Ni Al Solid Solution Research Articles

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni33Al33Cr34 (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10−5 to 10−3 s−1. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10−4 s−1. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for 63Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.

Nanocrystalline Ni 3Al alloy produced by mechanical alloying of nickel aluminides and hot-pressing consolidation

The reduction of grain size to the nanometer scale can improve the mechanical properties of materials, including intermetallic compounds. In the present study, the mechanical alloying process followed by hot-pressing consolidation has been used to obtain nanocrystalline Ni 3Al intermetallic compound. Nanocrystalline powders of Ni(Al) supersaturated solid solution containing 25 at.% of Al were prepared by ball milling of AlNi or Al 3Ni 2 alloys with addition of Ni powder. Subsequently, the milling products were sintered at 1000 °C under a pressure of 7.7 GPa. The XRD investigations of the consolidated pellets revealed that: (I) ordering of Ni(Al) solid solution and its transformation into a Ni 3Al intermetallic compound occurs during sintering and (II) the material remained nanocrystalline after sintering. The microhardness of Ni 3Al intermetallic produced in this work is of about 980 HV 0.1, the density of compacted materials is nearly 100% of theoretical value and their open porosity is negligible.

On the change of a Ni 3Al phase in a Ni–12 wt.%Al MCFC anode during partial oxidation and reduction stages of sintering

It is examined what changes the Ni 3Al phase, one of the two equilibrium phases in a Ni–12 wt.%Al anode, will go through during partial oxidation and reduction stages of sintering. The purpose of this study is to get basic data with which we can predict the final phase composition and eventually the creep behavior of the anode after sintering. A new phase of Ni–Al solid solution has been formed during sintering due to outward migration of Al, and coexists with the pre-existing Ni 3Al phase. The amount of the solution phase increases with time, rapidly at an early stage but with an ever-decreasing rate as time passes. It also increases with temperature, but in this case exponentially. After the stage of partial oxidation or reduction, the specimen shows the morphology of a network structure with small particles scattered on its surface. The particle consists of a mixture of NiO–Al 2O 3 solid solution and NiAl 2O 4 phases after partial oxidation, but of a single Al 2O 3 phase after reduction.

APFIM investigations on site occupancies of the ternary alloying elements Cr, Fe, and Re in NiAl

The site occupancies of the transition metals Cr, Fe, and Re dissolved in NiAl of stoichiometric composition have been determined by atomic layer resolved atom probe field-ion microscopy (APFIM). The investigations were supported by X-ray diffraction studies to evaluate the lattice parameters. These are influenced by atomic size effects and constitutional lattice defects like Ni antistructure atoms in the Al sublattice and vacancies in the Ni sublattice. The APFIM results were compared with ALCHEMI data and calculated site preference energies published in the literature. Chromium additions to stoichiometric NiAl with 0.8at% in solid solution exhibit a strong preference for Al sites. The lattice parameter of NiAl(Cr) solid solution is decreased. Iron atoms dissolved in higher concentrations of 5at % in NiAl are almost equally distributed within both sublattices. They are possessing a weak preference for Al sites, which causes a lattice expansion of NiAl(Fe) solid solution. ALCHEMI results and site preference energy data show a strong site preference of Cr atoms for the Al sublattice. In contrast, iron atoms exhibit a weak site preference for Ni sites depending upon the stoichiometry of the NiAl host lattice.Re solutes in low concentrations of about 0.2at % in NiAl possess a strong site preference for the Ni sublattice. The increase of the lattice parameter of NiAl(Re) is due to the pronounced size effect of Re atoms. For these species no ALCHEMI and site preference energy data are available in the literature.

Atom probe field ion microscopy investigations on the intermetallic Ni49.5Al49.5Re1 alloy

AbstractThe hypoeutectic intermetallic alloy Ni49.5Al49.5Re1 exhibits promising mechanical properties for high‐temperature applications up to 1250°C, therefore atom probe field ion microscopy (APFIM) investigations were performed in the as‐cast state and after various heat treatments (at 1000°C for different times up to 1000 h and at 1300°C for 1000 h). The imaging properties and the effect of various heat treatments on the solubility and precipitation of Re within the NiAl matrix have been investigated. The field ion micrographs of Ni49.5Al49.5Re1 alloy reveal characteristic imaging features of the NiAl–Re system. Single Re atoms dissolved in the NiAl solid solution matrix were observed as bright imaging species. This is due to preferential retention of Re atoms, as well as the high ionization rate caused by adsorption of Ne image gas atoms by Re atoms. The adsorption of Ne image gas atoms is proved by the occurrence of ReNe3+ and ReNe2+ ions detected in the atom probe mass spectra. Rhenium precipitates of different shapes and several tens of nanometres in size were imaged, as well as small clusters of a few atoms. Conglomerations of a few Re atoms have been detected and the site occupancy of Re atoms in the NiAl superlattice was determined by means of APFIM measurements. Copyright © 2004 John Wiley & Sons, Ltd.

The effect of milling parameters on the synthesis of Ni3Al intermetallic compound by mechanical alloying

Abstract Ni and Al elemental powder mixtures with composition Ni75Al25 were mechanically alloyed in a planetary ball mill under different conditions. The structural changes of powder particles were studied by X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Mechanical alloying (MA) first resulted in a Ni(Al) solid solution which transformed to the disordered Ni3Al intermetallic compound with nanocrystalline structure on further milling. The rate of this transformation was found to be dependent on milling variables such as rotation speed of mill and ball-to-powder weight ratio.

The (γ + γ′)/γ′ phase boundary in the Ni–Al phase diagram from 600 to 1200°C

Abstract The phase boundary separating the γ + γ′ and γ′ regions of the Ni–Al phase diagram (the γ solvus) was established over the temperature range 600–1200°C. The γ phase is the Ni–Al solid solution, and γ′ is (non-stoichiometric) Ni3Al. Cast alloys containing 22.0, 22.2, 22.4, 22.6, 22.8 and 23.0 at.% Al, all compositions being within ±0.03 at.%, were used in the experiments. At temperatures from 1000 to 1200°C, the γ solvus was determined using aging treatments designed to eliminate the dendritic structure in the as-received castings. At lower temperatures, from 600 to ≈ 900°C, dissolution experiments were conducted. The method takes advantage of the absence of a barrier to the dissolution of a dispersed phase. Specimens of the supersaturated γ′ phase were aged at low temperatures, 600 or 650°C, for long times (up to 633 h), to produce fine dispersions of the coherent γ phase precipitates. These specimens were then re-aged at 10–20°C intervals of temperature and re-examined to determine whether the γ...

Creep behavior of Ni-12 wt.% Al anodes for molten carbonate fuel cells

Ni-12 wt.% Al anodes are fabricated for use in molten carbon fuel cells by tape casting and sintering. Sintering is performed in three steps, first at 1200 °C for 10 min in argon, then at 700 °C for 2.5 h in a partial oxidation atmosphere ( P H 2 / P H 2O =10 −2), and finally at 950 °C for 5 min, 30 min or 1.5 h in hydrogen. Three anodes with different phases or microstructures are produced at different reduction times. One anode contains three phases, namely Ni–Al solid solution, Ni 3Al, and Al 2O 3. The amount of Al 2O 3 is extremely small at 5 min. A second anode also contains the three phases with the amount of Al 2O 3 comparable with that of Ni 3Al at 30 min. Third anode contains two phases, i.e. Ni–Al solid solution and Al 2O 3 formed at 1.5 h. The creep strains measured for the three anodes after a 100-h creep test are practically the same with an average value of 0.85%.

X-ray studies on the kinetics of microstructural evolution of Ni3Al synthesized by ball milling elemental powders

Abstract Synthesis of intermetallic compound of Ni 3 Al by ball milling elemental blends of Ni and Al in the proportion of 3:1 has been done with a view to study the mechanism of alloying during ball milling. Systematic study on the kinetics of transformation in the intermediate stages of milling from 1 to 64 h has been contemplated by characterizing the microstructures in terms of various defect-related parameters viz. stacking fault, crystallite size, root mean square (rms) strain, dislocation densities, percentage of different phases formed as a function of milling time. The methodology used is the powerful Rietveld’s whole X-ray profile fitting technique. The results reveal the formation of two phases, solid solution of Ni-Al transforming gradually to Ni 3 Al (disordered). Complete transformation is achieved after 57 h of milling. The second phase forming disordered Ni 3 Al has smaller crystallite size (nano-order), larger rms strain, higher stacking fault and dislocation density compared to those of Ni-Al solid solution. Study of grain structure by transmission electron microscopy (TEM) further corroborates the value of the nano-order crystallite diameters as evaluated from the X-ray study.

The Ni-Ni3Al phase diagram: thermodynamic modelling and therequirements of coherent equilibrium

The Ni-rich portion of the Ni-Al phase diagram is examined in light of dataon coherent equilibrium between the γ (Ni-Al solid solution) andγ' (Ni3Al) phases, as well as on the thermodynamicrequirements of coherent equilibrium. The model of Ardell and Maheshwari wasused to calculate the difference between the incoherent and coherentsolubility limits over the temperature range of 400-800 °C. New dataon the elastic constants of Ni3Al as a function of temperatureand composition, as well as the Gibbs free energy functions of the most recentthermodynamic model of the Ni-Al alloy system, were used in the calculations.With these input parameters the predicted differences between the incoherentand coherent solubility limits are very small, and inconsistent with recentexperimental data on the variation of the coherent solubility with theγ' volume fraction. The inconsistency is a consequence of the largecurvature of the Gibbs free energy function for the γ phase obtainedfrom the thermodynamic model. These findings are discussed in the context ofthe accuracy of thermodynamic modelling of phase diagrams.

Temperature and composition dependence of the elastic constants of Ni3Al

The stiffness constants, c ij , of monocrystalline Ni3Al of three different compositions, 23.2, 24.0, and 25.0 at. pct Al, were measured over the temperature range from 300 to 1100 K using the rectangular parallelepiped resonance (RPR) method. The bulk modulus, as well as the shear modulus, Young’s modulus, and Poisson’s ratio for randomly oriented polycrystalline stoichiometric Ni3Al, were derived from the stiffness constants. The data indicate that c 44 is essentially independent of composition, decreasing slightly with increasing temperature for all three alloys. The values of c 11 and c 12, however, decrease with increasing aluminum content, the difference being small at room temperature but becoming larger at higher temperatures. We find that c 11 and c 12 are not as sensitive to aluminum concentration as is implied by previous results. A comparison of different shear moduli of Ni3Al and the saturated Ni-Al solid solution in equilibrium with it indicates that the ordered phase is generally elastically stiffer than the solid solution over the range of temperatures at which coarsening of the Ni3Al precipitate has been heavily investigated.

Determination Of Coherency Strains Around Particles In Ni-Al Alloys By HREM And CBED

Quantitative evaluation of coherency strains in Ni-12 at.% Al alloys has been performed by means of high resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED) techniques. The alloy Ni-12 at.%Al is composed of two phases, the disordered fee Ni-Al solid solution and the coherent Ni3Al (γ') particles with an LI2 structure. The presence of γ' particles can impart excellent mechanical properties to these alloys. However these particles coarsen if subject to high temperature with an important reduction of mechanical properties. Coarsening of coherent particles in solid alloys is driven by the decrease of surface and elastic energies. The effect of surface energy is well understood but that of the elastic energy still requires experimental investigation. The source of elastic interaction between particles is the lattice parameter mismatch between particles and matrix. Evaluation of the elastic strain fields around particles can give some insight into the elastic interaction between particles and thus into the coarsening mechanism from an experimental viewpoint.

Measurement of γ′ precipitate morphology by small angle neutron scattering

Modern Ni-base superalloys contain large volume fraction of ordered Ni{sub 3} (Al, Ti) precipitates ({gamma}{prime} phase) which are coherently embedded in a Ni-Al solid solution ({gamma} phase). Superalloys are strengthened by the {gamma}{prime} precipitates and it is known that the precipitate morphology, their distribution in the matrix and their volume fraction strongly influence the strength of the alloys. In this paper the authors results from the measurements by small angle neutron scattering (SANS) and use special data analysis procedures to quantify various {gamma}{prime} morphological parameters. The authors use complementary measurements by scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) to verify and augment the results of SANS. They find SANS is a very useful tool for these kind of measurements as it measures over a large volume of specimen. Further, the 3-Dimensional microstructural parameters could be obtained quantitatively from measurements on a single specimen with the help of a cradle used to in-situ align the specimen with respect to the neutron beam.

Compatibility between Aluminium Nitride and Nickel.

In relation to the joining of aluminium nitride ceramics to metal, the reaction products and the reaction mechanism between AlN and Ni have been investigated under a nitrogen or an argon atmosphere at temperatures of 1273-1673K. Using AlN-Ni powder mixtures, reaction rates were determined by thermogravimetric analysis and the reaction products were examined by X-ray diffraction. The reaction between AlN and Ni occurred above 1250K under both N2 and Ar atmosphere. Under N2 atmosphere, the reaction product was only a Ni-Al solid solution. Under Ar atmosphere, at higher temperatures and on prolonged heating, the reaction product was changed in the following order: Ni-Al solid solution, Ni-Al (s.s.)+Ni3Al, Ni3Al, Ni3Al+NiAl and finally NiAl. The initial rates of the reaction obeyed a linear rate law. Under both N2 and Ar atmosphere, the rate constants and the activation energies (224 and 210kJ/mol) showed comparable values. The reaction rate is thought to be controlled by a chemical process, such as the dissociation of AlN. When the Ni particles were covered with a reaciton layer, the rate obeyed a parabolic rate law. The rate constant under N2 atmosphere was 2 orders of magnitude lower than that under Ar atmosphere. In the range of stability of Ni-Al(s. s.), the activation energy under N2 atmosphere (124kJ/mol) was almost identical to that under Ar atmosphere (127kJ/mol). Under Ar atmosphere, the activation energy for Ni3Al formation was 342kJ/mol. The kinetics are probably determined by solid-state diffusion through the reaction layer.

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni3Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl2O4, and an inner layer of Al2O3, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al2O3 layer at the grain boundaries which favor oxygen penetration through the grain interior.

Evidence for Significant Short-Range Order Effects on Surface Segregation in Ni-Al Solid Solution

Aluminum surface segregation in Ni-9% Al as studied by low-energy Auger electron spectroscopy is characterized by quite low equilibrium levels that increase with temperature, in contrast to predictions of Bragg-Williams--type theories. The free-energy cumulant expansion approach, adapted for surface segregation, reveals that short-range order can induce significant suppression of solute segregation, depending mainly on the solvent-solute interaction strength. The diminution of the effect with increasing temperature leads to anomalous increase of equilibrium segregation, in agreement with the Ni-Al experiments.

Ordering kinetics of disordered Ni 3Al synthesized by mechanical alloying

The Ni[sub 3]Al compound belongs to that class of intermetallics whose virtual critical temperature for the order-disordered transformation is higher than the melting temperature. Chemically disordered Ni[sub 3]Al can be prepared by several methods including high energy ball milling (BM) of the ordered compound. Another possibility is offered by mechanical alloying (MA) of elemental Ni and Al powders. Aluminum and nickel powders in the 3:1 atomic ratio were mechanically alloyed in an air-cooled SPEX mill for times up to 40 hours. EDS microanalysis after MA showed an Fe contamination of about 2 and 4 at.% in the powders milled for 20 and 40 hours respectively. The structural characterization of the samples was performed by x-ray diffraction and their thermal stability was tested in a Perkin-Elmer DSC7 calorimeter. The diffraction patterns of the samples milled for at least 10 hours were very similar and showed the presence of a supersaturated fcc Ni(Al) solid solution whose diffraction peaks were considerably broadened with respect to those of the starting Ni powder.

Resistivity recovery in Ni-Al alloys after ultrasonic and static loading at 4.2 K

Results are presented from investigations of the recovery of resistivity in two Ni-Al solid-solution alloys (containing 7.5 or 5.5 at. pct Al) subjected to ultrasonic and static deformation treatments in liquid helium. The alloy samples were annealed for 4 hr at 1370 K in a vacuum, then furnace cooled. The variations of the resistivity during the isochronal annealing of low-temperature deformed Ni-Al solid solutions was interpreted in terms of defect annihilation and short-range ordering. The shape of the resistivity curve was found to depend on the method of deformation (ultrasonic or static deformation). 12 refs.

Oxidation of Single-Crystal γ′-Ni3A1 at Low Oxygen Partial Pressure

ABSTRACTSingle crystals of γ′-Ni3Al((001)-oriented) were oxidized at 1223 K under an oxygen partial pressure of ∼4 ×10−19 atm for times ranging from 0.1 to 50 hours. Microstructural development of the oxide scale and subscale metal was studied by electron microscopy. A special technique permitted the reproducible and efficient preparation of TEM cross section specimens. Initially, a fine-grained γ-Al2O3 scale formed with a preferred orientation. Depletion of Al from the γ′-Ni3Al resultedin a Ni-Al solid solution zone between the oxide scale and the intermetallic. After 20 hours oxidation, a discontinuous α-A12O3 layer between the γ-A12O3 and the metal was observed. The α-A12O3 layer exhibited a much larger grain size than that of the γ-A12O3 and was continuous after 50 hours oxidation. Formation of the α-A12O3 layer correlated with a decreasing parabolic oxidation rate constant kp, as measured by thermogravimetric analysis (TGA).

X-ray diffuse scattering investigation of different states of local order in Ni-Al solid solutions

The local atomic arrangement in the γ Ni-Al solid solution and its evolution with temperature and concentration are investigated in quenched samples by X-ray diffuse scattering. The value of the first Warren-Cowley parameter indicates strong short range order, but the other parameters decrease rapidly. Both the amount and range of the short range order increase with aluminium content and decrease with increasing temperature. The pair interaction potentials are determined in the Ising model through an inverse cluster variation method (ICVM). From these pairwise interaction potentials the virtual ordering temperature for Ni 3Al is estimated, as well as antiphase boundary energies. They are compared to available experimental results.