Ni-Al Phase Diagram Research Articles

Thermo-Calc determination of phase diagram and thermodynamic properties of Ni-Al binary system

Ni-Al alloys have good corrosion-resistant properties making them preferable as a material of choice compared to ordinary metals. A good understanding of the phase diagram and thermodynamic properties of the Ni-Al alloy binary system is essential in determining relevant industrial applications of the system. The Ni-Al phase diagram is being reassessed since those found in the literature are based on old experimental studies. The phase diagram and thermodynamic properties of the Ni-Al alloy is determined using the Thermo-Calc 2022b software over a temperature range of 200 K to 2000 K and 1 atm of pressure. A total of nineteen fields (including elemental Al and Ni) comprising five single solid phases, one liquid phase, and twelve double regions were obtained from the equilibrium calculation. The thermodynamic calculation has six invariant reactions containing two eutectic reactions (914.83 ± 0.025 K and 1641.80 ± 0.025 K), three peritectic reactions (1123.55 ± 0.025 K, 1400.72 ± 0.025 K and 1642.65 ± 0.025 K), and one peritectoid reaction (913.96 ± 0.025 K). The solubility of Al in Ni is also determined. The Gibbs free energy, the molar enthalpy and the activities of Al in Ni are all calculated at temperatures 913 K, 1642 K, and 1953 K. The Thermo-Calc temperature – composition of the Ni-Al alloy is compared with the existing experimental studies to identify the physical properties of the different thermodynamic properties of the Ni-Al alloy binary system.

Accurate prediction of the solid-state region of the Ni-Al phase diagram including configurational and vibrational entropy and magnetic effects

The solid-state region of the Ni-Al phase diagram is predicted from first-principles calculations and Monte Carlo simulations through the cluster expansion formalism. In addition to the formation enthalpy and to the configurational entropy, the vibrational entropy and the magnetic enthalpy are included to calculate the Gibbs free energy of each phase. The computed phase diagram is in excellent agreement with the experimentally accepted phase diagram and provides information about the phase boundary between AlNi3 and Ni below 300 K. These results demonstrate the potential of this methodology to determine accurately the phase diagram of alloys of technological interest. Finally, the contributions of vibrational entropy and magnetic effects to the overall stability and solubility of the different phases are analyzed independently.

Facile synthesis of micro-sized Ni–Al alloy powders through low-temperature chemical alloying

Micro-sized Ni–Al alloy powders were synthesized using different quantities of Ni and Al powders as well as AlCl3 as an activator in a quartz batch reactor at ≤773 K. This method, named low-temperature chemical alloying (LTCA), which allows Al atoms to diffuse into micro-sized nickel particles mediated by aluminum chloride (chemical promotor), is distinct from conventional processes, such as cast-and-crush, and gas atomization. As-synthesized Ni–Al alloys were characterized using different analytical techniques that included X-ray diffraction (XRD), particle size analysis, and field emission scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (FESEM-EDS), to confirm the formation of alloy phases, such as Ni3Al, NiAl, Ni2Al3, and NiAl3. The analytical results showed that the crystalline phase compositions of the Ni–Al alloys were highly dependent upon the initial amounts of Ni and Al powders employed at the given alloying conditions (alloying temperature, 773 K; alloying time, 20 h; amount of AlCl3, 1.2 wt%). As a result of the thermal treatment of Ni–Al powder mixtures with the Al contents of (5, 15, 30, and 50) wt.% under continuous powder mixing by rotation, each powder was found to have (i) Ni solid solution + Ni3Al, (ii) Ni3Al, (iii) NiAl, and (iv) Ni2Al3 + NiAl3 phases, respectively, corresponding to the equilibrium states of the Ni–Al phase diagram. The cross-sectional analyses showed that the alloy structures of the heat-treated powders exist in a single-phase or core-shell form, depending on the number of crystalline phase compositions predicted from the phase diagram. In particular, the Ni–50 wt% Al powder has a unique Ni2Al3@NiAl3 core-shell structure. We further evaluate the performance of the as-developed Ni–Al alloy powders as oxidation-resistant materials and template materials for high-surface area (∼60 m2/gcat) nickel catalysts.

A first-principles phase field method for quantitatively predicting multi-composition phase separation without thermodynamic empirical parameter

To design tailored materials, it is highly desirable to predict microstructures of alloys without empirical parameter. Phase field models (PFMs) rely on parameters adjusted to match experimental information, while first-principles methods cannot directly treat the typical length scale of 10 μm. Combining density functional theory, cluster expansion theory and potential renormalization theory, we derive the free energy as a function of compositions and construct a parameter-free PFM, which can predict microstructures in high-temperature regions of alloy phase diagrams. Applying this method to Ni-Al alloys at 1027 °C, we succeed in reproducing evolution of microstructures as a function of only compositions without thermodynamic empirical parameter. The resulting patterns including cuboidal shaped precipitations are in excellent agreement with the experimental microstructures in each region of the Ni-Al phase diagram. Our method is in principle applicable to any kind of alloys as a reliable theoretical tool to predict microstructures of new materials.

Formation of a Diffusion Zone at the Interface of Nickel/Aluminum Bimetal

In this paper, a study of a diffusion zone at the nickel/aluminum interface is carried out. Bimetallic samples constructed from pressed plates of Nickel and Aluminum have been annealed in a vacuum furnace at a temperature above the melting temperature of aluminum. Formation patterns of diffusion zones are investigated using the samples with different intervals of annealing time. Sintered samples have been cut across the boundaries of the section; the cut surface has been polished.Analysis of experimental results shows that the increase of diffusion process thermal activation time leads to the increase diffusion zone width. For this zone, the measured microhardness is a function with its maximum that indicates the nature of the phase components distribution at the Nickel/Aluminum interface. According to the Ni-Al phase diagram, the widest homogeneity range and, consequently, greater probability of formation of intermetallic compounds, have NiAl and Ni3Al phases. The diffusion zone itself is a macroscopic object at the interface of Nickel/Aluminum bimetal, thereby, showing the macroscopic diffusion of diffusants.Measured diffusion parameters of the diffusion zone at the interface of bimetallic sample demonstrate the increase of diffusion coefficient with the increase of exposure time at 700 0C. Apparently, this behavior is due to the formation of branched intergranular and interphase boundaries during the synthesis of intermetallic compounds in the diffusion zone. The combination of intermetallic phases and solid solutions determines the strength properties of the diffusion zone, and, consequently, the adhesion strength of the bimetallic compound.DOI 10.14258/izvasu(2018)1-04

Analysis of crack mechanism in FGH 98 superalloy during the laser solid forming process

In this study, FGH 98 superalloy parts were fabricated by laser solid forming (LSF) and the microstructure and defects of the LSF part were revealed using optical microscope and scanning electron microscope. It was found that the main defect in the LSF part was cracks. In order to analyze the phase transition process and crack mechanism, the temperature and stress fields were simulated. Based on the simulated result, Ni-Al phase diagram and morphologies of cracks, it can be concluded that the micro segregation caused by the rapid heating and cooling rates was the main reason for the crack defect.

Identification of a kinetic length scale which dictates alloy phase composition in Ni-Al interfaces on annealing at low temperatures

Ni-aluminides are an important class of intermetallics from technological point of view. Ni-Al phase diagram has been studied in detail experimentally as well as theoretically. It is known that if annealed at low temperature, the first alloy phase is usually NiAl3 according to Bené's rule. It is also understood that heat of formation may get modified by local densities of the constituents forming the alloy. In this regard, it is important to identify a kinetic length scale for defining “local density” in a system. We have deposited ultrathin multilayers of Ni and Al of layer thickness in tens of nanometres with Ni:Al stoichiometric ratio as 3:1 and 1:3, respectively. Considering these stoichiometry, Ni3Al and NiAl3 are the thermodynamically favoured alloy phases in these samples. We used x-ray reflectivity, polarized neutron reflectivity, x-ray diffraction, and secondary ion mass spectroscopy to follow the alloy formation after annealing and identified the alloy phases at interfaces with nanometre resolution. Diffusion length of Ni and Al was obtained using Darken's law. Our results predict that ‘diffusion length’ is the unique length scale that connects kinetics to local density. In another interesting observation, using “virtual Kirkendall markers” at the interfaces, we showed asymmetry in consumption of Al for alloy formation, at Al on Ni (Al/Ni) and Ni on Al (Ni/Al) interfaces by comparing as-deposited and annealed states with respect to the markers.

Thermodynamic Justification for the Ni/Al/Ni Joint Formation by a Diffusion Brazing

A theoretical model for the joint formation was developed for the diffusion brazing. The phenomena of dissolution and solidification were included into the model. A thermodynamic justification for the isothermal soldering occurrence in meta-stable conditions was developed. It involved the application of the criterion of higher temperature of the solid / liquid (s/l) interface. The dissolution of the filler metal in the substrate was described by the solute concentration within the dissolution zone (liquid film) situated at the substrate surface. The selection of the parameter was justified by the Thermocalc calculation of the Ni-Al phase diagram for meta-stable equilibrium. According to the model assumptions, the solidification was accompanied by undercooled peritectic reactions resulting in formation of the intermetallic phases. The average Al – solute concentration measured across a given Al 3 Ni 2 /Al 3 Ni/Al 3 Ni 2 joint confirmed that the solute concentration was conserved within the joint sub-layers. The Ni-Al phase diagram for meta-stable equilibrium referred to the solidification was also calculated by means of the Thermocalc Software. It allowed to locate the solidification path, s/l interface path and redistribution path onto the mentioned diagram. Superposition of both calculated phase diagrams was also given to show that the joint formation occurred cyclically under the meta-stable conditions. The measured parameter was introduced into the proposed model as an initial condition in order to solve the formulated differential equation describing solute redistribution after solidification. Isothermal formation of the Ni/Al/Ni joints has been performed at different temperatures. The following temperatures have been applied: 700, 750, 800, 850, 900, 950, 1000 0 C. The solidification was arrested and the actual morphologies frozen. It allowed to make a measurement of the Al-solute concentration across each joint by means of the EDAX micro-analyzer to estimate its average concentration, . Regardless the - temperature , the solidification path was always the same.

Effect of Al content on porous Ni–Al alloys

Porous Ni–Al alloys with different nominal compositions ranging from Ni–10 wt.% Al to Ni–40 wt.% Al were fabricated through reactive synthesis of Ni and Al elemental powders. It had been found that the volume expansion, maximum pore size and permeability of porous Ni–Al alloys increased with increasing the Al content, indicating that the nature of the pores can be manipulated through changing the Al content. In addition, detailed structural characterizations showed that the fabricated porous Ni–Al alloys can have three crystalline phases (i.e., Ni 3Al, NiAl, and Ni 2Al 3) when using different compositions, in good agreement with the Ni–Al phase diagram. Mechanical strength tests of the fabricated porous Ni–Al alloys were also performed. Both of the bending strength and tensile strength of porous Ni–Al alloys were in the range of 5–80 MPa, and decreased with increasing the Al content.

Temperature dependence of the pre-wetting transition at the (1 1 1) anti-phase boundary in Ni3Al: an atomistic study

Chemical composition and ordering at the (1 1 1) anti-phase boundary (APB) in the Ni3Al-based γ′ phase have been studied by semi-grand canonical Monte Carlo simulations at temperatures from 700 to 1300 K. The atomic interactions are modeled with an embedded-atom potential reproducing the relevant part of the Ni–Al phase diagram. In a certain range of bulk compositions and temperatures within the γ′ phase stability domain, the APB undergoes a pre-wetting transformation by becoming a layer of disordered γ phase. The pre-wetting transformation line is mapped onto the bulk phase diagram allowing predictions of the APB state under various thermochemical conditions.

Development of an interatomic potential for the Ni-Al system

We construct an interatomic potential for the Ni-Al system within the embedded-atom method formalism. The potential is based on previously developed accurate potentials for pure Ni and Al. The cross-interactions are fitted to experimental cohesive energy, lattice parameter and elastic constants of B2-NiAl, as well as to ab initio formation energies of several real or imaginary intermetallic compounds with different crystal structures and chemical compositions. The potential accurately reproduces a variety of physical properties of the NiAl and Ni3Al phases, and shows reasonable agreement with experimental and ab initio data for phase stability across the Ni-Al phase diagram. Most of the properties reproduced by the new potential were not involved in the fitting process, which demonstrates its excellent transferability. Advantages and certain weaknesses of the new potential in comparison with other existing potentials are discussed in detail. The potential is expected to be especially suitable for simulations of heterophase interfaces and mechanical behavior of Ni-Al alloys.

Solidification behavior of γ′-Ni 3Al-containing alloys in the Ni–Al–O system

The chemical activities of Al and Ni in γ′-Ni 3Al-containing alloys were measured using the multi-cell Knudsen effusion-cell mass spectrometry technique, over the composition range 8–32 at.% Al and temperature range T = 1400 to 1750 K. From these measurements a better understanding of the equilibrium solidification behavior of γ′-Ni 3Al-containing alloys in the Ni–Al–O system was established. Specifically, these measurements revealed that (i) γ′-Ni 3Al forms via the peritectiod reaction, γ + β (+Al 2O 3) = γ′ (+Al 2O 3), at 1633 ± 1 K; (ii) the { γ + β + Al 2O 3} phase field is stable over the temperature range 1633–1640 K; and (iii) equilibrium solidification occurs by the eutectic reaction, L (+Al 2O 3) = γ + β (+Al 2O 3), at 1640 ± 1 K and a liquid composition of 24.8 ± 0.2 at.% Al (at an unknown oxygen content). When projected onto the Ni–Al binary, this behavior is inconsistent with the current Ni–Al phase diagram and a new diagram is proposed. This new Ni–Al phase diagram explains a number of unusual steady-state solidification structures reported previously and provides a much simpler reaction scheme in the vicinity of the γ′-Ni 3Al phase field.

The Fe–Ni–Al phase diagram in the Al-rich (>50 at.% Al) corner

The ternary Fe–Ni–Al phase diagram between 50 and 100 at.% Al was investigated by a combination of powder X-ray diffraction (XRD), differential thermal analysis (DTA) and electron probe microanalysis (EPMA). Ternary phase equilibria and accurate phase compositions of the respective equilibrium phases were determined within the isothermal section at 850 °C. The crystal structure of τ1 (Fe 4− x Ni x Al 10) and τ2 (Fe 2− x Ni x Al 9) was determined by means of single crystal X-ray diffraction. The decagonal quasi-crystalline phase q (Fe 4.9Ni 23.4Al 71.7) was found to be stable between 850 °C and 930 °C. All experimental data were combined to yield a ternary reaction scheme (Scheil diagram) involving 10 ternary invariant reactions in the investigated composition range, and a liquidus surface projection was constructed based on DTA results.

Modeling heterogeneous interaction during SHS in the Ni-Al system: A phase-formation-mechanism map

Heterogeneous reaction Ni + Al → NiAl in strongly non-isothermal conditions typical of SHS has been modeled for planar geometry (unit bilayer Ni/Al film). The model used for simulation is based on the traditional quasi-equilibrium concept of the diffusion-controlled growth of a continuous solid product layer, NiAl, between molten Al and solid (or liquid) Ni-based solution. The basic idea is to analyze the competition between the growth of NiAl and its dissolution in parent phases due to variation in solubility limits with temperature. In calculations, a fragment of the equilibrium Ni-Al phase diagram is used along with real values of the interdiffusion coefficient in NiAl instead of determining the activation energy and preexponent by fitting the calculated and experimental results of the SHS wave temperature profile (as was done in other modeling works). The temperature dependence of phase densities is also accounted for. Simulation has revealed the existence of critical heating rates that correspond to the onset of the quasi-equilibrium dissolution-precipitation route with an intermediate solid NiAl film separating two liquid phases and then to non-equilibrium crystallization (when this film dissolves completely). Thus, the existence of non-equilibrium interaction mechanisms during SHS in the Ni-Al system, which were observed experimentally, is confirmed theoretically by the ex adverso method. A map of phase formation mechanisms is plotted which permits determining the reaction route in non-isothermal conditions.

Modeling nonisothermal interaction kinetics in the condensed state: A diagram of phase formation mechanisms for the Ni–Al system

Nonisothermal interaction in condensed Ni–Al system takes place during combustion synthesis (CS) or the so-called self-propagating high-temperature synthesis, which is characterized by a high heating rate, high final temperature, fast accomplishment of interaction, and unconventional, nonequilibrium phase formation mechanisms. In this work, a kinetic model for phase formation during CS of nickel monoaluminide in a layered Ni–Al system is developed basing on the diffusion-controlled growth of a solid product layer in strongly nonisothermal conditions. The model includes competition between the growth of NiAl and its dissolution in the parent phases (Al-base melt and solid or liquid Ni-base solution). For numerical calculations, real values of diffusion parameters in NiAl and the Ni–Al phase diagram are used. Simulation has revealed the existence of critical heating rates that correspond to a transition from the common diffusion-controlled growth mechanism to quasiequilibrium dissolution-precipitation pattern and then to nonequilibrium crystallization route. Thereby the existence of uncommon interaction pathways during CS of intermetallics, which were observed experimentally and debated in literature, is proved theoretically ex contrario. A map of phase formation mechanisms for reaction Ni+Al→NiAl in nonisothermal conditions is constructed, which defines the applicability domains of previously known theoretical approaches and permits predicting the interaction route.

Atomistic modeling of the γ and γ ′-phases of the Ni–Al system

A new embedded-atom potential has been developed for Ni 3Al by fitting to experimental and first-principles data. The potential describes lattice properties of Ni 3Al, point defects, planar faults, as well as the γ and γ ′ fields on the Ni–Al phase diagram. The potential is applied to calculate the energies of coherent Ni/Ni 3Al interphase boundaries with three different crystallographic orientations. Depending on the orientation, the interface energy varies between 12 and 46 mJ/m 2. Coherent γ/γ ′ interfaces existing at high temperatures are shown to be more diffuse and are likely to have a lower energy than Ni/Ni 3Al interfaces.

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...

Site preference of ternary Fe addition in Ni75Al25

An extensive investigation of structure, surface morphology andcomposition in Ni75−xFexAl25 alloys withnominal composition x = 0,5, 10 and 20, deliberately ‘prepared’ in different states of site disorder, has beencarried out by x-ray diffraction, scanning electron microscopy, x-ray fluorescence(energy dispersive absorption of x-rays) and inductively coupled plasma opticalemission spectroscopy. The results of an elaborate analysis of the present x-raydiffraction data on Ni75−xFexAl25(Ni-deficit) alloys and those reported previously on Ni75Al25−xFex(Al-deficit) alloys vindicate the recent theoretical prediction that the compositionof the alloy plays a decisive role in determining the site preference of Fe by clearlydemonstrating that Fe has nearly exclusive Ni (Al) site preference in the Ni-deficit(Al-deficit) alloys and thereby resolves a long-standing controversy surroundingthe site preference of the ternary Fe addition in the γ′-Ni3Alphase in the Ni–Al phase diagram. However, contrary to the recent theoreticalclaim that entropy promotes Al site preference of Fe in Ni-deficit alloys, we findthat Fe exclusively occupies the Ni sites in those Ni-deficit alloys that have agreater degree of site disorder.

Processing and microstructural characterization of sputter-deposited Ni/Ni3Al multilayered thin films

The crystallographic texture, orientation relationships, coherency stress, and thermal stability of sputter-deposited Ni/Ni3Al multilayered thin films were studied as a function of bilayer period (Λ) as well as processing parameters such as substrate type, deposition temperature, and prebake conditions. Deposition onto oxidized Si or single-crystal Cu(001), NaCl(001), or KBr(001) substrates near room temperature produces multilayers with a [111] crystallographic texture along the Ni/Ni3Al interface normal and a disordered face-centered cubic structure for the Ni3Al phase. In contrast, deposition at 673 K onto NaCl(001) or KBr(001) substrates that are prebaked in vacuum at 693 K produces a chemically ordered L12 structure for the Ni3Al phase and (001) epitaxial growth. X-ray diffraction measurements of (001) multilayers with equal volume fraction of Ni and Ni3Al reveals a transition from a nearly incoherent state at Λ=40 nm to a semicoherent one at Λ 40 nm. Remarkably, (001) multilayers were observed to solutionize at 1373 K, which is approximately 100 K below that predicted by the Ni–Al phase diagram.

Reassessment of Ni-Al and Ni-Fe-Al solidus temperatures

Solidus temperatures of the B2 NiAl phase have been determined by high-temperature differential thermal analysis for binary melt compositions NixAl100−x (45<x<57) and for ternary alloys FeyNi50−yAl50 (0≤y≤50). It was shown that the melting temperature of the stoichiometric Ni50Al50 phase is 1681 °C, which is 43 K higher than some literature data. The solidus line at the Ni-rich side of the Ni-Al phase diagram exhibits a steeper slope than that reported previously. Substituting Fe for Ni, the decrease of solidus temperature along the isoplethal section with 50 at.% Al of the ternary Ni-Fe-Al phase diagram exhibits a steep initial slope of −13 K/at.% Fe for small Fe-fractions, which changes into a nearly linear decrease with an average slope of −8.5 K/at.% Fe.