Characterisations of HVOF sprayed NiCrBSi coatings on Ni- and Fe-based superalloys and evaluation of cyclic oxidation behaviour of some Ni-based superalloys in molten salt environment

Microstructure plays a predominant role in determining material behaviour. Increasing microstructure uniformity has long been considered a fruitful means of improving thermal, chemical and mechanical properties of the materials. High velocity oxy-fuel (HVOF) is one of the emerging technologies among the thermal spraying techniques, for producing uniform and dense coatings, having high hardness and good adhesion values. In this study, HVOF technique was used to deposit NiCrBSi coatings, approximately 250-300 μm thick, on the Ni- and Fe-based superalloys for hot corrosion applications. The coatings were characterised in relation to coating thickness, porosity, microhardness and microstructure. The hot corrosion behaviour of the coatings deposited on nickel-based superalloys after exposure to molten salt (Na₂SO₄-60% V₂O₅) at 900 °C under cyclic conditions was also studied. The techniques used in the present investigation include X-ray diffraction, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and electron probe microanalysis (EPMA). The thermogravimetric technique was used to establish kinetics of corrosion. The structure of the as sprayed NiCrBSi coating mainly consisted of γ-nickel solid solution containing small fraction of Cr₇C₃ and Ni₃B phases. Very weak peaks of NiCr₂O₄ spinel oxides were also formed during spraying of the coatings. Some porosity (less than 1.4%) and inclusions were observed in the structure of the coatings. Coating microhardness values were found to be in the range of 750-930 Hv (Vickers Hardness) on different substrates. The NiCrBSi coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. The hot corrosion resistance imparted by NiCrBSi coatings may be attributed to the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromium.     

Evaluation of hot corrosion behaviour of HVOF sprayed NiCrAl coating on superalloys at 900 °C

In the present investigation, NiCrAl coating was deposited on Ni- and Fe-based superalloy substrates by using high velocity oxy-fuel (HVOF) process to study the hot corrosion behaviour in molten salt (Na₂SO₄–60% V₂O₅) environment at 900 °C under cyclic conditions. The mass gain measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopic analysis (SEM/EDS) and X-ray mapping techniques were used to analyse the corrosion products. The bare superalloys experienced higher weight gain. The NiCrAl-coated Superni 750 alloy (SN 750) provided a better protection among the coated superalloys investigated. The formation of oxides and spinels of nickel, chromium and aluminum may be contributing better resistance to hot corrosion.     

Elimination of μ phase and improvement of hot corrosion resistance of a nickel-base superalloy by laserglazing

Laser irradiation of a wrought nickel-base superalloy with high tungsten and molybdenum contents can eliminate acicular μ. phase, which has a tendency to precipitate out in the matrix or beneath the aluminized layer after prolonged exposure to high temperature. It is shown that laserglazing refines the microstructure and homogenizes the composition distribution, markedly improving the hot corrosion resistance.     

Characterization and hot corrosion performance of LVPS and HVOF-CoNiCrAlYSi coatings

The CoNiCrAlYSi coatings were produced by using low vacuum plasma spray (LVPS) and high velocity oxy-fuel (HVOF) techniques on the Inconel-738. Hot corrosion behavior and microstructure characterization was investigated by exposing the sample to a molten film of Na₂SO₄-20? wt NaVO₃ at 880 °C for up to 560 h. The hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. The result of weight change measurements showed better hot corrosion resistance for HVOF-CoNiCrAlYSi coatings. This was attributed to the α-Al₂O₃ nucleation during the HVOF coating process and replacement of non-protective oxide due to a fluxing mechanism. It was also observed that the non-protective and porous oxides such as (Co,Ni)Al₂O₄ and (Co,Ni)Cr₂O₄ were formed on the both types of coatings due to large β-depleted zone after long exposure time of hot corrosion testing.     

Constitutive models for high-temperature flow behaviors of a Ni-based superalloy

The high-temperature deformation behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rate of 0.001–1 s⁻¹and temperature of 920–1040 °C. The experimental results show that the deformation behaviors of the studied superalloy are significantly affected by the deformation temperature, strain rate and strain. The flow stress increases with the increase of strain rate or the decrease of deformation temperature. The flow stress firstly increases with the strain to a peak value, showing the obvious work hardening behaviors. Then, the stress decreases with the further straining, indicating the dynamic flow softening behaviors. Considering the coupled effects of deformation temperature, strain rate and strain on the hot deformation behaviors of the studied Ni-based superalloy, the phenomenological constitutive models are established to describe the work hardening-dynamic recovery and dynamic softening behaviors. In the established models, the material constants are expressed as functions of the Zener–Hollomon parameter. The established constitutive models can give good correlations with the experimental results, which confirm an accurate and precise estimation of the flow stress for the studied Ni-based superalloy.     

碳对镍基高温合金AM3热腐蚀性能的影响

为了更充分地了解碳对镍基高温合金热腐蚀性能的影响,提高合金的耐热腐蚀性能,本文研究了不同碳含量镍基高温合金AM3在850℃条件下,经75%Na2SO4+25%NaCl饱和混合盐溶液热腐蚀5 h的行为。利用扫描电镜(SEM)、X射线衍射(XRD)等测试方法分析了合金热腐蚀后的组织形貌和腐蚀产物。研究表明,腐蚀5 h过程中,碳含量(质量分数)为0、0.045%、0.15%的合金持续增重,碳含量为0.085%的合金在2~4 h发生氧化膜与盐膜的碱性熔融,有失重现象存在。合金加入碳后,促使合金表层腐蚀层变薄且与基体结合力变好。含碳的镍基高温合金腐蚀层产物以氧化物为主,主要有NiO、TiO2、Al2O3、Cr2O3。碳含量(质量分数)为0.085%、0.15%的合金由于腐蚀层氧化物较薄,可检测到Al4CrNi15、Ni3Al相的存在。综合分析发现碳含量为0.085%时,合金耐热腐蚀性能达到最优。     

A review of factors controlling the gas turbine hot section environment and their influence on hot salt corrosion test methods

Environmental and operational factors that influence the formation and deposition of corrosive species on hot section components in gas turbine engines are reviewed. In addition to air and fuel impurities, combustion gas chemistry, velocity, pressure and temperature are identified as key operational factors affecting the formation and deposition of Na₂SO₄ salt (the primary corrosive species) when trace amounts of sodium and sulphur are entrained into combustors. Test methods for ranking the resistance of different materials to hot salt corrosion are reviewed and compared in terms of the type of damage they produce. The methods considered range from a simple furnace or crucible test to burner rig and engine tests. It is shown that high velocity burner rigs, running at atmospheric pressure, allow all the relevant operational factors to be simulated, and they produce realistic hot corrosion damage similar to that observed on engine parts in service.     

Forming behavior and workability of Hastelloy X superalloy during hot deformation

The hot deformation behavior of Hastelloy X superalloy has been characterized using hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during hot working. Hot workability of this alloy has been analyzed by calculating flow localization parameter and construction of workability map for occurrence of shear band. In addition, on the basis of flow stress data obtained as a function of temperature and strain rate in compression, power dissipation map and instability map for hot working have been developed.     

Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy

The low cycle fatigue behavior of a directionally solidified nickel-based superalloy DZ125 was examined at 850 °C in air using bare and salt-coated specimens. Experimental results show that the salt-coated specimen showed relatively low fatigue life compared with the bare specimen, and this effect accelerated with the increased applied maximum stress. Damage of hot corrosion in fatigue life was found to be associated with the reduction of the bare area and the early crack initiation from the weaken grain boundaries of recrystallized grains.     

The effect of SO3 on vanadate-induced hot corrosion

Unrefined fuel oils may contain considerable amounts ofboth sulphur and vanadium. Despite this, exposure to combustion gases seldom yields deposits which consist ofboth vanadates and sulphates. Calculations for the sodium sulphate/vanadate system show that this is due to thermodynamics ofthe system. Sodium sulphate cannot exist in equilibrium with fused vanadate unless the sulphur trioxide pressure in the ambient atmosphere is high or the vanadium to sodium ratio in the deposits is less than one.

Thermogravitmetric studies which delineate the conditions for simultaneous sulphate- and vanadate-induced corrosion at 650 to 8OO°C have been performed. NiCrAlY coatings with K-sodium vanadyl vanadate deposits and the deposit alone have been exposed to oxygen containing 4% sulphur dioxide at 650 to 8OO°C. The results generally confirm the calculations from available thermodynamic data, but the solubility ofsulphur oxide in fused sodium vanadate is higher than expected from the literature values. The ' corrosion mechanism changes from initial vanadate-induced to essentially sulphate-induced hot corrosion when the sodium trioxide pressure is high enough that sodium sulphate may be formed.     

钛合金微等离子体氧化陶瓷膜结构与耐酸腐蚀

在铝酸钠溶液中,利用微等离子体氧化技术,在TC4钛合金表面原位生长复合氧化物陶瓷膜,研究了陶瓷膜的相组成、形貌和陶瓷膜对钛合金耐酸腐蚀的影响。陶瓷膜由Al2TiO5,α-Al2O3和金红石型TiO2构成;整个膜层由致密层和疏松层组成。在盐酸和硫酸中陶瓷膜使得钛合金耐酸腐蚀特性提高了5倍左右。带有陶瓷膜的钛合金在硝酸中的腐蚀速率为膜层的溶解速率。     

Comparative hot corrosion studies of EB-PVD and plasma sprayed coatings*

The use of directly fired heat engines, i. e. diesel and gas turbine engines, is likely to grow in the future. However, economic requirements will necessitate the development of highly efficient engines that operate at high temperatures and utilize low-grade fuels. An increase of combustion gas temperature and the utilization of low-grade fuels contaminated with, amongst others, sulphur and vanadium, may be accomplished by careful selection of the materials, e.g. by applying corrosion-resistant coatings on heatresistant alloys. The main purpose of the present work has been to test various coatingsubstrate combinations in a highly corrosive environment. The comparison of various coating techniques, e.g. electron beam physical vapour deposition (EB-PVD), air plasma spraying (APS), and low pressure plasma spraying/vacuum plasma spraying (LPPS/VPS), has been emphasized.     

Fatigue effect of WC coatings thermal sprayed by HVOF and laser treated, on medium carbon steel

Tungsten carbide coatings thermal sprayed by High Velocity Oxy-Fuel (HVOF) could be employed to obtain very hard surfaces and for the recovery of shafts or pieces with worn zones, like bearing supports, retaining rings, and so on. The aim of this study is to analyse the effects of fatigue in the WC thin coatings (about 0.1 mm thick) sprayed by HVOF on medium carbon steel substrate.Rotational bending fatigue tests were carried out with test specimens, having applied the coating over the raw materials (without anchorage layer), and the different resistance to fatigue was analysed on the uncoated material, laser surface hardened samples, the samples with the WC coating thermal sprayed by HVOF and last with the same WC coating treated by laser. Results show the possibility of predicting the fatigue life of materials with this coating and these treatments and the huge differences between them.     

预氧化对一种镍基单晶高温合金的热腐蚀影响

为了解预氧化对镍基单晶高温合金的短时热腐蚀影响,采用涂盐法研究了一种高温合金经预氧化后在Na₂SO₄-NaCl盐膜条件下的热腐蚀行为,测量了不同预氧化处理时间合金的腐蚀动力学曲线,借助扫描电子显微镜(SEM)观察了合金腐蚀层截面的微观组织,并利用X-射线衍射仪(XRD)确定相结构.结果表明：未经过预氧化处理的合金表面腐蚀严重,热腐蚀增重质量最多达到0.033 mg/cm²,其腐蚀动力学曲线遵循抛物线规律,而经过预氧化处理后合金表面腐蚀明显改善;合金在900 ℃下的热腐蚀符合硫化氧化模型,腐蚀产物出现了分层结构;合金的主要腐蚀产物是NiO、Al₂O₃以及少量NiCr₂O₄、TiO₂、NiS₂等, 其中Al₂O₃、TiO₂等氧化物位于氧化层最外层;预氧化生成的Al₂O₃层对高温合金在Na₂SO₄-NaCl盐膜下的短时热腐蚀有明显的保护作用,腐蚀层厚度从170 μm降低至80 μm.致密的氧化物保护层能够显著阻止硫元素的扩散,提高合金的耐热腐蚀性能.     

Stacking faulted P phase in a grit-blasted Ni-based single-crystal superalloy after thermal exposure

In the present work, direct and detailed investigation was carried out with respect to the precipitation in the grit-blasted single-crystal superalloy of DD6 after thermal exposure at 1100°C for 8?h. The precipitation was confirmed to be an orthorhombic topologically close-packed phase of P. In the phase, abundant atomic stacking faults were clearly revealed by bright field transmission electron microscope imaging and high-resolution HAADF imaging. Furthermore, statistical analysis for the 42 P phase particles observed by electron backscattered diffraction revealed that they had a mean size of 0.21?µm and volume percentage of 0.172% in the recrystallised region. The orientation distribution was essentially random besides very weak textures of {511}<0-32> and {921}<0-15>.     

不同Ru含量的镍基高温合金热腐蚀研究

为了解Ru对高温合金热腐蚀性能影响规律,采用了浸盐方法研究了不同Ru含量镍基高温合金的热腐蚀过程,采用扫描电镜观察了合金腐蚀层截面的微观组织,并利用X-射线衍射仪确定相结构.结果表明:在900℃浸盐条件下,Ni基高温合金中加入质量分数3%的Ru可以显著提高合金自身的热腐蚀抗力,并且随着Ru含量增加,合金的热腐蚀抗力进一步增强;合金中加入Ru后,能促使合金表层腐蚀层变薄且与基体结合力好,从而提高了合金的热腐蚀抗力.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Abstract

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃ C₆ and Ni₃ B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.     

Improving the hot corrosion resistance of plasma sprayed ceria–yttria stabilized zirconia thermal barrier coatings by laser surface treatment

Ceria–yttria stabilized zirconia (CYSZ) thermal barrier coatings (TBCs) were deposited by air plasma spraying on NiCoCrAlY-coated Inconel 738LC substrates. After that, the surface of plasma sprayed CYSZ TBCs were glazed using a pulsed Nd:YAG laser. The effects of laser glazing on hot corrosion resistance of the coatings were evaluated in presence of 45 wt%Na₂SO₄ + 55 wt%V₂O₅ corrosive molten salt at 1000 °C. The results revealed that the hot corrosion resistance of plasma sprayed CYSZ TBCs were enhanced more than twofold by laser surface glazing due to reducing specific reactive area of the dense glazed surface layer and consequently, decreasing the reaction between molten salt and zirconia stabilizers.     

AZ91D镁合金表面电沉积n-ZrO2/Ni复合镀层及耐蚀性研究

在AZ91D镁合金表面化学镀Ni-P的基础上电沉积n-ZrO2/Ni复合镀层和纯Ni电镀层,并对其表面形貌、截面形貌和耐蚀性进行了对比分析。结果表明AZ91D镁合金表面电沉积纳米复合镀层比纯镍镀层组织更细小,镀层更致密、平整。n-ZrO2/Ni复合镀层具有明显的钝化区,具有良好耐蚀性。