Intermetallic NiAl Research Articles

Ab initio calculation of point defect energies and atom migration profiles in varying surroundings in L12-ordered intermetallic compounds

ABSTRACTFormation energies of antisite defects and vacancies were derived for the L12-ordered intermetallics Ni3Al, Ni3Ga, Pt3Ga, and Pt3In by a supercell ab initio approach. A thermodynamic treatment of point-like defects was then used for the calculation of temperature-dependent defect properties. Energy profiles for atom jumps in Ni3Al in systematically varied atomic neighborhoods were calculated by statically displacing the jumping atom or by using a nudged elastic band method. It is discussed how a kinetic Monte-Carlo model can be modified so that the jump barrier height reflects the strongest neighborhood influences.

Atomistic simulations of Paidar–Pope–Vitek lock formation in Ni 3Al

The thermally activated process of Paidar–Pope–Vitek (PPV) lock formation in the L1 2 intermetallic Ni 3Al is simulated using the “nudged elastic band” method with an embedded atom potential. It is found that the one-step PPV lock is intrinsically unstable, but it can be stabilized by a large enough Escaig stress. The two-step PPV lock can be formed with a low activation energy when the one-step lock is involved as an intermediate step. In this case, the overall activation energy is about 1 eV, in reasonable agreement with the range of 0.5–0.9 eV expected from experimental observation of the flow stress anomaly in Ni 3Al. High Escaig stresses are required to stabilize the one-step lock, and the absence of such high stresses in the micro-strain regime is thought to be the underlying reason for the absence of the flow stress anomaly in micro-strain conditions.

Formation of intermetallic compounds in the Ni–Al–Si ternary system

Phase equilibria in the Ni–Al–Si system have been studied in the temperature range 550 °C using a ternary diffusion couple experiment with pure Ni and an Al–Si eutectic alloy. A number of binary and ternary intermetallic compounds have been observed. The intermetallics NiAl 3, Ni 2Al 3, NiAl and Ni 3Al form as solid layers just above pure Ni. Each of the binary intermetallics has some solubility for Si, which replaced Al in the ordered lattices. Two ternary intermetallics τ 1 and τ 3 have been observed. The τ 1 phase has a composition very close to Ni 4AlSi and τ 3 (Ni 3(Al 1− x Si x ) 7) has been analyzed to have a lower Si content. A new ternary phase Al(Ni)Si 4(τ 4) has been observed in the diffusion couple heat treated for longer times at 550 °C. The morphology of the binary and ternary phases is presented.

Synthesis and Characterization of Ni-Al Intermetallics Compounds with Grain Refinement Emphasis

Synthesis and Characterization of Ni-Al Intermetallics Compounds with Grain Refinement Emphasis

HRTEM study of the microstructure of TiC 0.6/NiAl composite

Intermetallic NiAl bonded sub-stoichiometric titanium carbide based composite of TiC 0.6/NiAl was fabricated by a two-step melt infiltration processing method, and its microstructure was characterized with scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Ordered Ni 2AlTi precipitated in NiAl matrix and a new long period modulated structure as a transition layer appeared at the phase boundary.

Aluminium Enriched Diffusion Layers On Nial Alloy

Materials based on Ni–Al intermetallics are prospective materials for high temperature applications. They often show good resistance to oxidation and lower specific gravity than nickel based superalloys. High temperature resistance of Ni–Al intermetallics to oxidation can often be improved by aluminium enriched surface layers. Microstructure and chemical composition of aluminium enriched layers on an Al–79 wt-%Ni alloy were investigated using SEM and local elemental analyses. The layers were prepared by diffusion annealing of the Ni–Al alloy in contact with an aluminium plate preferentially at temperatures 480–500°C, for 5 h. In these conditions continuous layers of the Ni2Al3 and NiAl3 phases originate. The region of the aluminium plate in contact with the nickel rich phases transforms into a two phase structure consisting of an aluminium matrix containing < 1 wt-%Ni and two morphological types of NiAl3 precipitates which have nominal compositions. Solid state diffusion and reactions in the Ni–Al system are also important phenomena in microelectronic devices.

Production-scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application: Part I. Production of master alloy remelt ingots and investment casting of combustor liner model panels

Several master alloy remelt ingots of about 100 kg each with nominal composition 45 at.% Ni, 45 at.% Al, 7.5 at.% Cr and 2.5 at.% Ta were produced by vacuum induction melting. These ingots were used for a series of investment castings with variation of casting parameters in order to develop and optimise the investment casting process for this new intermetallic NiAl-based alloy. The microstructures of the remelt ingots and of various investment castings were characterised by light optical and scanning electron microscopy (SEM). The compositions of the constituent phases were determined by energy dispersive and wavelength dispersive analysis. The 0.2% yield stress, secondary creep rates and oxidation rates were determined and compared with those of materials produced on laboratory scale. Except of a decreased toughness of the production scale material the properties were otherwise comparable to those of the laboratory scale material. Though properties of the cast alloys are found not to be sensitively affected by impurities such as oxygen, carbon, iron, hafnium and zirconium, silicon was found to promote segregation during casting which is believed to result in embrittlement of the production scale material. Silicon contaminations could be avoided by usage of a ceramic-free cold-wall induction melting aggregate, but due to the coarse resulting microstructures this did not improve toughness of the production scale material.

Thermal cycling behaviors of thermal barrier coatings on intermetallic Ni 3Al based superalloy

Ni 3Al based alloy IC6, a high performance cast alloy with approximately 14 wt.% Mo addition, has been developed for turbine blades and vanes of advanced aero-engines and other high temperature structural components. However, because of the high Mo content, during high temperature use, Mo will diffuse into the protective coating layer, which will affect the thermal cycling performance directly. In order to block the diffusion and improve the thermal cycling behavior of thermal barrier coatings (TBCs), a Si chemical gradient MCrAlYSi bond coat was prepared by changing the Si content from 0.35 wt.% at the interface of substrate and bond coat to almost 0 near to the surface of the bond coat. For comparison, conventional two-layered structural TBCs with MCrAlY as bond coat were also prepared. The fabricated coatings were evaluated for the microstructures and the composition distribution properties, such as thermal stability and resistance to oxidation. For the bond coat without Si addition, Mo content near the surface was higher than 7 wt.% after isothermal oxidation at 1373 K for 60 h and 12.5 wt.% after thermal cycling tested at 1373 K for 300 h (600 cycles). However, in the Si chemical gradient bond coat, only 0.4 wt.% Mo was detectable after isothermal oxidation at 1373 K for 100 h and approximately 0.66 wt.% after thermal cycling tested at 1373 K for 340 h (680 cycles).

Production scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application: Part II. Powder metallurgical production of bolts by hot isostatic pressing

A series of cylindrical bolts of 80 mm diameter and 115 mm height each weighing about 3.5 kg were produced by hot isostatic pressing (HIP) from pre-alloyed powder of the new intermetallic NiAl–Ta–Cr alloy for high-temperature application with nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Al (at.%). The HIP parameters (temperature, pressure, and time) were chosen according to production scale processing conditions for obtaining specific microstructures. To avoid reactions between the NiAl–Ta–Cr alloy and the Ni capsule, a special coating technique was employed. The microstructure of the compacts was characterised by light optical and scanning electron microscopy and the compositions of the constituent alloy phases were determined by energy dispersive analysis. The temperature dependence of the 0.2% yield stress and the stress and temperature dependence of creep were checked and compared with those of the corresponding laboratory scale material.

Properties of Ni–Cr–Al Alloys Obtained by a Powder Metallurgy Method

Alloys of practically 100% density were formed by hot pressing mixtures of nickel, chromium and aluminum powders. The principal phases formed are solid solutions based on chromium, on nickel, and also the intermetallic NiAl. These correspond to phases in the phase equilibrium diagram of theNi ― Cr ― Al system. The strength and wear resistance of the alloys are determined by the ratio of phase components.

TEM Investigation on Microstructural Characteristics in Nanostructured Al-Mg Alloy

ABSTRACTThe microstructural evolution in a nanostructured Al-Mg alloy fabricated by cryogenic mechanical alloying (cryomilling), was investigated using transmission electron microscopy (TEM). Nanostructured Al-Mg powders were first synthesized by a mechanical alloying under liquid nitrogen media (cryomilling), and the powders were subsequently degassed, hot isostatically pressed, and extruded into a full dense, bulk form. The results showed that Si containing phases and (Fe,Ni)Al intermetallics existed in as-extruded Al-Mg alloy. In addition, the extrusion temperature has a strong influence on the formation of microstructural anisotropy. A lower extrusion temperature yields a microstructure that is more anisotropic relative to that present at the higher extrusion temperature. More specifically, at the lower temperature, the nano-sized Al grains have a tendency to rotate towards the &lt;111&gt; direction, along the extrusion direction.

Fabrication of intermetallic NiAl by self-propagating high-temperature synthesisreaction using aluminium nanopowder under high pressure

By using aluminium nanopowder prepared by wire electrical explosion, puremonophase NiAl compound with fine crystallites (≤10μm)and good densification (98% of the theoretical green density) was successfullyfabricated by means of self-propagating high-temperature synthesis (SHS)under a high pressure of 50 MPa. Investigation shows that, due to thephysical and chemical characteristics of the nanoparticles, the SHS reactionmode and mechanism are distinct from those when using conventionalcoarse-grained reactants. The SHS reaction process depends on the thermalconditions related to pressure and can occur at a dramatically lowtemperature of 308̂C, which cannot be expected in conventional SHSreaction. With increasing pressure, the SHS explosive ignition temperature(Tig)of forming NiAl decreases due to thermal and kinetic effects.

Reactive sintering of nickel aluminide-(alumina or silicon carbide) composite materials

Nickel aluminide intermetallic compounds possess attractive properties that make them good candidates for high temperature structural applications. The production of these materials in short processing times and low energy via reaction synthesis routes has been reviewed in this article including most recent novel reactive methods applied to Ni–Al intermetallics (e.g. microwave combustion synthesis and hot extrusion reaction synthesis). Future directions in this field have been suggested.

Nickel aluminides: a step toward industrial application

Intermetallics based on nickel aluminide are subjects of an ongoing development of new high-temperature materials for application as heat shields for combustion chambers and as first-row vanes in industrial gas turbines. Due to the complex geometry of these parts and the complicated machinability of NiAl intermetallics, these components have to be manufactured by near-net-shape casting. This casting technique required an adaptation of the investment casting process to the special properties of NiAl alloys. For this purpose, a special ceramic mold system was developed to prevent casting defects and excessive mold–metal reactions. Suitable casting parameters were established for the production of grain-refined, crack-free heat shields. Accompanying numerical simulations were performed to optimize the investment casting process for the NiAl-alloy helped to save development time and to reduce the number of casting trials noticeably. These modeling results for various casting conditions (mold and pouring temperature, cooling rate) were compared with outcomes of casting trials in aspects of porosity, mold filling and temperature-field development during the casting process.

Synthesis of intermetallic NiAl by SHS reaction using coarse-grained nickel and ultrafine-grained aluminum produced by wire electrical explosion

Nickel aluminide NiAl was synthesized by using aluminum nanoparticles via the thermal explosion mode of the self-propagating high temperature synthesis (SHS). Differential scanning calorimetry (DSC) was used to investigate the reaction process and mechanism. It was found that the mode and mechanism of reaction strongly depended on the charateristics of aluminum nanoparticles that have small grain size, high surface areas, high stored-energies and low melting point. The initial reaction started from 470 °C. The SHS reaction was controlled by a competition between thermal and kinetic processes. With increasing green density and heating rate, the ignition temperature decreased. With proper heating rates, a fine (<10 μm), monophased, homogeneous NiAl compound was obtained.

The enhancement of tensile strength and elongation in two phase (NiAl+Ni 3Al) intermetallics of Zr-doped Ni–20Al–27.5Fe (at.%) alloys

Microstructure and tensile properties in the nickel intermetallics, Ni–20Al–27.5Fe and Ni–20Al–27.5Fe–1Ti (at.%), were investigated in order to understand the effect of zirconium addition. It was found that the addition of zirconium causes formation of a thin layer along the phase boundary of NiAl and Ni 3Al. The thin layer was identified as an ordered fcc phase with 6.9 Å in lattice parameter. The compressive deformation caused by hot extrusion results in fine precipitates and a mixture of subgrains within the matrix phase in zirconium-doped alloy. Additional zirconium gives significant improvement in the room and high temperature fracture strength and strain. The extruded zirconium-doped alloy exhibited a high tensile elongation of above 10%, and also a relatively high tensile strength of 1350 Mpa at room temperature, keeping it as high as 600 Mpa at 700°C.

611 アルミナ/NiAl 複合材料の表面自己組織化による強靱化

Tri-layer composites consisting of surface layers with residual compressive stress and a stress-tolerant inner layer have been fabricated by the oxidation of alumina-matrix composites dispersing intermetallic NiAl. The residual surface stress is generated by the volume expansion due to the formation of NiAl_2O_4. The starting Al_2O_3-NiAl powder was prepared by heating the precursor mixture of α-Al_2O_3 powder for the matrix and (Ni, Al)-hydradino-benzoate precipitate of at 1200℃ in Ar. The fracture toughness of. the 10vol%NiAl-Al_2O_3 composites was 6.3MPa・m^<1/2>, indicating the suitability as the inner layer material applied under tensile stress. The measured surface compressive stress for the tri-layer composites was more than 120MPa and this stress improved the fracture toughness to 1.5 times of the dispersion composites at the degree of oxidation of 85%. However, when the surface oxidation progressed to more than 90%, much defects formed at the boundary of the surface and inner layers to relax the stress and degrade the toughness.

Corrosion resistance of the intermetallic compound, NiAl, in a molten carbonate fuel cell environment

The corrosion resistance of intermetallic NiAl, superalloy GH217 and an 18-8 stainless steel in molten carbonates at 923 K was determined by weight loss and electrochemical measurements. Morphology and structure of the corrosion products were characterized using a combination of electron probe and X-ray diffraction (XRD). The corrosion resistance of NiAl is the best among the materials investigated. NiAl produces more protective corrosion products and forms more complete oxide films on the surface than GH217 or 18-8 stainless steel.

High velocity oxy-fuel spraying of a NiCoCrAlY and an intermetallic NiAlTaCr alloy

The high velocity oxy-fuel (HVOF) spray systems of the third generation can be used to process some oxidation sensitive materials such as MCrAlYs, due to possible lower melting degrees and higher kinetic energies of the powder particles. In the study presented here, a NiCoCrAlY and an intermetallic NiAlTaCr were sprayed using the HVOF process. The influence of the spray parameters on the oxidation during spraying was investigated. Both the O/H ratio and the spray distance could significantly influence the oxygen content of the coatings. Coatings containing low oxygen contents were developed. The microstructures of the coatings were studied in as-sprayed as well as heat-treated states. The NiCoCrAlY coating showed a homogeneous two-phase structure after the heat-treatment. The NiAlTaCr coating consisted of the intermetallic NiAl phase and an intermetallic phase of Ta, Ni, Al and Cr. The high temperature oxidation behaviour of the coatings was also tested. The oxide scales on the NiCoCrAlY and NiAlTaCr coatings formed at 1100°C consisted mainly of Al 2O 3. Traces of some intermetallic phase components could be detected in the oxide scale on the NiAlTaCr coating after 100 h oxidation.

Electronic mechanism of impurity?dislocation interactions in intermetallics: NiAl

The nature of impurity-dislocation interactions is one of the key questions governing the strength and plasticity of solid-solution materials. To investigate the influence of impurities on the mechanical properties of intermetallic NiAl, the electronic structure and energy of NiAl with a <100>{010} edge dislocation and transition-metal impurities was calculated using the real-space tight-binding linear muffin-tin orbital method. The localized electronic states, appearing in the core of the dislocation, are found to lead to strong impurity-dislocation interactions via two mechanisms: firstly, chemical locking, due to strong hybridization between impurity electronic states and dislocation localized states; secondly, electrostatic locking, due to long-range charge oscillations caused by the electron localization in the dislocation core. The results obtained explain qualitatively why the solid-solution hardening effect in NiAl correlates with the electronic structure of impurities rather than with size misfit, as expected according to traditional views.