Effects of pack composition on the formation of aluminide coatings on alloy steels at 650°C

This is a detailed study aimed to understand the effects of pack composition on the formation and growth of aluminide coatings on alloy steels by pack aluminisation at 650°C, a temperature below the melting point of Al (660°C), using pack powders consisting of Al as depositing source, a halide salt as an activator and Al₂O₃ as inert filler. The packs activated by AlCl₃, NH₄Cl, AlF₃ and NH₄F were used to investigate the effects of the type of halide salt on the coating formation and growth process and subsequently to identify the most suitable activator for pack aluminising alloy steels at 650°C. The effects of pack Al content on the rate of coating growth were then studied by varying the pack Al content from 1.4 wt% to 10 wt% whilst fixing the pack activator content at 2 wt%. It was observed that among the halide salts studied, AlCl₃ is the only suitable activator for pack aluminising alloy steels at 650°C and the rate of coating growth increases with the pack Al content. The equilibrium partial pressures of vapour species generated at the deposition temperature in packs activated by different types of halide salts were calculated and the results were discussed in relation to the observed deposition tendency of packs activated by different types of activators. A vapour phase transportation model was applied to elucidate the relationship between the rate of coating growth and the pack Al content. It was also demonstrated that by combining the low temperature pack aluminising parameters identified in this study with electroless or electro Ni plating, coherent nickel aluminide coatings free of microcracking can be produced on alloy steels at 650°C.     

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.     

Nickel aluminide coating produced on γ-TiAl alloy by high activity aluminising process

Abstract

Ni aluminide diffusion coatings on the surface of γ-TiAl alloy were produced by electroplating a Ni layer followed by a single step high activity aluminising carried out in Ar+H₂ atmosphere with a mixture of Al, NH₄Cl and Al₂O₃ powders at 1000°C for 5 h. The effect of initial thickness for Ni layer on microstructure of produced Ni aluminide coating was highlighted. The thickness of initial Ni layer was changed to 4–20 μm. In the case of the Ni layer with thickness of 4 μm, only a little amount of NiAl phase was formed in a TiAl₃ matrix. However, the microstructure of coating, in the case of the Ni layer with thickness of 8 μm, consisted of an outer layer of two phases (NiAl+TiAl₃), an intermediate layer of TiAl₃ and an interdiffusion layer. For thicker initial Ni layers (16 and 20 μm), beside the latter coating microstructure, a continuous surface layer of NiAl phase was observed. Isothermal oxidation tests on these aluminide coatings reveal that the oxidation resistance of the aluminide coatings increases with increase in initial thickness of Ni layer.     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

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.     

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

Abstract

An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na₂SO₄/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.     

A new way to produce Al + Cr coating on Ti alloy by vacuum fusing method and its oxidation resistance

An Al + Cr coating successfully produced by the vacuum fusing method was proposed for improving the oxidation resistance of Ti–6Al–4V alloy specimens. Unlike powder pack cementation, this technique was much more effective and cheap without the need for long time diffusion, and the Al concentration and the thickness of vacuum fusing coating layer could be controlled by adjusting powder mixing ratio and adding the immersed times, respectively. Compared with the specimen with an Cr-modified aluminide coating, the oxidation resistance of the specimens with vacuum fused Al + Cr coating was about two times than that of the specimens with Cr-modified aluminide coating. During oxidation, Cr additions suppressed metastable θ-alumina formation. It is only one phase -alumina scale that developed on the vacuum fused Al + Cr coating surface, while a single metastable θ-alumina scale formed on the Cr-modified aluminide coating surface.     

Correlation of processing technique and microstructure of platinum aluminide bond coats with performance of thermal barrier coatings deposited on nickel base superalloy

Abstract

We show that the performance of thermal barrier coating systems is critically dependent upon the processing technique and microstructure of platinum aluminides utilised as bond coats. It is demonstrated by thermal exposure tests at 1150°C in air with 24 h cycling period to room temperature that the average useful life of a coating system employing zirconia–7 wt-% yttria as top coat and alloy MAR M002DS as substrate is increased from 192 to 480 h by replacing a three-layer bond coat aluminised by conventional pack cementation with a two-layer bond coat aluminised by chemical vapour deposition. Before each aluminising process, the superalloy has been electroplated with a platinum layer about 7 μm in thickness. Microstructural characterisation using scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron-probe microanalysis, transmission electron microscopy and X-ray diffraction indicates that the superior performance provided by the two-layer bond coat is related to its higher thermal stability enhancing the adhesion of the thermally grown oxide. However, both coating systems are found to fail by the same mechanism involving loss of adhesion between the thermally grown oxide and bond coat.     

The effect of yttrium content on the oxidation-induced degradation of NiCoCrAlY coatings and the influence of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings

Abstract

It has been found that the yttrium content of NiCoCrAlY coatings affects the useful lives of such coatings during cyclic oxidation. In particular, NiCoCrAlY coatings with 0.1wt% yttrium have more than twice the lifetime at 1100°C compared to NiCoCrAlY coatings with 0.5wt% yttrium. The mechanism by which the yttrium concentration influences the degradation of NiCoCrAlY coatings will be described. It has also been observed that the adverse effect of yttrium can be inhibited by reducing the roughness of the coating surface. The influence of surface condition on the oxidation of yttrium in NiCoCrAlY coatings will be examined in detail and the effects of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings will be compared. Finally the effects of yttrium in the substrate alloy on the oxidation of platinum aluminide coatings will be discussed.     

Development of process for aluminising nickel alloy 718 strips for tube bundle support structures in liquid sodium–water steam generators

Abstract

The tubes in liquid sodium–water steam generators of the Indian prototype fast breeder reactor (PFBR) will be supported by corrugated nickel alloy 718 strips. Aluminisation of nickel alloy 718 strips has been chosen for this application because of the excellent performance of aluminide coatings in reducing impact fretting wear of the tubes due to flow induced vibrations and compatibility of the coating with liquid sodium at the operating temperature of the steam generators. Aluminisation of nickel alloy 718 strips for steam generator tube bundle support structures has been developed using a procedure involving thermal spraying of aluminium followed by diffusion heat treatment in vacuum atmosphere. One of the advantages of the technique is that it will coat only the desired surfaces of the strips, whereas in conventional pack cementation process, significant precautions have to be ensured. Furthermore, this process has enabled aluminisation to be carried out at a much lower cost than the conventional process of pack aluminising. The problems encountered during the initial trials and technology development, such as coating thickness and distortion, are discussed. A process flow chart for this procedure to take the job on an industrial scale is also reported. This process (under patenting) has been adopted for the aluminisation of corrugated strips for the support structures of one steam generator module and the steam generator for a test facility during the technology development phase, as also of all the steam generators being fabricated for the PFBR.     

Micromechanical modelling of layered systems containing titanium alloy and titanium MMC subjected to bending

Abstract

The mechanical behaviour of layered systems, consisting of layers of monolithic titanium alloy, (IMI834) and titanium metal matrix composite, (Ti-MMC) is examined. A finite element based micromechanical model was developed to investigate the mechanical response of the systems subjected to four point bending. Two types of layered systems were examined: the first designated as C type with Ti-MMC in the centre of the beam, and IMI834 on the outside, and an S type which consists of a Ti-MMC layer on the outside and IMI834 on the inside. A representative volume element (RVE)was defined and boundary conditions imposed to simulate bending of the layered systems. While the S type system exhibited higher stiffness and linear behaviour up to fracture, the C type system exhibited more ductile behaviour prior to fracture. The overall predicted behaviour of the two systems was in good agreement with experimental results.     

Cyclic oxidation of copper doped Ti-48Al-2Cr-2Nb

The widespread use of titanium aluminide alloys will require effective joining techniques for primary fabrication and repair. One such technique is Transient Liquid Phase (TLP) bonding, which has been used to join titanium aluminide alloys. A successful TLP bonding process uses a copper-containing composite interlayer and thus introduces a small amount of copper into the alloy. Although even relatively small alloying additions can be detrimental to the oxidation resistance of titanium aluminide alloys, the amount of copper added to the alloy during the TLP bonding process has previously been shown to be neutral or beneficial to the isothermal oxidation resistance of the alloy. In this paper, a small amount of copper introduced during TLP bonding is shown to have no detrimental effect on the cyclic oxidation of Ti-48 at% Al-2 at% Cr-2 at% Nb.     

Dwell sensitive fatigue in a near alpha titanium alloy at ambient temperature

The ambient temperature fatigue performance of the near alpha titanium alloy IMI834 was evaluated using laboratory specimens manufactured from two material sources: rolled bar stock and an isothermally forged compressor disc. The effect of dwell periods imposed at peak stress and R value were assessed. Significant differences were defined between the two variants with respect to their sensitivity to dwell loading. Variations in microstructural form together with a localised texture within the disc material are considered responsible for controlling the dwell performance in each case. The findings are consistent with a previously proposed model for facet development in this class of material.     

Cyclic-Oxidation Behavior of Multilayered Pt/Ru-Modified Aluminide Coating

Multilayered Pt/Ru modified aluminide coating for thermal barrier coating(TBC) systems has been investigated.2 μm Pt+2 μm Ru+2 μm Pt was first deposited on nickel-base superalloy DZ125 by electrodeposition,and then the coating was treated by annealing and a conventional pack-cementation aluminizing process.The cyclic oxidation tests were carried out at 1423 K in air.It was found that the thermal cyclic oxidation resistance of Pt/Ru-modified aluminide coating was comparable to that of Pt-modified aluminide coating,which was much better than simply aluminized DZ125.The addition of Ru to Pt-modified aluminide coating increased the resistance to rumpling.The microstructures and phase constitutions of the coating before and after oxidation were investigated.     

Electron back scattered diffraction (EBSD) analysis of quasi-cleavage and hydrogen induced fractures under cyclic and dwell loading in titanium alloys

Evidence for sub-surface fatigue crack initiation is often reported for near alpha titanium alloys such as the coarse grained IMI685 and the fine duplex structured IMI834. In such materials with a typical as received hydrogen concentration of 40–60 ppm the initiation site is invariably characterized by quasi-cleavage facetting. Similar facetting is also associated with the low temperature dwell sensitive fatigue response in the same alloys. For IMI685, it is reported that this failure mechanism is replaced by / interface cracking when the alloy contains a relatively high concentration of interstitial hydrogen. The present paper characterises the local grain orientation and microstructural conditions associated with these various forms of failure through the use of a microtextural analysis technique based upon electron back scattered diffraction (EBSD) measurements. The observations are related to an existing model to account for facet formation based upon the pile-up of dislocations at grain-boundaries. The implications for further use of this technique with titanium alloys are discussed.     

600 ℃高温钛合金发展现状与展望

钛及钛合金具有比强度高、耐腐蚀性能和低温性能好、热强度高等优点,是航空航天工业中重要的结构材料。同时,相比于铝、镁轻合金,钛合金高温性能优异,因而在航空发动机耐高温部件中也有着相当大的应用潜力。1954年,美国研发出了第一种实用型高温钛合金Ti-6Al-4V,高温长时使用温度为300～350℃,综合性能良好,在之后的很长一段时间内被广泛使用。随着航空航天工业的不断发展,尤其是航空发动机的发展,其他各国也都相继研发出了一些使用温度更高的高温钛合金,直至1984年,英国开发出了世界上第一个使用温度达600℃的高温钛合金IMI834。IMI834的典型特点是在原有的近α型高温钛合金Ti-Al-SnZr-Mo-Si体系中加入了0.06%C,扩大了两相区的加工窗口,优化了组织。在此之后,美国于1988年在原有高温钛合金Ti-6542S的基础上通过调整一些合金元素的含量也获得了一种实用温度为600℃的高温钛合金Ti1100。1992年,俄罗斯在BT18Y的基础上用5%的高熔点W代替1%Nb也开发出了一种达600℃的高温钛合金BT36。而国内高温钛合金起步相对较晚,前期以仿制为主,后逐渐形成了以添加稀土元素为特色的高温钛合金体系,典型的有中科院金属研究所和宝钛集团研发的Ti60和西北有色金属研究院自主研发的Ti600,它们的实际使用温度均为600℃,综合性能优异。总体来说,目前高温钛合金的使用温度很难突破600℃,主要是由于使用温度高于600℃时合金的热强性与热稳定性难以匹配协调,并且合金的抗氧化性急剧下降,表面氧化严重,导致合金热稳定性以及疲劳性能下降,甚至可能使航空发动机高压压气机部位的零部件存在"钛火"的风险。本文综述了国内外600℃及600℃以上的高温钛合金的发展现状。重点介绍了美国的Ti1100、英国的IMI834、俄罗斯的BT36、中国的Ti60、TG6和Ti600(600℃高温钛合金)以及中国的Ti65和Ti750(600℃以上高温钛合金)。总结了各国发展高温钛合金的思路,指出了限制高温钛合金向更高使用温度发展的瓶颈并提出了可能的解决途径。从控制α2相大小、形态、含量以及改善热加工工艺的角度对未来高温钛合金的发展进行了展望,以期为进一步提高高温钛合金的使用温度、优化高温钛合金性能提供指导。     

Oxidation Behavior of Pd-Modified Aluminide Coating at High Temperature

(Ni,Pd)Al coating, prepared by low pressure pack cementation on the Ni-base superalloy M38 where Pd-20 wt pct Ni alloy was predeposited, consists of a single β-(Ni,Pd)Al phase. The initial isothermal oxidation behavior of (Ni,Pd)Al coating was investigated by TGA, XRD, SEM/EDS at 800～1100℃. Results show that oxidation kinetics accord preferably with parabolic law at 800, 900 and 1100℃, but not at 1000℃.θ-Al2O3 was observed at 800～1100℃. It is found that Pd plays an important role in accelerating the diffusion of Ti from the substrate to the coating surface in the aluminide coating.     

Aluminide coating formation on nickel-base superalloys by pack cementation process

A detailed study was carried out to investigate the effects of pack powder compositions, coating temperature and time on the aluminide coating formation process on a superalloy CMSX-4 by pack cementation. With the aid of recently developed thermodynamic analytical tools, powder mixtures that are activated by a series of fluoride and chloride salts were analysed and the effectiveness of these activators in transferring and depositing Al was evaluated at a range of coating temperatures. The Al chloride vapours formed at coating temperatures from 900°C to 1100°C were also analysed thermodynamically as a function of Al concentration in the original pack for the powder mixtures activated by 4 wt% CrCl₃·6H₂O. Based on the thermochemical calculations, a series of coating experiments was carried out. Aluminide coatings were formed at temperatures from 850°C to 1100°C for periods varying from 4 hours to 8 hours using powder mixtures activated by NH₄Cl, NaCl and CrCl₃·6H₂O and AlF₃. The effects of changing Al concentration as well as adding small quantities of Cr in the powder mixtures on the coating formation process were also investigated. The aluminide coatings were analysed using a range of techniques including SEM, EDX and XRD. The relationships between the mass gain and coating thickness and structure were investigated. The experimental results were compared with the predictions from thermochemical calculations. Based on the understandings established, an effective approach to control the aluminide coating parameters and structures was identified, which made it possible to optimise powder mixture compositions and coating conditions for different coating requirements.     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni-CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni-CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.     

Oxidation behavior of aluminide coatings on carbon steel with and without electrodeposited Ni–CeO2 film by low-temperature pack cementation

A CeO₂-dispersed aluminide coating was fabricated through aluminizing the electrodeposited Ni–CeO₂ nanocomposite film on carbon steel using pack cementation method at 700 °C for 4 h. The isothermal and cyclic oxidation behavior of the CeO₂-dispersed aluminide coating at 900 °C, including the growth of oxide scale and the microstructure of the coatings, have been investigated comparing with the aluminide coating on carbon steel. The results show enhanced oxidation performance of the CeO₂-dispersed aluminide coating, which is concerned with not only CeO₂ effect on the microstructure and oxidation, but also decreased interdiffusion between the aluminide and the Ni film. The CeO₂ benefit effects and interdiffusion are discussed in detail.