Secondary Reaction Zone Research Articles

Suppressing the formation of SRZ in a Ni-based single crystal superalloy by RuNiAl diffusion barrier

NiAl-based bond coats applied in thermal barrier coating (TBC) systems containing Ru and Pt have been studied in recent years. Interdiffusion between the coatings and the underlying superalloy substrates usually results in degradation of the mechanical properties of the substrates. In this paper, a NiAl/Ru coating was deposited onto a single crystal superalloy DD6 by electroplating and electron beam physical vapor deposition (EB-PVD). Interdiffusion behavior of the NiAl/Ru and NiAl coated specimens at 1100°C was comparatively investigated. For the NiAl coated specimen, after 100h vacuum annealing, beneath the coating there was a 50μm thickness secondary reaction zone (SRZ) with some needle-like topologically closed packed (TCP) phases. However no SRZ was observed in the NiAl/Ru coated specimen. This may be attributed to the RuNiAl that acted as a diffusion barrier to prevent the inward diffusion of Al from the coating and the outward diffusion of refractory elements from the substrate, and hence suppressing the formation of TCP phases and SRZ in the superalloy.

Understanding the role of sapwood loss and reaction zone formation on radial growth of Norway spruce (Picea abies) trees decayed by Heterobasidion annosum s.l.

Allocation of photosynthates to defence responses at the expense of biomass increase is a common strategy amongst plants to cope with stress factors. Trees reduce the spread of decay by creating a secondary metabolite-rich reaction zone as fungal ingresses the sapwood. Reaction zone formation implies a sacrificial conversion of sapwood, thus, as decay progresses, the sapwood area of the tree is reduced. The relative contribution that reaction-zone formation and sapwood loss make to radial growth decrease is unclear. To answer this question we reconstructed radial-growth patterns in 100 Norway spruce (Picea abies) trees with a range of reaction zone and sapwood disruption. Basal area increment (BAI) between 1960 and 2007 and its relationship with sapwood reduction and reaction zone formation was assessed using structural equation models (SEM). BAI data showed that over 10years, trees with small or no decay columns (<40%) and a reaction zone shifted from a growth rate that was similar to trees without a reaction zone towards low growth rate similar to trees with large decay columns. The fitted SEM indicated that: (i) the effects of decay on growth would begin with the formation of the reaction zone, and (ii) the smaller sapwood in decayed trees as compared with healthy trees would not reduce radial growth, but would be in part the result of previous periods of low growth due to reaction-zone formation.

アルミナイズを施した Ni 基単結晶超合金の加熱履歴に伴う微細組織変化

The effect of the thermal history on microstructural changes in aluminized and Pt-aluminized Ni-based single-crystal superalloys was investigated. The superalloy substrates were first electropolished to get rid of the residual surface strain. Then, Pt was electrodeposited and vacuum annealing was conducted for some of the substrates, and aluminized and Pt-aluminized specimens were prepared using the conventional aluminizing process. It was found that in the aluminized specimens, voids were formed in the vicinity of the substrate/coating interfaces during thermal cyclic heating, whereas secondary diffusion zones (SDZ) were formed by isothermal heating. These different microstructural changes of the aluminized specimens can be explained by the kinetics of diffusion between the coating layer and the substrate during the heating/cooling processes. In the Pt-aluminized specimen, on the other hand, secondary reaction zone (SRZ) formation was observed after both thermal cyclic and isothermal treatments. These results can be explained by the polycrystallization of the substrate surface during the annealing process, which promotes interdiffusion, resulting in the formation of an SRZ. [doi:10.2320/matertrans.M2013205]

Effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys

An effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys was investigated. Aluminized fourth generation alloys, having different amounts of Cr in substrates, were examined at 1100°C up to 300h. After aluminizing, all alloys exhibited similar phase constitutions, however, they showed distinctive morphologies of secondary reaction zones (SRZs): an alloy containing the highest Cr content developed continuous SRZs under an inter-diffusion zone (IDZ), whereas the other alloys hardly developed SRZ formation. The highest Cr containing alloy particularly exhibited higher coverage of SRZ after aluminizing, and this suggests higher Cr in substrate encourages SRZ formation. All alloys developed more SRZ formation after 300h, and higher Cr containing alloys precipitated more topologically close-packed (TCP) phases in substrate. All alloys demonstrated increases of SRZ coverage, whereas penetration depth of SRZ in each alloy was not significantly different after 300h. Higher Cr containing alloys precipitated TCP phases in substrate after 300h, and this prevents further growth of SRZ.

Secondary Reaction Zone Formations in Pt-Aluminised Fourth Generation Ni-Base Single Crystal Superalloys

Fourth generation superalloys are characterised by the addition of Ru which contributes to improved creep resistance whilst improving the microstructural stability. However, Ru additions have a negative effect on coated Ni-base superalloys, promoting Secondary Reaction Zone (SRZ) formation. Formation of a layer of SRZ beneath an aluminised or Pt-aluminised coating has the potential to reduce the effective cross section of a blade by in excess of 100 μm or 10% of the wall thickness. In this paper the effects of alloy composition on the formation of the SRZ in Pt-Aluminised fourth generation alloys were investigated systematically. A series of experimental fourth generation alloys was used having two distinct compositions of Co, Mo, W and Ru and conforming to a four factorial 'Design of Experiments' model. These alloys showed significant and consistent changes in the SRZ depending on alloy composition. These were in distinct contrast to the effects of these elements on stability in the bulk. Mo was demonstrated to be by far the most effective element suppressing SRZ formation, followed by Co. In contrast, both W and Ru enhance SRZ formation.

Precipitation phases in the nickel-based superalloy DZ 125 with YSZ/CoCrAlY thermal barrier coating

Interdiffusion behavior of the thermal barrier coating (TBC) with the CoCrAlY bond coat (BC) and directionally solidified Ni-based superalloy DZ 125 was investigated. Severe inward-diffusion of Al, Co and Cr from the BC to the superalloy and outward diffusion of Ni and refractory elements such as W from the superalloy occur during annealing at 1050 °C in air. After 100 h annealing, a ∼30 μm thick inter-diffusion zone (IDZ) forms between the BC and superalloy, and a ∼35 μm thick secondary reaction zone (SRZ) forms beneath the IDZ. The IDZ mainly consists of β phase and γ matrix. Besides, small amount of Ta and Hf containing carbides are also observed in the IDZ. Needle and fine granular topologically close-packed (TCP) phases, characterized as rhombohedral μ phases, are abundant in the SRZ. The formation of SRZ is mainly due to the precipitation of refractory elements such as W and Mo from the γ matrix and β phase. The formation mechanism of SRZ and μ-TCP phase is discussed.

Cyclic oxidation and interdiffusion behavior of a NiAlDy/RuNiAl coating on a Ni-based single crystal superalloy

A novel NiAlDy/RuNiAl coating was deposited onto a Ni-based single crystal superalloy DD3. The cyclic oxidation and interdiffusion of the NiAlDy/RuNiAl coating and a NiAlDy coating at 1100 °C were investigated. For the NiAlDy coating, secondary reaction zone (SRZ) consisting of γ′, γ and σ-topologically-closed-packed phase (σ-TCP) needles was formed in the superalloy after 100 h annealing. The NiAlDy/RuNiAl coating effectively suppressed the formation of SRZ as the RuNiAl layer worked as a buffer layer. Premature scale spallation was observed on the NiAlDy coating after about 80 h cyclic oxidation, whereas the NiAlDy/RuNiAl coating exhibited improved oxidation-resistance property.

Processing and microstructure of NiRuAl diffusion barrier coating on Ni-based superalloy

Thermal barrier coatings (TBCs), consisting of ceramic topcoat and metallic bond coat, are applied onto Ni-based superalloy hot components in turbine engines to protect the components from high temperature gas. Detrimental phases in the superalloy substrate such as topologically close-packed phase (TCP) and secondary reaction zone (SRZ), resulting from interdiffusion between the bond coat and the substrate, significantly deteriorate the desired mechanical properties of the superalloy substrate. In this paper, a NiRuAl coating was produced onto Ni-based superalloy to inhibit the interdiffusion between the coating and substrate. During the processing, a dense and continuous Ru film of around 4 μm thickness was first electrodeposited onto a K3 superalloy at 70°C using a current density of 1·2 A dm−2. The NiRuAl coating was formed by pack aluminisation of the Ru coated specimen in argon atmosphere at 900°C. The coating shows a two-layered structure: the top layer NiAl and the bottom layer NiRuAl, with the potential functions of diffusion barrier and high temperature oxidation resistance.

Effect of H2S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H2S/O2 under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ=0.5), stoichiometric (Φ=1.0), to fuel-rich (Claus condition, Φ=3.0) are examined. The results revealed that the presence of H2S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H2 and H2S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH3 radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S8 (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S2) during its condensation at low temperatures.

Ni–W diffusion barrier: Its influence on the oxidation behaviour of a β-(Ni,Pt)Al coated fourth generation nickel-base superalloy

A Ni–W base diffusion barrier (DB) has been developed to limit interdiffusion between a fourth generation Ni-base superalloy (MCNG) and a Pt-modified nickel aluminide bondcoat. After long term oxidation, the DB layer permits to reduce the Al depletion in the coating and to delay the phase transformations in the coating. But despite this result, the oxidation behaviour of the system with DB is slightly worse than without the DB. This difference may be caused by the addition of S and/or W in the coating of the system with the DB. The DB layer also delays the Secondary Reaction Zone (SRZ) formation. Nevertheless, the propagation of the SRZ is similar in systems with and without a DB, with growth kinetics which are driven by interdiffusion.

Secondary Reaction Zone Formations in coated Ni-base Single Crystal Superalloys

Ruthenium (Ru) has been added to the latest 4th Generation Ni-base superalloys to improve phase stability and modify creep life. Various coatings are routinely applied to these advanced alloys to protect the turbine blade at elevated temperature, however, this creates several problems such as the precipitation of brittle Topologically Close-Packed (TCP) phases and the formation of Secondary Reaction Zones (SRZ). The SRZ forms under the plat-aluminized coating of turbine blades and consists of γ, γ and TCP phases growing into substrate by the migration of high-angle grain boundaries. Surface residual stress and chemical super-saturation of alloying elements are associated to SRZ formation. In the thin sections of high-pressure turbine blades this is critical in determining blade performance and longevity. It is essential to know how Ru additions affect coating and SRZ morphologies during exposure. In this study, we focus on the effects of three variables on the SRZ formation: Ru concentration, alloy composition in Ru-containing alloys and surface finish. A series of Platinum-Aluminised superalloys containing 2–5wt% Ru and having various surface finishes was studied after isothermal exposure at 1100°C for up to 500h. The alloys were classified into two groups by their distinctive SRZ morphology. At the lowest Ru levels sporadic formation of SRZ was observed, whilst a continuous SRZ was formed in the higher Ru alloys. EBSD analysis revealed that the latter group have a higher nucleation rate of individual SRZ grains and also showed more rapid SRZ growth. The precipitation of TCPs in the substrate also inhibited the growth of the SRZ towards the end of the exposure further reducing the penetration of the SRZ into the substrate. It is concluded that Ru-additions to Ni-base superalloys are effective in impeding TCP phase formation in the substrate, but increase both the extent and the rate of SRZ formation beneath coating.

Formation of secondary reaction zone in ruthenium bearing nickel based single crystal superalloys with diffusion aluminide coatings

Several Ru bearing experimental single crystal superalloys were coated with β-NiAl and Pt–Hf modified γ–γ′ coatings and subjected to elevated temperature exposure. The microstructure, oxidation resistance and the propensity for the formation of secondary reaction zones (SRZ) were investigated for these coatings. The presence of significant amounts of Ru in the superalloys did not prevent the cellular transformation leading to the formation of SRZ beneath the β-NiAl coating. Cyclic oxidation exposure of the β-NiAl coated alloys at 1100°C led to a significant further growth of the SRZ. In contrast, the γ–γ′ coating did not induce the undesirable cellular transformation, even after prolonged high temperature exposure. The γ–γ′ coating also provided good oxidation resistance to the superalloys. The driving forces for SRZ formation in case of the β-NiAl coating as compared to the γ–γ′ coating are discussed.

A concept for the EQ coating system for nickel-based superalloys

Nickel-based single-crystal superalloys with high concentrations of refractory elements are prone to generate a diffusion layer called a secondary reaction zone (SRZ) beneath their bond coating during long exposure to high temperatures. The SRZ causes a reduction of the load-bearing cross section and it is detrimental to the creep properties of thin-walled turbine airfoils. In this study, a new bond coat system, “EQ coating,” which is thermodynamically stable and suppresses SRZ has been proposed. Diffusion couples of coating materials and substrate alloys were made and heat treated at 1,100°C for 300 h and 1,000 h. Cyclic oxidation examinations were carried out at 1,100°C in air and the oxidation properties of EQ coating materials were discussed. High-velocity frame-sprayed EQ coatings designed for second-generation nickel-based superalloys were deposited on fourth-and fifth-generation nickel-based superalloys, and the stability of the microstructure at the interface and creep property of the coating system were investigated.

Anisotropy of secondary reaction zone formation in aluminized Ni-based single-crystal superalloys

Distribution of the secondary reaction zone (SRZ) on the (1 0 0) plane was investigated in the aluminized Ni-based single-crystal superalloy TMS-75. Preferential formation of the SRZ was observed along the {0 0 1}〈1 1 0〉 directions, whereas the SRZ was not formed along the {0 0 1}〈1 0 0〉 directions. This result can be explained by the anisotropy of recrystallization in the single-crystalline substrate.

Formation of Secondary Reaction Zones in Diffusion Aluminide-Coated Ni-Base Single-Crystal Superalloys Containing Ruthenium

The formation of secondary reaction zones (SRZs) beneath aluminide coatings in several Ru-bearing single-crystal Ni-base superalloys has been investigated. The presence of significant amounts of Ru in the superalloys did not prevent the formation of the secondary reaction zone. However, the Ru content of the alloys affected the type of refractory element-rich phase formed during the transformation. As the Ru content increased, the phase involved in the transformation shifted from the orthorhombic P to the β-RuAl phase. A differential tendency to SRZ formation was observed between the dendritic and interdendritic regions of the alloys. Significant growth of the SRZ was also observed during the high-temperature oxidation exposure of the coated alloys. The effects of the alloying elements on SRZ formation are discussed.

Pt 添加 &amp;beta;-NiAl コーティング超合金の高温酸化に対するRe-W-Cr-Ni 系 Diffusion Barrier の効果

The oxidation behavior of a fourth generation Ni-based single crystal superalloy with diffusion barrier coating system was investigated under thermal cycling conditions at 1100°C. The coating consisted of outer Pt-modified β-NiAl with and without inner σ- Re-W-Cr-Ni diffusion barrier. After 100 one-hour oxidation cycles β-NiAl was retained on the diffusion barrier coated sample. However, in the non-barrier Pt-modified β-NiAl sample the Al reservoir changed to γ′-Ni3Al. Also without the diffusion barrier a secondary reaction zone (SRZ) formed extensively. The σ- Re-W-Cr-Ni diffusion barrier proposed in this study effectively suppressed Al and Pt diffusion toward the substrate, and also reduced diffusion of alloying elements into the Al reservoir.

溶射 EQ コーティングの組織安定性および機械的特性

EQ coating, which is a new and stable bond coat system, suppresses the diffusion of alloying elements between the substrate and coating and the formation of secondary reaction zone (SRZ). In this study, high velocity frame sprayed (HVOF) EQ coatings designed for 2nd generation Ni-base superalloy were deposited on 4th and 5th generation Ni-base superalloys. The stability of microstructure at the interface and creep property of the coating system were investigated. Less than 50 μm-thick of the interdiffusion zone and no SRZ were observed in EQ coating system after 300 h heat treatment at 1100°C, in contrast to beyond 90 μm-thick of diffusion zone and 140 μm-thick of SRZ in the conventional CoNiCrAlY coating system. Creep strength of Al-diffusion and CoNiCrAlY coated 4th generation superalloys showed decrease in thin creep specimens, but EQ coated 4th generation superalloy showed equivalent creep strength for bare material.

Development of EQ coating for new TBC system in Ni based superalloys

Abstract Ni based single crystal superalloys containing high concentrations of refractory elements are prone to generation of a diffusion layer called secondary reaction zone (SRZ) beneath their bond coating during exposure at high temperatures. The SRZ causes a reduction of the load bearing cross-section and is detrimental to the creep properties of thin wall turbine airfoils. In the present study, a new bond coat system, 'EQ coating' which is stable and suppresses formation of the SRZ is proposed. The characteristic of EQ system is that the coating stays in equilibrium state and never reacts with the substrate. Diffusion couples of coating materials and substrate alloys were made and were heat treated at 1100°C for 300 and 1000 h. The concentration profiles of alloying elements in these diffusion couples were analysed by electron probe microanalyser to investigate the existence of the diffusion zone. Cyclic oxidation examinations were carried out at 1100°C in air and the oxidation properties of EQ coati...

Development of New Bond Coat System in Ni-Base Alloys

Ni-base single crystal (SC) superalloys containing high concentrations of refractory elements are prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new coating system – “EQ coating”, which is in thermodynamic equilibrium with the substrate, has been proposed and the formation behavior of SRZ beneath bond coat materials was investigated on the 5th generation Ni-base SC superalloy developed by NIMS. Diffusion couples of several alloys were made and were heat treated at 1100°C for 300 h, 1000 h. The concentration profiles were analyzed by EPMA. Also, cyclic oxidation tests were carried out at 1100°C in air.

Isothermal Oxidation of Pt Modified and Ru Modified Aluminide Coating on a Fourth Generation Single Crystal Superalloy

Isothermal oxidation behavior of a 4th generation Ni-base single crystal superalloy with Pt-modified and Ru-modified aluminide coating was examined in a temperature range 1223 to 1373 K in air. Both Pt and Ru modification improve the oxidation resistance of a simple aluminide coating, especially above 1273 K. They allow thin protective and continuous Al2O3 scales to be intact for at least 500 h at temperatures up to 1323 K. However, the Pt modification drastically accelerates the formation of a secondary reaction zone (SRZ). This suggests that Pt promotes the formation of a topologically close-packed phase by lowering the solubility of refractory elements in γ-Ni. In contrast, the Ru modification reduces the SRZ, and is expected to enhance the phase stability under the coating by preventing the depletion of Ru due to its outward diffusion.