Ni Ga Research Articles

Effect of γ precipitates on martensitic transformation and magnetic properties in aged Mn–Ni–Ga alloys

Multiple martensitic transformation peaks were observed from differential scanning calorimetry curves in aged Mn50Ni25Ga25 alloys in which transformation temperature varied with aging temperature. The magnetization and coercivity were enhanced as a result of aging above magnetic transition temperature. A maximum magnetization up to 54emu∕g and a magnetization difference between two phases of 28emu∕g was achieved in the alloy aged at 873K. However, coercivity as large as 1.2kOe was obtained in 573K aged alloy, with a high saturation field, which can be understood in terms of domain wall pinning.

Structures and magnetic properties of Co–Ni–Ga melt-spun ribbons

It was found that Co50Ni20Gax (x=0–40) melt-spun ribbons were ferromagnetic, but Co50Ni20Ga40 melt-spun ribbon was paramagnetic at room temperature. The addition of Ga to Co–Ni melt-spun ribbon significantly reduced the saturation magnetization. It was also found that the Curie temperature of the β phase in Co50Ni20Gax (x=20–30) melt-spun ribbons was around 300K and that of the β phase in Co50Ni20Ga40 melt-spun ribbon was 110K. Unlike the other Co–Ni–Ga melt-spun ribbons, Co50Ni20Ga30 melt-spun ribbon exhibited a coercivity of 1.0kOe after annealing at 773K for 1h.

Microstructure and mechanical properties in Co–Ni–Ga–Al shape memory alloys with two-phase structure

Abstract The fcc second phase (γ) is commonly encountered in some new magnetic shape memory alloys, such as Ni–Mn–Fe–Ga, Co–Ni–Ga, Co–Ni–Al and Ni–Fe–Ga systems. The introduction of the γ phase is greatly favored to the enhancement of ductility and its volume fraction is generally determined by the content of Ga or Al. In this study, we attempt to change the amount of the γ phase by partially substituting Al for Ga in Co–Ni–Ga alloys. The results show that not only the amount but also the phase structure and the solidified morphology of the second phase are strongly affected by the addition of Al. The non-facet γ phase is transitioned to the sharp-edge γ′ ordered-structure phase when 2 at.% Ga is replaced by Al. Additionally, the martensitic transformation and compression stress–strain behavior in two-phase structure Co–Ni–Ga–Al alloys are investigated. The martensitic transformation temperatures increase with increasing Al content and decreasing the second phase. However the ultimate strength and ductility decrease by the decrease of the second phase.

Effect of composition and heat treatment on the martensitic transformations in Co–Ni–Ga alloys

A series of Co–Ni–Ga samples was investigated with regard to the structural, thermal, magnetic and mechanical properties. Optical and transmission electron microscopies revealed two types of two-phase polycrystalline alloys: (a) martensite + γ precipitates and (b) parent phase + γ precipitates. X-ray and electron diffraction methods were used to identify the martensitic phase as body-centered tetragonal structure with about c 0/ a 0 ≈ 1.2. The structures of the parent and the γ phase were determined as B2 and face-centered cubic, respectively. Influence of heat treatment on microstructure, martensitic transformation temperatures and the Curie temperature were reported. Annealing at 1073 K for 20 h causes dispersion of precipitation and led to grain boundary decoration, since, quenching from 1223 K results in a considerable increase of martensitic start and Curie temperatures. Additionally, compression tests were performed.

Ferromagnetic shape memory alloys: Alternatives to Ni–Mn–Ga

This paper intends to review the state-of-the-art in the field of ferromagnetic shape memory alloys different than Ni–Mn–Ga, developed in the last years. It starts with the effects of additions of different fourth elements to Ni–Mn–Ga and continues with the review of other Heusler (or B2) Ni-based and Co-based alloy systems, like Ni–Mn–X, Ni–Fe–Ga, Co–Ni–Al and Co–Ni–Ga alloys. Finally, ferromagnetic Fe-based alloys, like Fe 3Pt and Fe–Pd and a new mechanism of magnetic actuation in Ni–Mn–In–Co, recently introduced, will be treated.

Orientation dependence and tension/compression asymmetry of shape memory effect and superelasticity in ferromagnetic Co 40Ni 33Al 27, Co 49Ni 21Ga 30 and Ni 54Fe 19Ga 27 single crystals

In the present study the effects of crystal axis orientation, stress state (tension/compression) and test temperature on shape memory effect and superelasticity of Ni 54Fe 19Ga 27(I), Co 40Ni 33Al 27(II), Co 49Ni 21Ga 30(III) (numbers indicate at.%) single crystals were investigated. The shape memory effect, the start temperature of superelasticity T 1 and the mechanical hysteresis Δ σ were found to be dependent on crystal axis orientation and stress state. Superelasticity was observed at T 1 = A f ( A f, reverse transformation-finish temperature) in tension/compression for [0 0 1]-oriented Ni–Fe–Ga crystals and in compression for [0 0 1]-oriented Co–Ni–Ga crystals, which all displayed a small mechanical hysteresis (Δ σ ≤ 30 MPa). An increase in Δ σ of up to 90 MPa in the Co–Ni–Al and the Co–Ni–Ga crystals lead to stabilization of the stress-induced martensite, and an increase in to T 1 = A f + Δ. The maximal value of Δ (75 K) was found in [0 0 1]-oriented Co–Ni–Al crystals in tension. A thermodynamic criterion describing the dependencies of the start temperature of superelasticity T 1 on crystal axis orientation, stress state and the magnitude of mechanical hysteresis is discussed.

Phase equilibria in the Ni-rich portion of the Ni–Ga binary system

The phase equilibria in the composition range from 0 to 60 at% Ga of the Ni–Ga system were determined by electron probe microanalysis (EPMA) using diffusion couples, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). It was found that while the phase equilibria between the α′ (L1 2: Ni 3Ga) and α (Ni-solid solution) or β (B2: NiGa) phases are basically in agreement with the diagram evaluated by Lee and Nash, those between γ (B8 1: Ni 13Ga 7), δ ( Cmmm: Ni 5Ga 3) and ɛ ( C2/ m: Ni 13Ga 9) are topologically different from that diagram. Three eutectoid reactions (γ → δ + ɛ, β → γ + ɛ, β → α′ + γ) and one peritectoid reaction (α′ + γ → δ) were confirmed and the temperatures and concentrations of those invariant reactions were determined.

Characteristics of the Si-doped ferromagnetic shape memory alloy Co50Ni21Ga29Six

In order to improve the properties of Co—Ni—Ga alloys, experiments of adding Si to Co50Ni21Ga29 alloys were carried out to examine the effect of Si in quaternary Co50Ni21Ga29Six (x=1—2) alloys on their structure, martensitic transformation behavior and magnetic properties. The Co50Ni21Ga29Six (x=1,2) single crystals were successfully grown. All the samples with low Si doping show a sharp martensitic transformation with increasing temperature. The single crystal samples show a completely recoverable two-way shape memory with a strain of 2.5% upon the thermoelastic martensitic transformation. A large magnetic-field-induced strain of 110 ppm was measured at room temperature. A small quantity of Si doping can down-shift the martensitic transformation temperature, reduce the thermal hysteresis, decrease the magnetization but increase the Curie temperature. Especially, the Si doping can increase the magnetic anisotropy of Co—Ni—Ga-alloys, which is available to obtain large magnetic-field-induced strain in a low magnetic field. An electronic resistance “jump" of 4.5% upon the thermoelastic martensitic transformation was also reported.

Crystal growth of Co 46Ni 27Ga 27 ferromagnetic shape memory alloys with large single-variant set

The present work proposes a directional solidification method based on liquid melt cooling (LMC) technique to prepare large grain with single-variant set in Co–Ni–Ga alloys. The competitive growth from equaixed grains to steady columnar crystals with 〈1 1 0〉 orientation along the axis was observed. The directionally solidified rod has a uniform chemical composition. It can be also found that the unidirectional lamellar martensitic variants were well aligned in a whole grain, forming a single-variant state. Furthermore, the needle-like Ni 3Ga-type γ′ precipitates were formed in alloy with lower growth velocity, and it exhibited the complicated microstructural evolution. At the lowermost part of rod-like crystal, a large number of precipitates were dispersed both in grain interiors and at boundaries but its amount decreased when the columnar crystals were formed and gradually increased again from bottom up to top in the whole rod.

Microstructure, superelasticity and fracture behavior in magnetic shape memory Co–Ni–Ga–Fe alloys with two-phase structures

It is shown that the martensitic transformation temperatures of Co 50 Ni 22− x Ga 28 Fe x ( x = 0, 1, 2, 5) alloys decrease as Ni is gradually substituted by Fe. The volume fraction of the second phase is also reduced with increasing Fe content, which impairs the mechanical properties . Two-phase Co–Ni–Ga–Fe alloys exhibit complete superelastic behaviors in a wide temperature interval between room temperature and 200 °C. Moreover, the stress hysteresis in two-phase alloy shows much stronger sensitivity to temperature than that in single-phase alloy.

Magnetic force microscopy observations of Co–Ni–Ga and Co–Ni–Al alloys with two-phase structures

This study investigated the magnetic domain morphologies of Co–Ni–Ga and Co–Ni–Al alloys with the β + γ two-phase structures by magnetic force microscopy. In the rapidly solidified state, the martensite in a grain refined Co 45 Ni 25 Ga 30 alloy exhibited sharp magnetic contrast, whereas the parent phase in a Co 38 Ni 34 Al 28 alloy with divorced eutectic structure had weak magnetic anisotropy. After annealing at 1173 K, the γ phase showed fulgurous magnetic domains in the annealed Co 45 Ni 25 Ga 30 , and stripe-like 180° domain walls in regularly eutectic Co 38 Ni 34 Al 28 .

Surface characteristics of Ni–Ga–Fe sputtered thin films

Chemical composition and surface morphology of Ni–Ga–Fe thin films grown under various sputtering powers and argon gas pressures by r.f. magnetron sputtering technique are studied. X-ray fluorescence results show that the sputtering power has obvious effect on the chemical composition of Ni–Ga–Fe thin films. The argon gas pressure has little effect on the composition of Ni–Ga–Fe thin films. The change in surface morphology of Ni–Ga–Fe films is discussed in terms of root-mean-square (rms) surface roughness values. The grain size and the rms of Ni–Ga–Fe thin films increase with the increase of sputtering power and argon gas pressure. The film deposited at room temperature has a cubic L2 1 structure and the annealed film shows martensitic structure.

Phase transformation behavior and microstructures of rapidly solidified Co–Ni–Ga alloys

Martensitic transformation behavior, magnetic properties and microstructures of Co–Ni–Ga alloy ribbons are investigated and contrasted with those of Co–Ni–Ga bulk alloy. The rapidly solidified 40–60 μm-thickness ribbons of the alloy were prepared by an electro-magnetically controlled single-rolled melt-spinning apparatus. Martensitic transformation characteristics of the Co–Ni–Ga ribbons resemble those of bulk samples. However, transformation temperatures in the ribbons are higher than those of the bulk, suggesting the formation of stress-induced martensites. Coercive force of the Co–Ni–Ga ribbons strongly depend on the direction of the applied magnetic field. Maximum coercive force is obtained at θ = 70° ( θ is the angle between the magnetic field and the ribbon plane). This anisotropy of the magnetic properties is considered to be characteristics of the rapidly solidified ribbons. Microstructures of the martensitic phase in the ribbons are similar to those in the bulk samples, that is, optical and scanning electron microscopies reveal a typical lenticular shape and transmission electron microscopy shows internal twins and specific streaks on selected area electron diffraction patterns.

Martensitic and magnetic transformations of Ni–Ga–Fe–Co ferromagnetic shape memory alloys

Effects of Co addition on the phase equilibria between the β (B2) and γ (A1) phases at 800 and 1000 °C and aging temperature dependence on the martensitic and magnetic transformation temperatures in Ni–Ga–Fe–Co alloys were examined with energy dispersion X-ray spectroscopy (EDX), differential scanning calorimetry (DSC) and vibrating sample magnetometry (VSM). The equilibrium compositions of Fe and Ga are little changed by the addition of Co, and Co tends to be distributed in the γ phase rather than in the β phase. The B2/L2 1 order–disorder and magnetic transformation temperatures increase with increasing Co content, while the martensitic transformation temperatures decrease. While the martensitic transformation temperatures monotonously decrease with decreasing aging temperature, the Curie temperature increases with the decrease of aging temperature in the high temperature region over 300 °C and then starts to decrease at low temperatures.

The elastic constants of FCC Ni–Ga and Ni–Ge alloys up to 1100K

The elastic constants , C ij , of monocrystalline Ni–12.16Ga and Ni–11.59Ge (at.%) were measured to 1100 K. C 44 is roughly equal for both solid solutions and Ni 3 Ga and Ni 3 Ge. Otherwise, Ni 3 Ga is elastically softer, while Ni 3 Ge is elastically harder above 900 K. Implications of this latter comparison are discussed.

High undercooling effect on magnetic shape memory Co–Ni–Ga alloys

The martensitic microstructure and crystal structure evolution of Co46Ni27Ga27 alloys under different undercooling condition was investigated. The increase of reverse martensitic transformation temperature was found with undercooling in 22 to 165 °C undercooling range, but decreased with further high undercooling. The reasons for these have been discussed using the elastic strain energy mechanism and grain refinement caused by internal stress during rapid solidification.

The microstructure and martensitic transformation of Co–Ni–Ga–Ta ferromagnetic shape memory alloys

Abstract The effect of the fourth element Ta on the microstructure, crystal structure and martensitic transformation of (Co 45 Ni 25 Ga 30 ) 1− x Ta x ( x = 0, 0.5, 2, 5) alloys was investigated. Some Ta-rich precipitates were observed in the higher Ta-addition samples, which lead to the decrease of the martensitic transformation temperatures and thermal hysteresis. In addition, 1423 K annealing temperature is not high enough to impede the presence of the γ phase in Co–Ni–Ga–Ta alloys, and 1473 K heat treatment temperature is needed.

Crystal structures and phase transitions in ferromagnetic shape memory alloys based onCo–Ni–Al and Co–Ni–Ga

Magnetization and AC permeability measurements show that the shape memory compoundsCo1.52Ni1.32Al1.16 and Co1.8Ni0.92Ga1.28 both order ferromagnetically below K and show a further transition at K. Neutron powder diffraction measurements have confirmed that the lowertransition is martensitic and have enabled the structures of the austenite andmartensite phases in both compounds to be established. The temperatures atwhich the transitions occur in the two alloys are similar, as are the structuresof both their austenite and martensite phases. The austenite phase ofCo1.52Ni1.32Al1.16 ispartially ordered in the B2 structure. The martensite phase in both compounds arises from a simpletetragonal distortion of the cubic cell. Unlike the related ferromagnetic shape memory compoundNi2MnGa, the martensitic transitions are not abrupt and there is a temperature range of atleast 100 K in which both tetragonal and cubic phases coexist. The widths of thecubic Bragg peaks are resolution limited but those associated with the tetragonalstructure show broadening consistent with the presence of anisotropic micro-strain.

Synthesis of NiGa Layered Double Hydroxides. A Combined EXAFS, SAXS, and TEM Study. 3. Synthesis at Constant pH

The present paper focuses on the direct synthesis of NiGa layered double hydroxides (LDHs) by concomitant addition of Ni and Ga chloride in solution with NaOH solution, at a constant pH value of 6.5. Various Ni/Ga ratios (Ni/Ga = 2, Ni/Ga = 4, or Ni/Ga = 6) were investigated, and samples were collected after successive additions. Each sample was then analyzed by small-angle X-ray scattering (SAXS) to derive information about the shape of the objects formed upon synthesis. In parallel Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) analysis were carried out on relevant samples to obtain information on the local structure of the species in suspension. SAXS curves reveal significant changes. At the beginning of the synthesis, for low added volumes (V(s)), the suspension appears to contain both monodimensional and bidimensional objects. For higher V(s), the monodimensional objects disappear and only platelike particles are detected in the suspension. EXAFS results show that all the added gallium atoms are involved in a solid Ni-Ga LDH phase throughout the whole synthesis. In contrast, some nickel cations do not precipitate in the Ni-Ga LDH. At the beginning of the synthesis, for low added volumes they are also involved in the formation of fibrous polycations. For higher added volumes, the fibers are not observed anymore and monomeric nickel species are then present in the suspension.

Synthesis of NiGa Layered Double Hydroxides. A Combined EXAFS, SAXS, and TEM Study. 2. Hydrolysis of a Ni2+/Ga3+Solution

Takovites are nickel-based layered double hydroxides (LDH) with a general formula that can be written as Ni(1-x)Al(x)(OH)2, A(z-)(x/z), yH(2)O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses on the hydrolysis of Ni(2+) and Ga(3+) ions, together in solution, carried out by base addition. The samples will be defined by their hydrolysis ratio R = [OH(-)]/([Ni(2+)] + [Ga(3+)]). Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to obtain information on the colloidal species size and shape on a large scale. Each hydrolyzed sample was also studied by Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the species in suspension. SAXS curves reveal the presence of bidimensional objects whatever the R values. The platelets formed for R values >/=1.1 are slightly thicker and smaller in size, which may be linked to their different natures. Complementary information is provided by TEM analysis: the first colloids formed have a structure very close to that of alpha-GaOOH, as shown by electronic diffraction. Those structures are progressively replaced by Ni-Ga LDH platelets with increasing hydrolysis ratio, which are the only species in suspension for R = 2.0, as shown by XRD. EXAFS results confirm the complete hydrolysis of gallium before the formation of Ni-Ga LDH phases.