Sulfur Segregation at Al2O3/γ-Νi + γ′-Ni3Al Interfaces: Effects of Pt, Cr and Hf Additions

The interfacial chemistry that developed as a result Al₂O₃-scale growth on γ-Νi + γ′-Ni₃Al alloys at 1150 °C was studied using scanning Auger microscopy after the oxide layer was scratched to spall under ultra-high vacuum. The extent of scale spallation was used to evaluate semi-quantitatively the interfacial strength. The alloys investigated were primarily γ′ in structure, containing 22 at.% Al plus further additions of Pt, Cr and/or Hf. In the case of the binary γ + γ′ alloy, it was found that a sub-monolayer of sulfur segregated at the alloy/scale interface. Platinum reduced and hafnium eliminated sulfur segregation, but chromium enhanced it through Cr–S co-segregation, even on Pt- and Hf-containing alloys. Platinum also segregated slightly at the alloy/scale interface. The interface strength was a strong function of the sulfur content. Beyond the effect of eliminating S segregation, Pt and Hf both showed additional beneficial effects on alumina scale adhesion.     

The effect of gas composition and contaminants on the lifetime of surface‐treated FeCrAlRE alloys

The degradation of a variety of alumina‐forming Fe20Cr5Al based alloys has been investigated at temperatures ranging from 1100°C to 1300°C for up to 4000 h (100 h/cycle) in different oxidising environments such as laboratory air, air + 10 vol% H₂O, air + 60 vol% H₂O and simulated automotive exhaust gas. Seven model alloys with controlled levels of impurities such as P, S and C and carefully controlled levels of additional elements (Y, Zr, Ti, Hf, Si, La, etc.) and two different commercial alloys (Aluchrom YHfAl and Kanthal APMT) were chosen for this study. The investigation showed that the model alloys containing La, Y + Si and Y with added C, and the commercial alloy APMT usually had the lowest initial oxidation growth rates, whereas model alloys containing Y plus Zr, and the commercial alloy YHfAl had the higher oxidation rates regardless of the different oxidising environments. Scale spallation was more prevalent in the case of alloys with low oxide growth rates but changing the levels of water vapour in the oxidising atmospheres had only a minor influence on the degree of spallation or the oxide morphology. The scales formed on the alloys containing La, Y + Si and high C spalled in an adhesive manner (at the scale/metal interface), whereas scales formed on alloys containing Y plus Hf, Zr or Ti cracked and spalled in a cohesive manner (within the scale). Inhomogeneities in the distribution of the alloying additions led to greater changes in the oxide morphologies than any difference in oxidising atmosphere, but the crystallographic textures of all the oxides were similar. This pilot study enabled us to rank the alloys according to their resistance to spallation and also to determine the influence of minor elements when added as tightly controlled single or multiple element additions. Some of these alloys were then used as mechanically weak PVD coatings on a strong (APMT) substrate, and further oxidation experiments confirmed that the coatings then oxidised in a similar way to the bulk alloys.     

Effect of Hf and Y alloy additions on aluminide coating performance

Iron- and Ni-base alloys, with and without Hf or Hf and Y alloy additions, were aluminized by chemical vapor deposition to study the potential for minor alloy additions to improve oxidation resistance of coated alloys. Compared to uncoated specimens, the coated specimens showed improved cyclic oxidation resistance at 1100° and 1150 °C. However, alumina scale spallation was observed at relatively short times and, particularly for the Ni-base alloy X, the aluminized lab-cast alloy with Hf tended to have poor coating performance compared to the commercial alloy without Hf. Internal oxidation of Hf at 1150 °C and rapid Al depletion in the relatively thin aluminide coatings contributed to the observed detrimental Hf effect. For the Ni-base alloys, the increased scale spallation could be attributed to much higher S contents (10-50 ppma) in the laboratory-cast alloys. Oxide scale spallation from the coating surface was minimized when Hf and Y were added to a casting and the [Y]/[S] content ratio was ∼ 1.     

Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part II: Cyclic oxidation tests

Several routes of yttrium introduction were applied to test the high temperature oxidation performance of a FeCrAl alloy. Isothermal oxidation tests were described in a previous paper (Part I of this paper in this journal, 2004, 55, 352). Cyclic oxidation tests were performed in air under atmospheric pressure on blank specimens, Y₂O₃ sol‐gel coated‐, Y₂O₃ metal‐organic chemical vapor deposited (MOCVD)‐, yttrium ion implanted‐alloys, as well as on a steel containing 0.1 wt. % of yttrium as an alloying element. For the 20 hours cycles, all the samples, except FeCrAl‐0.1Y, exhibit weight losses after a few cycles, indicating drastic spallation of the oxide scales. The MOCVD coated specimen has the highest weight loss. The oxidation kinetics of the FeCrAl‐0.1Y alloy obey a parabolic law, indicating that the alumina scale formed on its surface is protective even after more than 1200 hours of oxidation (> 50 cycles). The 100 hours cycle oxidation tests give similar results. The FeCrAl‐0.1Y alloy exhibits the best oxidation behavior with very little spallation after more than 2000 hours (85 days) of oxidation at 1100°C (20 cycles). Most of the other samples exhibit severe oxide scale spallation followed by an increase of their oxidation rate related to the formation of non‐protective iron oxides.     

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti–34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al₂O₃ barrier being formed. This is probably due to the doping of those elements in TiO₂. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al₂O₃ layer as the second layer from the outer surface and the relatively massive precipitation of Al₂O₃ in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.     

Atomic site location by channelling enhanced microanalysis (ALCHEMI) in γ′-strengthened Ni- and Pt-base alloys

The additions of alloying elements to Ni- and Pt-base alloys influence the microstructure and thereby the creep properties, whereas the mechanism is uncertain. Therefore atomic site location by channelling enhanced microanalysis (ALCHEMI) was used to determine the site partitioning of ternary and quaternary alloying elements in the L1₂-ordered γ′-phase. Two ternary Ni–Al alloys with Cr and Ti additions were investigated. The measured site partitioning showed that Cr and Ti atoms prefer the Al-sublattice sites. For a ternary Pt–Al–Cr alloy, it was found that Cr atoms occupy Al sites. The influence of Ni as a fourth alloying element in a Pt–Al–Cr–Ni alloy on the site partitioning was also investigated. The detected results give evidence that in the quaternary alloy Cr and Ni atoms prefer the Pt sublattice. First principles calculations were used to support the experimental data.     

The oxidation behavior of CoCrAI systems containing active element additions

The effect of small amounts of yttrium (up to 1 wt. %) and hafnium (up to 1.5 wt.%) on the oxidation behavior of Co-Cr-Al alloys in the temperature range 1000–1200°C for times up to 1000 hr in air has been studied. The major portion of the study has been concerned with Co-10Cr-11Al base alloys. Both isothermal and cyclic tests have been carried out; the cycle used consisted of 20 hr at temperature, followed by cooling to room temperature. Both additions reduce the overall oxidation, Hf somewhat more so than Y. In part, this is due to the improved adhesion between scale and alloy reducing scale spallation at temperature, and in part due to possible modification of the Al₂O₃ grain size. The former factor is far more critical under thermal cycling conditions. Under isothermal conditions the oxidation rate increases with increasing Hf content with all but the 1.5 wt.% alloy oxidizing more slowly than the Hf-free alloy; increase in Y content has the reverse effect. Under thermal cycling conditions the 0.3 and 1.0 wt.% Hf alloys show the lowest overall weight gain. Metallographic evidence suggests that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al₂O₃ into the alloy. However, if the intrusions are too large, they can act as initiators of scale failure.     

Influence of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ni–Al Alloys

Ni–Al coating alloys, which are commonly used in gas turbine engines operating in marine environments, are highly susceptible to hot corrosion attack. The effect of alloy composition and exposure conditions on the development of a protective alumina scale, which is important for the hot corrosion resistance of the alloy, and how they affect the transition of alumina from the θ to the α polymorph have been evaluated. A series of Ni–Al model alloys with a base composition of Ni–36 at.% Al, and 5 at.% additions of Cr, Pt and Si were exposed in dry air and in air–10%H₂O at 900 °C. The presence of water vapor in the gas led to higher oxidation rates and retarded the θ- to the α-Al₂O₃ transformation. The oxidation behavior of the alloys and the alumina polymorph which formed differed depending on the alloying element considered. Additions of Cr accelerated the θ to α transformation, while Pt and Si retarded it.     

Formation of Ni-aluminide Coating Containing Hf by Molten-Salt Electrodeposition and Cyclic-Oxidation Resistance

The formation of a coating layer consisting of Ni-aluminide containing Hf on a Ni–10at.%Cr–8at.%Al alloy substrate was attempted by the electrodeposition of Hf, Ni, and Al. The cyclic-oxidation resistance for the alloy covered with this coating was then evaluated in air at 1,423 K. Ni was deposited by aqueous solution electrolysis. Hf and Al were deposited by molten-salt electrolysis. For the sample first treated with the Hf-deposition, subsequently treated with the Ni-deposition, followed by Al-deposition, a coating consisting of Ni₂Al₃ of about 40-μm thickness was uniformly formed on the alloy. At the center region of this coating, a Hf-concentration layer was formed. The cyclic-oxidation test showed that, for the untreated sample and the sample with only Ni and Al depositions, a mass reduction due to spallation of a scale took place during the initial oxidation period. On the contrary, for the sample with Hf, Ni, and Al depositions, a mass reduction due to spallation of a scale scarcely took place. The cross-sectional observation using SEM showed that, for the sample with Hf, Ni, and Al depositions after the oxidation test, an adhesive scale having a spiked shape was formed. This scale mainly consisted of α-Al₂O₃, and contained HfO₂ particles. It was postulated that, for this sample, the Hf in the Hf-concentration layer diffused into the surface region of the Ni-aluminide layer, contributing to improvement in the exfoliation resistance of the scale.     

Effects of alloying elements on thermal expansions of γ-Ni and γ′-Ni3Al by first-principles calculations

The effects of alloying elements (Al, Cr, Hf, Pt, Y, and Zr) on the coefficients of thermal expansion (CTE) of face-centered-cubic Ni (γ) and L1₂-Ni₃Al (γ′) phases have been investigated in terms of the first-principles quasi-harmonic approach by considering both the vibrational and thermal electronic contributions. By combining the present CTE data and the compositions and phase fractions of γ and γ′, the CTE for some Ni-Al-base alloys are predicted and compared with available experimental measurements in the literature, showing good agreement. It is observed that the addition of Pt is effective in reducing the CTE of both γ and γ′ phases, resulting in lower expansion mismatch between the bond coat and the thermally grown oxide alumina layer, as shown by the predicted CTE of Ni-22 Al-x Pt (at.%) alloys (x = 5, 10, 15, 20, 25, 30).     

Influence of small Pt additions on Al2O3 scale adherence

The effects of small Pt additions (1 or 3 wt.%) on the oxidation behavior of Co-10Cr-11Al and a similar alloy containing Hf have been studied. An intermetallic phase was present in the alloy containing Hf and Pt but not in that containing Pt alone. The size and distribution of the intermetallic was comparable to that of similar alloys containing oxide dispersions produced by a controlled internal oxidation treatment. As a consequence it promoted the formation of inwardly growing Al₂O₃ pegs that helped key the surface scale to the substrate and improve the scale-metal adhesion in both isothermal and cyclic oxidation tests. The improvement in overall oxidation resistance relative to an addition-free alloy was considerable, and similar to that of the best oxide dispersion-containing alloys.Formerly Department of Metallurgy and Materials Science, University of Liverpool, Liverpool, England.     

Thermal cycling behavior of EBPVD TBC systems deposited on doped Pt-rich γ–γ′ bond coatings made by Spark Plasma Sintering (SPS)

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1 h cycles at 1100 °C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based systems.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Synthesis and oxidation behavior of platinum-enriched γ + γ′ bond coatings on Ni-based superalloys

Simple Pt-enriched γ + γ′ coatings were synthesized on René 142 and René N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment at 1150-1175 °C. The Al content in the resulting γ + γ′ coating was in the range of 16-19 at.% on superalloys with 13-14 at.% Al. After oxidation testing, alumina scale adherence to these γ + γ′ coatings was not as uniform as to the β-(Ni,Pt)Al coatings on the same superalloy substrates. To better understand the effect of Al, Pt and Hf concentrations on coating oxidation resistance, a number of Ni-Pt-Al cast alloys with γ + γ′ or β phase were cyclically oxidized at 1100 °C. The Hf-containing γ + γ′ alloys with 22 at.% Al and 10-30 at.% Pt exhibited similar oxidation resistance to the β alloys with 50 at.% Al. An initial effort was made to increase the Al content in the Pt-enriched γ + γ′ coatings by introducing a short-term aluminizing process via chemical vapor deposition or pack cementation. However, too much Al was deposited, leading to the formation of β or martensitic phase on the coating surface.     

Effect of Cr,Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

The oxidation behavior of Ni₃Al+2.90 wt.% Cr, Ni₃Al+3.35 wt% Co, and Ni₃Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni₃Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni₃Al alloys was the existence of small -Al₂O₃ particles in the middle of the -Al₂O₃ scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.     

Elemental partitioning of platinum group metal containing Ni-base superalloys using electron microprobe analysis and atom probe tomography

The partitioning of platinum group metal (PGM) additions in polycrystalline Ni-base superalloys has been investigated. Alloys with a baseline composition of 15Al–5Cr–1Re–2Ta–0.1Hf (at.%) with systematic variations in Pt, Ir, Ru and W were cast and heat-treated to produce bimodal two-phase γ–γ′ microstructures. Electron microprobe analysis was used to measure the composition of coarse γ′ particles. Selected alloys were studied using atom probe tomography. Pt and Ru weakly partition to the γ′ and γ phases, respectively, whereas Ir partitions equally between both phases. Pt and Ru do not change partitioning behavior appreciably with additional W and Re. Interestingly, Ir lowers γ′ volume fraction and reduces partitioning of W, Re, Ru, Al and Pt. Tungsten partitioning coefficients exhibit an unusually strong temperature dependence, with increased partitioning of W to the matrix at lower temperatures. The thermodynamics of these PGM-containing systems are analyzed and the implications for partitioning are discussed.     

Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H₂O was investigated. The oxidation kinetics of Ni and the alloys in air + H₂O were very similar, but the mass gains of Ni and each alloy were smaller in air + H₂O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al₂O₃ precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H₂O. The NiO scale formed in air + H₂O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H₂O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H₂O.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

Effects of Hf additions on high-temperature mechanical properties of a directionally solidified NiAl/Cr(Mo) eutectic alloy

In order to improve the high-temperature mechanical properties of NiAl/Cr(Mo) alloys, the effects of Hf additions on microstructure and mechanical properties were systemically examined. Two directionally solidified alloys with composition of Ni-32Al-28Cr-(6−x)Mo-xHf (x = 0.2 and 0.5 at.%, respectively), named as 0.2Hf and 0.5Hf hereafter, were prepared. The Hf additions disturbed the cellular structure. The 0.2Hf alloy consisted of dendritic structure, while the 0.5Hf alloy had an intercellular structure. In the 0.5Hf alloy, the Ni₂AlHf and Ni₁₆Hf₆Si₇ precipitates were also confirmed. The high-temperature strength and brittle-to-ductile transition temperature (BDTT) increased with increasing of Hf additions, due to the different strengthening mechanism. In contrast, the ductility and creep resistance decreased with increasing of Hf because of the disturbance of cellular structure.     

Development of Re‐based diffusion barrier coatings on nickel based superalloys

A diffusion barrier type coating with a duplex layer structure, an inner σ‐(Re, W, Cr, Ni) as a diffusion barrier and outer Ni‐aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS‐82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room temperature and 1423 K for up to 360 ks. The inner σ layer with a composition (at%) of (35–40) Re, (15–20) W, (15–25) Cr and (15–25) Ni was produced by electrodeposition of Ni‐70Re and Ni‐20W films from aqueous solutions followed by Cr‐pack cementation at temperatures between 1473 and 1573 K, and the outer Ni‐aluminides of β‐(Ni,Cr)Al + γ′‐(Ni,Cr)₃Al was formed by electrodeposition of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both σ layer and alloy substrate were retained with little change. Furthermore, there was little Al in the σ layer. It could be concluded that the Re‐based alloys such as σ (Re(W),Cr,Ni) are very promising candidates as a diffusion barrier between the outer Al‐reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)‐Cr‐Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.