Ni-Al Phase Diagram Research Articles

Formation of intermetallic layers at high intensity ion implantation

The experimental results are presented on a study of intermetallic phase formation in the surface zone of metal target at high intensity ion implantation. High intensity ion implantation allow to obtain the surface-alloyed layers of a much greater thickness in comparison with ‘ordinary’ ion implantation. Pure polycrystalline nickel was chosen as the target. The nickel samples were irradiated with the aluminum ions using the vacuum-arc ion beam and plasma flow source ‘Raduga-5’. The RBS and TEM were used for the investigations presented. It was established that the fine dispersed intermetallic precipitates are formed in the surface alloyed nickel layer. The alloyed layer thickness is equal to 150 nm and more, while the ion projected range that is equal to 70 nm. Compositions of these intermetallic precipitates are close to Ni 3Al and NiAl phases. The solid solution of aluminum in nickel is also formed. The depth dependence of the formation of intermetallic phases can be deduced from the Ni–Al phase diagram.

Formation of stable and metastable phases in Ni–Al–Nb and Ni–Al–Me–C (Me=Ti, Nb or V) powder systems during mechanical alloying and thermal treatment

A series of quaternary Ni–Al–Me–C (Me=Ti, Nb or V) alloys and two Ni–Al–Nb alloys from various areas of the Ni–Al phase diagram were prepared by the mechanical alloying (MA) method starting from pure powder mixtures. X-ray diffraction and differential scanning calorimetry were used to investigate the powder alloys. It was found, that MA results in the formation of partially ordered metastable B2-type solid solutions in the case of carbon-free systems. On the other hand, the presence of carbon in the powder alloy results in the formation of ordered intermetallic phase and carbide during the thermal treatment. The grain size of carbide and intermetallic phase were kept within the nanosize range even after heating up to 700°C.

Electronic structure and thermodynamics of defects inNiAl3

We present first-principles calculations of the structural and electronic properties of orthorhombic ${\mathrm{NiAl}}_{3},$ including the intrinsic point defects. The energies obtained from these calculations are employed in a statistical mechanics model to investigate the role of ${\mathrm{NiAl}}_{3}$ in the Ni-Al phase diagram. Our calculations show that the preferred constituent defect mechanisms for ${\mathrm{NiAl}}_{3}$ off stoichiometry are the $8d$ Ni antisites on the Ni-rich side and Ni vacancies on the Al-rich side. The calculated free energy is in agreement with the observation that ${\mathrm{NiAl}}_{3}$ is a stoichiometric compound.

Formation of aluminide coatings on nickel by a fluidised bed CVD process

The present work concerns the formation and modelling of aluminide diffusion coatings onto nickel and nickel alloys by means of a fluidised bed chemical vapour deposition (FBCVD) process. In this process, the treated object is suspended within the FB reactor, which contains the treating agent. The latter is a mixture of powders of the donor of the coating-forming element (Fe x Al y ), and of a filler material (Al 2O 3). As an activator of the process, a halide compound is used (NH 4Cl). The bed is fluidised by inert gas (Ar). The precursor vapours of the element to be deposited are formed in situ, by the reaction of the donor with the activator. The produced coatings are characterised by optical microscopy, SEM and EPMA. The modelling of the nickel–aluminide coating growth on nickel, has been based on the diffusion equations of aluminium in pure nickel, considering the intermetallic compounds predicted by the Ni–Al phase diagram. The model predicts the aluminium composition profiles as functions of the time and the concentration of Al in the donor, for the aluminisation conditions under consideration. The model is in satisfactory agreement with experimental results.

Solid state reactions in Ni–Al–Ti–C system by mechanical alloying

Several Ni–Al–Ti–C compositions from different areas of the Ni–Al phase diagram containing various amounts of Ti and C were prepared by mechanical alloying. Synthesized samples were investigated by X-ray diffraction and differential scanning calorimetry methods. Solid state reactions in Ni–Al–Ti–C powder mixtures with an equiatomic Ni/Al ratio lead to the formation of NiAl and TiC equilibrium phases. Metastable solid solution of Ni(Al,Ti,C) and non-stoichiometric TiC x are formed in the mixtures with C Ni/C Al ratio of 1.7–3. The grain size of metastable intermetallic phases is less than 5 nm, if Ni and Al content in the mixtures corresponds to the dual-phase and Ni 3Al-areas of the Ni–Al phase diagram.

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.

Effects of composition on microstructures of rapidly solidified Ni–Al alloys

Microstructures of melt-spun Ni–Al alloys with compositions from 61–85 at% Ni were studied by means of transmission electron microscopy, X-ray diffraction analysis and optical microscopy. The microstructures of as-quenched ribbons exposed to cooling rates of the order of 106 K s-1 reflect the transition from primary β-NiAl to γ-Ni solidification with increasing nickel content. In 70 at% Ni alloy ribbons, martensitic NiAl grains were detected near the wheel-side surface contrasting with anomalous and lamellar eutectic microstructure in the top part. Directly ordered Ni3Al grains with single (or large) antiphase domains (APDs) and a minor eutectic fraction were observed in 75 at% Ni alloy ribbons. Samples containing 80 at% Ni exhibit mainly single-phase Ni3Al grains with 10–20 nm sized APDs indicating sequential ordering. Weak L12 ordering was even detected in 85 at% Ni ribbons which displayed ordered antiphase zones of 1 nm size. Disordered γ-(Ni) films on grain boundaries can be discounted for 80 at% Ni ribbons, but occurred near the top of 85 at% Ni samples. The results are explained in terms of the reassessed Ni–Al phase diagram employing recent corrections near to the Ni3Al composition and new results on phase formation in undercooled Ni–Al melts. © 1998 Kluwer Academic Publishers

High temperature thermal analysis of Ni-Al alloys around the γ′ composition

The region of the Ni-Al phase diagram close to the Ni{sub 3}Al ({gamma}{prime}) compound has been analyzed several times recognizing that {gamma}{prime} has an incongruent melting and its field of existence is narrow, bounded by the equilibria with the {gamma} terminal solid solution and the NiAl ({beta}) phase. With the availability of High Temperature Differential Scanning Calorimetry (HTDSC) as a sensitive technique for quantitative thermal analysis, it appeared useful to re-examine the portion of the phase diagram around {gamma}{prime}. In this paper the authors report data confirming Hilpert`s version of the phase diagram. In addition, the HTDSC analysis provides information on the enthalpy change during fusion not yet available in the literature, which also has implications in the analysis of the order-disorder transition in Ni{sub 3}Al. Furthermore, a detailed knowledge of the DSC behaviour of these alloys on melting and solidification is relevant as a reference basis to interpret data on analogous transformations in superalloys and intermetallics.

Resistometric investigation of phase transformations in NiAl alloys

The paper represents a contribution to the martensitic transformation and to the existence regions of Ni 2Al and Ni 5Al 3 phases in the Ni–Al phase diagram. Resistivity, optical microscopy and differential thermal analysis (DTA) investigations were performed on 63.4, 64.8 and 66.2 at.% Ni alloys in the temperature region between −190 and 850°C. The martensite–austenite transformation (reversible for the alloy with the lowest Ni content) takes place between 60 and 300°C according to the nickel content of the alloy. In the 63.4 at.% Ni alloy, the Ni 2Al phase originates above 350°C and exists up to 580°C. After an incubation period, the Ni 5Al 3 phase starts to nucleate from austenite above 640°C. The Ni 5Al 3 phase then dissolves above 740°C forming Ni 3Al+NiAl phases in a peritectoid reaction. In Ni-richer alloys, martensite rapidly changes into the Ni 5Al 3 phase which does not transform martensitically. Only very rapid heating to 600°C retains the possibility of martensitic transformation in these alloys.

Phase transformations in nickel rich NiAl using electrical resistivity measurements

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. This phase is stable in a wide composition range and shows thermoelastic martensitic transformations in a region supersaturated in nickel. It is known that formation of Ni{sub 5}Al{sub 3}, Ni{sub 2}Al, Ni{sub 3}Al phases influences the martensitic transformation and, consequently, the shape memory effect. The aim of this paper is to throw more light upon structure changes of the mentioned phases using resistivity measurements during heating and cooling of the nickel rich NiAl phase specimens within a relatively broad temperature region ({minus}180 C to 900 C). Identification of the phases was done using the Ni-Al phase diagram, their experience in SEM investigation of the structural changes in this system, and electrical properties of the phases.

The thermal stability of TiB2 in Ni-43 a% Al

The intermetallic compound NiAl has been suggested for use as a matrix for high temperature composites. Interest in this compound is derived from its low density compared to conventional superalloys, high melting point, and excellent oxidation resistance. It has been demonstrated that a composite of stoichiometric NiAl and TiB[sub 2] is stable when exposed at elevated temperatures for extended periods of time. However, it has been shown that a composite consisting of Ni[sub 3]Al and TiB[sub 2] is not stable in the same temperature range. In the case of Ni[sub 3]Al, the TiB[sub 2] particles dissolve and the matrix is enriched with Ti and transforms to Ni[sub 3](Al,Ti) and NiTi. If stoichiometric NiAl reinforced with TiB[sub 2] is used in a high temperature oxidizing atmosphere, then there is also a possibility that the Al may be depleted as it is oxidized. If this occurs, the composition of the matrix may shift to nickel-rich NiAl or even to Ni[sub 3]Al. This work was conducted to determine if TiB[sub 2] is used in a high temperature oxidizing atmosphere, then there is also a possibility that the Al may be depleted as it is oxidized. If this occurs, the composition of the matrixmore » may shift to nickel-rich NiAl or even to Ni[sub 3]Al. This work was conducted to determine if TiB[sub 2] is stable in nickel-rich NiAl. The Ni-Al phase diagram is shown, and the composition of NiAl used in this study is indicated.« less

Theoretical study of phase stability in Ni-Al and Ni-Ti alloys

The thermodynamic stability of the bcc and fcc based ordered phases in Ni-Ti and Ni-AI has been studied with a highly accurate first-principles electronic structure method. The occurrence of a martensitic transformation in Ni-AlB2 ordered intermetallic alloys is discussed with relation to the existence of intermediate structures between bcc and fcc based phases. It is shown that closely related ordered structures can exist on fcc and bcc lattices in the composition range where the transformation occurs. The Ni-rich side of the Ni-Al phase diagram has been computed, and a comparison with a recent assessment is made. In addition, the rather unusual appearance of the NiTi B2 ordered structure in the phase diagram is discussed.

First-principles determination of the Ni-Al phase diagram

Phase stability in the Ni-Al binary system is investigated using linear muffin-tin orbitals total energy (LMTO) calculations. They provide total energies for the different existing compounds and, using Connolly-Williams inversion, the many-body interactions occurring in the FCC and BCC lattices. These interactions are used in conjunction with the cluster variation method (CVM) to calculate the phase diagram. The computed phase diagram agrees very well with the experimental one.

Tight-binding calculations of the Ni-Al phase diagram.

Tight-binding electronic band-structure calculations were combined with a free-energy expression given by the cluster-variation method. The effective pair interactions used in the cluster-variation method were evaluated by the cluster-Bethe-lattice method. They are restricted to the first-nearest neighbors in fcc-based structures and to first- and second-nearest neighbors in bcc-based structures; s and d orbitals have been used for Ni, while s and p orbitals were included for Al. The phase diagram has been constructed incorporating both fcc and bcc lattices and the occurring ordered compounds. The liquid part has been evaluated using the same Hamiltonian, chemical short-range order being calculated in an isotropic environment. The calculated diagram agrees reasonably well with the one determined experimentally.

Two—Phase Nickel Aurunides

AbstractThe as—extruded microstructures of two alloys in the two phase field consisting of Ni3Al and NiAl in the Ni—Al phase diagram exhibit fibrous morphologyand consist of Ll2 Ni3Al and B2 NiAl. These as—extruded microstructures can be modified dramatically by suitable heat treatments. Martensite plus NiAl or martensite plus Ni3Al microstructures are obtained upon quenching from1523 K. Aging of martensite at 873 K results in the recently identified phase Ni5Al3 whereas aging at 1123 K reverts the microstructures to Ni Al plui NAl. The microstructures with predominantly martensite or Ni5Al3 phases are brittle in tension at room temperature. The latter microstructure does not deformplastically even in compression at room temperature. However, some promise of room temperature tensile ductility is indicated by the Ni3Al plus NiAl phase mixtures.

Modeling of γ/γ′ phase equilibrium in the nickel-aluminum system

A theoretical model is proposed for the determination of phase equilibrium in alloys taking into consideration dissimilar lattice parameters. Volume dependent pair interactions are introduced by means of phenomenological Lennard-Jones potentials and the configurational entropy of the system is treated in the tetrahedron approximation of the cluster variation method. The model is applied to the superalloy relevant nickel-rich, γ/γ′ phase region of the Ni-Al phase diagram. The model predicts reasonable values for the lattice parameters and the enthalpy of formation as a function of composition, and the calculated phase diagram closely approximates the experimental diagram.

Photoelectric work functions of Ni$z.sbnd;Al alloys: Clean surfaces and adsorption of CO

Abstract The work function Φ, the emission constant M , and the increase in work function after CO adsorption ΔΦ have been measured as a function of the overall composition of well annealed Ni-Al alloy films. The work function of the Ni-Al alloys is not constant within the several miscibility gaps of the Ni-Al phase diagram, and is not lower than the work function of the respective pure metals, as was found with Ni-Cu and Pt-Au alloy films within the miscibility gap. Of all compositions studied only the 25% Ni alloy (NiAl 3 ) distinguishes itself by an increased Φ-value. The higher Φ is explained by the higher atomic density of the NiAl 3 compound compared with pure Al and Ni 2 Al 3 . As with other metal alloy systems, an enrichment of the surface by the component with the lower surface energy (Al) is also found with the ordered Ni-Al alloy system. From ΔΦ measurements it appears that up to about 40% Ni the surface of the Ni-Al alloy is composed entirely of Al. The behavior of Φ and ΔΦ in the Ni-rich part is explained by the enrichment of the surface by Al. The tendency to form the surface from Al atoms is strengthened by the existence of suitable crystallographic structures. The emission constant does not distinguish any of the intermetallic compounds and resembles that of the Pt-Au and Pd-Ag alloy films.