Fe-Ni Invar Research Articles

Microstructure and Plastic Deformation of the As-Welded Invar Fusion Zones

The as-welded Invar fusion zones were fabricated between cemented carbides and carbon steel using a Fe-Ni Invar interlayer and laser welding method. Three regions in the as-welded Invar fusion zones were defined to compare microstructures, and these were characterized and confirmed by scanning electron microscopy and X-ray diffractometry. The structure and plastic deformation mechanism for initial Invar Fe-Ni alloys and the as-welded Invar fusion zones are discussed. (1) After undergoing high-temperature thermal cycles, the microstructure of the as-welded Invar fusion zones contains γ-(Fe, Ni) solid solution (nickel dissolving in γ-Fe) with a face-centered cubic (fcc) crystal structure and mixed carbides (eutectic colonies, mixed carbides between two adjacent grains). The mixed carbides exhibited larger, coarser eutectic microstructures with a decrease in welding speed and an increase in heat input. (2) The structure of the initial Invar and the as-welded Invar is face-centered cubic γ-(Fe, Ni). (3) The as-welded Invar has a larger plastic deformation than initial Invar with an increase in local strain field and dislocation density. Slip deformation is propagated along the (111) plane. This finding helps us to understand microstructure and the formation of dislocation and plastic deformation when the Invar Fe-Ni alloy undergoes a high-temperature process.

Enhancement of Curie Temperature (T c) and Magnetization of Fe-Ni Invar alloy Through Cu Substitution and with He+2 Ion Irradiation

The magnetic properties of ternary Fe-Ni-Cu invar alloys are affected by ion irradiation, which goes on increasing with increasing ion fluence (Φ), and by increasing Cu content. In the present study, the ions used are He+2 with 2 MeV energy and with 1 × 1013 cm−2, 1 × 1014 cm−2, 5 × 1014 cm−2, 1 × 1015 cm−2 and 5 × 1015 cm−2 fluence (dose) for irradiation purpose. The face centered cubic structure of the alloy was investigated after ion irradiation using x-ray diffraction (XRD) and found unchanged. However, the peaks become broader with increasing ion dose. Additionally, the lattice fluctuations were observed in XRD study. Curie temperature (Tc) is also increased after irradiation. Many factors are considered here for the reason for increasing Tc, such as the stopping of incident ions, atomic mixing effect at micro scale level owing to ion irradiation, which might change local concentration and ordering already reported in diffuse scattering, and as a result the Fe-Fe interatomic distance and the Fe-Fe coupling are changed. A comparative study shows that the effect of irradiation on Tc and magnetization with increasing ion fluence is more distinctive than the addition of Cu.

Fiber laser welding of WC-Co to carbon steel using Fe-Ni Invar as interlayer

A fiber laser procedure was developed for joining cemented carbides to carbon steels (3-mm-thick plates) in the butt joint configuration using fcc Fe-Ni invar as interlayer. The optimized welding parameters gave a required weld width and penetration. The as-welded microstructure was characterized as a function of welding speed through fiber laser welding. In fusion zone, non-equilibrium fcc (γFe, Ni) was the predominant phase with α-WC and some mixed carbides such asFe3W3C and Co6W6C. In heat affected zone (HAZ) of cemented carbide, the phases consisted of α-WC, (γFe, Ni) and mixed carbides. Bct–martensite was observed in HAZ near steel side. Which were confirmed by XRD, TEM, and nickel-iron phase diagram calculate using Thermo Cal Classic software. Thick and large sized η-phases were inhibited in the HAZ of cemented carbide and replaced by η-phases with carbon depletion. A higher energy input produced more non-equilibrium (γFe, Ni) and martensite. Small amounts of η-phases, non-equilibrium (γFe, Ni) besides the martensite. The thermal cycles associated with laser welding led to a rather small fusion zone, which usually inhibited η-phases formation due to the inhibition of normal grain growth and the decrease of iron/nickel diffusion across WC/Co interface. The results indicated that the maximum bend strength was 980.90MPa. Due to the formation of martensite/carbides and an excess of which led to embrittlement, welded-joint samples were broken at the HAZ or in the fusion zone instead of the base metal. The cracks exhibited the characteristics of hot crack ((γFe, Ni) in the fusion zone) and cold crack (α-WC at the HAZ of cemented carbide).

Effects of fluence and fluence rate of proton irradiation upon magnetism in Fe65Ni35 Invar alloy

Curie temperature, TC, of the Fe-Ni Invar alloys increase due to irradiation with electron and some kinds of ions. In this study, proton irradiation effects upon magnetism in an Fe65Ni35 alloy have been investigated. It is found that the increment of TC, ∆TC, increases with increasing fluence. The magnetic hysteresis curve of the alloy was found to be unaffected by irradiation. Comparing ∆TC and the calculated energy transfer from the ions to the sample, it seemed that ∆TC was found to be related to the number of vacancies formed in nuclear collision events. In addition, ∆TC was influenced by the fluence rate, i.e., the deposited energy per unit time.

Effect of Nano Grain Growth on Coefficient of Thermal Expansion in Electroplated Fe-Ni Invar Alloy

The aim of this paper is to consider the effect of annealing on the coefficient of thermal expansion (CTE) of electroplated Invar Fe-Ni alloy. The CTE of the as-electroplated alloy is lower than those of alloys annealed at <TEX>$400^{\circ}C$</TEX> and <TEX>$800^{\circ}C$</TEX>. XRD peaks become sharper as the as-electroplated alloy is annealed, which means the grain growth. The average grain sizes of as-electroplated and as-annealed alloys at <TEX>$400^{\circ}C$</TEX> and <TEX>$800^{\circ}C$</TEX> are 10 nm, 70 nm, and <TEX>$2{\mu}m$</TEX>, respectively, as determined by TEM and EBSD analyses. The CTE variation for the various grain sizes after annealing may come from the magnetostriction effect, which generates strain due to changes in the magnetization state of the alloys. The thermal expansion coefficient is considered to be affected by nano grain size in electroplated Fe-Ni Invar alloys. As grain size decreases, ferromagnetic forces might change to paramagnetic forces. The effect of lattice vibration damping of nano grain boundaries could lead to the decrease of CTE.

Low-Temperature Magnetism of Metals in the Dynamic Spin-Fluctuation Theory

The low-temperature behavior of magnetization in ferromagnetic metals is studied by means of the dynamic spin-fluctuation theory. An expression is derived for the dynamic magnetic susceptibility, which extends the one in the random phase approximation. The transverse susceptibility is shown to have the spin-wave poles, which yield the asymptotic T 3/2 law for magnetization. An explicit expression for the coefficient in the T 3/2 law is derived on the basis of the multiband Hubbard Hamiltonian and real band structure. The temperature dependence of magnetization is demonstrated by the example of Fe and disordered Fe-Ni Invar.

Effects of energetic ion irradiation on the magnetism of Fe–Ni Invar alloy

The magnetic properties of Fe–Ni Invar alloys are significantly affected by ion irradiation. Au3+ with the energy of 16MeV irradiation effects on the magnetism of Fe66Ni34 have been reported in this paper. Considering from the temperature variations of AC susceptibility of irradiated Fe66Ni34, Curie temperature of a part of sample increase with increasing incident ion fluence, and the magnetization of irradiated Fe66Ni34 is also increase. The FCC structure of Fe66Ni34 is not changed by ion irradiation; however peaks become broader with increasing ion fluence. It means that lattice fluctuations are generated owing to ion irradiation. However it cannot be considered that lattice fluctuations observed X-ray diffraction measurements are enough to increase the Curie temperature observed in AC susceptibility measurements. Then, we suggest as the considerable origin of increasing TC, atomic mixing effects owing to the ion irradiation. It might change the chemical ordering reported in the diffused scattering, such as Fe–Fe coupling.

인바합금 도금층의 물성에 영향을 미치는 도금인자에 관한 연구

The experiments were carried out in the variation of current density, pH, temperature, and duty cycle to investigate the influence of electroplating parameters on the properties of Ni-Fe invar alloys. When the current density and temperature were changed, the composition of invar alloy was varied, however, duty cycle and pH hardly affected on the composition of electrodeposited alloys. However, as the duty cycle was increased, microstructure was changed and the decrease of hardness was also observed.

Catalyst for methane vapor reforming fabricated on the basis of natural oxides and stable in H2S medium: Activity, selectivity, and nanostructure evolution

This work presents the results of studies on the activity of an iron-nickel catalyst fabricated on the basis of mixed oxides separated from layered vermiculite in the process of methane vapor reforming. It has been demonstrated that the degree of methane transformation in the conversion process is close to the equilibrium one, whereas the catalyst manifests stability to coke formation and the presence of hydrogen sulfide in the vapor-gas mixture. The structure and phase state of the active components of the system have been investigated using the XRD and Moessbauer spectroscopy methods. It has been shown that, during the formation of the catalyst and the catalysis itself, the system surface hosts solid-phase transformations of deposited active components with iron ions present in the substrate. The formation of superparamagnetic clusters of iron oxide strongly bound to the substrate structure on the surface of the catalyst (particles of invar alloy FeNi) determined its polyfunctional properties: high activity in both methane vapor reforming and hydrogen sulfide oxidation decomposition up to elementary sulfur.

Magnetic EXAFS study of Fe-Ni invar alloy under high pressure using nano-polycrystalline diamond anvils

The magnetic EXAFS technique was applied to study the local magnetic structure in Fe-Ni Invar alloy under high pressures. Nano-polycrystalline diamond was used to apply pressure in a diamond anvil cell. As a result, we succeeded in recording a glitch-free spectrum with a high accuracy by preventing Bragg diffraction from the diamonds. As the pressure was increased, the ferromagnetic correlation was strongly reduced, which was more remarkable for Fe atoms than for Ni atoms. The canting effect of the magnetic moment may be crucial for a rapid decrease in the ferromagnetic correlation. This local magnetic structure in the Invar alloy is consistent with a model based on the non-collinear spin structure.

Formation of boron solid solution in Fe-Ni invar upon severe plastic deformation

Mossbauer spectroscopy, X-ray diffraction, electron microscopy, and magnetic susceptibility measurements have been used to study the process of mechanical synthesis of the solid solution of boron in the matrix of an Fe-Ni alloy. The internal effective field, the Curie temperature, and the lattice parameter of the Fe-Ni austenite were found to increase after severe plastic deformation in Bridgman anvils, which is related to the incorporation of boron into the matrix and the formation of a crystalline supersaturated solid solution coexisting with metastable borides.

Extended dynamic spin-fluctuation theory of metallic magnetism

A dynamic spin-fluctuation theory that directly takes into account nonlocality of thermalspin fluctuations and their mode–mode interactions is developed. The Gaussianapproximation in the theory is improved by a self-consistent renormalization of the meanfield and spin susceptibility due to the third- and fourth-order terms of the free energy,respectively. This eliminates the fictitious first-order phase transition, which is typical forthe Gaussian approximation, and yields a proper second-order phase transition. The effectof nonlocal spin correlations is enhanced by taking into account uniform fluctuations in thesingle-site mean Green function. Explicit computational formulae for basic magneticcharacteristics are obtained. The extended theory is applied to the calculation of magneticproperties of Fe–Ni Invar. Almost full agreement with experiment is achieved for themagnetization, Curie temperature, and local and effective magnetic moments.

Radiation-induced separation of solid solution in Fe–Ni invar

Radiation-induced solid solution separation (RISSS) in the Fe–34.7 at.% Ni invar alloy was investigated by residual resistivity and linear thermal expansion coefficient (TEC) measurements. The alloy was irradiated with 5 MeV electrons at 245–570 K and isochronous annealed. It was found that RISSS leads to an increase of 20% in the residual resistivity and an increase of 10 −5 K −1 in the TEC for an electron irradiation dose of 5 × 10 18 cm −2. The kinetics of RISSS are the same, but the irradiation efficiency coefficient increases approximately by a factor of 10 when the irradiation temperature rises in the range of 240–570 K. Vacancy clusters, which are formed under irradiation, retard the decomposition. It was shown that dissociation of the vacancy clusters at temperatures of 320–500 K was followed by the free migration of the formed vacancies, and leads to further RISSS.

Microstructure characterisation and CTE study of Fe—42Ni—Nb invar alloys

Invar alloy has low coefficient of thermal expansion; however, the week strength always limits its application. To circumvent this problem, the authors have designed Fe—42%Ni—Nb invar alloy using midfrequency vacuum induction melting technique. The microstructures of this alloy were characterised by means of XRD, SEM and TEM. The multicomponent composition (NiFeCMnNb) is chosen such that upon chill casting of the alloy the liquid undergoes a metastable reaction, forming a strengthening NbC precipitated phase on γ-(Fe, Ni) solid solution. As a result, the alloy achieves the low coefficient of thermal expansion α30–100=6·14 × 10−6 K−1 ranged from room temperature to 100°C. The results show: in NiFe invar alloys, the primary phases are γ-(NiFe), FeNi3) Ni and Fe2C; and with the addition of carbon, manganese and rare earth niobium, NiFeCMnNb invar alloy achieves such primary phases as γ-(NiFe) and NbyCx; and TEM structure consists of superfine structure and its matrix is γ-(NiFe).

Noncollinear Spin Structure in Fe–Ni Invar Alloy Probed by Magnetic EXAFS at High Pressure

To examine theoretical models of the Invar effect, X-ray magnetic circular dichroism and magnetic extended X-ray absorption fine structure (MEXAFS) measurements are performed under high pressures at the Fe and Ni K -edges in 35.4 at. % Ni–Fe alloy. An oscillatory MEXAFS signal is observed up to 6 GPa. Its amplitude significantly decreases with increasing pressure. The magnetic component of the radial distribution function, obtained by taking the Fourier transform, shows a different reduction in the ferromagnetic correlations of Fe and Ni absorbing atoms. The present results are favorable to the noncollinear spin structure picture rather than the Fe 2γ-state model.

Laves-phase(Zr,Nb)Fe2alloys as model Invar systems without magnetic frustration: Comparison to Fe-Ni Invar

We calculate the spontaneous volume magnetostriction and exchange interaction in C15 Laves-phase $({\text{Zr}}_{1\ensuremath{-}x}{\text{Nb}}_{x}){\text{Fe}}_{2}$ alloys, which exhibit a distinctive Invar-type anomaly in their thermal expansion. Our first-principles study is based on the disordered local-moment approximation and the magnetic force theorem applied in the framework of the local spin-density approximation and the Korringa-Kohn-Rostoker band-structure method. The theory presented is able to predict the qualitative difference in the thermal expansion between the systems considered. The exchange interactions in $({\text{Zr}}_{1\ensuremath{-}x}{\text{Nb}}_{x}){\text{Fe}}_{2}$ and in classical Fe-Ni Invar alloys have been recalculated and compared in a wide range of volumes. We find that the magnetic interactions in $({\text{Zr}}_{1\ensuremath{-}x}{\text{Nb}}_{x}){\text{Fe}}_{2}$ do not become frustrated at lower volumes contrary to the case of Fe-Ni Invar alloy. Thus, it is explicitly shown that antiferromagnetism and magnetic noncollinearity at low volumes are not a prerequisite for an Invar anomaly to occur.

Spin-fluctuation theory beyond Gaussian approximation

A characteristic feature of the Gaussian approximation in the functional-integral approach to the spin-fluctuation theory is the jump phase transition to the paramagnetic state. We eliminate the jump and obtain a continuous second-order phase transition by taking into account high-order terms in the expansion of the free energy in powers of the fluctuating exchange field. The third-order term of the free energy renormalizes the mean field, and the fourth-order term, responsible for the interaction of the fluctuations, renormalizes the spin susceptibility. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar.

Hyperfine interactions and local environment effects in Fe xNi 100− x ( x = 55–67) Invar alloys: 57Fe Mössbauer spectroscopy data at 5 K

A systematic study of the magnetic hyperfine field distribution for 57Fe in the Invar alloys Fe x Ni 100− x ( x = 55, 60, 63, 65 and 67) have been performed by Mössbauer spectroscopy technique at 5 K. The composition dependences of the magnetic hyperfine fields ( B hf), isomer shifts and relative intensities of the magnetic subspectra were measured and analyzed. The reliable data on the correspondence between the B hf magnitude and the type of locale atomic configuration were obtained. It is confirmed that the Fe–Ni Invar alloys in their ground state are predominantly collinear ferromagnets with well-defined atomic magnetic moments. Particular emphasis has been placed on the low-field (LF) component of the distribution ( B hf = 1.4 and 2.5 T) which is considered as corresponding to the Fe sites with the antiferromagnetic (AFM) alignment of the magnetic moment. The most striking feature of the LF component is the anomalous positive isomer shift that corresponds to a large reduction of the local electron density at the Fe sites. It may be proposed that the volume effect is one of the plausible reasons for the increase in the IS value for LF components. In this case, we should suggest that the formation of the AFM sites leads to some increase in the local atomic volume. The possible influence of the competition between energetically satisfied and unsatisfied exchange bonds on the stability of the AFM states of the Fe atoms is briefly discussed.

The effect of temperature on the structural and magnetic behaviour of Fe-Ni Invar alloys

The crystal structure and magnetic properties of polycrystalline Fe1−xNix alloys with x = 0.30, 0.325 and 0.375 were investigated in the temperature range 20K<T<900K with the use of X-ray diffraction, magnetostatic and Mössbauer effect methods. Two structural transitions were observed above 600K. In the first one bcc (α) → fcc (γ) and in the second one fcc (γ) → fcc (γ́) with a lower lattice constant. It was found that below 700K the lattice constant of the γ phase does not depend on temperature. The transition α → γ changes the local environment of the Fe atoms and affects the magnetic properties of the Fe-Ni alloys. The disappearance of the ferromagnetic order in γ-Fe0.70Ni0.30 alloy can be explained by the presence of Fe atoms in the low (0μB) and high (2.8μB) spin state. The Fe atoms with zero magnetic moment can destroy the long range magnetic order in the investigated Fe-Ni alloys.

Beyond Gaussian approximation in the spin-fluctuation theory of metallic ferromagnetism

A characteristic feature of the Gaussian spin-fluctuation theory is the jump transition into the paramagnetic state. We eliminate the jump and obtain a continuous second-order phase transition by taking into account the high-order terms of the free energy of electrons in the fluctuating exchange field. The third-order term of the free energy yields a renormalization of the mean field, and fourth-order term, responsible for the interaction of the fluctuations, gives a renormalization of the spin susceptibility. The extended theory is applied to the calculation of magnetic properties of Fe-Ni Invar.