Oxide structure on aluminide coatings

A comparative study of MOS structures metallized in planar and cylindrical magnetron systems and in a conventional d.c. diode sputtering system was conducted. In all cases the influence of the discharge led to distortion and hysteresis in high frequency C-V curves owing to the generation of neutral traps spatially distributed in the oxide. The effective concentration of the traps was about 1.4 × 10¹² eV^?1cm^?2 in the central part of the band gap. These defects can easily be removed by conventional low temperature annealing. The necessity of post-metallization annealing of MOS structures prepared in magnetron systems can be avoided if the substrates are isolated from ground and heated to 300 °C during the sputter deposition of the metal layer.     

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.     

Surface morphology of diffusion aluminide coatings

The surface structure and morphology of aluminide coatings can be important in their performance both in oxidation or hot corrosion resistance and in their mechanical behavior. However, little information is available on the effects of aluminide coating deposition conditions on the coating surface. A study of the effects of aluminide processing parameters on both pure nickel and a nickel-base superalloy, IN 738, revealed a systematic change in surface morphology from a rough, serrated, fine-grain surface which occurs with a high aluminum activity deposition process to a smooth coarse-grain structure for the low aluminum activity process. A systematic change in coating color was also observed for both the pure metal and the alloy with changing coating aluminum contents. Results are reported and related to the coating phases present during the deposition process.     

Factors in the degradation of ceramic coatings for turbine alloys

Internal stresses introduced during the processing of ceramic coatings for turbine components can lead to coating failure. The sources of such stresses include not only the thermal expansion mismatch between ceramics and metals but also elastic anisotropy and thermal gradients introduced within the ceramic during plasma spraying or phase transformations (e.g. in ZrO₂). However, the ceramic coatings can be made more resistant to fracture (or spalling) by control of their microstructure. The ability of ceramic coatings both to resist corrosion and to protect the metallic components presents additional challenges to the development of coatings.     

Resulting morphologies on quenching of titanium aluminide alloys

Abstract

The effect of alloying elements (aluminium, silicon, niobium, and zirconium) on the mechanism and morphology of the allotropic transformation Ti-β(bcc) → Ti-α(hcp), occurring during the quenching of binary, ternary, and quaternary titanium aluminide alloys, has been studied. The alloys investigated were (at.-%) Ti–16Al, Ti–16Al–1Si, Ti–16Al–3·5Si, Ti–14Al–1Si–1Nb, Ti–14Al–1Si–1Zr, Ti–22Al, Ti–22Al–1Si, Ti–22Al–3·5Si, Ti–20Al–1Si–1Nb, and Ti–20Al–1Si–1Zr. The allotropic transformation in these alloys presented a very narrow temperature range for the formation of all possible α morphologies resulting from quenching. The different morphologies of α phase observed have been correlated with the competing mechanisms of β decomposition. The morphological observations indicated that small variations in thermodynamic and kinetic conditions in the β phase might account for changes in the mechanisms of formation and growth of the α phase. Additionally, the effect of the alloying elements on the ordering reaction α → α₂ occurring during quenching has been investigated. Silicon addition promoted the formation of columnar α₂ domains during quenching.     

Atomic structure of nanoclusters in oxide-dispersion-strengthened steels

Oxide-dispersion-strengthened steels are the most promising structural materials for next-generation nuclear energy systems because of their excellent resistance to both irradiation damage and high-temperature creep. Although it has been known for a decade that the extraordinary mechanical properties of oxide-dispersion-strengthened steels originate from highly stabilized oxide nanoclusters with a size smaller than 5 nm, the structure of these nanoclusters has not been clarified and remains as one of the most important scientific issues in nuclear materials research. Here we report the atomic-scale characterization of the oxide nanoclusters using state-of-the-art Cs-corrected transmission electron microscopy. This study provides compelling evidence that the nanoclusters have a defective NaCl structure with a high lattice coherency with the bcc steel matrix. Plenty of point defects as well as strong structural affinity of nanoclusters with the steel matrix seem to be the most important reasons for the unusual stability of the clusters at high temperatures and in intensive neutron irradiation fields.     

Influence of constitutional undercooling on grain refinement in titanium aluminide alloys

In this present work, the influence of constitutional undercooling on the grain size of titanium aluminide alloys and the dependence on boron content have been investigated. By means of the TTTIAL1 thermodynamic database of the Thermo-Calc programme, the values of the growth restriction factor for a series of alloys have been calculated. It can be shown that grain size decreases with increasing growth restriction factor. A comparison of calculated and published experimental results shows that the constitutional undercooling significantly influences the grain size in titanium aluminide alloys, but most likely, it is not the unique mechanism of grain refinement alone, in so far as not all of the experimental results may be explained by the influence of constitutional undercooling.     

通过离子掺杂剂界面偏析来提高氧化物弥散强化合金的机械性能

氧化物弥散强化钨合金(ODS-W)因具有显著提升的抗辐照能力、高温强度以及抗蠕变性能而展现出巨大的高温应用潜力.然而,分散在合金中的氧化物第二相颗粒容易在钨晶界处团聚并长大(甚至到微米尺寸),这大大抑制了它们对钨合金的强化效果.目前,如何有效细化和分散钨晶界处的氧化物第二相颗粒一直是人们面临的巨大挑战.在本文中,我们通...     

A study of the structure and surface properties of nanostructured biocompatible coatings on Ti alloys

This paper presents a study of the stability of biocompatible nanostructured TiO₂ surfaces prepared on a Ti6Al4V alloy by anodic oxidation, and it considers changes in the contact angles of ethylene glycol and distilled water. In the first stage, we measured the changes in the contact angles of the TiO₂ surfaces before and after sterilization. Then we examined the changes in the temporal contact angles of TiO₂ surfaces in different environments. The surface energy was calculated from the measured contact angles according to the Owens - Wendt model. Stability is significant for providing and innovating biocompatible coatings.The morphology of the deposited layers was observed using a scanning electron microscope (SEM).     

Zirconium influence on microstructure of aluminide coatings deposited on nickel substrate by CVD method

Influence of Zr on the microstructure and phase characteristics of aluminide diffusion coatings deposited on the nickel substrate has been investigated in this study. The coatings with and without zirconium were deposited by CVD method. The cross-section chemical composition investigations revealed that during the coatings formation, there is an inward aluminum diffusion and outward nickel diffusion in both types of coatings (with and without zirconium), whereas zirconium is located far below the coating surface, at a depth of ~17 μm, between β-NiAl phase and γ ^′-Ni₃Al phase. XRD examinations showed that β-NiAl, γ-NiAl and γ ^′-Ni ₃Al were the main components of the deposited coatings. β-NiAl phase is on the surface of the coatings, whereas γ-NiAl and γ ^′-Ni₃Al form deeper parts of the coatings. Zirconium is dissolved in NiAl on the border between β-NiAl and γ ^′-Ni₃Al.     

Tribotechnical properties of coatings on titanium alloys

We compare the antifriction properties of different types of coatings on titanium alloys under conditions of boundary friction as applied to parts of the hydraulic cylinders of an aircraft. We show that gasthermal titanium carbide coatings cladded with nickel and with both copper and nickel have better antifriction characteristics than ones obtained by chrome electroplating, nickel chemoplating, thermooxidation, anodization, etc. Kiev International University of Civil Aviation; J.S.S.T.C. “Antonov,” Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 2, pp. 55–62, March–April, 1998.     

Microstructural study of aluminide surface coatings on single crystal nickel base superalloy substrates

Abstract

Aluminide diffusion coatings are frequently employed to enhance the oxidation resistance of nickel base superalloys. However, there is a concern that the presence of an aluminide coating could influence the properties of the coated superalloy, especially in respect of fatigue behaviour. To understand the nature of the effects of surface coatings on the fatigue properties of superalloys, an understanding of microstructural development within both the coating and the coating/substrate interfacial zone during high temperature fatigue testing is necessary. This paper is concerned with microstructural changes in aluminide diffusion coatings on single crystal γ′ strengthened superalloy substrates during the course of high temperature fatigue testing. The ‘edge on’ transmission electron microscopy technique is employed to study cross-sections of two stage (aluminisation plus diffusion treatment) coated superalloy samples. The paper examines the degradation of the coating produced by phase transformations induced by loss of aluminium from the coating and/or aging of the coating. Aluminium removal both by interdiffusion with the substrate and by oxidation of the coating surface is considered. Microstructural development in the portion of the substrate influenced by interdiffusion with the coating is also discussed.

MST/1694     

Exothermic synthesis of cast nickel aluminide alloys with tungsten and molybdenum carbides

Cast NiAl–Mo₂C and NiAl–WC alloys have been prepared by self-propagating high-temperature synthesis involving the reduction of starting metal oxides. Synthesis conditions have been found experimentally. The elemental and phase compositions of the alloys have been determined and their microstructure has been studied. The composites have been shown to have higher microhardness than do NiAl intermetallic alloys containing Mo and W inclusions.     

Effects of processing parameters on aluminide morphology in aluminium grain refining master alloys

Abstract

An investigation has been made of the effects of processing parameters on the microstructure of Al–Ti and Al–Zr alloys produced by the reduction of potassium fluorotitanate and potassium fluorozirconate by aluminium. Blocky aluminides (TiAl₃ and ZrAl₃ respectively) are produced when the reduction is performed in the liquid+solid phase region and flaky aluminides are produced when it is performed in the liquid phase region. The use of Na₂TiF₆ rather than K₂TiF₆ has no apparent effect on the alloy structure. The microstructure and the size of aluminide blocks or flakes produced are further influenced by the subsequent cooling rate experienced by the alloy. Post-fabrication heat treatment of Al–5Ti–0·2B (wt-%) alloys (produced by combined reduction of potassium fluqrotitanate and potassium fluoroborate by aluminium) causes boride particles to precipitate on the aluminide surfaces. These borides grow in size and decrease in number with heat treatment time.

MST/1355     

Highly resistant icephobic coatings on aluminum alloys

Aluminum alloys are widely used for outdoor structures such as ground wires and phase conductors of overhead power lines, as well as aircrafts wings and fuselage. To protect these surfaces against excessive ice accumulation, icephobic coatings must be highly reliable and durable. New coatings with icephobic characteristics and excellent mechanical properties have been developed. The method consisted in depositing an extremely adherent poly(tetrafluoroethylene) (PTFE) coating on an Al₂O₃ underlayer produced by anodizing in a phosphoric acid electrolyte followed by an oxide etching step to enhance surface roughness. PTFE impregnation was carried out at low temperature (320 °C) and coating adhesion was assessed using tape and bend tests. Some of these coatings showed superhydrophobic properties; ice adhesion was around four times lower than bare aluminum. As well, they remained effective after ten ice-shedding events using an aggressive centrifugal technique. Moreover, no sign of PTFE degradation after 14 ice removals was noted and the coatings remained extremely adherent and very hydrophobic. This technique therefore shows very good potential and could be applied to new high-voltage overhead aluminum cables as protection against excessive ice or snow accumulation.     

Novel alginate based coatings on Mg alloys

Coatings on yttrium doped magnesium (Mg4Y) alloy substrates were prepared using alginate hydrogels by dip coating method to improve the surface bioactive properties of the substrate. Furthermore, composite coatings containing nano-sized calcium phosphate corresponding to hydroxyapatite (HA) phase entrapped within alginate hydrogel were also synthesized on the Mg4Y substrates. Surface characteristics of these coated substrates have been investigated using FTIR-ATR, SEM and EDS. The results show that the coatings with alginate alone are not stable in vitro; however, incorporation of NanoCaPs slightly improves the stability of these coatings. In addition, these composite coatings showed cell attachments with fibronectin incorporation. These results indicate that alginate hydrogels have the potential to be used as bioactive coating materials for different biofunctional applications.     

Deposition of silicon modified aluminide coatings on nickel base superalloys by pack cementation process

Abstract

The equilibrium partial pressures of vapour species generated in halide activated pack powder mixtures at high temperatures were calculated for a series of compositions using thermochemical analysis tools. The results obtained were applied to identify suitable activators and pack compositions for codepositing Al and Si to form diffusion coatings on nickel base superalloys by the pack cementation process. The calculation results suggested that compositions of the packpowder mixtures activated by CrCl₃.6H₂O may be adjusted to create deposition conditions favourable for codepositing Al and Si, but, those activated byAlF₃or AlCl₃ may only deposit Al.A series of coating deposition experiments were also carried out at 1000 ° C and 1100 ° C and the results obtained confirmed that, with adequate control of pack compositions and deposition conditions, codeposition of Al and Si can be achieved with CrCl₃.6H₂O activated pack powder mixtures. A mixture of elemental Al and Si powders may be used as a depositing source instead of using Al-Si master alloy powders as conventionally recommended. The coatings could be formed either through the inward diffusions of Al and Si or through the outward diffusion of Ni together with other substrate elements such as Cr and Co, depending on the deposition temperature used. Prolonged deposition at 1100 ° C ledtothe formationofa coatingwith amultilayeredstructure consistingofanouter nickelsilicide layerand a middle Simodified NiAl layer followed by a diffusion zone. The pack compositions and deposition conditions may be adjusted to control the microstructure of the coatings formed by the codeposition process.