Pure NiAl Research Articles

Dislocation processes during the deformation of NiAl–0.2at.%Ta

Macroscopic compression tests and in situ straining experiments in a high-voltage electron microscope were performed on NiAl–0.2at.%Ta at room temperature and at elevated temperatures. At room temperature in soft orientations, dislocations of a〈100〉 Burgers vectors bow out between jogs. In contrast to pure NiAl, the dislocations move in a viscous way between the pinned configurations. At 475°C in a hard orientation, dislocations with a〈110〉 Burgers vectors move in a viscous way in configurations strongly depending on the respective slip plane. Preferred orientations of dislocations are of mixed character, most pronounced as very straight dislocations oriented along 〈111〉 directions on {110} planes. These configurations cannot be explained on the basis of the existing atomistic theories. The flow stress is interpreted in terms of the back stress of the dislocations bowing out between jogs at room temperature, the statistical theory of solid solution hardening, and the formation of atmospheres containing Ta atoms at elevated temperatures.

The effect of fe on high temperature oxidation of NiAl

A study was conducted to observe the oxidation of NiAl+3.5at.%Fe alloy inβ-NiAl phase field at air temperatures of 1000, 1200 and 1400°C, and these results were compared with those of pure NiAl alloys. The primary effects of the Fe-addition in NiAl were found to be: 1) decrease in oxidation resistance and adherence of scales during both isothermal and cyclic oxidation tests, 2) enhancement of phase transformation rate from θ toα-AL2O3, 3) more rapid formation of characteristically ridgedα-A12O3 scales during initial oxidation stages, and 4) partial sealing of voids formed at the scale-substrate interface and dissolution of Fe inside the alumina scale by the outward diffusion of Fe from the substrate alloy.

Smart Oxygen Diffusion Barrier Based on IrAl Alloy

ABSTRACTIr should be used as an effective oxygen diffusion barrier (ODB) for ultrahigh temperature structural materials since If exhibits extremely low oxygen diffusivity. Oxidation resistance of Ir is, however, not good due to formation of gaseous oxide, IrO3, over 1390K. In this study, the improvement of oxidation resistance was aimed through alloy design of alloying with Al. IrAl is expected to form a self-healing multifunctional layered structure composed of Ir layer as ODB and A12O3 layer as a protective oxide (PO) on the Ir layer. Ar arc-melted IrAl alloy was crushed into powder, followed by hot pressing and heat treatment to remove Ir formed by eutectic reaction. Oxidation behavior was measured using thermogravimetry (TG) and differential thermal analysis (DTA) under the conditions of (1) dynamic heating of 0.167K/s and (2) isotherms at 1273K to 1673K in O2 environment. It was found that oxidation resistance is much improved by alloying with At and that the designed structure (PO/ODB) is formed on the IrAl substrate. Compressive mechanical properties were investigated from RT to 1873K: both the strength as a function of normalized temperature and specific strength are higher than those of pure Ir, NiAl, TiAl and Ni3A1. IrAl is promising as a advanced smart coating material equipping good oxidation resistance as well as high temperature strength.

Deformation processes at crack tips in NiAl single- and bicrystals

Pre-cracked single- and bicrystals were examined in order to understand the quasi brittle behaviour of NiAl single crystals and the strength of grain boundaries. Single crystals were tested in-situ within a modified scanning force microscope (SFM). Sequences of elastic deformation, dislocation emission, followed by discontinuous crack growth on a submicron scale were observed within the SFM. In addition the fracture toughness of bicrystals with stoichiometric and off-stoichiometric composition pre-cracked along and perpendicular to the boundary was measured. The results proved that NiAl is inherently ductile. However, depending on purity, discontinuous crack growth occurred with local crack opening displacements (COD) of less than 100 nm and slip band lengths of less than 10 μm, showing clearly that local dislocation pinning can initiate brittle crack growth after blunting. In all cases the toughness of grain boundaries was more than a factor of two lower than the toughness of the adjacent component crystals. This proves that grain boundary brittleness is an inherent property of pure NiAl. However, boundaries with a surplus in Ni were tougher. For cracks precracked perpendicular to the boundaries dislocation-boundary interaction can either increase or decrease the toughness depending on the orientation of the adjacent grains.

He + scattering from Ni, Al, and their alloy. Model calculation of the charge exchange

The changes in the He + neutralization probability when scattered from pure Al and Ni targets and from a NiAl alloy are analyzed. The neutralization probability is calculated using a time-dependent quantum formalism which allows incorporation of several electronic bands of localized and extended nature in the description of the target band structure. The parameters of the interaction Hamiltonian are obtained from an ab-initio model based on a pair-bonds superposition. We find that for He + scattering from Al atoms in NiAl the neutralization is less probable than for He + scattering from pure Al, while for the scattering from Ni atoms there is no change in the neutralization probability when going from pure Ni to the alloy target. These results are in agreement with the experimental trends which show a matrix-dependent signal only for the scattering from Al. The marked difference in the neutralization behavior of He + can be explained by taking into account the differences in the electronic structure between pure Al and NiAl alloy, and the presence of the corelike Al-2p state which promotes the charge exchange process at low primary He + energies.

Fracture Toughness, Fracture Planes and BDT in Stoichiometric Feal and Nial Single Crystals

AbstractFracture toughness data of stoichiometric FeAl and NiAl single crystals were measured in four-point-bending tests and the crystallographic orientations of the resultant fracture surfaces were determined.For FeAl single crystals unexpectedly high fracture toughness values were measured even at low temperatures (17 MPa [001] at 77K). However, these crystals failed in a brittle manner for temperatures up to 300K. The crystallographic orientation of the fracture surfaces were strongly dependent on the environment due to hydrogen embrittlement.The NiAl specimens were precracked and the fracture toughness was measured in the weakest crystal orientation. The fracture surfaces were similar to those obtained in previous investigations. Although the room temperature (RT) KIC-value was the highest for pure weak-oriented NiAl (7.5 MPa [001]) the BDT temperature was found in the same region where it occurs for the hard orientation (673K to 693K). The onset of the BDT will be discussed.

Anomalous Oxidation of Intermetallics

AbstractMoSi2, (γ-NiAl and TiAl with Cr additions are of interest for high temperature applications in oxidizing environments, where an oxide layer such as SiO2 or Al2O3 should form to protect the base material. At elevated temperatures (600-1700°C), a protective SiO2 layer forms on MoSi2, while near 500°C pesting and/or accelerated oxidation could disintegrate the material to powder as Mo and Si oxidize to form a complex, thick, non-protective oxide layer. Use of γ -TiAl is limited by poor oxidation resistance, whereby layered mixed oxides of TiO2 and Al2O3 form. With the addition of Cr from 4 to 34 at%, results are varied: protective Al2O3 formation, mixed oxide formation as with TiAl or more rapid oxidation than TiAl. NiAl is currently used as a diffusion coating on Ni-based superalloys and is being considered for use as a structural material itself because of its excellent oxidation resistance, i.e. forming α-alumina above 1000°C. Recent work indicates that pure NiAl oxidized under low oxygen partial pressures in a contained atmosphere develops nonprotective oxide scales similar to accelerated oxidation of MoSi2. This study explores the parameters defining protective behavior of these intermetallics and attempts to describe and explain anomalies at low temperatures and pressures.

ＭＡ法によるＮｉアルミナイドの製造とその機械的性質

As representative Ni aluminides, NiaA1 (26, 27at%Al), NiiAl+NiAl (32, 38at%Al) and NiAl (42, 46at%Al) intermetallic compounds were produced by mechanical alloying (MA) process for the purpose of refining microstructures. MA was performed in the attritor mill using pure nickel and NiAl prealloyed powders. MA treated powders were consolidated using the vacuum hot pressing. Produced aluminides showed fine grain microstructures with average grain size under 4 pm and had relative density above 97%. Each of them, however, contained a few percent of alumina which was made by oxidation of Al during MA treatment. This causes the decrease of Al content of at most 2 at%A1 compared with blend composition. High strengths over 1000 MPa and large fracture strain of more than 15% at room temperature were found for all of the alloys. Furthermore, it was confirmed that the flow stress of two phase alloy was larger than that of mono-phase alloys in the temperature range from room temperature to 1073K. The high strain rate sensitivity exponent (m value) of more than 0.3 was obtained for Ni3Al at 1073K and this is considered to be associated with the development of superplastic deformation.

Microstructures and Mechanical Properties of Ni-Nb Aluminides Produced by MA Process.

Of all the intermetallics that belong to Ni-Nb-Al system, (Ni3Al+Ni3Nb), (NiAl+NiAlNb), (NiAl+Ni3Al), NiAl and Ni3Al were produced by mechanical alloying (MA) process with a view to refining microstructures. MA was carried out in the attritor mill using pure nickel, pure niobium powder and NiAl prealloyed powder as starting materials. MA powders were consolidated using the vacuum hot pressing.Sintered aluminides showed fine grain structures with average grain diameter under 5 μm and relative density above 96%. The combination of high strength and large fracture strain of more than 10% at room temperature was confirmed by compression test for each aluminide except for both (NiAl+NiAlNb) and (Ni3Al+Ni3Nb). At high temperatures, these aluminides exhibited a steady state deformation and no fracture during the testing. The strain rate sensitivity exponent (m value) of more than 0.3 was obtained for alloys except NiAl. Especially the highest value of m (>0.6) was observed for (Ni3Al+Ni3Nb) at 1 173 K. Such large m value more than 0.3 is believed to be associated with the development of superplasticity.

An18O tracer study on the growth mechanism of alumina scales on NiAl and NiAlY alloys

The effect of Y additions on the oxidation mechanism of NiAl at 1270 K has been investigated. Mass transport in the alumina scale was examined with18O tracers. Proton activation analysis (18O(p, α)15N) and SIMS were used to measure the18O distribution in the scale. On pure NiAl and low-doped (0.07% Y) NiAl mainly outward scale growth was observed. Addition of 0.5% Y induces oxide formation at both interfaces of the scale. Larger quantities of Y added to the alloy (>0.5%) do not dissolve completely in the NiAl, but form Y-rich segregates. Internal oxidation of these intergranular segregates was observed. The relation between the modification of the scale-growth mechanism and the improved adherence of the scale to the alloy due to the addition of Y is discussed.

Effects of γ irradiation and lithium doping on the catalytic activity of nickel-aluminum mixed oxides

The catalytic activity of pure NiO and Ni-Al mixed oxides subjected to various doses of γ irradiation was investigated using the oxidation of CO with O 2 at various temperatures. The effect of lithium doping on the catalytic activity of Ni-Al was also investigated. The results obtained revealed that γ irradiation of pure NiO using doses up to 10 Mrad effected a decrease of 45% in its catalytic activity which suffered a further slight change when the dose was increased to 20 Mrad. The activity of Ni-Al mixed oxide was modified in a somewhat complicated manner by γ irradiation. Doses of 5 – 10 Mrad produced an increase of 34% in its catalytic activity and a dose of 15 Mrad caused a drastic decrease in its activity of 57%. Increasing the dose of 20 Mrad produced a slight increase in the catalytic activity of the mixed oxide solid. The modifications in the catalytic activity of pure NiO and mixed oxide solids arising from γ irradiation were attributed to an induced removal of chemisorbed O 2 in the case of pure NiO and to a possible increase in the [Ni]/[Al] ratio, especially in the outermost surface layers, of the mixed oxide catalyst. Lithium doping was found to cause a significant increase in the catalytic activity of Ni-Al mixed oxide which was attributed to the increase in the amount of chemisorbed O 2 and to an increase in the [Ni]/[Al] ratio.

Support effects in the catalytic nitroxidation of toluene into benzonitrile on nickel oxide based catalysts

Pure NiO and NiAl 2O 4 (spinel) are rather selective and initially active catalysts towards conversion of toluene by NO into benzonitrile at 440°C, but they deactivate with time on stream. Coprecipitated aerogels NiO-Al 2O 3 and NiO-SiO 2 are very selective catalysts and their activity is stable with time. An interaction leading to crystallized nickel aluminate or amorphous nickel silicate is detected by XRD or IR. Mechanical mixtures of NiO or of NiAl 2O 4 (spinel) with silica aerogel exhibit the same selectivity and initial activity as pure compounds and also deactivate with time on stream. Admixed silica behaves therefore as a physical diluent of the active phase and the catalytic properties are not improved. When the previous crystallized compounds are mechanically admixed with alumina aerogel (instead of silica) a higher catalytic stability with time on stream is registered for NiO, while the deactivation is no longer observed for NiAl 2O 4.

The influence of reducing conditions on the sodium sulfate-induced corrosion of pure and alloyed βNiAl

This study concerns the corrosion of the β-NiAl phase (pure or with additions of Co and Cr) by Na 2SO 4 in reducing atmospheres ( P o 2 = 10 −16 atm), at 850°C. With pure NiAl, an alumina scale impregnated with Na 2S is formed at the surface, whereas in the alloy, aluminum depletion leads to the formation of a very porous scale of the γ′Ni 3Al phase. With chromium additions, sodium chromite is observed on the alumina scale and sodium thiochromite at the γ′/oxide interface. A high corrosion rate is observed, which is probably due to the formation of a liquid sulfide filling pores in the γ′ layer.

Activities and their relation to cation distribution in NiAl 2O 4MgAl 2O 4 spinel solid solutions

The activity of NiAl 2O 4 in NiAl 2O 4MgAl 2O 4 solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl 2O 4 + Al 2O 3(α)/CaOZrO 2/Ni + Ni x Mg 1− x Al 2O 4 + Al 2O 3(α). Pt, in the temperature range 750–1150°C. The activities in the spinel solid solutions show negative deviations from Raoult's law. The cation distribution in the solid solutions has been calculated using site preference energies independent of composition for Ni 2+, Mg 2+, and Al 3+ ions obtained from crystal field theory and measured cation disorder in pure NiAl 2O 4 and MgAl 2O 4, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, α, of Ni 2+ ions on tetrahedral sites for 1> x>0.25, observed by Porta et al. [ J. Solid State Chem. 11, 135 (1974)] but do not support their tentative evidence for an increase in α for x < 0.25. The measured excess free energy of mixing can be completely accounted for by using either the calculated or the measured cation distributions. This suggests that the Madelung energy is approximately a linear function of composition in the solid solutions. The composition of NiOMgO solid solutions in equilibrium with NiAl 2O 4MgAl 2O 4 solid solutions has been calculated from the results and information available in literature.

The intrinsic low temperature magnetic and transport properties of NiAl

The magnetic susceptibility, resistivity, magnetoresistance, and NMR properties of very pure stoichiometric NiAl samples have been measured. The Kondo-like phenomena previously observed in this material are absent suggesting that the intrinsic properties agree with a simple Fermi surface as predicted by APW calculations.