFe2Al5) during aluminizing should be avoided as they tend to embrittle the aluminide layer andreduce its oxidation resistance.     

Formation of nickel aluminide/nickel hybrid coatings on alloy steels by two step process of nickel plating and low temperature pack aluminisation

Abstract

The present study investigates the conditions required for forming a hybrid coating consisting of an outer nickel aluminide layer and an inner nickel layer on alloy steels. A commercial alloy steel of 9Cr–1Mo was used as a substrate. Electroless and electronickel plating processes were used to form an initial nickel layer on the steel. The AlCl₃ activated packs containing pure Al as a depositing source were then used to aluminise the nickel deposit at temperatures ≤650°C. The effect of phosphorus or boron content in the initial nickel layer deposited with the electroless nickel plating solutions using hypophosphite or boron–hydrogen compound as reducing agent was investigated in relation to the spallation tendency of the coating either immediately after the aluminising process or during the thermal annealing post-aluminising process. Under the aluminising conditions used, the outer nickel aluminide layer formed was Ni₂Al₃. For the electroplated nickel deposit, the growth kinetics of the outer Ni₂Al₃ layer during the pack aluminising process was found to obey the parabolic rate law with a parabolic rate constant being 12·67 μm at 650°C for 2 wt-%AlCl₃ activated pack containing 4 wt-% pure Al as a deposition source.     

The structure and properties of a hard alloy coating deposited by high-velocity pulsed plasma jet onto a copper substrate

Using a plasmatron operating in specially calculated regimes, tungsten carbide (WC) based coatings were deposited onto a copper crystallizer plate. It was found that a local hardness of the WC-Co coating may reach up to 1.3×10₄ N/mm² and the coating adhesion to substrate may be as high as 270 MPa. The elemental and phase compositions of coatings were studied by Rutherford backscattering spectroscopy, X-ray diffraction, and transmission electron microscopy with electron diffraction. The surface morphology and depth-composition profiles of the coatings were studied by optical and scanning electron microscopy. The coating is composed of WC crystal grains with hexagonal close packed (hcp) lattice, α-and β-Co grains, and cubic WC grains. The average size of the hcp WC grains is 0.15 μm and that of the cobalt particles is about 25 nm. In addition, the grain boundaries contain W₃Co₃C particles with an average size of 15 nm.     

Ni–P amorphous phases obtained by Nd–YAG pulsed laser alloying of deposited Ni–P coating with aluminum

Ni–P electroless deposited coating with crystalline morphology was prepared on aluminum substrate. A Nd–YAG pulsed laser was used to alloy the materials at the condition of power density 5.36 × 10⁹ W/m² and scanning speed 3.0 mm/s. In the laser alloyed layer, Ni–P amorphous phases were found by means of TEM. Electron diffraction patterns and X-ray diffraction results showed that these Ni–P amorphous phases were decomposed as Ni and Ni₃P equilibrium phases when tempered at temperature over 300 °C.     

Mechanical behavior of segmented oxide protective coatings

Mechanically and thermally induced fractures were examined in sputtered coatings consisting of an NiCrAlY underlayer, either a thin or a thick transition layer grading from NiCrAlY to ZrO₂, and an outer ZrO₂ layer. A pronounced columnar (fibrous) microstructure was obtained, although the columnar boundaries in the ZrO₂ layers and in the thick transition layers were much more open than in the NiCrAlY, effectively producing a more segmented structure. Some coatings also included a continuous fine-grained outer NiCrAlY sealing or close-out layer.For this complex metal and ceramic coating, a stress applied parallel to the layer plane always resulted in fracture perpendicular to the layer plane along open columnar boundaries. If the transition layer was thick, no fractures other than those at columnar boundaries were observed. If the transition layer was thin, fracture parallel to the layer plane occured in the ZrO₂-rich portion of this layer. When stress was perpendicular to the layer plane, fracture parallel to the layer plane occurred first at the outer ZrO₂(NiCrAlY) interface if an outer NiCrAlY sealing layer was present. Otherwise the ZrO₂-rich portion of the transition layer failed first, followed by fracture in the ZrO₂ layer.     

Effect of Shell Coating Techniques on Dynamic Response of Photoconductive Core/Shell Nanorod Arrays

Efficient carrier collection in the core/shell nanowire (nanorod) arrays requires a high quality interface between core and shell materials. A highly conformal shell layer around nanorods can lead to fast dynamic response in photoconductive devices by a radial charge flow. Therefore, choice of the deposition technique for the conformal shell layer becomes crucial. In this study, the dynamic response of indium sulfide (In₂S₃) nanorods/silver (Ag) core/shell devices is compared in which Ag shell layers are deposited by different physical vapor deposition (PVD) techniques. In₂S₃ nanorods are fabricated by glancing angle deposition. The core/shell devices with Ag shell sputtered at a relatively high working gas pressure (≈3 × 10⁻² mbar) produce the highest photocurrent compared to other devices in which more directional incident flux (with working gas pressure of ≈3 × 10⁻³ mbar) is utilized for Ag shell layer. The reduced transit times indicate a conformal shell achieved by the high pressure sputtering technique that has a wide angular distribution flux. In addition, a more directional flux yet with a small angle (≈30°) incidence with respect to the substrate surface normal also helps increase the photocurrent. Such simple and scalable PVD techniques are shown to offer alternative fabrication approaches in producing high quality core/shell nanostructures.     

Pack cemented silicon carbide interlayer for plasma sprayed yttria over graphite

Thermodynamically stable and non-reactive yttrium oxide is proposed to be the coating material on high-density graphite (HDG) crucible for melting and consolidating uranium in the pyrochemical reprocessing of spent metallic nuclear fuels. Yttria deposited over HDG by atmospheric plasma spray (APS) process without interlayer cannot withstand temperatures above 1400°C because of large thermal expansion mismatch and high-temperature partial oxidation of underlying graphite substrate through pores and openings in the plasma sprayed yttria layer. To overcome this limitation, a stable oxidation-protective intermediate layer of SiC was developed over graphite by conventional pack cementation process using two different precursor powders. The coated samples were characterized with respect to coating thickness, microstructure, and phase composition. Experimental results revealed that the optimum pack composition for obtaining thick and diffused coating was 70 wt. % Si – 20 wt. % C – 10 wt. % Al₂O₃. Subsequently, yttria top coat was deposited over pack cemented HDG coupons by APS process with optimized process parameters. The interlayer of SiC developed by pack cementation for yttria top coat facilitated to extend the thermal cycling life of the coating in inert argon environment significantly. The onset of micro-cracking in the top coat with SiC interlayer occurred after 45, 25 and 13 thermal cycles at 1400°C, 1450°C and 1500°C, respectively.