Ni-Al System Research Articles

Nanoindentation of NiAl and Ni3Al crystals on (100), (110), and (111) surfaces: A molecular dynamics study

Molecular dynamics simulations were performed to study the nanoindentation of NiAl and Ni3Al crystals on three surfaces: (100), (110), and (111). The calculated load-displacement curves show discrete drops at certain indentation depths, indicating dislocation bursts during indentation. The hardness values for the two materials were found to depend significantly on the indented crystallographic plane: the (100) surface is the softest for NiAl and the hardest for Ni3Al. We also found distinctive deformation activities in the subsurface region in Ni3Al crystals, while dislocation loops propagate deep into the substrate in NiAl systems.

SHS in the Ni-Al system: A TRXRD study of product patterning

Combustion of Ni-Al mixtures was explored by time-resolved XRD within the homogeneity range for NiAl. Unconventional behavior of the strongest (110) NiAl peak was observed: with decreasing temperature, the initially narrow peak became broadened and then split into several components that merged during subsequent cooling down. This observation is thought to reflex the complicated kinetics of the phase transformation from the high-temperature (around 1600°C) disordered [Ni,Al] solid solution (space group Im3m) to the low-temperature fully ordered NiAl phase (space group Pm3m).

Ni-Al Nanoscale Energetic Materials: Phenomena Involved During the Manufacturing of Bulk Samples by Cold Spray

It has been shown that the cold-gas dynamic spraying process, or simply cold spray, is a suitable technique to manufacture nanoscale energetic materials with high reactivity and low porosity. The current study focuses on the Ni-Al system, for which the reactivity has been increased by an initial mechanical activation achieved by the ball-milling technique, leading to lamellar nanostructured composite particles. The consolidation of this nanoscale energetic material using the cold-gas dynamic spray technique permits to retain the feedstock powder nanoscale structure in the coatings, which in turn retain the high reactivity features of the powder. However, it has been noticed that the stagnation temperature during the spray can lead to partial reaction of the highly reactive feedstock powder, which directly influences the reactivity of the coatings. In this study, different stages of the spray process were investigated: (i) the in-flight behavior of the nanoscale energetic material (powder) at different stagnation temperatures (from 300 to 800 °C); (ii) the substrate-temperature evolution as the function of gas temperature; and (iii) the impact of the powder on the substrate, related to particle’s velocity and its influence on the nanostructure of the particles.

The Gd–Ni–Al system: Phases formation and isothermal sections at 500 °C and 800 °C

Phase relations in the Gd–Ni–Al ternary system have been established for two isothermal sections (500 and 800 °C) based on X-ray powder diffraction analysis (XRD), Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) coupled with Energy dispersive Microprobe Analysis (EPMA) on about 31 annealed alloys in the 50–100 at.% Al region. Seven intermetallic phases have been identified in samples annealed at 500 °C and 800 °C: Gd3Ni5Al19 (oS108–Gd3Ni5Al19), Gd4Ni6Al23 (mS66–Y4Ni6Al23), GdNiAl4 (oS24–YNiAl4), GdNiAl3 (oP20–YNiAl3), Gd3Ni7Al14 (hP72–Gd3Ni7Al14), GdNiAl2 (oS16–CuMgAl2) and GdNi2Al3 (hP18–GdNi2Al3). One additional ternary phase GdNi3Al9 (hR78–ErNi3Al9) has been found only in samples annealed at 800 °C. The isothermal sections have been determined and then compared, the crystal structures of the ternary phases have been confirmed and lattice parameters calculated.

The configurations of nanoalloy by impact deposition: atomistic simulation on Ni–Al system

The use of energetic particles can change the growth mode and provide control of nanoalloy morphology and properties. The impact deposition of Ni (or Al) on the truncated octahedral nanoparticle of Al (or Ni) is studied. The embedded atom method is used for the description of the interatomic interactions in combination with molecular dynamics method for the growth simulation. Three configurations of Ni–Al nanoparticle are obtained depending on incident energy and deposition sequence. A perfect core–shell nanoparticle with Ni-core/Al-shell is obtained as Al atoms are deposited over Ni nanoparticle. For the deposition of Ni atoms on Al nanoparticle, an onion-like nanoparticle at smaller incident energy, and a configuration with Al-shell and alloyed Ni–Al core at larger incident energy are observed, respectively. The formation energies show that the latter is energetically favorable.

High-pressure phase transformations, microstructure, and magnetic properties of the hypereutectic alloy 10Ni-90Al

Phase transformations of the hypereutectic alloy 10Ni-90Al have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ≅100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al3Ni. At high pressures, the solid-state decomposition of the supersaturated solid solution of nickel in aluminum occurs at 622°C and yields an (Al) + Al3Ni mixture. We assume that, during cooling of the alloy at high pressure, intermetallic particles form in three steps. Barothermal analysis data have been compared to the canonical t-x phase diagram of the Al-Ni system. As a result of melting and crystallization at high pressure, the microcrystalline structure of the as-prepared alloy transforms to a macrocrystalline, dendritic structure and the micropore concentration increases. X-ray diffraction data indicate an increase in the unit-cell volumes of the Al and Al3Ni in the alloy crystallized at 100MPa. We have studied the magnetic properties of the alloy after barothermal analysis.

Mathematical modeling of metallochemical reactions in a two-species reacting disperse mixture

A heterogeneous model of gasless combustion of binary disperse systems is proposed. The model combines the description of microscale processes of interphase interaction in the cell of the mixture with the macroscale description, which allows physicochemical transformations to be described on the basis of the continuum approach of mechanics of continuous media and the phase composition of the final products to be calculated. Schemes of metallochemical reactions of formation and decomposition of intermetallic phases are proposed, based on an analysis of the state diagram of the Ni-Al system. A problem of SHS wave propagation and evolution of the spatial distribution of concentrations of intermetallic phases during combustion of a mixture of Ni and Al powders is solved numerically. A two-wave structure of the combustion wave is obtained, and comparisons with experimental data are performed.

Liquid phase formation in the system Al2O3–Y2O3–AlN: Part II. Thermodynamic assessment

The mixing parameters of liquid phase in the Al2O3–Y2O3–AlN system were assessed based on differential thermal analysis (DTA) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX) investigations of selected compositions. Phase diagram of the Y2O3–AlN system was calculated. Liquidus surface of the Al2O3–Y2O3–AlN system was constructed and compared with experimental results on primary crystallisation fields. Calculated temperatures of invariant reactions were in agreement with DTA results. Vertical sections of the Al2O3–Y2O3–AlN system were calculated and compared with experimental data

The Specific Heat Capacity and Oxidation Kinetics of NiAl, FeAl and TiAl Alloys

The specific heat capacity was calculated by DSC and the kinetics of oxidation was determined for alloys from the NiAl, FeAl and TiAl phase equilibrium system. The Ti48Al alloy contained an addition of 2 at.% of chromium and niobium. The highest value of heat capacity had the Ti-Al and Fe-Al alloys. At temperatures above 673K, the Ti48Al alloy showed an anomaly, that is, a lower value of the heat capacity. The change in heat capacity was related with an exothermic reaction. The oxidation of alloys was running according to either parabolic or cubic law. Alloys characterised by high thermal capacity showed a higher gain in weight. The slowest to oxidise was the Ni3Al intermetallic phase.

Combinatorial Approach to Turbine Bond Coat Discovery

A nondestructive method for the high-throughput screening of novel bond coat materials has been developed. By using a suite of characterization techniques, including Raman spectroscopy, fluorescence spectroscopy, and X-ray diffraction, a rapid determination of thermally grown oxide phases and their protective capability over a continuous composition spread sample can be obtained. The methodology is validated with the Ni-Al system. The procedure developed in this work results in the rapid identification of bond coat composition regions in which the preferred thermally grown oxide, α-Al2O3, is nucleated thus significantly reducing the amount of phase space that needs to be explored in subsequent studies.

Studying exothermic reactions in the Ni-Al system at rapid heating rates using a nanocalorimeter

Heats of reaction and heat capacity changes were measured using scanning nanocalorimetry for a nickel and aluminum bilayer where initial heating rates of 104 K/s were achieved. Multiple exotherms were observed on the initial heating, but the number of intermediate exotherms decreased with increasing heating rate. The final phase was the B2 NiAl intermetallic. Results from the nanocalorimeter were compared with a conventional differential scanning calorimeter (operating at 0.7 K/s) to understand the effect of significant (10 000×) increases in heating rate on the phase transformation sequence. The high heating rate in the nanocalorimeter delays reaction initiation, causes the exothermic peaks to shift to higher temperatures, and appears to suppress the formation of intermediate, metastable phases. Potential explanations for this apparent suppression are discussed.

FORMING OF INTERMETALLIC PHASES DURING INFILTRATION OF NICKEL RODS AND POWDERS WITH MOLTEN ALUMINIUM

Reaction kinetics in the Al-Ni system was investigated considering the fact that Al-Ni composites have been found as attractive candidates for a number of industrial sectors. Al-Ni composites were prepared by gas-assisted infiltration of either Ni rods or Ni powders with molten Al. Microstructural analysis revealed that various Ni aluminides were grown and their formation and the morphology at the interfacial zones can be effectively controlled by the time of infiltration. Furthermore, we pointed to the instability in the Al-Ni system that was caused by the presence of the pure Al metal. This could be completely abolished by the additional thermal treatment.

Thermal explosion in various Ni-Al Systems: Effect of mechanical activation

Thermal explosion (ignition) of various Ni-Al samples—such as powder blends, rolled and magnetron sputtered films, compacts of mechanically activated and non-activated mixtures—was explored with special emphasis on the role of mechanical activation. The critical ignition temperature (Tc) was found to diminish from 650°C for starting (non-activated) Ni-Al mixtures down to 280°C for properly activated samples. For the thin films of alternating Ni and Al layers prepared by magnetron sputtering, Tc turned out to be as low as 230°C.

Molecular dynamics simulation of surface segregation, diffusion and reaction phenomena in equiatomic Ni-Al systems

The molecular dynamics method is used to provide fundamental insights into surface segregation, bulk diffusion and alloying reaction phenomena in equiatomic Ni-Al systems. This knowledge can serve as a guide for the search and development of economic routes for controlling microstructure and properties of the intermetallic compound NiAl. This paper gives an overview of recent molecular dynamics simulations in the area along with other theoretical calculations and experimental measurements.

Microstructure development during NiAl intermetallic synthesis in reactive Ni–Al nanolayers: Numerical investigations vs. TEM observations

Heterogeneous reactions leading to the formation of intermetallic compounds in nanometric Ni–Al multilayer system are examined both numerically and experimentally. On the numerical side, the reactivity of a layered Ni–Al–Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method (EAM) potential. The mechanism of nucleation and growth of the intermetallic phase is determined. Four main stages in the reactive process, which lead to the formation of rounded shape grains of intermetallic phase at the Ni–Al interface, are delineated. On the experimental side, TEM imaging of quenched samples revealed the behavior of the Ni–Al reactive foils and showed the typical morphology of the reaction products. Both approaches exhibit a good qualitative agreement and provide new insights in the understanding of metallic nano-foils reactivity. Obtained results can be used for rational design of energetic materials.

Analysis of softening alloys of the Al-Ni system containing particles of variable dispersity

Regularities of the deformation strengthening and softening of aluminum alloys containing second-phase Al3Ni particles 0.3 to 2.2 μm in size with a volume fraction from 0.03 to 0.1 are investigated during cold deformation and subsequent annealing at 0.6tm. It is shown that the largest hardness increment is observed for alloys with a maximal fraction of fine particles (d = 0.3 μm) after rolling deformation larger than 0.4. Fine particles prevent the development of crystallization upon true deformation up to 2.3, thereby effectively inhibiting softening. An increase in the particle size to 1.2–2.2 μm stimulates nucleation during recrystallization, substantially accelerating this process. For example, in order to ensure recrystallization uniformly over the entire sheet volume at d = 2.2 μm, cold deformation with ɛ = 0.4 is sufficient.

Free-energy based pair-additive potentials for bulk Ni-Al systems: Application to study Ni-Al reactive alloying

We present new numerical pair-additive Al, Ni, and Al-Ni potentials by force-matching (FM) ionic force and virial data from single (bulk liquid) phase ab initio molecular dynamics (MD) simulations using the Born-Oppenheimer method. The potentials are represented by piece-wise functions (splines) and, therefore, are not constrained to a particular choice of analytical functional form. The FM method with virial constraint naturally yields a potential which maps out the ionic free-energy surface of the reference ensemble. To further improve the free energetics of the FM ensemble, the FM procedure is modified to bias the potentials to reproduce the experimental melting temperatures of the reference (FCC-Al, FCC-Ni, B2-NiAl) phases, the only macroscopic data included in the fitting set. The performance of the resultant potentials in simulating bulk metallic phases is then evaluated. The new model is applied to perform MD simulations of self-propagating exothermic reaction in Ni-Al bilayers at P = 0-5 GPa initiated at T = 1300 K. Consistent with experimental observations, the new model describes realistically a sequence of peritectic phase transformations throughout the reaction and at a realistic rate. The reaction proceeds through interlayer diffusion of Al and Ni atoms at the interface with formation of B2-NiAl in the Al melt. Such material responses have, in the past, been proven to be difficult to observe with then-existing potentials.

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ∼100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 1010 cm−3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 1011 cm−3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al3Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al3Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.

Spontaneous ignition temperature for the compacted mixture of Ni–Al system: Experiment, theory, and comparisons

Spontaneous ignition of compacted mixture has been examined not only experimentally but also theoretically, relevant to materials synthesis for Ni–Al system. Spontaneous ignition temperature, determined from the inflection-point of the temporal variation of surface temperature, is found to decrease at first, reach the minimum, and then increase gradually, with increasing size ratio, being defined as the ratio of compact and particle diameters and reported to be useful in correlating experimental results. The lowest temperature observed is as low as that, more than 200K below the melting point 934K of Al, at the size ratio of c.a. 700. While its decrease is attributed to the increase in the particle surface per unit spatial volume of compacted mixture, as reported in the literature, its increase found in the course of the present study can fairly be correlated to an enhancement of heat loss from each particle in the compacted mixture, by conducting a theoretical consideration. As for the dependence of spontaneous ignition temperature on the mixture ratio, a shifting occurs from a decreasing trend with increasing mixture ratio at small size ratios, less than about 700, to an insensitive trend to the mixture ratio, had not been reported in the literature. In addition, a fair degree of agreement, shown in experimental comparisons with theoretical results, indicates that the present formulation has captured the essential features of the spontaneous ignition of compacted mixture, especially for size ratios with large values. Since this kind of particle size effects, relevant to the spontaneous ignition of the compacted mixtures, has not been captured in the previous studies, its elucidation can be considered not only notable but also useful, in manipulating combustion process in materials synthesis, especially, in choosing a specific condition for the lowest spontaneous ignition temperature by varying the size ratio.

Investigation of solidification in undercooled Al-rich Al-Ni alloy systems

Dendrite growth velocities as a function of undercooling on the Al-rich side of the Al-Ni system have recently been measured by electromagnetic levitation. They have shown an anomalous behaviour, which so far cannot be theoretically described. The present work investigates whether properties of the phase diagram could explain this effect. For this purpose two simplified qualitative models of sharp interface theory, one treating the forming AlNi (B2) phase as a solid-solution, one treating it as an intermetallic phase, have been developed. To treat the concentration range between 50 and 70 at% Al, sets of thermodynamical data for six different compositions have been created. Results are compared to the experimental data. In both models low undercoolings at low Ni concentrations lead to very limited velocities, because the constitutional undercooling ΔTC overrides the kinetic undercooling ΔTk. The experimental results for low undercoolings seem to contain an effect which cannot be described by a model of this type.