On the initiation of cyclic oxidation-induced rumpling of platinum-modified nickel aluminide coatings

The evolution of the surface roughness of an initially flat platinum-modified nickel aluminide (NiPtAl) coating on a single crystal superalloy with cyclic oxidation has been measured by optical profilometry and correlated with the aluminide microstructure. The roughness evolution is dominated by the relative motion of individual grains on thermal cycling, with the larger sized grains moving up and the smaller grains moving down with respect to the average surface. Detailed crystallographic analysis indicates that the grains in the coating are randomly distributed and have no crystallographic relationship with the underlying single crystal superalloy. Furthermore, there is a correlation between the number of sides a grain has and the propensity for its center to move relative to the average surface plane: grains with six or more sides bow up, whereas the surfaces of those grains with fewer than six sides bow down. It is proposed that rumpling initiates due to the vertical displacement of the smaller grains having fewer than six sides. Once initiated, rumpling proceeds driven by the strain energy in the thermally grown oxide.     

Factors affecting the microstructure of platinum-modified aluminide coatings during a vapor phase aluminizing process

Platinum (Pt)-modified aluminide coatings were developed by electroplating a thin layer of Pt followed by an industrial vapor phase aluminizing process. The goal of this work was to systematically investigate the effect of critical coating process parameters (such as the electroplated Pt thicknesses, Al contents in Cr-Al nuggets, diffusion heat treatments) and substrates on the final Pt-modified aluminide coatings. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Optical Microscope, Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Experimental results showed that the Al and/or Pt increase shall favor the formation of ξ-PtAl₂ phase; transformation of ξ-PtAl₂ into β-(Ni,Pt)Al phase can be obtained via a heat treatment process; Cr, Co elements in the studied Ni-base superalloy substrates did not show significant influence on coating outer layer microstructure; while substrate elements affect the microstructure of the coating interdiffusion layer.     

Vapor deposition of platinum alloyed nickel aluminide coatings

Platinum-doped NiAl coatings are widely used to increase the oxidation resistance of superalloys. These coatings are usually synthesized by a solid state reaction-diffusion process conducted at high temperature. It requires the chemical vapor deposition of aluminum on a nickel rich superalloy substrate that has been pre-coated with several microns of electrodeposited platinum. Here, we show that an electron beam directed vapor deposition (EB-DVD) technique can be used to deposit well bonded, structurally and chemically homogeneous NiAlPt bond coats of any composition onto superalloy substrates. The approach utilized a high voltage, rapid scan frequency electron beam to independently heat elemental nickel, aluminum and platinum melt pools to create three closely spaced vapor plumes. These vapor plumes were then entrained in an inert gas jet flow, which mixed and directed them to a substrate. By adjusting the electron beam current applied to each elemental source, homogeneous, dense, Pt alloyed β-phase NiAl coatings could be synthesized at substrate temperatures of 1050 °C. The width of the substrate-coating interdiffusion zone was controlled by the deposition temperature and time.     

Orientation relations in aluminide coatings on single crystals of nickel superalloys

The crystallographic orientation of NiAl refractory coatings on the surface of single crystals of high-temperature nickel alloy ZhS32 is studied. The orientation relation between single-crystal substrates based on an fcc γ-phase and coatings based on a bcc β-phase is studied.     

Heat resistance of aluminide coatings on nickel obtained by mechanochemical heat treatment

Mechanochemical heat treatment (MCHT) is a new direction in the creation of protective coatings with a controlled micro- and macrostructure. The use of MCHT provides diffusion coatings with an anomalously large thickness and a structure that is most favorable for the operating conditions. Results of a study of high-temperature aluminide coatings on nickel deposited by an MCHT method are presented. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 13–15, October, 1998.     

Effect of chromium on the protective properties of aluminide coatings

Aluminide and chromaluminide diffusion coatings on nickel and a nickel-base superalloy, EI 867, were subjected to different corrosion tests consisting of oxidation under isothermal and thermal cycling conditions, and oxidation in the presence of fused sodium sulfate. It was found that chromium present in the surface layers of aluminide coatings has a beneficial effect on their resistance to oxidation and hot corrosion.     

Boriding of nickel aluminide

Boriding of Ni₃Al was carried out with Ekabor-Ni powders at temperatures between 1073 and 1223 K for 2, 4, 6 and 8 h. The characterization of boride layer formed on the surface of nickel aluminide substrates were identified by optical microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and wave length dispersive spectroscopy (WDS). The presence of boride Ni₃B and Ni₄B₃ on the Ni₃Al substrate was confirmed by X-ray diffraction (XRD) analysis. Microhardness and thickness of boride layers were measured. Its hardness was found to be in the range of 780 to 1150 ± 15 HV. Depending on process temperature and boriding time the thickness of coating layers ranged from 12 to 70 ± 3 µm. A parabolic relationship was observed between the layer thickness and the process time. The activation energy for process was calculated as 188, 8 ± 14.4 kJ/mol.     

Synthesis of Hf-modified aluminide coatings on Co-base superalloys

Hf-modified and Hf-Pt-modified aluminide coatings on Haynes 188 Co-base alloy coupons were developed and furthermore, Hf-modified aluminide coatings on MAR-M-509 Co-base aircraft turbine engine vane segments were produced for potential industrial applications. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and Glow-Discharge Mass Spectrometry (GDMS). Experimental results showed that Hf was successfully incorporated and localized Hf-rich precipitated phase particles were observed on the coating surface and within the coating additive layer. The Hf-rich particles and the effect of heat treatment on the particles are also discussed.     

Synthesis of Hf-modified aluminide coatings on Ni-base superalloys

With small additions of Reactive Elements (RE), such as hafnium (Hf), to aluminide coatings applied on aircraft turbine engine components, the adherence of the protective oxide scale to the coatings and the oxidation resistance of the coatings at high temperatures can be significantly improved. At SIFCO, Hf-modified aluminide coatings, Hf-Pt-modified aluminide coatings on Ni-base superalloys and the industrial-scale production of Hf-modified aluminide coatings on different aircraft turbine engine components were successfully developed. Surface morphology and cross-section microstructure of the developed coatings were inspected and compared by using a Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS), and Glow-Discharge Mass Spectrometry (GDMS). Experimental results showed that Hf was successfully incorporated and precipitated Hf-rich particles were observed on the coating surface, along grain boundaries within the additive layer, and at the interface between the additive layer and the interdiffusion layer. Finally, Hf-rich particles as well as Hf solubility are discussed.     

铁铝含量对Fe-Cr-Ni合金渗铝涂层及其防结焦性能的影响

采用固体粉末包埋渗铝法,在Fe-Cr-Ni合金表面制备渗铝涂层,并研究了其因渗剂中铁铝含量的不同对Fe-Cr-Ni合金渗铝、氧化及结焦行为的影响。通过对各组渗铝以及渗铝并氧化后的试样表面及截面微观形貌、相组织进行研究,并结合结焦增重曲线发现,铁铝比例的增加能够促使合金表面形成平整致密的渗铝涂层,提高氧化膜的质量,但也会降低渗层的厚度,影响表面氧化膜的连续性。选用合适的铁铝含量可以获得较高质量的氧化铝涂层,进而达到防结焦效果。     

Low temperature formation of aluminide layers on nanocrystalline nickel

The study of the formation of aluminide coating on nanocrystalline and bulk nickel indicated that on bulk nickel a single layer of Al₃Ni₂ phase was formed, whereas on nanocrystalline nickel a single layer of Al₃Ni₂ and double layers of Al₃Ni and Al₃Ni₂ phases were formed in short- and long-time aluminizing, respectively. The parabolic growth rate constant of single layer formed on nanocrystalline nickel was approximately 2–3 times greater than the bulk nickel.     

Chemical synthesis of micrometer-sized nickel aluminide powders

A new process has been developed to directly produce micrometer-sized fine Ni₃Al powders from a chloride chemical precursor. The reaction can be carried out in several media, including the fused-salt medium described in this study. The Ni₃Al powders produced from chloride precursors had an L1₂ structure and contained 0.5% excess aluminum over stoichiometry. Because of their small grain size, the particles may prove instrumental in improving the properties of intermetallic components, thus accelerating commercialization of nickel aluminide.     

Machinability of a nickel aluminide intermetallic alloy

This article reports the results of an experimental study on the machinability of a nickel aluminide intermetallic alloy. Machining was conducted at various speeds, and results indicate low material removal rates. Chips collected for each test run were measured for thickness for shear angle calculation and were subsequently observed microscopically. Chip segmentation was observed microscopically, and a fluctuation in the shear angle was evidenced. A parameter characterizing the severity of the machining process, the chip reduction coefficient (K), was calculated from chip thickness measurements. The calculated values ofK were found to be low, indicating a low severity of the metalcutting process. This, however, is in contract with the observed low rates of metal removal and low tool life. Thus, conventional metalcutting characterization parameters require re- examination in terms of machining high-strength materials.     

Iron aluminide coatings on ferritic steels by CVD-FBR technology

The deposition of Al on ferritic steels coupons by chemical vapor deposition in fluidized bed reactors (CVD-FBR) was studied. Before performing the experiments, thermochemical calculations were used to obtain the initial parameters of processes. These calculations were made by means of the Thermocalc software. The thermodynamic system studied consisted of Al powder as donor, HCl and H₂ as gaseous activators and Ar as fluidizing inert gas. The results indicate that the most important precursor aluminide chlorides formed in this system, in the temperature range from 400 to 600 °C, were AlCl₃, Al₂Cl₆, AlHCl₂ and AlCl. Aluminium diffusion coatings were obtained on ferritic steel (P-92). The effect of heat treatment under inert atmosphere was also been studied. Diffusion heat treatment was performed at 700 °C for 2 h in order to allow the phase transformation of Fe₂Al₅ to phases with lower aluminium content. Morphology and composition of coatings were characterized by different techniques, such as optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). The results are discussed in relation to the diffusion occurring during the heat treatment of Al coatings obtained at low deposition temperatures and short deposition times.     

Mechanisms of formation of diffusion aluminide coatings on nickel-base superalloys

Studies on microstructures and compositions of model diffusion aluminide coatings coupled with existing information on diffusion in nickel-aluminum intermetallic compounds indicate that only two basic types of these coatings can be formed on nickel-base superalloys. The first type is formed by inward diffusion of aluminum from coating media of sufficiently high aluminum activity to cause formation of the (Ni₂Al₃) phase. Subsequent stabilizing heat treatments required for practical use of this type of coating cause some outward diffusion of nickel to form a three-zone (NiAl) coating. The second type is formed by outward diffusion of nickel from the substrate alloy to react with aluminum of suitable activity to stabilize that (NiAl) phase of composition through which only nickel motion can occur. This process produces a two-zone (NiAl) coating. It follows that the structural type of coating to be expected from any given coating process can, in principle, be predicted from knowledge of the aluminum activity present during formation of the coating. The initial stages of diffusional degradation of these coatings involve continued outward diffusion of nickel from the substrate through the (NiAl) coating with the resultant dilution of aluminum content of the coating; little inward diffusion of aluminum occurs.     

Formation and characterization of aluminide coatings on alloy 800 substrate

Fe-Ni-Cr based Alloy 800 substrates were pack aluminized at 1273 K for 8 hours in argon atmosphere. The cross-section of aluminized substrates indicated formation of multilayer of aluminides as revealed by microscopy, electron probe microanalysis and X-ray diffraction. The outermost layer (~ 150 μm) was found to consist of FeAl + Fe₂Al₅ type phases, while the adjoining layer (~ 250 μm) was composed of an FeAl type phase. The innermost layer (~ 60 μm), which was a solid solution zone, was found to consist of ~ 43 at.% Fe, 38 at.% Cr, 11 at.% Ni containing about 6 at.% Al. In microhardness test, a hardness variation of 213 to 1098 Knoop hardness number along the cross-section was obtained. Scratch test along the cross-section at load levels ranging from 0.9 to 10 N with a loading rate of 30 N/min showed a maximum penetration depth of 12 μm indicating a good adherence of aluminide coatings.     

Creep and corrosion testing of aluminide coatings on ferritic-martensitic substrates

Pack and chemical vapor deposited (CVD) aluminide coatings on commercial ferritic-martensitic Fe-9Cr-2W steel are being investigated by creep and corrosion testing at 650 °C. Results from different coating thicknesses show that the coated region makes no contribution to the creep strength. The creep behavior of uncoated material was studied after various heat treatments to simulate the coating process and typical secondary heat treatments. Alternating creep and corrosion exposures showed little effect on the creep strength of uncoated material but coated materials became progressively weaker. The coatings were protective in wet air at 650 °C after creep testing.