Fe-Ni Invar Alloys Research Articles

Effect of C Content on the Microstructure and Physical Properties of Fe-36Ni Invar Alloy

Thermodynamic equilibrium of Fe-36Ni-base Invar alloy containing C contents from 0.25 to 0.38 weight per cent during solidification has been investigated in this study. From the thermodynamic simulation using ThermoCalc®, it has been revealed that equilibrium phases which can be formed are two kinds of MC-type precipitates, i.e. (Nb, Mo, V)C and MoC, and M2C carbides. The MoC carbide was expected to be formed at relatively lower temperature below 770oC. Microstructure observation revealed the coarse precipitates in the as-cast ingots, which was Mo-rich non-equilibrium phase and could be resolved by the successive heat treatment. With increasing C contents from 0.25 to 0.38 wt.%, tensile strength of Invar alloy increased from 950 to 1100MPa after hot rolling and thermal expansion coefficient showed a trivial increase. Hot-rolled Invar alloy was successfully drawn at room temperature from 6 to 4 mm in diameter, which dramatically decreased thermal expansion coefficient.

Shape-dependent magnetism of bimetallic FeNi nanosystems

Shape-dependent magnetic properties of a bimetallic system have been studied for FeNi nanoparticles with well-defined concave cubic and octahedron shapes. The alloy composition was chosen to be close to that of an Invar FeNi alloy (with 35% Ni content) but with the coexistence of both bcc and fcc phases, in order to investigate the role of phase combinations in controlling the magnetic properties for the first time. Different saturation magnetization and coercivities were observed for the resulting FeNi alloy nanoparticles, and these differences have been correlated with surface anisotropy and formation of different two-phase combinations. The role of two-phase combinations in governing the magnetic properties has also been studied for both bulk and nanoalloys by large-scale Density Function Theory (DFT) calculations using VASP, which provides a new complementary approach to understanding the magnetic properties of alloy materials.

Influence of gaseous hydrogen on the tensile properties of Fe–36Ni INVAR alloy

Abstract No loss in tensile ductility was found for Invar 36 alloy tested in a gaseous hydrogen atmosphere (1 MPa, −50 °C). Fractography revealed no indication of hydrogen assisted damage. Deformation mechanisms of Invar 36 are published in the open literature. Comparing the tensile test results as well as the deformation mechanisms with those of other iron based stable austenitic alloys indicate that the inherent deformation mechanism of Invar 36 comprising of a high portion of dislocation cross slip is an important reason for the negligible loss in tensile ductility under the presence of hydrogen.

Effect of Y and Mg on the Solidification Structure of Fe-36Ni Invar Alloy

AbstractThe effect of Y and Mg on the solidification structure of Fe-36Ni invar alloy melted in a carbon tube furnace was investigated using digital camera, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that Y or Mg addition could remarkablly refine the solidification structure and increase the fraction of equiaxed grains. Great amount of high-melting point compounds (Y2O2S, MgS and MgS-MgO) formed in the alloy with the addition of Y and Mg respectively. Based on the theory of lattice misfit, the effectiveness of high-melting point compounds as heterogeneous nuclei of γ-matrix was calculated. The calculation of the lattice misfit between Y2O2S, MgS and γ-matrix, which were far less than 12%, showed that Y2O2S and MgS acted as heterogeneous nuclei were highly effective. However, MgO could not serve as the heterogeneous nuclei because the lattice misfit between it and γ-matrix exceeded 12%.

Influence of Heating Conditions on the Oxidation Behavior of Fe-36Ni Invar Alloy

AbstractThe oxidation of Fe-36Ni invar alloy at 1273–1473 K in 15%CO2-(0, 3, 5, 6)%O2-N2 atmosphere and the effect of the grain structure on the compressed cracks were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and a microcomputer control universal material testing machine. The results showed that the oxygen content and the oxidation temperature had an obvious influence on the morphology of the external scales, which changed into only two-layer structures from three stratified structures when the oxygen content was 0% or the oxidation temperature was below or equal 1423 K. However, the subscales still consisted of a typical structure of the intergranular subscale and the intragranular subscale with different oxygen contents or oxidation temperatures. The growth rate of the scales did not depend on the partial pressure of oxygen when oxygen existed. The oxidation temperature had an obvious influence on the growth rate of the scales, the growth rate of the scales was slower below or equal 1423 K and faster over 1423 K. A linear relationship existed between the thickness of the scales and the square root of oxidation time, moreover, the estimated activation energy for the growth of the external scale, the intragranular scale and the intergranular scale were 165, 159 and 131 kJ/mol, respectively. The higher apparent activation energy corresponded to the thinner scale at each oxidation temperature. In addition, the compressed cracks were hard to propagate and grow in the specimen with equiaxed grain structures, which would result in the narrower and shallower compressed cracks and the better surface quality during the compressed process after oxidation.

Effects of Ti-Ce refiners on solidification structure and hot ductility of Fe-36Ni invar alloy

Effects of Ti-Ce refiners on the solidification structure and the hot ductility of Fe-36Ni invar alloy were investigated, the corresponding mechanisms were also discussed. The results showed that the solidification of the alloy was remarkably refined with the addition of 0.05%Ti-0.01%Ce refiners. Not only did the columnar grains become shorter and thinner, but the growth pattern of them changed into staggered growth from linear growth. The alloy had a bad hot ductility below 1050 °C, which was mainly attributed to weaker boundaries and the presence of grain boundary sliding. However, the hot ductility of the alloy was highly enhanced at 850–1000 °C as the addition of 0.05%Ti-0.01%Ce refiners could refine grain sizes, thus hinder grain boundary sliding, strengthen the grain boundary and promote the grain boundary movement. The alloy had a good hot ductility over 1050 °C, dynamic recrystallization occurred and was found to be responsible for the better hot ductility. In addition, the average coefficient of thermal expansion of the alloy decreased a little with the addition of 0.05%Ti-0.01%Ce refiners, which met the requirement of material properties.

Effect of Two Kinds of Refiners on the Solidification Structure and Property of Invar Alloy

AbstractThe solidification structure of two 500 g cylindrical ingots of Fe-36Ni invar alloy, which were melted in a carbon tube furnace and cast adding Ce and Zr-Ti refiners respectively, were investigated. The results showed that the grain size became smaller. With the increase of refiner content, the ratio of equiaxed grains of ingot increased and the equiaxed grain size decreased. As for the 100 kg flat ingot, which was melted in a vacuum induction furnace and cast with Ce and Zr-Ti refiners respectively, the situation was different slightly. When adding Ce the dendritic grain diameter of the flat ingot became finer and the equiaxed grain ratio remained low; however, the mechanical property deteriorated. While adding Zr-Ti complex refiners, the solidification structure of the alloy was improved dramatically. Therefore, the mechanical properties at room temperature of as-cast flat ingots were improved. Furthermore, its high temperature ductility range expanded and the expansion coefficient of its hot-rolling plate remained almost the same.

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.

Fabrication of Low CTE Metal Masks by the Invar Fe-Ni Alloy Electroforming Process for Large and Fine Pitch OLED Displays

Low CTE and fine pitch metal masks were fabricated by the invar Fe-Ni alloy electroforming process. The invar Fe-Ni alloy metal masks obtained had finer 10µm x 30µm rectangular open areas with a thickness of about 10µm. The CTE of the electroformed Fe-Ni alloy fine pitch metal mask was 10ppm/K for the as-deposited mask and higher than the melted invar alloys. On the other hand, the CTE of the mask with annealing at 873K exhibited approximately 3ppm/K and was 4 times less than that of conventional electroformed Ni and Ni-Co alloys. The electroformed invar Fe-Ni alloy metal masks obtained are anticipated for use in the vapor deposition process for large and fine pitch OLED displays.

Effect of Ultrasonic Melt Treatment on Solidification Structure of Fe-36Ni Invar Alloy

AbstractThe effect of ultrasonic treatment on the solidification structure of Fe-36Ni invar alloy was investigated. The experiment results showed that the ultrasonic treatment before its solidification had no significant effect on the solidification structure. However, when ultrasonic was inputted into the molten alloy during its solidification process, the primary dendrites were broken up into lots of fragments and solidification structure was refined significantly. When ultrasonic treatment was applied in the melt doped with yttrium before its solidification, ultrasonic cavitation could break up precipitates into many small ones, which could refine its solidification structure as nucleation cores. In samples containing yttrium treated by ultrasonic at 1753 K, the number of the precipitates was 623/mm2 and its average size was 2.18 µm; while at 1803 K, they were 604/mm2 and 2.34 µm respectively. The ultrasonic cavitation had a similar effect at two different temperatures. The solidification structure refined greatly at 1753 K was due to its low pouring temperature.

High-Temperature Oxidation Behavior and Formation Mechanism of Rolling Cracks of Fe-36Ni Invar Alloy

AbstractThe oxidation of Fe-36Ni invar alloy in 5%O2-15%CO2-80%N2 atmosphere at 1200 °C for 4 h and the cause of rolling cracks have been investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Gleeble thermal simulation machine. The result showed that the scales consisted of the external scale and the subscale. The external scale had stratified structures of Fe2O3/Fe3O4 + NiO/Fe3O4 + FeO + NiO from outside to inside. The subscale had a typical substructure of the intra-granular subscale and the inter-granular subscale. Oxidation particles of (Fe3O4 + FeO) were isolated in FeNi3 matrix in the intra-granular subscale zone. The grain boundary was the preferential short-circuit diffusion paths of the diffusing O and was first to be oxidized. The inter-granular subscale led to the subscale and was the origin of rolling cracks. The protective heating measure was put forward to prevent rolling cracks.

Effect of 50-keV proton irradiation on the magnetism of a Fe66Ni34 Invar alloy

The magnetism of Fe–Ni Invar alloys is very sensitive to the lattice constant, stress, and the number of nearest-neighbor Fe–Fe atomic pairs. Ion irradiation is a useful tool to alter the local atomic structure of a given material. Therefore, the effects of low-energy and light-ion irradiation on the magnetism of a Fe66Ni34 Invar alloy were investigated in this study. The Fe66Ni34 Invar alloy was irradiated with 50-keV protons at a fluence of 1×1015ions/cm2 at room temperature. The Curie temperature was found to increase from 465K (before irradiation) to 535K (after irradiation). The X-ray absorption analysis of the fine structure of the alloy revealed that irradiation had no effect on the atomic structures surrounding Fe and Ni.

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.

Fabrication of Low CTE Metal Masks by the Invar Fe-Ni Alloy Electroforming Process for Large and Fine Pitch OLED Displays

not Available.

Experiment Study of Adiabatic Shear Critical Conditions in Orthogonal Cutting of Fe-36Ni Invar Alloy

Orthogonal cutting experiments of Fe-36Ni invar alloy are performed. The change of chip morphology with cutting conditions are investigated through metallurgical observation, and the critical cutting speed of adiabatic shear for Fe-36Ni invar alloy at different cutting depths and rake angles are given. In addition, the characteristic of chip deformation before the occurrence of adiabatic shear is also analyzed. The results show that the critical cutting speed decreases with the increase of cutting depth and hardness, but increases with the increase of rake angle. The deformation coefficient tends to a constant value with the increase of cutting speed.

Study on Constitutive Relationship of Fe-36Ni Invar Alloy

By electronic universal testing machine and Split Hopkinson Pressure Bar, the mechanical properties data of Fe-36Ni invar alloy are gained at a range of temperature from 20°C to 800°C and strain rate from 10-3 /s to 104/s. An improved Johnson-Cook model is presented to describe the mechanical behavior of Fe-36Ni invar alloy at high temperature and high strain rate, and verified by experimental results.

A study of thermal stability in electrodeposited nanocrystalline Fe–Ni invar alloy

The grain growth of an electrodeposited nanocrystalline Fe–Ni invar alloy was studied with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results showed very limited grain growth below the temperature of 0.36 T m , and the activation energy was 40 ± 3 kJ/mol in the low temperature range, which implied the possible mechanism of grain growth as re-ordering of grain boundaries; an abrupt grain growth happened above 0.36 T m with a DSC exothermic peak detected, of which the heat release was about 14 ± 2 J/g; at temperatures above 0.36 T m , the grain growth activation energy was obtained through Kissinger analysis and isothermal kinetics analysis, both of the results suggested the grain growth mechanism as grain boundary diffusion.

Experimental Study and Theory Prediction on Adiabatic Shear Critical Conditions of Fe-36Ni Invar Alloy

Orthogonal cutting experiments of Fe-36Ni invar alloy are performed to investigate the influence of cutting conditons on adiabtic shear, which occurs in the process of chip formation of many materials. It is found that the cutting speed, cutting depth and rake angle all have influence on adiabatic shear and there is a critical cutting speed at which the adiabatic shear appears. By metallurgical observation, the critical cutting speed under different cutting depth and rake angles are given. A model based on linear pertubation analysis is used to predict the adiabatic shear critical ctting conditions of Fe-36Ni invar alloy. The comparison of prediction results and that of expriments shows that this prediction model is available.

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.