Ni Ga Research Articles

Mixing enthalpies of liquid nickel–gallium alloys

The partial mixing enthalpies of melt components in the Ni–Ga system are measured using hightemperature isoperibolic calorimetry at 1770 ± 5 K in a wide composition range. The partial mixing enthalpy of gallium in liquid nickel is –96 kJ/mol. for infinite dilution, while the same function of nickel in liquid gallium is –75 kJ/mol. The integral mixing enthalpy of liquid alloys in this system is calculated from partial enthalpies for the entire composition range (∆m H min = –32 kJ/mol. at xNi = = 0.5). The mixing enthalpies ∆m H of Ni–B (Al, Ga, In) binary alloys are compared. It is shown that the indicated systems are arranged in the Ni–In → Ni–Ga → Ni–B → Ni–Al series according to the increase in interaction energy of dissimilar components. The ∆m H min values of liquid Ni–B(Al, Ga, In) alloys are compared with the values of electrochemical and size factors.

Mechanism of Ohmic Cr/Ni/Au contact formation on p-GaN

Detailed investigation of Ohmic Cr/Ni/Au based contact formation to p-GaN was realized by scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy and compared to the Cr/Au bilayer metallization scheme. The authors found that it is essential to introduce a nickel film in Cr-based contacts and anneal the trilayer structure in air in order to suppress the Shottky barrier and thus obtain the Ohmic contact. Our findings also indicate that oxygen behaves as a dopant dispersed in chromium nitride matrix. Thus Ohmic trilayer Cr/Ni/Au contact to p-GaN annealed in air is formed by Ni–Ga–Au alloy mixed with Au–Ga-doped Cr2N crystalline composites. Possible ways for improvement of such types of contact are discussed.

A less expensive NiMnGa based Heusler alloy for magnetic refrigeration

We present a study of the substitution of Mn by Cu on the compound Ni2Mn1-xCuxGa0.9Al0.1, showing that the substitution of a small amount of Al on the Ga site does not affect the magnetic and magnetocaloric potential compared to Ni2(Mn,Cu)Ga alloy. The samples were prepared with 10% substitution of Al and with Cu concentrations of x = 0.0, 0.2, and 0.3. Magnetization measurements as a function of temperature performed from 10 to 400 K, with an applied field of 0.02 T showed a ferromagnetic state, with critical temperature Tc = 295 and 300 K for the samples with Cu, x = 0.2 and 0.3, respectively. For the sample without Cu, a complex behavior is observed at Tc = 370 K, with martensitic transition at 220 K and a premartensitic at 250 K. Analysis of x-rays diffractograms at room temperature show a L21 structure for x = 0.0, while for x = 0.2 a mixture of L21 and martensitic is present, and the sample with x = 0.3 it is in a fully martensitic phase. Heat capacity measurements were performed in order to calculate magnetocaloric effect in the samples. The results indicate that in Ni(Mn,Cu)Ga alloys, a partial substitution of Ga by Al still produce a high refrigerant capacity while reducing the costs of fabrication.

Mineral and chemical composition of the new iron meteorite Javorje from Slovenia

Abstract– The single‐piece iron meteorite Javorje, with a mass of 4920 g, is the heaviest and largest meteorite found in the territory of Slovenia. The meteorite Javorje is a medium octahedrite with kamacite bandwidth of 0.85 ± 0.26 mm. The bulk composition of Ni (7.83 wt%), Co (0.48 wt%) and trace elements Ga (25 μg/g), Ge (47 μg/g), Ir (7.6 μg/g), As (5.8 μg/g), Au (0.47 μg/g), and Pt (13.4 μg/g) indicates that the meteorite Javorje belongs to the chemical group IIIAB. Mineral and bulk chemical compositions are consistent with other reported group IIIAB meteorites. The presence of numerous rhabdites, carlsbergite, sparse troilite, and chromite and abundance of daubréelites are in accordance with low‐Ni and low‐P IIIAB iron meteorites. The severely weathered surface and secondary weathering products in the interior of the meteorite suggest its high terrestrial age.

A magnetic Compton scattering study of Ga rich Co–Ni–Ga ferromagnetic shape memory alloys

The temperature dependent spin momentum densities of Co1.8NiGa1.2 and Co2Ni0.76Ga1.24 alloys have been measured using the magnetic Compton scattering technique. The individual contributions of constituents in the formation of the total spin moment are also calculated using Compton line shape analysis. The magnetic Compton data when compared with the magnetization data obtained using a vibrating sample magnetometer show a negligible orbital contribution. The spin moments deduced from the experimental Compton data are compared with the theoretical results obtained from the full potential linearized augmented plane wave method and are found to be in good agreement. The origin of the magnetism in both alloys is also described in terms of the eg and t2g contributions of Ni and Co.

Ternary intermetallics Hf13.0Ni40.8Ga30.9 and Zr13.0Ni40.6Ga31.0

Ternary compounds Hf13.0Ni40.8Ga30.9 and Zr13.0Ni40.6Ga31.0 were synthesized from the pure elements in arc-melting reactions; their structures were characterized through X-ray diffraction of single crystals. Each compound adopts a hexagonal structure of type Y13Pd40Sn31 and crystallizes in space group P6/mmm (No. 191). The cell parameters of Hf13.0Ni40.8Ga30.9 are a=17.895(3)Å; c=8.2434(16)Å; V=2286.0(6)Å3; R1/wR2=0.0299/0.0598. The cell parameters of Zr13.0Ni40.6Ga31.0 are a=17.964(3)Å; c=8.2757(17)Å; V=2312.7(7)Å3; R1/wR2=0.0348/0.0686. These structures comprise polyhedra with diverse coordination numbers – Hf 12–15, Ni and Ga 6–12. Analysis of the structures reveals layer frameworks based on structures of type CaCu5, accompanied with atomic substitution and elimination on the layers and decreased values of the concentration of valence electrons. Calculated electronic structures reveal a strong contribution from a Ni–Ga interaction and the characteristics of a polar intermetallic phase.

Investigating the effects of bulk supercooling and rapid solidification on Co–Ni–Ga ferromagnetic shape memory alloys

In this study, Electromagnetic Levitation (EML) technique was utilized to explore the effect of bulk supercooling and rapid solidification in alloys with Co46Ni27Ga27 and Co48Ni22Ga30 (at.%) compositions. The effects of γ + β on the martensitic and austenitic transformation temperatures and magnetic properties were investigated. The presence of γ phase was found to suppress the martensitic and austenitic transformations to below room temperature. Bulk supercooling and rapid solidification led to the formation of homogeneous martensitic phase from the hyperperitectic Co46Ni27Ga27 alloy. In contrast with pure martensite phase in Co48Ni22Ga30, the hyperperitectic martensite in supercooled Co46Ni27Ga27 sample showed no grain boundaries microsegregation and embrittlement that caused deep cracks along grain boundaries. The sample had a high Curie temperature about 400 K and good directional magnetic properties.

A1-L12 interfacial free energies from data on coarsening in five binary Ni alloys, informed by thermodynamic phase diagram assessments

The data on coarsening of γ′-type precipitates (Ni3X, with the L12 crystal structure) in Ni–Al, Ni–Ga, Ni–Ge, Ni–Si, and Ni–Ti alloys are re-evaluated in the context of recent (TIDC) and classical (LSW) theories of coarsening, with the objective of ascertaining the best values possible of interfacial free energies, σ, of the γ/γ′ interfaces in these five alloy systems. The re-evaluations include fitting of the particle size distributions, reanalyzing all the available data on the kinetics of particle growth and kinetics of solute depletion, and using thermodynamic assessments of the binary alloy phase diagrams to calculate curvatures of the Gibbs free energies of mixing. The product of the work is two sets of interfacial free energies, one set for the analysis using the recent TIDC theory and the other for the analysis using the classical LSW theory. The TIDC-based analysis yields lower values of σ by about a factor of 2/3. All the interfacial energies are considerably larger, by factors ranging from ~4 to 10, than those previously reported, which were for the most part calculated from data on coarsening assuming ideal-solution thermodynamics. In the TIDC theory the width of the interface, δ, is allowed to increase with particle size, r. A simple equation relating σ to the ratio of the gradient energy and δ is used to show that σ can remain constant even though δ increases with r. Published work supporting this contention is presented and discussed.

A novel route for growing single-crystal and internal-stress-induced martensitic transformation of ferromagnetic shape memory alloys Co 50Ni 20Ga 30

As a new single-crystal preparation method, undercooling directional solidification has been proposed for the case of the ferromagnetic shape memory Co 50Ni 20Ga 30 rod. The as-grown single crystal has strong [1 1 1] orientation and a very homogenous composition along the rod axis, which confirms that the undercooling directional solidification is a feasible and effective approach for preparing Co–Ni–Ga single crystals. Additionally, the drastically increased transformation temperatures upon undercooling was ascribed to the internal stress induced martensitic transformation.

Effect of aging on martensitic transformation, microstructures and mechanical properties in off-stoichiometric Mn–Ni–Ga alloys

(Mn,Ni) 4Ga-type γ precipitates were obtained in Mn 53Ni 25Ga 22 alloys aged at 573–973 K. Aging greatly affects the transformation temperature by modifying the matrix composition and changing the unit cell volume. When the aging temperature is 573 K, needle-like γ precipitates 70–80 nm in length can be seen by transmission electron microscopy. Rotational twin martensite variants were discovered to be related with an angle rotation of 15° in the [1 1 1] direction. A crystalline orientation relationship between the tetragonal martensites and face-centered cubic γ phases was observed as [1 1 1] γ//[1 1 0] M and ( 0 2 2 ¯ ) γ// ( 0 0 2 ¯ ) M. A substructure of ( 1 ¯ 1 1 ¯ ) type-I twins for γ phase was found in the 973 K/3 h aged sample in which the coarsened precipitates reveal a plate-like morphology. Compression tests show that a compressive strength of 1850 MPa with a fracture strain up to 25.6% can be achieved in 773 K/10 min aged samples due to a substantial dispersion strengthening from small γ precipitates.

Effect of high undercooling on Co–Ni–Ga ferromagnetic shape memory alloys

Co 45Ni 25Ga 30 ferromagnetic shape memory alloys were treated by glass fluxing combined with superheating cycling. The effect of high undercooling on solidified microstructure and transformation temperatures was investigated. The martensite lath is obviously refined and a mass of sub-grains are produced with the increase of undercooling. Undercooling rapid solidification introduces a number of dislocations and large internal stress, which give rise to the production of sub-grains in recovery. During the following annealing process, these sub-grains can gradually recrystallize and grow up with an amount of γ′ phase precipitated. Meanwhile, transformation temperatures of undercooled alloys are greatly elevated compared with those of as cast, which results from the large internal stress, and gradually reduce with the heating time.

Microstructure evolution of the undercooled Co–Ni–Ga magnetic shape memory alloy

Microstructure evolution of the martensite Co 50Ni 22Ga 28 magnetic shape memory alloy was investigated by the method of glass fluxing combined with superheating cycling. A maximum bulk undercooling of 220 K is achieved and a mass of sub-grains are introduced by the undercooling. A number of dislocations and large internal stress, due to the undercooling rapid solidification, give rise to the production of sub-grains during the recovery process. These sub-grains, as well as martensite variants, are remarkably refined with the increase of undercooling. The recrystallization and growth of sub-grains during the following annealing are also discussed.

Properties of Ni doped and Ni–Ga co-doped ZnO thin films prepared by pulsed laser deposition

ZnNiO and Zn(Ni,Ga)O thin films were prepared on glass substrates by pulsed laser deposition. The obtained films are of good crystal quality and have smooth surfaces, which have a hexagonal wurtzite ZnO structure with a highly c-axis orientation without any Ga or Ni related phases. Hall-effect measurements showed that the ZnNiO film is n-type, in which the carrier concentration would be greatly enhanced by the addition of Ga. Room temperature ferromagnetism is observed for the ZnNiO and Zn(Ni,Ga)O films. The addition of Ga into the ZnNiO films increases the electron concentration but weakens the room temperature ferromagnetism.

Anisotropy of the electrical transport properties in a Ni2MnGa single crystal: Experiment and theory

Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ=1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR=31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Ni 3d and Ga 4p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation.

Microstructures and shape memory characteristics of dual-phase Co–Ni–Ga high-temperature shape memory alloys

The influence of microstructure on mechanical properties and shape memory characteristics of Co–Ni–Ga high-temperature shape memory alloys were investigated in this study. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to detect the microstructures. We found that these alloys were composed of dual phases, a non-modulated tetragonal L1 0 martensite and a face-centered cubic (fcc) γ phase. The martensite was twinned and well self-accommodated. The γ phase was a Co-based solid solution with 30% lower hardness than martensite. Although the fracture mode was intergranular, the strength and plasticity of the alloys increased markedly with the increasing volume fraction of the γ phase. The presence of the γ phase in grain boundaries rather than in the martensite is favorable to shape memory recovery. This was revealed by the maximum shape recovery strain over 5.0% that was obtained in the Co 46Ni 25Ga 29 alloy, with the γ phase formed mainly in grain boundaries.

Microstructure, transformation temperatures, hardness and magnetic properties of Co 36.4+ xNi 33.3− xGa 30.3 ferromagnetic SMA

Microstructure, martensitic transformation temperatures, hardness, and magnetic properties have been studied in Co–Ni–Ga alloys for different compositions of Co and Ni. Hardness, transformation temperatures and thermal hysteresis show a decrease with Co content. On the contrary, maximal magnetization, and magnetic hysteresis both grow with cobalt composition.

Martensitic transformation and magnetocaloric properties of Sn doping Mn–Ni–Ga alloys

The effects of Sn addition on phase transformation behavior and magnetocaloric properties of Mn 50Ni 25Ga 25− x Sn x ( x=0, 0.1, 0.5, 1 and 2 at%) alloys were investigated in this work. The results show that the addition of Sn reduces the structural transformation temperatures. It is found that the second phase exists in the austenite matrix of the as-casted alloys at room temperature. After being annealed at 1073 K for 48 h, the precipitates totally soluted into the matrix. Magnetization measurements indicate that the saturation magnetizations of the alloys increase significantly with increase in Sn contents. In addition, the Δ M/Δ S obviously increases with increase in the Sn contents, implying the higher efficiency shift of the martensitic transformation temperature under the magnetic field.

Structure, lattice dynamics and Fermi surface of the magnetic shape memory system Co–Ni–Ga from first principles calculations

Advanced magnetic shape memory materials like the prototypical Ni–Mn–Ga alloy system are limited to operating temperatures that are too low for many practical applications. To overcome this problem, an intensive search for new magnetic shape memory compounds has been started. One interesting system, showing magnetic as well as conventional shape memory behavior, is CoNi–Ga. In this work we report systematic studies of stoichiometric Co–Ni–Ga based alloys in the full and inverse Heusler structure by means of density functional theory. A prediction of the martensitic transition temperatures can be obtained by the structural energy differences calculated for different crystal structures. In prototype, near-stoichiometric Ni–Mn–Ga, the (pre-)martensitic transformation is accompanied by an anomalous softening of one transversal acoustic phonon branch along the [110] direction which has been frequently linked to nesting features of the Fermi surface in the past. In order to clarify this aspect for the Co–NiGa system, we will discuss the influence of structure on the phonon dispersions determined from first principles and investigate whether the Fermi surface of the Co–Ni–Ga compound reveals nesting features as well.

Ordering in mechanically alloyed Ni(Ga) and Ni(Ga, C) solid solutions during heating

Nonequilibrium Ni(Ga) and Ni(Ga, C) solid solutions are fabricated by mechanical alloying. The ordering in the solid solutions, which is accompanied by the formation of the Ni3Ga intermetallic compound and the Ni3GaC x carbide, is studied by X-ray diffraction, differential scanning calorimetry, and calculating model X-ray diffraction patterns. A procedure is proposed for determining the carbon content in the binary Ni3GaC x carbide with a perovskite structure (structure type E21) from the intensity ratio of superlattice lines. The equilibrium carbon content in the carbide synthesized by mechanical alloying is shown to be 0.25 atom per unit cell.

Magneto-microstructural coupling during stress-induced phase transformation in Co49Ni21Ga30 ferromagnetic shape memory alloy single crystals

The present study reports on direct magneto-microstructural observations made during the stress-induced martensitic transformation in Co49Ni21Ga30 alloy single crystals with optical, scanning electron, and magnetic force microscopy (MFM). The evolution of the microstructure and the associated magnetic domain morphology as a function of applied strain were investigated in the as-grown condition and after thermo-mechanical training. The results demonstrated that the stress-induced martensite (SIM) evolves quite differently in the two conditions and depending on the martensite formation mechanisms, the magnetic domain configuration was dissimilar. In the as-grown crystals two twin-related martensite variants were formed and the growth of these twin variants resulted in large strain. After thermo-mechanical training a morphology similar to a self-accommodating martensite structure was present at the initial stages of the transformation and thereafter martensite reorientation (MR) was the main transformation mechanism. The magnetic domains were found to be superimposed on the nano-scaled martensite twins in the as-grown condition, whereas training brought about the formation of domains on the order of a few microns without showing the one-to-one correspondence between domains and the twin structure. After the thermo-mechanical training detwinning at high-strain levels led to the formation of stripe-like domain structures. The ramifications of the results with respect to the magneto-microstructural coupling that may cause the magnetic shape memory effect (MSME) in Co–Ni–Ga alloys under constant external stress is addressed.