Ni Al Solid Solution Research Articles

Corrosion Assessment of Fe-Ni-Al Based Alloy in Saline Solution

The corrosion assessment of a Fe-Al-Ni alloy in saline solution was carried out by using electrochemical techniques. The Fe-Al-Ni alloy was produced by casting in an induction furnace. It was microstructural and chemically characterized by SEM, EDS XRD. The microstructural characterization results of the casting Fe-Ni-Al alloy showed a homogeneous microstructure morphology, with some porosities formed during the solidification process. The Fe-Ni-Al base alloy presented a B2-(Fe,Ni)Al solid solution phase and non-third phase was observed. The potentiodynamic results showed a more positive Ecorr value in the solution at pH 5. At the pH 7, the linear polarization resistance results showed the highest values of the icorr kinetic. Whereas, the best performance against corrosion was observed at pH 5, where the lowest icorr kinetic values were observed. This was associated to the formation of a more stable and dense scale of corrosion products at that condition.

Calculation of open and closed system elastic coefficients for multicomponent solids

Thermodynamic equilibrium in multicomponent solids subject to mechanical stresses is a complex nonlinear problem whose exact solution requires extensive computations. A few decades ago, Larch\'e and Cahn proposed a linearized solution of the mechanochemical equilibrium problem by introducing the concept of open system elastic coefficients [Acta Metall. 21, 1051 (1973)]. Using the Ni-Al solid solution as a model system, we demonstrate that open system elastic coefficients can be readily computed by semigrand canonical Monte Carlo simulations in conjunction with the shape fluctuation approach. Such coefficients can be derived from a single simulation run, together with other thermodynamic properties needed for prediction of compositional fields in solid solutions containing defects. The proposed calculation approach enables streamlined solutions of mechanochemical equilibrium problems in complex alloys. Second order corrections to the linear theory are extended to multicomponent systems.

Preparation, microstructure and mechanical properties of multicomponent Ni3Al-bonded cermets

Ni3Al intermetallic compound was synthesized by mechanical milling mixture of elemental Ni and Al powder after 20h milling; mechanical alloying (MA) first generated a Ni(Al) solid solution that transformed to Ni3Al intermetallic compound on further milling. Ti(C,N)–xNi3Al cermets were prepared through conventional powder metallurgy route at different sintering temperatures. The microstructure and mechanical properties of Ni3Al-bonded cermets sintered at different temperatures with different contents of Ni3Al were systemically studied. The dark core-gray rim carbide grains and/or dark core-white inner rim-gray outer rim carbide grains embedded in light-gray Ni3Al based binder phase were observed. Abnormal grains growth happened in all cermets and these grains tend to be lath like grains in this research. Moreover, with the rise of sintering temperature, the phenomenon of abnormal grains growth becomes more obvious, resulting in rapid growth of individual grains. Generally, transverse rupture strength (TRS) of cermets increased with the ascent of Ni3Al content and sintering temperature, however, it reached a peak value 1131±80.3MPa, when sintering temperature was 1450°C. HRA of cermets ascended firstly and then dropped down with the increase of binder content and sintering temperature. Fracture toughness (K1C) increased with Ni3Al content and got a peak value 16.1±0.55MPam1/2.

Cross-slip process in model Ni(Al) solid solution: An embedded-atom method study

The cross-slip process of a screw dislocation in model Ni(Al) random solid solution is studied using the climbing image nudged elastic band method with an embedded-atom method potential. The average stacking fault energy of model Ni(Al) solid solution decreases with increasing Al concentration from 0 to 10 at.%. However, the average activation energy under zero stress shows an initial increase and then plateaus with increasing concentration of Al. The excess activation energy is determined by comparing with results from linear-elastic continuum theory. The short-range repulsive solute atom pair and its interaction with the dislocation provide the excess activation energy in the cross-slip process of model Ni(Al) solid solution.

Phase Equilibrium between B2 Aluminide NiAl and Nb-Mo bcc Solid Solution Phase

In order to understand the composition of the Nb-Mo bcc phase suitable to introduce B2-NiAl phase for the improvement of oxidation resistance as an Al reservoir layer for maintaining Al2O3 surface layer, a NiAl-Nb-Mo section of the quaternary Al-Nb-Ni-Mo phase diagram is studied. It is found that the ternary τ2 phase appears in wide composition range of alloys, both the hardness and cracking tendency decreases with increasing the Mo concentration of the alloys.

New method for low temperature fabrication of Ni–Al alloy powder for molten carbonate fuel cell applications

Abstract Lump-free Ni-5 wt% Al alloy powder was successfully prepared using an AlCl 3 activator at 400 °C under vacuum. The AlCl 3 activator served as the catalyst, lowering the fabrication temperature by 1000 °C compared with the temperature required for the conventional process. The Ni–Al alloy was formed by the following steps: the formation of NiAl by the reaction of the Ni surface with AlCl 2 or AlCl produced by the reaction between Al and AlCl 3 , the formation of Ni 3 Al by Al diffusion and reaction, and the formation of a Ni–Al solid solution by Al diffusion into the Ni matrix until the solubility limitation was reached. Although lowering the alloying temperature lengthens the reaction time, the time could be reduced by controlling the amount of AlCl 3 . A single cell test and a creep test were also conducted using a green sheet of as-prepared Ni–Al alloy powder as an anode of a molten carbonate fuel cell (MCFC).

Mechanochemical synthesis and compaction of nanocomposites based on Ni3Al intermetallic compound containing carbon and carbide-forming elements

Ni(Al, C) mixed (substitutional and interstitial) solid solutions have been synthesized by mechanical alloying; the atomic ratio of Ni and Al was constant at 3: 1, but the concentration of carbon was varied. A carbide-forming element CE (Ti, Nb, or Hf) was added to the formed solid solutions with nanocrystalline substructures; the size D of coherent scattering regions was 4–6 nm. These mixtures Ni(Al, C) + CE were subjected to mechanical activation (short-term milling) and subsequent hot isostatic pressing. According to X-ray phase analysis, compacted samples exhibited three phases: Ni3AlCx antiperovskit, Ni(Al) solid solution with D = 50–100 nm and corresponding metal monocarbides (TiC, NbC, or HfC; D ≈ 7 nm) located at boundaries. These results were confirmed by scanning electron and optical microscopy. Thus, we obtained the analog of the conventional high-temperature γ/γ′ alloys hardened by grain-boundary nanoscale carbides, which prevent recrystallization.

Fabrication of Nanorod Array Structure by Using Aluminizing and Internal Oxidation of Ni with Micro-Channels

A metallurgical method for preparing oxide nano-rod array structure was developed with internal oxidation of Ni(Al) solid solution obtained by aluminizing Ni with a pack cementation technique. The present method was applied in order to fabricate nano-rod array on the micro-channel surface of microreactor. A micro-channel with 1 mm in depth, 1mm in width and 10 mm in length was fabricated on the Ni disk by an electric discharge machining and electropolishing. The Ni disk with a micro-channel was annealed at 1300°C for 12 h in vacuum. The sample was aluminized by a pack cementation using pack powder mixture consisting of 10 mass% of Ni3Al, 88 mass% of Al2O3 and 2 mass% of NaCl at 1100°C for 12 h in a flow of argon gas. The sample was oxidized from 1000 to 1200°C for 6 h with the Co/CoO buffer. Ni matrix in internally oxidized zone was removed by electropolishing to expose nano-rods. Al2O3 nano-rod array located on the micro-channel surface was successfully fabricated with the proposed technique. Diameter and height of nano-rods were ranged from 100 to 300 nm and from 1.5 to 3 µm, respectively.

Nanocrystalline Ni3Al intermetallic produced by hot-pressing consolidation of mechanically alloyed powders

In the present work bulk nanocrystalline Ni3Al alloys were produced by mechanical alloying of powdered Al3Ni or Al4Ni96 alloys mixed with Ni or Al respectively, followed by a hot-pressing consolidation. In the case of both mixtures a Ni(Al) solid solution was a product of ball milling. The powders of Ni(Al) were consolidated at 1000 °C for 180 s under the pressure of 7.7 GPa. The XRD investigations have revealed that during consolidation the disordered solid solution transforms into the ordered nanocrystalline Ni3Al intermetallic compound with an average mean crystallite size of ∼30 nm. The microhardness of the produced samples is 1033 and 1093 HV0.1 and is relatively high in comparison to literature data. The density of the obtained samples is close to the theoretical value of the Ni3Al intermetallic phase and the open porosity of the compacts is negligibly small.

A fabrication method for vertical oxide nano-rod array by internal oxidation of alloys at high-temperatures

A simple metallurgical process for fabricating vertical oxide nano-rod array structures with internal oxidation of alloys is described. Oxidation at high-temperatures and under low oxygen partial pressures can develop internally rod-like oxide precipitates in some dilute alloys such as Ni(Al) solid solutions. The vertical oxide nano-rod arrays can be developed on the metal substrates or components by removing the metallic matrix of the internal oxidation zone. Al2O3 or NiAl2O4 spinel nano-rod arrays were prepared by using M(Al) solid solutions. Pack cementation process to develop Ni(Al) solid solution zone on a metal surface was used for the fabrication of vertical nano-rod arrays on Ni substrates with desired shape. Near-net shape Ni substrates with vertical oxide nano-rod array structures on their surfaces can be prepared by using pack cementation and internal oxidation. Fabrication of vertical nano-rod arrays with Co(Al) and Fe(Al) solid solutions were also described briefly.

Internal Oxidation of Ni(Al) Solid Solution Zone Prepared by Pack Cementation of Pure Ni

Pure Ni was aluminized with an Ni3Al, NaCl and Al2O3pack powder mixture at 1100°C for 12 h. The Ni(Al) solid solution formed in aluminized Ni was internally oxidized at 1000°C for 2 to 12 h with a Co/CoO buffer. The internal oxidation zone consisted of 2 regions: granular precipitate zone near the surface and rod-like zone near the oxidation front. The growth of internal oxidation zone deviated from the parabolic law due to the non-constant Al profile developed by the aluminizing process. The granular zone grows linearly from the surface of the alloy.

The Interface between γ and γ’ in Ni-Al Alloys by HREM

Ni Base superalloys owe their excellent mechanical properties to the presence of particles of γ’ phase (Ni3Al with an L12 struc­ture) in a g matrix (Ni–Al solid solution with an fcc structure). Besides Al, other elements are used to im­part either a higher strengthening or improved corrosion properties at high temperature. The interface between γ and γ’ becomes of absolute importance for the resulting mechanical properties and technological application. Especially by considering the consequences that diffusion driven coarsening brings about to the parti­cle distribution either with or without the influence of an applied stress or strain. In this work the interface between γ and γ’ phases is characterized by means of measurements on phase images obtained from high resolution transmission electron microscopy images (HRTEM). Phase images represent the sample structure much more accurately than typical HREM experimental images and allow correction of spherical aberration and other residual aberrations. The investigation is performed by using a binary Ni-Al alloy as well as technical Ni base superalloys (MC2 and MCNG). While a sharp interface is developed during stress free coarsening in Ni-Al alloys, a wider volume needs to be considered when alloying elements are introduced. Measurements of lattice spacings on phase images and chemical composition from energy dispersive spectroscopy are used to show the interface characteristics in the alloys under consideration. The interface in the binary Ni-Al alloy can be described by micro­mechanics as a typical misfitting inclusion. In the technical alloys, the presence of concentration gradients changes the expected lattice strains in a given volume around the particles.

Molecular dynamics study of reaction pathways in an Al-coated Ni nanoparticle

Using molecular dynamics simulation in combination with an embedded atom method potential we analyze the alloying reaction in an Al-coated Ni nanoparticle with equi-atomic fractions and a diameter of about 9.5 nm. The first stage of the alloying reaction is controlled by interdiffusion between the f.c.c. Al-shell and f.c.c. Ni-core. Then, the large driving force for further alloying of Ni and Al promotes solid state amorphization of a supersaturated metastable f.c.c. Ni–Al solid solution in the vicinity of the interface region and, eventually, the whole shell of the nanoparticle. It is shown that there are at least two further possible pathways of the reaction. The first pathway occurs through crystallization of the amorphous shell into an Al-rich B2-NiAl phase before the complete dissolution of the f.c.c. Ni-core followed by interdiffusion between the Al-rich B2-NiAl shell and the f.c.c. Ni-core. A prediction is made that under certain conditions, interdiffusion between an Al-rich B2-NiAl shell and an f.c.c. Ni-core in such a nanoparticle may result in the formation of a hollow B2-NiAl nanoparticle. The second pathway leads to the complete dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell. In order for this scenario to be realised the rate of temperature increase at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell should be higher than in the previous case. We also demonstrate some possibilities to control the self-heating rate at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell, which is important for a switching of the reaction pathways in a desired direction.

Oxide nano-rod array structure via a simple metallurgical process

A simple method for fabricating oxide nano-rod array structure via metallurgical process is reported. Some dilute alloys such as Ni(Al) solid solution shows internal oxidation with rod-like oxide precipices during high-temperature oxidation with low oxygen partial pressure. By removing a metal part in internal oxidation zone, oxide nano-rod array structure can be developed on the surface of metallic components. In this report, Al2O3 or NiAl2O4 nano-rod array structures were prepared by using Ni(Al) solid solution. Effects of Cr addition into Ni(Al) solid solution on internal oxidation were also reported. Pack cementation process for aluminizing of Ni surface was applied to prepare nano-rod array components with desired shape. Near-net shape Ni components with oxide nano-rod array structure on their surface can be prepared by using the pack cementation process and internal oxidation,

Synthesis of nanocrystalline Ni 3Al by mechanical alloying and its microstructural characterization

Nanocrystalline Ni 3Al intermetallic compound was successfully synthesized by mechanical alloying of the mixed powders of elemental Ni and Al. The phase evolution and microstructural changes of the powders during mechanical alloying were investigated by means of X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and energy dispersive spectrometry. It was found that supersaturated Ni(Al) solid solution was formed during the early stage of mechanical alloying process. Upon further milling, Ni(Al) solid solution was transformed into ordered Ni 3Al intermetallic nanocrystalline. It is imperative to adopt a long enough ball milling time (120 h in the present work) so as to ensure the complete transformation of Ni(Al) solid solution to Ni 3Al intermetallic nanocrystalline and the formation of well ordered Ni 3Al nanocrystals composed of fine grains with a size of about 11 nm.

Surface nanocrystallization of Ni 3Al by surface mechanical attrition treatment

The mechanical property and microstructure evolutions of Ni 3Al intermetallic compound subjected to surface mechanical attrition treatment (SMAT) were investigated in relation to surface nanocrystalization. Grain size in topmost surface of SMATed Ni 3Al alloy was refined to a minimum size of about 10 nm, and then increased with the enhancement of the depth from surface to matrix. The original ordered L1 2 phase transformed to Ni (Al) solid solution with a disordered face-centered cubic structure. The maximum nanohardness of the deformed Ni 3Al alloy was near 12 GPa. The microstructure evolution including the variation of defects during the SMAT as well as post-annealing processes showed that the surface nanocrystallization of Ni 3Al intermetallic compound was predominantly controlled by dislocations which divided the coarse grains. The different microstructures at each sublayer illustrated that the nanocrystallization process was decided by the accumulated energy resulted from plastic strain.

The effect of Al addition on the prevention of Ni sintering in bio-ethanol steam reforming for molten carbonate fuel cells

This paper investigates the effect of modifying an Ni catalyst on the prevention of sintering of the catalyst at high temperatures, without causing a reduction in catalytic activity. The Ni catalyst was modified by adding Al through a solid–gas–solid reaction at a low temperature to produce an Ni–Al solid solution. This process allows for low-cost production of the modified catalyst. An activity test of the catalysts was carried out at 650 °C and 1 atm to simulate direct internal reforming of a molten carbonate fuel cell (MCFC). Experimental results showed consistent activity of the Al-modified catalyst, even after aging under severe conditions (900 °C) to simulate accelerated sintering. TEM data did not show any significant physical changes even after aging. Addition of Al appeared to have successfully prevented Ni from sintering without reducing its catalytic activity in reforming bio-ethanol. In addition, an Ni–Al/MgO catalyst, integrated into the anode of an MCFC, was successfully tested for over 2000 h without any significant performance degradation.

Chemical diffusion in hypostoichiometric Ni 3Al from data on coarsening of Ni–Al solid solution precipitates

Data on chemical diffusion in γ′ (Ni 3Al) containing on average 22.67 at.% Al were obtained from the analysis of the rate constants for coarsening of γ phase (Ni–Al solid solution) precipitates in a γ′ matrix. The data on coarsening were obtained over the temperature range 923–1073 K and values of the chemical diffusion coefficient were extracted from the rate constants using the thermodynamic model of Ikeda et al. [T. Ikeda, H. Numakura, M. Koiwa, Acta Mater. 46 (1998) 6605–6613] to obtain the curvature of the free-energy of mixing of Ni 3Al, in conjunction with reliable estimates of the other necessary thermo-physical parameters. The data so obtained are in excellent agreement with data on chemical diffusion from more conventional high-temperature diffusion experiments conducted over the temperature range 1400–1600 K. The results indicate that the high-temperature data can be extrapolated to lower temperatures with great confidence.

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni3Al matrix was investigated in alloys containing 22.0–22.8at.% Al aged at 650–800°C for times exceeding 1800h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1±16.6kJmol−1, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.

Preparation of Ni–Al alloys at reduced temperature for fuel cell applications

Ni–Al alloys have been fabricated from the physical mixture of nickel and aluminum powders at temperatures below the melting point of aluminum using AlCl 3 vapor as an activator. Pure Ni 3Al intermetallic compound has been synthesized from the Ni–Al powder mixtures containing 15 wt% aluminum at 600 °C. From the powder mixture containing 5 wt% aluminum, a compound consisting of Ni–Al solid solution and Ni 3Al was produced at 600 °C, but this compound was converted entirely to a Ni–Al solid solution after annealing at 700 °C. The particle size and morphology of the Ni–Al solid solution were not greatly different from original nickel powders. It was observed that oxidation resistance was significantly enhanced by alloying of nickel with aluminum. The Ni–Al alloys synthesized in this study can be used as anode materials for high-temperature fuel cells which need improved resistance toward creep, sintering and redox cycling.