Ni Aluminides Research Articles

Ni Porous Preforms Compacted with Al2O3 Particles and Al Binding Agent.

This work presents an energy-efficient, cheap, and rapid production method of a metal-ceramic preform with open porosity suitable for liquid metal infiltration and filtration applications. It is based on cold isostatic pressing of a mixture of relatively hard Ni and Al2O3 powders with the addition of small amount of Al powders, acting as a binding agent. Open porosity is primarily controlled by Al2O3 particles partially separating Ni particles from mutual contacts. Cold isostatic pressed green compacts were subjected to thermal oxidation by heating in air to 600 °C, 700 °C, and 800 °C. The weight gain and open porosity of oxidized compacts were examined. The chemical composition and microstructure were analyzed by SEM-EDS and XRD techniques. The stability of preforms and the effect of thermal cycling on the open porosity were tested by thermal cycling in an inert Ar atmosphere in the temperature range up to 800 °C. It appeared that, in addition to NiO being an expected product of oxidation, Ni aluminides and spinel particles also played an important role in inter-particle bonding formation. Ni-NiO porous composites resist chemical corrosion and exhibit structural and chemical stability at higher temperatures and admixed Al2O3 particles do not deteriorate them. After subsequent infiltration with Al, it can offer a lower density than other materials, which could result in lower energy consumption, which is highly needed in industries such as the automotive industry.

Aluminide Diffusion Coatings on IN 718 by Pack Cementation.

This paper addressed the issues of both direct and indirect synthesis of Ni aluminides by pack cementation (pure Ni and IN 718 superalloy). On the Al-Ni diffusion twosome under pressure, at temperatures below and above the Al melting temperature, the appearance and evolution of diffusion porosity because of the Kirkendall–Frenkel effect manifestation was highlighted. It has been confirmed that, as the temperature rises above the Al melting temperature, the porosity decreases. Nickel-based superalloys, and in particular IN 718, significantly increase their performance by increasing the aluminides proportion in the top diffusion coating. This is made possible by changing the value of the Al and Ni weight percentage ratio in this area (noted as Al/Ni). In the case of the diffusion twosome between IN 718 and pack aluminizing mixtures, having in their composition as active components Al powder, Ferroaluminum (FeAl40) or mixtures of Al and Fe powders, at processing temperatures above the Al melting temperature, by modifying the active component of the mixture, substantial changes in the Al/Ni values were observed, as well as in the maximum %Al in the diffusion coating and of its thickness. It was found that, when switching from Al to FeAl40 or powder mixture (Al + Fe), the Al/Ni value changes between 3.43 and 1.01, from initial subunit values. The experiments confirmed that the highest %Al in the top aluminized diffusion coating, for IN 718, was obtained if the powder mixture contained 66.34 wt.% Al.

Al and Ni nanoparticles as precursors for Ni aluminides

Aluminides, intermetallic compounds of Al with at least one additional element, are promising materials particularly for high-temperature applications due to their physical and chemical properties. They are typically prepared starting from the elements using compacted, micrometer sized powders or multilayer systems, while the application of nanoparticles as precursors is much more uncommon although the high interface area can have advantages in the reactivity of the materials. We prepared Ni aluminides starting from submicron Al particles and Ni nanoparticles applying a self-propagating reaction. Upon applying separately prepared particles multiphase products containing Ni, Ni3Al, NiAl, Ni2Al3 as well as NiAl3 were observed depending on the sample size and heating rates. The preparation of single phase NiAl was possible applying particle mixtures synthesized by a two-step protocol. In a first step, submicron Al particles were synthesized via thermal decomposition of triisobutylaluminum. On the surface of the formed Al particles nanocrystalline Ni was deposited in a second step via thermal decomposition of bis(cycloocta-1,5-dien)nickel(0) resulting in the formation of well-mixed Ni–Al particle blends. Heating of powder compacts or loose powders under an atmosphere of Ar resulted in the formation of single phase NiAl at low as well as at high heating rates with no other intermetallic phases being present.

The assessment of growth kinetics in the intermetallic layers of Al-AlxNiy-Ni laminate composites

Ni aluminides have technologically attracted much attention as oxidation protective layers on Ni-superalloys for high-temperature and harsh environments applications as well as for reinforcements in metal-matrix composites. Among the Ni aluminides, the AlNi compound exhibits the best combination of oxidation-protective characteristics and hardness; thus there is a progressive demand to produce it particularly through convenient in-situ fabrication processes. Therefore, the evaluation of growth kinetics in AlxNiy layers is of crucial importance in determining an optimum compound formation process. To this purpose, Al-Ni intermetallic laminate composites were produced through cold roll bonding and subsequent annealing of aluminum and nickel sheets. The microstructure of the intermetallic layers was investigated in order to specify the controlling mechanisms and subsequently the growth model of the different phases. The Al3Ni layer was kinetically the first to appear but started to decompose at the expense of the AlNi compound when the direct source of Al disappeared for the reactive diffusion couples. The Al3Ni layer growth was initially controlled by bulk diffusion, but then at T≥525°C was modified as a function of competition between formation and consumption, whereas the AlNi growth was governed strongly by the interfacial reaction. The time dependence of the growth rate revealed different behaviors of linear and parabolic kinetics. The overall assessment revealed a bulk diffusion-controlled growth for all of the intermetallic layers. Arrhenius parameters could be derived for the Al3Ni layer, while it was impossible for the AlNi phase because the formation of this layer was caused by a mixture of diffusion mechanism.

Formation of the diffusion barrier at the interface of Cr20Ni80 alloy - Ni-Cr-Al coating

The effect of thermal influence conditions on the structure and chemical composition of the self-organizing diffusion barrier formed during high-temperature (1100 °C) long-term (up to 500 h) heating between the Cr20Ni80 alloy and the coating of the Ni-Cr-Al system is investigated. It is shown that the formation of a barrier, which is a Cr(Al) solid solution of variable composition, is due to the limited solubility of Cr in Ni aluminides. It is established that the growth of the thickness of the diffusion barrier is mainly due to the alloy Cr20Ni80, and the intensity of its growth and the content of chromium in it are determined by the temperature and time conditions of heating. With a diffusion barrier thickness of 55-60 μm, its further growth is stopped by blocking the diffusion flux of Ni from the alloy Cr20Ni80 into the coating.

Room Temperature Mechanical Properties of A356 Alloy with Ni Additions from 0.5 Wt to 2 Wt %

In recent years, the influence of Ni on high-temperature mechanical properties of casting Al alloys has been extensively examined in the literature. In the present study, room temperature mechanical properties of an A356 alloy with Ni additions from 0.5 to 2 wt % were investigated. The role of Ni-based compounds and eutectic Si particles in reinforcing the Al matrix was studied with image analysis and was then related to tensile properties and microhardness. In the as-cast condition, the formation of the 3D network is not sufficient to determine an increase of mechanical properties of the alloys since fracture propagates by cleavage through eutectic Si particles and Ni aluminides or by the debonding of brittle phases from the aluminum matrix. After T6 heat treatment the increasing amount of Ni aluminides, due to further addition of Ni to the alloy, together with their brittle behavior, leads to a decrease of yield strength, ultimate tensile strength, and Vickers microhardness. Despite the fact that Ni addition up to 2 wt % hinders spheroidization of eutectic Si particles during T6 heat treatment, it also promotes the formation of a higher number of brittle Ni-based compounds that easily promote fracture propagation.

An XPS Study with Depth Profiling for the Surface Oxide Layer Formed on Aluminides Produced on Superalloy 690 Substrates

Superalloy 690 substrates containing mainly Cr and Ni aluminides on the uppermost surface, formed by atmospheric plasma spraying and heat treatment, were oxidized at 1273 K in air for 2 h. Quantitative X-ray photoelectron spectroscopy (XPS) analyses indicated that the outermost surface layer formed on aluminides is composed of ~ 21.0 at.% Al+3 (as Al2O3), 17.0 at.% Al0 (elemental aluminium), 1.4 at.% Cr+3 (as Cr2O3) and 60.5 at.% O (in Al2O3 and Cr2O3 and also includes oxygen contaminant). Surface sputtering for 5 min exhibited splitting of Cr2p3/2 peak into a doublet comprising Cr+3 (0.9 at.%) and Cr0 (0.4 at.%) with the presence of 1.15 at.% Ni0 in the surface layer that mainly contained ~ 37.3 at.% Al+3, 7.3 at.% Al0 and 52.9 at.% O. Surface sputtering for 15 min indicated surface composition similar to surface sputtered for 5 min but with a marked reduction in ratio of Al+3/Al0 (32.2 at.% Al+3/11.90 at.% Al0) in the surface layer.

Comparison of in-situ oxide formation and post-deposition high temperature oxidation of Ni-aluminides synthesized by cathodic arc evaporation

Thin coatings were synthesized by cathodic arc evaporation of powder metallurgically fabricated Al-Ni targets with the chemical compositions of Al75Ni25, Al67Ni33 and Al52Ni48 atomic percent. The coatings were produced both either from pure metallic vapour or in reactive oxygen atmosphere. Phase transformations and chemical composition at the target surface were investigated by X-ray diffraction (XRD) and by energy-dispersive X-ray spectroscopy (EDS) techniques and compared with the phases composition obtained in the coating. The deposition in non-reactive mode produces intermetallic coatings containing different Ni aluminides. The formation of the aluminides can be controlled by the target composition and is by trend predictable from the Al-Ni phase diagram. The coatings produced in reactive mode are composed of both Al3Ni and Al3Ni2 with the additional formation of AlNi and Al2NiO4 in all the coatings. The coatings composed of intermetallics show high indentation hardness in the range of 10 GPA while the coatings composed of both oxides and intermetallics demonstrate exceptional high hardness of about 30GPa. All coatings have been annealed in ambient air up to 1200°C and were investigated by in-situ XRD analysis in order to follow in detail their oxidation process. For all coatings, the formation of α-Al2O3 and Al2NiO4 could be observed after annealing forming a thin high temperature stable protective layer for remaining AlNi. The coatings deposited in reactive atmosphere contain additional NiO, independently of their original composition.

FORMING OF INTERMETALLIC PHASES DURING INFILTRATION OF NICKEL RODS AND POWDERS WITH MOLTEN ALUMINIUM

Reaction kinetics in the Al-Ni system was investigated considering the fact that Al-Ni composites have been found as attractive candidates for a number of industrial sectors. Al-Ni composites were prepared by gas-assisted infiltration of either Ni rods or Ni powders with molten Al. Microstructural analysis revealed that various Ni aluminides were grown and their formation and the morphology at the interfacial zones can be effectively controlled by the time of infiltration. Furthermore, we pointed to the instability in the Al-Ni system that was caused by the presence of the pure Al metal. This could be completely abolished by the additional thermal treatment.

Evaluation of Ni–Al coatings processed by plasma transferred arc

Higher efficiency processes are strongly dependent on the ability to process materials/components that can sustain more aggressive service conditions. β-NiAl intermetallic alloys are known for exhibiting superior high temperature properties, but are difficult to process. This study evaluated Ni–Al coatings processed in situ by plasma transferred arc (PTA) welding. Coatings processed with mixtures of Ni and Al powders, Al content within the range 15–50 wt-%, were analysed. In situ development of Ni aluminides was confirmed by X-ray diffraction analysis. Increasing aluminium (Al) content in the deposited mixture increased the penetration depth. The microstructure of coatings depended on the amount of Al in the deposited powder mixture associated with the exothermal synthesis of the β-NiAl intermetallic compound. The present study presents an important step towards the protection of components for equipment of higher efficiency processes, as results validated the potential of PTA hardfacing as a processing technique for surface tailoring.

Thermal Barrier Systems and Multi-Layered Coatings Fabricated by Spark Plasma Sintering for the Protection of Ni-Base Superalloys

Aeronautic gas turbine blades, vanes and combustion chambers are protected against high temperature oxidation and corrosion by single or multilayered coatings. These include environmental coatings, generally based on Pt-modified Ni aluminides or MCrAlY overlays (where M = Ni and/or Co), thermal barrier coating (TBC) systems including a ceramic thermally insulating layer, and abradable seals. The present work shows the ability of the Spark Plasma Sintering technique to rapidly develop new coatings compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short production step. Fabrication of MCrAlY overlays with local Pt and/or Al enrichments is shown, as well as fabrication of coatings made of -PtAl2, -PtAl, α-AlNiPt2, martensitic and (Ni,Pt)Al or Pt-rich ’ phases, including their doping with reactive elements. The fabrication of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a bond-coating is also demonstrated, as well as the fabrication of a CoNiCrAlY-based cermet coating for abradable seal application. Difficulties of fabrication are reviewed, such as Y segregation, risks of carburization, local over-heating, or difficulty to coat complex shaped parts. Solutions are given to overcome these difficulties.

Pt-modified Ni aluminides, MCrAlY-base multilayer coatings and TBC systems fabricated by Spark Plasma Sintering for the protection of Ni-base superalloys

Pt-modified Ni aluminides and MCrAlY coatings (where M = Ni and/or Co) are widely used on turbine blades and vanes for protection against oxidation and corrosion and as bond coatings in thermal barrier coating (TBC) systems. The present work shows the ability of a new fabrication technique, the Spark Plasma Sintering, to develop rapidly new coating compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short experiment. Fabrication of MCrAlY overlays with local Pt and/or Al enrichment is shown, as well as fabrication of coatings made of ζ-PtAl 2, ε-PtAl, α-AlNiPt 2, martensitic β - (Ni,Pt)Al or Pt-rich γ/γ′ phases. The realization of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a γ/γ′ low mass bond coating is also demonstrated. Difficulties of fabrication are reviewed and discussed, such as Y segregation, risks of carburization, local overheating, or difficulty to coat complex shape parts. Finally, some first results of cyclic oxidation are given.

Three-dimensional study of Ni aluminides in an AlSi12 alloy by means of light optical and synchrotron microtomography

The three-dimensional (3D) microstructure of an AlSi12Ni alloy in as-cast and in solution-treated conditions is characterized by light optical and synchrotron tomography. Eutectic Al–Si alloys containing 1 wt.% Ni in as-cast condition present networks of connected Si lamellae as well as complex 3D shapes of Ni-containing aluminides. The eutectic Si networks disintegrate during solution treatment in the binary Al–Si alloy while they remain connected in the Al–Si–Ni alloy. The contiguity between eutectic Si and Ni-containing aluminides is maintained, when the alloy is solution treated at 540 °C for 24 h. The sphericity of the aluminides is only slightly increased by the solution treatment. The reinforcing role of eutectic Si together with the Ni-containing aluminides is governed by a stable interconnectivity and contiguity of these rigid phases accumulating ∼20 vol.%. The 3D data obtained by synchrotron tomography quantify connectivity, shape and volume fraction of eutectic Si and aluminides, whereas their contiguity is verified by light optical sectioning tomography.

Hydrolysis behaviour of magnides and aluminides and its application to the synthesis of nanoparticles

The hydrolysis behaviour of magnides and aluminides and the thermodynamic analysis of the hydrolysis have been systematically investigated. Two compounds in the Mg–Ni binary system (Mg 2Ni and MgNi 2) both exhibit hydrolysis behaviour at room temperature. Ni nanoparticles are the product of the hydrolysis of these two compounds. The size of the Ni nanoparticles synthesized by the hydrolysis of MgNi 2 had a mean diameter of 16.1 nm, which was slightly coarser than that for the Ni nanoparticles prepared by hydrolysis of Mg 2Ni (mean diameter of 11.7 nm). Using MgNi 2 to produce the Ni nanoparticles is more efficient and economical than using Mg 2Ni. The hydrolysis of magnides can be used to produce a range of transition metal nanoparticles such as Cu, Co, Au, Ag, and Pd. Al 3Ni amongst the Ni aluminides only exhibited spontaneous hydrolysis behaviour in water at room temperature. However, Ni nanoparticles were difficult to obtain because of Ni dissolution during removal of Al(OH) 3 by an acid. The hydrolysis of the Al-richest compounds has the potential to produce chemically inert transition metal nanoparticles such as Ag and Au.

Recubrimientos contra la corrosión a alta temperatura para componentes de turbinas de gas

Hot section gas turbine components are made of superalloys, developed to withstand high temperatures in conditions in which mechanical stresses are high and that require high surface stability. However, during the 50s it became evident that compositions resulting in high mechanical strength for these materials were not compatible with those offering optimal protection from the working environments. The idea of employing protective coatings over materials with high mechanical strength resulted therefore from this situation. Presently, both aeronautic and power generation turbines operate at temperatures within the 900-1400 °C, thanks to these coatings, indispensable for their correct operation. In this work, the principal degradation mechanisms in these operating conditions and the different type of coatings presently employed by the industrial sector are described, beginning by the oldest Ni or Co aluminides, following with the addition of other metals such as Pt, Cr, etc. to the former coatings in order to increase their useful life, continuing with the overlay MCrAlY coatings and finishing with the thermal barrier coating systems. Moreover, the corresponding deposition techniques industrially employed to deposit these coatings are described, and finally, an insight of the latest research lines currently being developed is also included.

Advanced Coatings on High Temperature Applications

To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures advanced coatings were developed, and they were classified into four groups, (1) the diffusion barrier coating with a duplex layer structure, an inner σ−(Re-Cr-Ni) phase as a diffusion barrier and outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-based alloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAl formed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activity pack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves three phases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the two step Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as a diffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(2050)at% Cr alloy changed from the δ-Ni2Al3 coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated at temperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). In the diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (or Hastelloy X) and Ni aluminides containing 1250at%Al when oxidized at 1423K for up to 1800ks. It was found that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermost and Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems that the inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and the outer/outermost layers.

The Role of Bond Coat in Advanced Thermal Barrier Coating

A novel diffusion barrier bond coat with a duplex layer structure, a sigma phase Re-Cr-Ni barrier and Ni aluminides as an aluminum reservoir was formed on a Ni based superalloy (TMS 82+) and Hastelloy X. The oxidation behavior of both alloys with and without the sigma- Re-Cr-Ni -phase as a diffusion barrier was investigated at temperatures of 1373K (Hastelloy X) and 1423K (TMS-82+) for up to 360ks. It was found that the Re-Cr-Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.

Effects of aluminizing on the oxidation and hot corrosion behaviors of two-phase nickel aluminides

In this work, aluminizing treatment to improve the oxidation resistance of two-phase [NiAl+Ni 3Al] Ni aluminides was performed and their surface characteristics were compared with the same alloys without aluminizing treatment in terms of microstructure, oxidation behaviors at 1100 °C and hot corrosion resistance at 900 and 950 °C. Two-step treatment of high Al activity low temperature (HALT) aluminizing was performed to form a NiAl compound layer of approximately 230 μm thickness on the surface. Isothermal oxidation test at 1100 °C showed that at the early stage of oxidation up to 2 h a metastable θ-Al 2O 3 phase was observed on the surface, which in turn would transform into dense equilibrium α-Al 2O 3 phase in the later stage of oxidation. Due to these dense phase formed on the surface, much improved oxidation resistance in the aluminized two-phase Ni aluminide, especially in the Ni–32 at.%Al–0.5 at.%B alloy could be obtained. Also improved resistance against hot corrosion up to 950 °C was resulted from aluminized two-phase Ni aluminides.

Wear behaviour of aluminum reinforced with nickel aluminide MMCs

In this work, the wear behaviour of different aluminum matrix composites reinforced with Ni aluminides was evaluated. The composite materials were obtained through P/M routes using different techniques: mechanical alloying, gas atomizing, cold compaction and hot extrusion. Aluminum matrix powders were alloyed mechanically, the aluminides used for reinforcing the matrix being: (i) those obtained through gas atomizing; or (ii) those generated `in situ' through nickel additions. Once the materials were mixed and cold compacted, the specimens were hot extruded and heat treated (T6 treatment). Wear tests were performed in both as-extruded and heat-treated conditions, using a `pin on disk' apparatus. The wear test variables were: the alumina counterpart (a ball of 6 mm diameter); environmental humidity lower than 30%; a track length of 1000 m; and a speed of 0.1 m/s. The friction coefficient was measured for all the materials and the wear behaviour determined through the volume loss of the disk. A complete metallographical study was done using scanning electron microscopy. The main advantage from the industrial point of view is the uncanned extrusion process that enables advance materials to be obtained by low cost processing. In this way, materials can be obtained with improved wear and mechanical properties at relatively low cost.

Transition-metal aluminide formation: Ti, V, Fe, and Ni aluminides

The heats of formation for binary and ternary $3d$ transition-metal (Ti, V, Fe, and Ni) aluminides are calculated from first principles within the local density approximation. The calculated heats for Ti and Ni aluminides are typically within $\ensuremath{\sim}0.02 \mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}$ of the experimental values, while the Fe aluminides heats appear to be overestimated by $\ensuremath{\sim}0.15 \mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}.$ This discrepancy appears to be related to the local density underestimation of the on-site magnetic energy in elemental Fe that enters the alloy heat of formation. The stabilities of selected ternary phases are also discussed, and it is shown that sublattice disorder may stabilize some ternary phases.