A phase-field study of the aluminizing of nickel

A quantitative phase-field approach for multiphase systems that is based upon CALPHAD free energies is used to model the aluminization of nickel wires, wherein vapour-phase alloying is used to deposit Al on the surface of the Ni wire and then the wire is annealed so that to remove all Al gradients and achieve a homogenous Ni-Al alloy. Both processes are modelled and numerical results are compared with experiments. It is found that the kinetics of both processes is controlled by bulk diffusion. During aluminization at 1273 K, formation and growth of intermetallics, Ni₂Al₃ NiAl and Ni₃Al, are strongly dependent on the Al content in the vapour phase. Ni₂Al₃ growth is very fast compared with NiAl and Ni₃Al. It is also found that an intermediate Al content in the vapour phase is preferable for aluminization, since the Ni₂Al₃ coating thickness is difficult to control. Ni₂Al₃ is found to disappear in a few minutes during homogenization at 1373 K. Thereafter, the NiAl phase, in which the composition is highly non-uniform after aluminization, continues growing until the supersaturation in this phase vanishes. Then, NiAl coating disappears concomitantly with the growth of Ni₃Al, which disappears thereafter. Finally, the Al concentration profile in Ni(Al) homogenizes.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni₃Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ_c, differ markedly from those of corresponding bulk alloys.The Ni₃Al ordered films form in a concentration range larger than the stability domain of the L1₂ Ni₃Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases.It also appears that the value of θ_c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ_c = 4.5 ML for a flat surface to θ_c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.     

Adsorption of methanol on Ni3Al(1 1 1) and NiAl(1 1 0): A high resolution PES study

The adsorption of methanol on Ni₃Al(1 1 1) and NiAl(1 1 0) has been studied using high resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT). Both methanol and methoxy are formed on these surfaces after the initial methanol exposure at low temperatures. Heating to 200 K leads to further formation of methoxy. On NiAl(1 1 0) two different methoxy species are observed where the first is formed upon methanol adsorption, and the other results from methanol decomposition during heating. The DFT calculations show that methanol and methoxy interacts with the Al atoms on both surfaces. Methanol is found to bond through the oxygen atom to the Al on-top site on Ni₃Al(1 1 1) and NiAl(1 1 0) with the C–O axis tilted with respect to the surface normal. On Ni₃Al(1 1 1) methoxy is situated in a 2Ni+Al hollow site, whereas on NiAl(1 1 0) the Al–Al bridge site is preferred.     

Growth of Ni-Al alloys on Ni(1 1 1): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

In this paper we describe the alloying process of ultra-thin Al layers (below 8 × 10¹⁵ Al/cm²) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni₃Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 1 1) planes for the thickest deposits. The Ni₃Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along 〈1 1 −2〉 directions. The Al content of the thin Ni₃Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface.     

Phase Characterization of Diffusion Soldered Ni/Al/Ni Interconnections

The formation and growth of intermetallic phases in the Ni-Al system during a novel joining process for Ni/Ni interconnections based on diffusion soldering has been studied. The Ni/Al/Ni bonds were accomplished by isothermal solidification and subsequent interdiffusion of Ni and Al in the Ni/Al/Ni joints held at a temperature of 720°C. Optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction analysis were used to characterize the microstructural changes as a function of the reaction time. The following phases appeared sequentially: liquid Al → Al₃Ni → Al₃Ni₂ → AlNi (stoichiometric) → AlNi (Ni-rich) → AlNi₃. At intermediate stages two to four phases coexisted. The NiAl phase occurred in two variants, namely a Ni-rich AlNi (60 at.% Ni) and stoichiometric AlNi. The joining process was completed after 30 h of reaction. Then only AlNi₃ was present in the Ni/Al/Ni interconnection zone. The quality of the resultant bond and the high melting point of the AlNi₃ phase (1360°C) indicate a great potential of the diffusion soldering for the joining of heat dissipating devices used in electronics and electrotechnics.     

Structure,stability and magnetic properties of (NiAl)n(n≤6) clusters

In this paper, density functional theory with generalized gradient approximation (GGA) for the exchange-correlation potential has been used to calculate the energetically global-minimum geometries and electronic states of (NiAl)_n(n≤6) clusters. Full structural optimizations, analysis of energy and frequency calculation are performed. The most stable structures of (NiAl)_n clusters are all three-dimensional structures except NiAl. The average bond lengths of (NiAl)_n clusters are larger than that of Ni_2n, and are smaller than that of Al_2n. The binding energy per atom of Ni_2n and (NiAl)_n has the same change trend, and that are larger than that of Al_2n. Stability analysis shows that Ni₈, (NiAl)₂ and Al₁₀ clusters have higher relative stability than other clusters. Mulliken analysis indicates that charges always transfer from Al atoms to Ni atoms, and the average charges of transfer from Al atoms to Ni atoms have a maximum at (NiAl)₆, implying the strong interaction between Al and Ni atoms in (NiAl)₆. The average atomic magnetic moments of (NiAl)_n are smaller than that of true Ni_2n. The analysis of the static polarizability shows that the electronic structures of (NiAl)_n clusters tend to be compact with the increase of atoms.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

Structure and phase composition of the intermetallide Ni3Al+0.5 at.% B synthesized under pressure

We have used x-ray structural analysis, together with transmission and scanning electron microscopy, to study the phase composition, structure, and elemental distribution in the intermetallide Ni₃Al+0.5 at.% B, produced by self-propagating high-temperature synthesis under pressure. It is shown that the synthesized material is an ordered alloy of the type L1₂. The phase composition of the alloy Ni₃Al+0.5 at.% B is represented by the phases Ni₃Al (the fundamental phase), grains of a layered NiAl composition, and inclusions of Ni₃B. The latter is found inside the Ni₃Al grains, in the form of surrounded particles, and at dislocations and at grain boundaries in the NiAl phase. Institute for the Physics of Strength and Materials Production. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 59–64, September, 1996.     

Effect of alkali leaching on the surface structure of Ni3Al catalyst

The surface structure of the alkali-leached single-phase Ni₃Al powder was investigated by X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area analysis, electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. It was found that fine Ni particles of several nm in diameter were formed on the outer surface layer of the Ni₃Al powder after the alkali leaching process. The surface of the Ni particles was covered with a thin layer of Ni oxides and hydroxide, Ni₂O₃, NiO and Ni(OH)₂, and these Ni oxides and hydroxide can be easily reduced by hydrogen to the metallic nickel that is catalytically active. The inside of the Ni₃Al powder remained as the original Ni₃Al ordered structure after alkali leaching. Having heat resistant properties, the Ni₃Al phase can serve as a support of the fine Ni particles and provide the structural and thermal stabilities to the fine Ni particles.     

Optical properties of ternary β electronphases based on NiAl

Optical constants of several ternary β Hume-Rothery phases have been investigated as a function of the valence electron concentration and defect structure. The composition of the alloys was based on the β phase NiAl, the absorption spectrum of which is dominated by two maxima at 2·5 and 4 eV. The intensity of the 2·5 eV-ε₂ peak is considerably increased with increasing valence-electron concentration, whereas that of the other peak is decreased. The valence electron concentration is varied by substituting Cu for Ni in Ni_1?yCu_yAl or Si for Al in NiAl_1?ySi_y. The 2·5 eV absorption peak disappears when Co is substituted for Ni in Ni_1?yCo_yAl. The absorption peaks are attributed to interband transitions of electrons and are discussed according to the rigid band model. The absorption in the infrared is explained by the scattering of electrons from lattice defects and phonons. The position of the d-band relative to the Fermi level is discussed in connection with s-d band scattering.     

Electron microscopic study on interfacial characterization of electroless Ni-W-P plating on aluminium alloy

The interface between electroless plating Ni-W-P deposit and aluminium alloy (Al) matrix at different temperature heated for 1 h was studied using transmission electron microscope. The results show that the interface between as-deposited Ni-W-P deposit and Al matrix is clear. There are no crack and cavity. The bonding of Ni-W-P deposit and Al matrix is in good condition. The Ni-W-P plating is nanocrystalline phase (5-6 nm) in diameter. After being heated at 200 °C for 1 h, the interface of Ni-W-P deposit and Al matrix is clear, without the appearance of the diffusion layer. There exist a diffusion layer and educts of intermetallic compounds of nickle and aluminium such as Al₃Ni, Al₃Ni₂, NiAl, Ni₅Al₃ and so on between Ni-W-P deposit and Al matrix after being heated at 400 °C for 1 h.     

Positron annihilation spectroscopy characterization of effect of intermetallic nanoparticles on accumulation and annealing of vacancy defects in electron-irradiated Fe–Ni–Al alloy

The effect of intermetallic nanoparticles like Ni₃Al and nanoparticles of an Fe-rich bcc phase on the evolution of vacancy defects in an fcc Fe–34.2 wt% Ni–5.4 wt% Al model alloy under electron irradiation at elevated temperatures (423 and 573 K) was investigated using positron annihilation spectroscopy. Nanosized (1–8 nm) particles, which are homogeneously distributed in the alloy matrix, cause a several-fold decrease in the accumulation of vacancies as compared to their accumulation in a quenched alloy. This effect depends on the size and the type of nanoparticles. The effect of the nanoparticles increases when the irradiation temperature increases. The irradiation-induced nucleation and the growth of intermetallic nanoparticles were also observed in an alloy pre-aged at 1023 K under irradiation at 573 K. Thus, a quantum-dot-like positron state within ultrafine intermetallic particles, which we revealed earlier, allows control of the evolution of coherent precipitates like Ni₃Al, along with vacancy defects, during irradiation and subsequent annealing. Possible mechanisms of the absorption of point defects by nanoparticles are discussed.     

Sputtering of ordered nickel-aluminium alloys: II. Preferential sputtering of NiAl single crystals and discussion

Atom-probe field-ion microscopy together with X-ray photoelectron spectroscopy and secondary ion mass spectrometry have been applied to the microanalysis of fully ordered NiAl single crystals subjected to 3 keV inert gas ion bombardment. As with the studies of Ni₃Al (the companion paper) aluminium was found to be preferentially sputtered by both argon and xenon bombardment. Comparisons between depth profiles through Ni₃Al and NiAl targets have provided information about the role of binding energies in the selective sputtering process. These data have also permitted conclusions to be drawn about the correct choice of bombarding species for sample cleaning and depth-profiling applications in surface analysis. Examination of field-ion images from specimens after bombardment suggests that the surface is microroughened and this has been confirmed using transmission electron microscopy.     

Sputtering-induced nanometre hole formation in Ni3Al under intense electron beam irradiation

The irradiation damage of polycrystalline Ni₃Al thin foils of stoichiometric composition by a stationary nanoscale 200?keV field emission gun (FEG) electron probe in a FEI Tecnai F20 (S)TEM has been investigated. At current densities greater than 10⁷?A/m², nanometre holes are produced quickly with both ?001? and ?110? incident electron beam directions. EDX spectra from the irradiated volume have been collected simultaneously during the hole forming process. From the EDX results, preferential surface sputtering of aluminium from Ni₃Al has been demonstrated. To understand the underlying physical process of sputtering, modelling based on a combination of molecular dynamics and Monte Carlo simulation has been performed. It appears to reproduce faithfully the overall film sputtering and hole formation processes, but is not capable of predicting the detailed geometry of the hole. It predicts that the sputtering cross-section of Al atoms is much higher than that of Ni atoms, resulting in a very small concentration of Al at the surface. This, together with the increase of surface area during hole formation, explains the preferential Al loss observed from the specimen. Calculated sputtering rates agree well with experiment, and are of the order of magnitude of 10^?8?atoms/electron.     

Sequence of phase formation during solid-state synthesis in Al/Ni films (Al: Ni = 60: 40 at %)

The structure formed during solid-state synthesis in thin bilayer Al/Ni films with the ratio Al: Ni = 60: 40 (at %) has been investigated. The films were obtained by thermal evaporation in vacuum with a residual pressure of 10^?5–10^?6 Torr. Solid-state synthesis was performed by diffusion reaction. The sequence of phase formation upon vacuum annealing of bilayer Al/Ni films has been established: Al + Ni → Al₃Ni + Ni (T _ann = 180°C) → Al₃Ni₂ (T _ann = 220°C).     

Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)

The growth and oxidation of a thin film of Ni₃Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 300 K, a 12 Å thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting in a thin film of Ni₃Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300 K of Ni₃Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina phases. By annealing up to 1000 K, a well ordered film of the Al₂O₃ film is formed which exhibits in the EEL spectra Fuchs-Kliewer phonons at 420, 640 and 880 cm⁻¹. The LEED pattern of the oxide shows a twelvefold ring structure. This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90° with respect to each other. The lattice constant of the hexagonal structure amounts to ∼2.87 Å. The EELS data and the LEED pattern suggest that the γ^′-Al₂O₃ phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.     

Adsorption of methanol and methoxy on NiAl(1 1 0) and Ni3Al(1 1 1): A DFT study

The adsorption of methanol and methoxy on NiAl(1 1 0) and Ni₃Al(1 1 1) has been investigated using density functional theory (DFT). Optimised adsorption geometries and core level shifts are presented. On both surfaces we find that methanol binds to the Al on-top site via its oxygen atom and with the C–O axis tilted away from the surface normal. Methoxy also shows a preference for Al-dominated sites. On NiAl(1 1 0), we predict that methoxy adsorbs with its oxygen atom in the Al–Al bridge site, while it is determined to be adsorbed with its oxygen atom in a 2Ni + Al hollow site on Ni₃Al(1 1 1), closer to Al than Ni. Surface and adsorbate induced binding energy shifts in the Al 2p states are calculated and found to be in good agreement with experimental high resolution photoelectron spectroscopy results.     

Glass formation and microstructure evolution in Al–Ni–RE (RE = La,Ce, Pr,Nd and misch metal) ternary systems

Glass formation has been systematically studied in melt-spun Al-rich Al–Ni–RE (RE?=?La, Ce, Pr, Nd and Mm; Mm?=?misch metal) alloys by navigating the compositions following the observation of microstructure evolution in the resulting ribbons. The optimum glass-forming regions are similarly located around Al₈₅Ni₁₀RE₅ and found in the centre of the composites with primary phase α-Al, Al₁₁RE₃ and Al₃Ni. The similarities in the critical cross-section below which a component is fully amorphous and the optimum compositions in these Al–Ni–RE systems are interpreted in terms of their competing crystalline phases and thermodynamic properties. Interestingly, it is indicated that Ni content is markedly higher than RE content in the best glass-forming alloys, which may be associated with strong interaction between Ni–Al atom pairs and the dense packing due to Ni-centred clusters.