Binary Ni-Al Alloys Research Articles

Three-dimensional phase-field simulations of coarsening kinetics of γ ′ particles in binary Ni–Al alloys

The coarsening kinetics of γ ′ precipitates in binary Ni–Al alloy is studied using three-dimensional (3D) phase-field simulations. The bulk thermodynamic information and atomic diffusion mobilities are obtained from databases constructed using the CALPHAD approach, while the experimental values for the interfacial energy, elastic constants and lattice mismatch are directly employed in the phase-field model. Specifically, we predict the morphological evolution, average precipitate size, and size distribution as a function of time for a given temperature and composition. Comparison of the phase-field simulation results with experiments shows good quantitative agreement in both time and length scales.

Ni and Fe tracer diffusion in the B2-ordered Ni 40Fe 10Al 50 ternary alloy

Ni and Fe bulk diffusion in the B2-ordered Ni 40Fe 10Al 50 ternary alloy is investigated by means of radiotracer measurements ( 63Ni and 59Fe tracers) and SIMS analysis ( 64Ni tracer) in an extended temperature interval from 1150 to 1550 K. Almost linear Arrhenius behavior was observed for both, Ni (pre-exponential factor D 0 ∗ Ni =1.3·10 −5 m 2/s and activation enthalpy Q Ni=270 kJ/mol) and Fe ( D 0 ∗ Fe =1.2·10 −4 m 2/s and Q Fe=279 kJ/mol) diffusion. The Fe diffusivity was found to be by a factor of three to five larger than that of Ni. Furthermore, Ni diffuses in Ni 40Fe 10Al 50 faster than in stoichiometric NiAl and the activation enthalpy of Ni diffusion in Ni 40Fe 10Al 50 is significantly smaller as compared to the stoichiometric binary NiAl alloy (by about 35 kJ/mol). The activation enthalpy of Ni diffusion in the Ni 50− x Fe x Al 50 ternary alloys was found to reveal a minimum for ternary compositions with about 10% of Fe addition. The apparent controversial behavior of creep resistance and self-diffusivity in the NiFeAl ternary alloys is discussed. The analysis of experimental data favors nearest-neighbor jumps (most likely the triple defect mechanism) as a dominant diffusion mechanism in the alloy.

Oxidation-induced cavity formation in binary β-NiAl alloys

Abstract Previously, it has been established that oxidation-induced cavitation of binary NiAl alloys occurs primarily when Nirich compositions are oxidized. In this work it is shown that at 1100 °C cavities form principally during the very early stages of oxidation for these compositions. During this time, the growing oxide is a transient, theta-alumina whereas on the Al-rich compositions the growing oxide rapidly transforms to alpha alumina. It is concluded that cavity formation is associated with tensile stresses generated by the flux divergence created by the outward diffusion of Al during the growth of theta-alumina and the inward diffusion of nickel. Once the transient oxide transforms to alpha alumina, the outward flux of aluminum is decreased and with it the flux divergence. Values for the inter-diffusion coefficients for the alloys investigated, derived from the diffusion profiles, are also reported.

Increasing length scale of quantitative phase field modeling of concurrent growth and coarsening processes

We present an approach for increasing the length scale of quantitative phase field modeling of concurrent growth and coarsening processes with neither growth nor coarsening kinetics altered. We modify locally the free energy hump between the two equilibrium phases, which determines interface thickness, while keeping the driving forces for growth and coarsening unchanged. The approach is applied to precipitate growth and coarsening in a binary Ni–Al alloy.

Quantitative interface models for simulating microstructure evolution

To quantitatively simulate microstructural evolution in real systems, we investigated three different interface models: a sharp-interface model implemented by the software DICTRA and two diffuse-interface models which use either physical order parameters or artificial order parameters. A particular example is considered, the diffusion-controlled growth of a γ ′ precipitate in a supersaturated γ matrix in Ni–Al binary alloys. All three models use the thermodynamic and kinetic parameters from the same databases. The temporal evolution profiles of composition from different models are shown to agree with each other. The focus is on examining the advantages and disadvantages of each model as applied to microstructure evolution in alloys.

Martensitic transformation behavior in Ni–Al and Ni–Al–Re melt-spun ribbons

The martensitic transformation behavior was investigated in rapidly solidified (melt-spun) Ni–(34–37)at%Al and Ni–(32–34)at%Al–2at%Re ribbons. The addition of Re increased the temperature for the formation of Ni 5Al 3, hence the martensitic transformations were observed at higher temperatures than those of binary Ni–Al alloys.

Effect of additions of Re in NiAl based alloys

An attempt to develop shape memory NiAl based alloys using macroalloying has been made. Re additions to the binary NiAl alloys result in the formation NiAl (B2), Ni 3 Al (L1 2 ), Ni 5 Al 3 , NiAl (Ll 0 ), and Re phase. The microstructure of these alloys was examined with using X-ray diffraction, optical microscope and resistance measurements. The binary NiAl alloy was investigated too. It seems Re is not dissolved in NiAl matrix. Comparison of NiAl and NiAlRe alloys reveals that additions of Re lead to the creation of the special pseudosubstructure. The NiAlRe system exhibits martensite transformation. The peculiarities of the pseudosubstructure allow expecting the influence on mechanical properties of the NiAl based alloys are discussed.

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5 at.% aluminum has been studied at 800 and 900°C under 1 atm O2. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.

Effect of Intrinsic factors on primary dendrite arm spacing in Ni-base binary alloys

Since the equilibrium distribution coefficient and solidification temperature range are determined when the initial solute concentration of a binary alloy is decided, researchers have not directed their attention to the effects of them on primary dendrite arm spacing, whereas there have been many studies on the effects of solidification rate and temperature gradient on primary dendrite arm spacing in past decades. In the present study, Ni-Al and Ni-Nb binary alloys were solidified unidirectionally in order to investigate the effects of intrinsic factors such as equilibrium distribution coefficient and solidification temperature range on primary dendrite arm spacing. It was found that the primary arm spacings of them increased linearly with the fourth root of the product of the equilibrium distribution coefficient and the solidification temperature range in the equilibrium solidification of each alloy. This result agrees with results obtained using previously proposed theoretical equations.

The coarsening kinetics of γ′ particles in nickel-based alloys

The present article describes a method for calculating the coarsening rate of γ′ in Ni-based superalloys, which has been applied to both binary Ni-Al alloys and a wide range of multicomponent alloys. A standard coarsening equation is utilized, but innovative methods for calculating the critical input parameters are presented. The article details methods of estimating interfacial energies and the effective diffusion coefficients that are key parameters for the coarsening model. Self-consistent calculations are made via a computer program in which the only input required is the composition of the alloys and the temperature of coarsening. The effects of coherent strain on the coarsening process have also been analyzed and discussed.

Drastically improved ductility of an Ir-base alloy by mixing Ir–Nb with Ni–Al

The fcc/L1 2 coherent structure formed in the quaternary Ir–Nb–Ni–Al alloys. The ductility of quaternary Ir–Nb alloy was drastically improved by mixing with Ni–Al binary alloy. The two investigated alloys exhibited higher high-temperature 0.2% flow stress (e.g. above 1000 MPa at 1200 °C) and better ductility (≈20% both at room temperature and 1200 °C). An excellent balance of high-temperature strength and ductility was obtained. The improvement of ductility was attributed to change the fracture mode from intergranular to transgranular.

An Introduction to the BFS Method and Its Use to Model Binary NiAl Alloys

We introduce the Bozzolo–Ferrante–Smith (BFS) method for alloys as a computationally efficient tool for aiding in the process of alloy design. An intuitive description of the BFS method is provided, followed by a formal discussion of its implementation. The method is applied to the study of the defect structure of NiAl binary alloys. The groundwork is laid for a detailed progression to higher order NiAl-based alloys linking theoretical calculations and computer simulations based on the BFS method and experimental work validating each step of the alloy design process.

The incoherent γ/γ′ solvus in Ni-Al alloys

Analysis of the kinetics of solute (Al) depletion during the coarsening of γ′ (Ni 3 Al)precipitates has been used in earlier investigations to provide values of the coherent solubility of the γ phase in binary Ni-Al alloys. As demonstrated in recent experimental investigations, the coherent solubility is a function of the total concentration of Al in the two-phase alloy. Using the model of Ardell and Maheshwari to analyze the data on coherent equilibrium, the authors estimated the incoherent equilibrium solubility of the γ′ phase over the temperature range 400 to 800 ° C. The calculated incoherent solubility limits are 3 to 5 % smaller than previously published measurements, the differences decreasing as the temperature increases. The authors argue that the calculations in this article provide the first estimates of the incoherent solvus of the γ′ phase in Ni-Al alloys.

Ni-Fe-Al 3元β相の力学的性質に及ぼすボロン添加の影響

The mechanical properties of the ordered intermetallic Ni-Fe-Al β phase alloys with and without boron has been studied as a function of composition and temperature by compression testing. The alloys without boron where Al is kept constant at 45 and 40 mol%, whereas that with boron where Al is kept constant at 45 mol% and addition of 0.05 mass% boron. The present work indicated that (1) the temperature dependence of strength behavior of (Ni, Fe)Al; (45 mol%Al) alloys is similar to bcc metals and binary NiAl alloys, characterized by three different stages. The alloys of stoichiometry in binary NiAl, FeAl and near the second stoichiometry in ternary (Ni, Fe)Al; (45 mol%Al) show plateaus distinctly and have a wide plateau temperature range, which becomes narrower and less distinct when the composition of the alloy is off stoichiometry. (2) Consisting with the hardness, the ternary alloys of near the second stoichiometry that with the β phase in the Ni-Fe-Al system show the lowest 0.2% flow stress, and an increase in iron content largely enhances the solid solution strength. (3) The boron improved the strength of (Ni, Fe)Al; (45 mol%Al) alloys, especially at low temperature, and with increasing temperature, the effect becomes weaker and disappears beyond 1250 K. (4) The brittle-ductile transition temperatures (BDTT) of (Ni, Fe)Al; (45 mol%Al) alloys were also largely decreased by a small amount of boron addition in comparison to the unalloyed (Ni, Fe)Al; (45 mol%Al) alloys, indicating a better high-temperature mechanical properties with a favorable of strength and fracture toughness.

Lattice misfits in four binary Ni-Base γ/γ1 alloys at ambient and elevated temperatures

High-temperature X-ray diffractometry was used to determine thein situlattice parameters,a γ anda γ′, and lattice misfits, δ = (a γ′, -a γ)/a γ, of the matrix (γ) and dispersed γ′-type (Ni3X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 °C. Concentrated alloys containing large volume fractions of theγ′ phase (∼0.40 to 0.50) were aged at 700 °C to produce large, elastically unconstrained precipitates. The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of ± 4 pct. The absolute values of the lattice constants of theγ andγ′ phases, at compositions corresponding to thermodynamic equilibrium at about 700 °C, are in excellent agreement with data from the literature, with the exception of Ni3Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, δ decreases to 0.00612 at 679 °C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, δ exhibits a stronger temperature dependence, changing to-0.00285 at 683 °C (Ni-Si) and to 0.00424 at 680 °C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in δ observed are attributable to differences between the thermal expansion coefficients of theγ andγ′ phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 δC. Empirical equations are presented that accurately describe the temperature dependences ofa γ,a γ′, and δ over the range of temperatures of this investigation.

H + processing of Ni/Al multilayer structures

The method of heat transfer in low pressure weakly ionized hydrogen plasma was used for the thermal processing of samples of an Ni/Al multilayer structure on well polished silicon substrates. The samples were exposed to hydrogen plasma under different conditions. The sample temperature increased typically at a rate of 10 °C s −1, remained constant for several minutes, and decreased at a rate of 10 °C s −1. The steady temperature was between 300 °C and 1000 °C. The samples were analyzed using Auger electron spectroscopy and extended X-ray absorption fine structure spectroscopy. The results show that the coating obtained after thermal processing is a mixture of nickel, aluminum and nickel aluminides, different from NiAl binary alloy. At high temperatures, evaporation of aluminum from the coating was observed.

Three stage yielding phenomenon in a NiAlFe alloy

The binary NiAl alloy is extremely brittle at ambient temperatures, so it is withdrawn from practical applications as structural materials or shape memory alloys. It has been found that iron is one of the effective elements in improving the ductility of the NiAl alloy. The NiAl-Fe alloys with ([gamma]+[beta]) dual-phase microstructures have good ductility and also good hot-working properties. These results provide the possibility for using the NiAl-Fe alloys for commercial use. However, there are still many experiments to be performed before these alloys can be practically used. The present paper reports a part of the authors work on the deformation behavior of a NiAl-Fe alloy with a ([gamma]+[beta]) microstructure.

1200 K Compressive Properties of N-Containing Nial

AbstractAs part of a series of experiments to understand the role of N on the strength of NiAl, a heat of NiAl was enriched with N by melting and atomization to powder in a nitrogen atmosphere. Following consolidation of the powder by hot extrusion, 1200 K compressive properties were measured in air. Within the range of strain rates examined, 10−3 to 10−9 s−1, the strength of the N-enriched NiAl was greater than that of a simple 15 µm grain size polycrystalline, binary NiAl alloy. For the most part the overall improvement in strength is ascribed to the fine grain size of the N-doped NiAl rather than the alloy chemistry; however, the alloy displayed a complex behavior exhibiting both weakening effects as well as strengthening ones.

Thermodynamical Estimations on the Amount of Liquid Phase During Reaction Synthesis of Nickel-Aluminides

AbstractThe amounts of intermetallic compounds and liquid phase are estimated as a function of temperature during reaction syntheses for Ni–Al binary alloys having arbitrary compositions under adiabatic conditions. It is assumed that the phases that would present during the process are only those appearing in the equilibrium binary phase diagram. Calculations are done by using available data on heat of formation of intermetallic compounds and those on enthalpy of liquid phase. The results obtained arc discussed in view of the estimation and control of the amount of liquid phase, which is significant for the densification and shape change of materials.

Effects of boron on the mechanical behaviour of a Ni-rich NiAl alloy

The NiAl based intermetallic alloys are currently of considerable interest as new structural materials because of their relatively low density, comparatively high melting temperature and excellent high temperature oxidation resistance. It has also been found that the Ni-rich (62-65 at % Ni) NiAl alloys with the [beta] (B2) structure undergo a thermoelastic martensitic transformation and exhibit the shape memory effect (SME). Unfortunately, the binary NiAl alloys are extremely brittle at ambient temperature and, therefore, are restricted in practical application. The micro-alloying and macro-alloying methods are commonly used to overcome the brittleness of the alloys. It is reported that boron is the most effective microalloying element in improving the ductility and suppressing the intergranular fracture of the Ni[sub 3]Al intermetallic compound containing aluminium less than 25 at %. However, whether boron has a beneficial effect on the binary NiAl alloy with the B2 structure is not clear. The present paper reports the effect of boron on the mechanical 1 and martensitic transformation in a NiAl alloy containing 37.5 at % Al with the B2 structure.