Invar-type Alloys Research Articles

Magnetic measurement of strain-induced martensite formation in Fe–30Ni steel

ABSTRACTThis investigation focuses on deformation-induced plasticity in Invar-type steel alloys. The effect is being studied in an austenitic model alloy, containing 30 wt-% of nickel. Its temperature dependent mechanical properties are being presented. Furthermore, the martensitic phase content has been determined by magnetic means in an alloy with two ferromagnetic phases for the first time. The results show that the α′-martensite formation within the austenitic phase with primarily wavy glide mechanism allows an increase in ductility of around 10% at the Msσ temperature of −5°C. This is the point of maximum uniform elongation. Near the Ms temperature, a microstructure of 70 vol.-% deformation-induced α′-martensite can be achieved.

Magnetovolume Effects in Strongly and Intermediately Correlated Electron Systems

Analyses of magnetovolume effects such as the forced volume magnetostriction and the thermal expansion anomaly can give estimation of local amplitude of spin fluctuations. We have shown that bcc Fe has a rigid magnetic moment and that the magnetic moment of Invar type alloys is rather soft. It is pointed out that the total energy difference between the magnetized state and the nonmagnetic state obtained by self-consistent energy band calculations using a local-spin-density approximation is much higher than the Curie temperature for rigid spin systems and comparable for soft spin systems. The effects of zero point spin fluctuations observed in the YMn2 system are discussed.

EFFECT OF HIGH TEMPERATURE DEFORMATION ON THE LOW THERMAL EXPANSION BEHAVIOR OFFE-29%NI-17%COALLOY

The effect of high temperature deformation on the low thermal expansion property of Fe -29 Ni -17 Co alloy was investigated in the compressive temperature range of 900~1300°C at a strain rate range of 25 ~ 0.01 sec.-1. The thermal expansion coefficient (α30~400) generally increased with increasing compressive temperature. In particular, α30~400increased remarkably as the strain rate decreased at temperatures above 1100°C. Note, however, that α30~400at low compressive temperatures (900°C and 1000°C) increased abnormally at high strain rates. Based on the investigation of various possibilities of change in low thermal expansion behavior, the experimental results indicated that both the appearance of the α phase and evolution of grain size due to hot compression clearly influenced the low thermal expansion behavior of this invar-type alloy. The correlation between the microstructural cause and invar phenomena and theoretical explanation for the low thermal expansion behavior of Fe -29% Ni -17% Co were also suggested.

Large magnetovolume effects and band structure of itinerant-electron metamagneticLa(FexSi1−x)13compounds

Among Fe-based ferromagnets, $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds are the first example realizing the itinerant-electron metamagnetic transition from the paramagnetic to the ferromagnetic state, accompanied by a marked magnetovolume effects. The effect of pressure on the Curie temperature is one order larger than that on the spontaneous magnetization in analogy with the results for Fe-Pt invar-type alloys. The large pressure coefficient of ${T}_{\mathrm{C}},$ $d\mathrm{ln}{T}_{\mathrm{C}}/dP,$ is attributed to the renormalization effect caused by spin fluctuations for free energy. The metamagnetic transition and the marked magnetovolume effects in the present compounds are related to the characteristic $3d$ band structures in analogy with Fe-Pt alloys in both the ferromagnetic and paramagnetic states.

The dissolution of intermetallic inclusions in atomic displacement cascades during neutron irradiation of dispersion-hardening alloys

The data of Mossbauer spectroscopy show evidence of a low-temperature (340 K) dissolution of dispersed intermetallic Ni3Ti particles in the fcc lattice of a Fe-Ni-Ti Invar-type alloy under the conditions of cascade-inducing neutron irradiation. The dissolution rate markedly increases with a decrease in the initial size of the intermetallic particles.

Redistribution of Alloying Elements in Electron-Irradiated Iron-Based Fcc Alloys Studied by Hydrogen-Probe Mechanical Spectroscopy

AbstractEffect of electron irradiation on substitutional ordering was studied in Fe-36.7 wt.% Ni invartype alloy and Fe-25.2 wt.% Cr-20.2 wt.% Ni austenitic stainless steel. It was shown that the fine structure of the hydrogen Snoek-type peak in invar changes after irradiation to a fluence of 1019 e−.cm−2 with electrons with an energy of 3 MeV as compared to the solution treated condition. Irradiation affects the temperature behavior of the shear modulus of invar. No significant effect of irradiation on hydrogen Snoek-type peaks was detected in stainless steel.

インバー合金の新しい展開 インバー合金の各種精密制御機器への応用

インバー合金の新しい展開 インバー合金の各種精密制御機器への応用

Heavy alloys with a low thermal expansion coefficient

The possibility of reducing the linear thermal expansion coefficient (LTEC) of heavy tungsten-base alloys by replacing the binder by one possessing Invar properties is studied. Use of Invar type alloys of the Fe-Ni and Fe-Co-Cr types as binders makes it possible to prepare heavy alloys with lower LTEC than for the base (tungsten). Tungsten dissolution in the binder is the main factor which restricts the LTEC of heavy alloys with an Invar binder.

Thermal and irradiation-induced phase separation in Fe-Ni based invar-type alloys

We summarize and review critically the existing experimental and theoretical evidence concerning both thermal and irradiation-induced high-temperature miscibility gaps in Fe-Ni based Invar-type alloys. Independent data regarding phase separation are obtained from studies on magnetic, low-expansion Invar-type alloys and model austenitic Fe-Ni based alloys studied for potential nuclear applications. The response of these alloys to long-term thermal aging is found to be inconsistent with that of single-phase alloys predicted by most accepted or proposed phase diagrams. These alloys show anomalies in numerous properties which suggest compositional or magnetic heterogeneities or both. We herein model the kinetics and thermodynamics of spinodal decomposition and nucleation in these alloys under thermal conditions. The absence of models for surface energy and gradient energy in systems with negative departure from ideality severely limits our analysis of the kinetics of both nucleation and spinodal decomposition. We can combine our calculations with those of others and also with experimental studies of decomposition to reach a conclusion that suggests a high-temperature miscibility gap for Fe-Ni alloys in the Invar regime. The gap is found to be very narrow at high temperatures but to be broadened at low temperatures by magnetic effects. Alloys in the Invar composition range have been subjected to a variety of high fluence irradiation treatments in the 725 to 1000 K temperature range. The result in most cases was large-scale decomposition into approximately 25 and 50 pet Ni phases. The apparent miscibility gap under irradiation is much wider than that observed thermally. We discuss these observations in the light of existing theories of irradiation-induced or irradiation-altered alloy decomposition. We conclude that although irradiation-enhanced diffusion speeds up phase separation, other processes must be operating to produce the greatly widened miscibility gap.

Characterization of the phase transition of Invar type alloys by magnetoelasticity

The nature of the magnetic phase transition of Invar type alloys was studied by precise measurements of elastic constants near T c . The bulk modulus, B , of Y c Fe 14 B exhibits a sharp λ type dip at T c . On the other hand, the bulk modulus of a Fe-35Ni Invar alloy does not show such a critical softening at T c while d B /d T shows a sharp peak at T c indicating a possible third order phase transition.

Magnetovolume Effects in Strongly and Intermediately Correlated Elecron Systems

Analyses of magnetovolume effects such as the forced volume magnetostriction and the thermal expansion anomaly can give estimation of local amplitude of spin fluctuations. We have shown that bee Fe has a rigid magnetic moment and that the magnetic moment of Invar type alloys is rather soft. It is pointed out that the total energy difference between the magnetized state and the nonmagnetic state obtained by self-consistent energy band calcula­ tions using a local-spin-density approximation is much higher than the Curie temperature for rigid spin systems and comparable for soft spin systems. The effects of zero point spin fluctuations observed in the YMn2 system are discussed.

Thermal disorder and magnetic fluctuations in invar type alloys

In invar type alloys, ferromagnetism depends strongly upon volume changes. Thus, at finite temperatures, thermal volume fluctuations induce localization of magnetization. Its spatial distribution, i.e. the mixing of paramagnetic and ferromagnetic states, is derived for itinerant ferromagnetism using the Debye model. Both states are accessible states.

Elastic properties of thin sheets of amorphous and crystalline invar-type alloys

Several kinds of thin sheets of Fe-Ni crystalline Invar alloys have been made by cold rolling, and Fe-B amorphous Invar-type alloys have been prepared from the melts by the rapid quenching method. At the fundamental mode, the pole effect becomes more pronounced with increasing magnetic field, but it almost vanishes below the length to thickness ratio of 100 even at the fundamental mode. The pole effect of Fe-Ni thinner sheets and Fe-B amorphous ribbons decreases with increasing the frequency mode and becomes nearly zero at the third tone mode. It was confirmed that Fe-Ni crystalline Invar alloys show the softening, and Fe-B amorphous Invar-type alloys, on the contrary, exhibit the stiffening in Young's modulus under the condition the pole effect is almost minimized.

Magnetic moment and high-field susceptibility of amorphous FeB Invar-type alloys

The saturation magnetization and high-field susceptibility of amorphous Fe 100− x B x (11 ⩽ x ⩽ 22) alloys have been measured at 4.2 K, 77 K and room temperature in magnetic fields up to 100 kOe. The concentration dependence of the saturation moment per iron atom shows a maximum around 14% boron, while the high-field susceptibility increases remarkably with decreasing boron content. The features are similar to those of crystalline FeNi Invar alloys.

Volume and shape magnetostriction of ferromagnetic invar type alloys

The volume and shape magnetostriction ω V ( H) and ω S ( H) of an Invar alloy and a technical constant-modulus alloy were measured with a strain gauge technique at room temperature. The considerable shape magnetostriction amounts to 0.1 of the volume effect. The itinerant model of ferromagnetism and Kanamori's formula for correlation effects is applied. The observed magnetostriction implies a deformation potential of about 0.3 eV.

The Effect of Variations in Chromium Content on the Physical Properties and Structure of Modified Invar-Type Alloys

The effect of variations in Cr content (5·25±0·25 wt.-%) on the physical and microstructural properties of ‘Ni-Span C’ alloy* has been investigated. Changes in microstructure hardness, modulus of elasticity, and Curie temperature after ageing are reported. Chromium increase in the Ni-Span composition range has been found to accelerate the formation of E precipitates and reduce the rate of hardening. It has also a more marked effect on the physical properties of Ni-Span C than that of binary Fe–Ni alloys.

Use of magnetic characteristics for investigations of internal stresses in invar-type alloys

Use of magnetic characteristics for investigations of internal stresses in invar-type alloys

Pressure Effect on the Curie Temperatures of Transition Metals and Alloys

We determined the variation of the Curie temperature with pressure for iron, cobalt, nickel, six iron-nickel alloys, and five cobalt-nickel alloys. A permeability method in zero magnetic field has been used in a belt-type apparatus at pressures up to 90 kbar and temperatures up to 1400 \ifmmode^\circ\else\textdegree\fi{}K. The pressure shifts measured for the Curie temperatures of iron and cobalt are zero; for nickel it is positive and markedly nonlinear. The Curie temperatures of face-centered-cubic iron-nickel alloys decrease very rapidly with pressure when the nickel content is low and show a progressively smaller decrease when nickel is added. They begin to increase when the nickel concentration is over 68 wt%. The existence of maxima on the curves of the Curie temperatures versus pressure or volume seems to be definitely established. pressure increases the Curie temperature of cobalt-nickel alloys in all cases, but here also in a nonlinear way. For strong ferromagnets with a few carriers a band theory has been previously developed. The shift of the Curie temperature is then given by $\frac{d\ensuremath{\theta}}{\mathrm{dP}}=\frac{5}{3}k\ensuremath{\theta}$ ($k=\mathrm{compressibility}$). For pure nickel and nickel-cobalt alloys with a cobalt content less than about 50 wt% good agreement with experiment is obtained at low pressures, but not at higher pressures. The Curie temperature shift of Invar-type alloys has been previously calculated using a weak-itinerant-electron-ferromagnet band theory. In this case we have been able to predict and observe a parabolic decrease of the Curie temperature with pressure: ${\ensuremath{\theta}}^{2}={\ensuremath{\theta}}_{0}^{2} (1\ensuremath{-}\frac{P}{{P}_{0}})$. In order to explain the shifts for the different fcc alloys of nickel with iron, use is made of a model where $\ensuremath{\gamma}$-iron is supposed to have two electronic levels and which gives account of many properties of these alloys at normal pressure.

Low-temperature thermal expansion and magnetostriction of Kovar

The temperature and magnetic field dependence of the thermal expansion coefficient and the magnetostriction of commercial Kovar, an Fe–Ni–Co glass-to-metal sealing alloy, has been measured below 50 K and well into the magnetic saturation region. The measured properties of Kovar show anomalous behavior similar to that found in other Invar-type alloys, and are markedly different from the properties of a pure alloy of about the same composition.

The Mössbauer Effect and the Anomalous Properties of the Invar Alloys

In order to investigate the cause of anomalous properties of the γ-phase iron-nickel alloys with Invar compositions, the Mossbauer effect spectra of these alloys were measured as a function of temperature. Especially, the influences of phase mixture (α+γ), cold-work and external magnetic field were examined. From these measurements, it was concluded that there exist transition states between the α and γ phases, and they cause a diversity of hyperfine fields in the Invar-type alloy. Many hyperfine fields thus produced make the Invar alloy an assembly of various magnetic transition points, which, as a result, gives rise to the anomalies of physical properties.