Ni Fe Ga Alloys Research Articles

Effect of Boron Microalloying on Microstructure and Thermoelastic Martensitic Transformation in Aged NiFeGa(B) Alloys

Effect of Boron Microalloying on Microstructure and Thermoelastic Martensitic Transformation in Aged NiFeGa(B) Alloys

Influence of Heat Treatments on Martensitic Transformations and Elastocaloric Effect in Two-Phase (β + γ) NiFeGa Alloys

Influence of Heat Treatments on Martensitic Transformations and Elastocaloric Effect in Two-Phase (β + γ) NiFeGa Alloys

Ultraslow calorimetric studies of the martensitic transformation of NiFeGa alloys: detection and analysis of avalanche phenomena

We study the thermal properties of a bulk Ni55Fe19Ga26\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ni}_{55}\\hbox {Fe}_{19}\\hbox {Ga}_{26}$$\\end{document} Heusler alloy in a conduction calorimeter. At slow heating and cooling rates (∼1Kh-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim {1}\\,\\hbox {K h}^{-1}$$\\end{document}), we compare as-cast and annealed samples. We report a smaller thermal hysteresis after the thermal treatment due to the stabilization of the 14 M modulated structure in the martensite phase. In ultraslow experiments (40mKh-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${40}\\,\\hbox {mK h}^{-1}$$\\end{document}), we detect and analyze the calorimetric avalanches associated with the direct and reverse martensitic transformation from cubic to 14 M phase. This reveals a distribution of events characterized by a power law with exponential cutoff p(u)∝u-εexp(-u/ξ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p(u)\\propto u^{-\\varepsilon }\\exp (-u/\\xi )$$\\end{document} where ε∼2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varepsilon \\sim 2$$\\end{document} and damping energies ξ=370μJ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi ={370}{\\upmu {\\rm J}}$$\\end{document} (direct) and ξ=27μJ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi ={27}{\\upmu {\\rm J}}$$\\end{document} (reverse) that characterize the asymmetry of the transformation.

Effect of Zr doping on precipitated phase and mechanical properties of NiFeGa alloy

Abstract The effect of Zr alloying on the precipitates and mechanical properties of magnetic shape memory alloy Ni55Fe12Ga34-xZrx (x=0,2,3,4,5,12 atomic number fraction, the same below) was investigated. The results showed that Ni54Fe12Ga34 alloy is a single martensitic phase with high brittleness. Ni55Fe12Ga34-xZrx (x=2, 3, 4, 5) alloy was composed of γ phase and eutectic structure. The compressive strength of Ni55Fe12Ga34-xZrx alloy increases with the increase of Zr content, which is caused by the precipitation of the second phase and eutectic structure. When Zr content was 12 at.%, the alloy consists of γ phase, eutectic structure and strip phase. Due to the appearance of strip brittle phase, the compression strength of the alloy decreases. With the increase of Zr content, the strip phase increases, and the compression strength decreases gradually. Ni55Fe12Ga34-xZrx series alloys have excellent superelasticity, and the alloy has the best superelasticity when Zr content is 5 at.%.

Cluster spin glass state caused by antiphase boundaries in NiFeGa alloys

We report an observation of magnetic glassy properties in NiFeGa ferromagnetic shape memory alloys. A cluster spin glass transition was observed at approximately 27 K in Ni50Fe21Ga29. The glassy property originates from the freezing of fine magnetic domains separated by antiphase boundaries (APBs). Due to the pinning effect of the APBs, depression of the magnetization just below the Curie temperature was observed. The pinned domains induce very large saturation fields and a nearly random distribution of magnetization in low field. Based on dynamic magnetic measurements, the frequency dependence of the spin-glass transition was investigated by using the Vogel-Fulcher law and a power law. Below the freezing temperature, an aging phenomenon was observed and described by stretched exponential functions. These theoretical investigations confirm the cluster spin-glass state, which originates from a strong interaction between the frustrated magnetic domains.

Entropy change of martensitic transformation in ferromagnetic shape memory alloys

A consistent analysis of the entropy change that accompanies the martensitic transformation (MT) of ferromagnetic shape memory alloys has been carried out using the Landau theory of phase transitions. The interrelation between the values of entropy change and the widths of the temperature intervals of MTs, observed in experiments with different alloys, was described. General theoretical expressions for the non-magnetic and magnetic parts of the total entropy change were obtained and then applied to the Ni–Mn–Ga and Ni–Fe–Ga shape memory alloys. For Ni–Mn–Ga alloys the theoretical values of the magnetic parts of entropy changes appeared to be close to the measured total entropy changes. This fact demonstrated the crucial role of the interaction between the magnetic and elastic subsystems of these alloys in the thermodynamic characteristics of MTs. A theoretical estimation of the magnetic entropy change has been obtained from the experimental temperature dependence of magnetization of a Ni–Fe–Ga alloy.

Influences of composition and annealing on the martensitic transformation in Ni-Fe-Ga alloys

A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X2YZ) were prepared through arc-melting suction-casting method. The dependences of the transformation behavior on the alloy composition and annealing treatment were studied in detail by an optical microscope, X-ray diffraction, and differential scanning calorimeters methods. The experimental results show that the martensitic transformation temperatures increase almost linearly with increasing Ni content in all the NiFeGa alloys. Annealing the Ni55.5Fe18Ga26.5 alloy at 100–500 °C for 3 h and at 300 °C for 1–10 h shifts the martensitic transformation start temperature by almost 20 °C to high temperature. The variations in the martensitic transformation temperatures in these alloys are discussed in terms of structural differences resulting from alloy composition and annealing treatment.

Shape memory effect and superelasticity in single crystal Ni54Fe19Ga27

Shape memory effect and superelasticity are studied in this paper. At room temperature, perfect strain plateaus of 3.3%, 2% and 3% are obtained in the [001], [110] and [111] directions. The energies required to induce the martensitic transformation in different crystallographic directions are estimated from the stress-strain results. Each of the magnetization curves in the martensitic phase indicates that a magnetic anisotropy energy density Ku = (1/2)MsHa =4.8× 105 erg/cm3, which is far smaller than the mechanical driving force for variants twinning. This interpretes why only a little magnetic field induced strain in NiFeGa alloy has been observed so far.

Magnetocaloric effect and magnetoresistance of Ni–Fe–Ga alloys

Heusler alloys with nominal composition Ni73−xFexGa27 (x = 17, 18, 19, 20, 21, 22) were prepared by arc melting and subsequent homogenization by annealing. The magnetic properties of the alloys were studied to analyse the magnetic entropy change as a function of temperature. The maximum entropy change was observed in the alloy (x = 19) where martensite–austenite structural and ferro–para magnetic transitions were very close to each other. The transport properties and magnetoresistance (MR) of the samples were also investigated up to a magnetic field of 8 T and within a temperature range 4.2–325 K. It was observed that the MR in the austenite state was higher than that of the martensite state irrespective of the composition of the sample. A large negative MR (∼9% at 8 T) was obtained for the sample Ni54Fe19Ga27 at 300 K which was very close to the martensitic transition temperature as well as the Curie temperature.

Temperature dependence of magnetic susceptibility in the vicinity of martensitictransformation in ferromagnetic shape memory alloys

Temperature dependences of low-field quasistatic magnetic susceptibility in the vicinity ofmartensitic transitions in an NiFeGa alloy are studied both by experiment and analytically.Pronounced reversible jumps of the magnetic susceptibility were observed near themartensitic transition temperature. A general description of the temperature dependencesof the susceptibility in ferromagnetic austenite and martensite phases and the susceptibilityjump at the transition is suggested. As a result, the main factors governing thetemperature dependences of the magnetic susceptibility in the magnetic shape memoryalloys are revealed. The magnetic susceptibility jump value is found to be related tochanges of: (i) magnetic anisotropy; (ii) magnetic domain wall geometrical constraints (thosedetermined by the alignment and size of twin variants) and (iii) mean magnetic domainspacing.

Micromechanical modeling of stress–strain behaviors with intermartensitic transformation in NiFeGa alloys

Magnetic shape memory alloys (MSMAs) NiFeGa are reported as good alternatives to Ni 2MnGa for their better mechanical behavior. Experimental data show its multistage transformation behavior. In spite of the recent progress in the development of material characteristics, no theoretical model has been developed to calculate the stress–strain behaviors of NiFeGa alloys. A model based on the micromechanical and the thermodynamic theory is presented for such a purpose. In the first plateau stage, the material is modeled as a three-phase composite, and a two-phase system for the second plateau stage. The end of the first plateau stage is considered as the origin of the second one. The kinetic equation is established in terms of the thermodynamic driving force derived from the reduction of Gibbs free energy of MSMA. The nonlinear and hysteretic strain response of NiFeGa is investigated under tensile load. The theoretical results are found to be in good agreement with experimental data.

Influence of Al addition on martensitic transformation and magnetic properties of Ni–Fe–Ga ferromagnetic shape memory alloys

The effect of the fourth element Al on the martensitic transformation and magnetic properties of polycrystalline Ni–Fe–Ga alloys is investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Al alloys was observed in a range of 309–394 K. Martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Al for Ga element: the addition of 1% Al can decrease the martensitic transformation temperature by about 68 K. On the other hand, the substitution of Al for Ga increases the saturation magnetization and decreases the saturation magnetic field.

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K 1 obviously.

Transformation temperatures in polycrystalline Ni 56.5− xFe 17 Ga 26.5Ge x ferromagnetic shape memory alloys

The effect of Ge addition on the microstructure and martensitic transformation of polycrystalline Ni 56.5Fe 17Ga 26.5 shape memory alloy was investigated using optical metallography technique, X-ray diffraction and differential scanning calorimeter. The results showed that the martensitic transformation temperatures decrease monotonously with the increase of fraction of Ge substitution for Ni. The decrease of the transformation temperatures should be related to the change of the electron concentration after the addition of Ge in NiFeGa alloys.

Effect of Ge addition on the martensitic transformation temperatures of Ni–Fe–Ga alloys

A systematical study of substitution of Ge for Ni, Fe, and Ga in the non-stoichiometric NiFeGa alloys was performed in this work. The effect of Ge addition on the structure and martensitic transformation in NiFeGa alloys was investigated by an optical microscope, X-ray diffraction, and differential scanning calorimeters. The results show that the transformation temperatures decrease almost linearly with increasing Ge content in all the three types from higher than 100 down to −90 °C and even lower. The decreases in rate of the martensitic transformation temperatures are different for the three cases. It is large for Ni substituted by Ge, slow for Ga and intermediate for Fe. The determined relationship will be significant for designing a suitable NiFeGa alloy with a required martensitic transformation temperature for application at specific temperature.

Effect of annealing and heating/cooling rate on the transformation temperatures of NiFeGa alloy

In this work, Ni(55)Fe(18)Ga(27) ferromagnetic shape memory alloy was prepared through a suck-casting method. The effects of annealing and heating/cooling rate on the martensitic transformation temperatures were investigated by differential scanning calorimetry (DSC). The results showed that the phase transformation temperatures increase with increasing the annealing temperature, upon the heating and cooling process. However, the start and the finish temperatures (M(s) and M(f)) of martensitic phase transformation increased firstly and then decreased upon the cooling process with the increase the annealing time at 300 degrees C. The start and the finish temperatures (A(s) and A(f)) of inverse phase transformation increased slightly upon the heating process with the increase of the annealing time. The results can be explained by the evolution of the microstructure after heat treatment. It was also found that the phase temperatures show great dependence on the heating/cooling rate of the DSC test, A, and At increased and Ms and Mf decreased with the increase of the heating/cooling rate. (C) 2008 Elsevier B.V. All rights reserved.

Phase equilibria in the Ni–Fe–Ga alloy system

Abstract The phase equilibria, A2/B2 and B2/L2 1 (or D0 3 ) order–disorder transitions and martensitic transformation on the Ni–Fe side of the Ni–Fe–Ga system were examined by electron probe microanalysis (EPMA) and differential scanning calorimetric (DSC) measurement. The equilibrium compositions of interrelations mainly among the α (A2), β (B2), β′ (L2 1 or D0 3 ), γ (A1) and γ′ (L1 2 ) phases were determined using diffusion triples which were fabricated by two-step diffusion coupling. It was confirmed that a bcc single-phase region composed of α, β and β′ at 850–1000 °C exists in a wide composition range and that the critical temperature of the B2/L2 1 order–disorder transformation in the Fe 3 Ga–Ni 3 Ga pseudo-binary section gradually increases with increasing Ni content. The existing composition region of the martensite phase at room temperature was also determined by the diffusion triple method.

Thermal and structural study of Ni–Fe–Ga ferromagnetic shape memory alloys

Abstract Some thermal and structural characteristics of Ni–Fe–Ga alloys were studied by means of differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), and X-ray diffraction techniques. It was observed that martensitic transition temperature rapidly decreases and Curie temperature increases with Fe content. The order–disorder transition temperature from a L21 to A2 structures grows linearly with iron content, as measured by TMA. TMA has been found as a useful tool to perform these characterizations.

Mechanical response of NiFeGa alloys containing second-phase particles

Tensile and compressive strain–temperature and stress–strain responses have been used for the first time to investigate the effects of second-phase particles on the deformation of NiFeGa shape memory alloys. Particles are conducive to the dissipation of stored energy; stress and thermal hysteresis increase three-fold in aged alloys. Although dissipation is severe, recoverable strain levels are as large as +12% and −6%, which match strains for the unaged alloys. These results demonstrate that second-phase particles can be used to tailor the hysteresis while maintaining large transformation strains.

Effect of precipitates on the stress–strain behavior under compression in polycrystalline Ni–Fe–Ga alloys

The stress–strain behavior under compression has been studied in three Ni 55− x Fe 18+ x Ga 27 ( x = 0, 1, 1.5 at.%) polycrystalline alloys with different distributions of γ phase precipitates. All the alloys present the superelastic effect at temperatures above the austenite-finish temperature and the mechanical tests confirm the linear dependence of the critical stress with temperature, as described by Clausius–Clapeyron-type equation. The ‘plateau’ in the stress–strain curves, associated to the stress-induced transformation, presents an increasing slope as samples contain a higher amount of inter-granular particles. This hardening effect is due to the enhanced difficulty to accommodate the deformation between adjacent grains. Intra-granular precipitates increase still further the hardening effect, as a consequence of the difficulties found by the growing martensitic plates to overcome the precipitates. Furthermore, the maximum observed stress-induced strain is notably reduced in samples containing higher fractions of γ precipitates. These precipitates also have an effect in the deformation of martensite, increasing the stress needed to rearrange the variants.