At the 10 at% limit of C and N interstitial solubility in γ-Fe the atomic volume crosses the critical volume separating the low-spin and high-spin states. This suggests that the lower energy state of the FCC solid solutions should be low-spin. This is confirmed by the temperature dependence of the thermal expansion coefficient of γ-Fe–C in the temperature range of stability of the FCC phase. γ-Fe–C exhibits anti-Invar behavior, which is an indication for the presence of moment–volume instabilities with the low-spin state of lower energy. Above the solubility limit the compounds have FM ground states. They show Invar behavior provided that their valence electron concentration, reduced to the metallic element concentration, lies in the range 8.6⩽(e/a)m ⩽9.2, as in Fe3C. The high solubility of C and N interstitials, particularly in FCC Fe, is discussed in relation to the magnetic properties.

Fe and its alloys exhibit magnetic properties that range from simple ferromagnetic ordering in the bce phase to moment-volume instabilities in the fee phase. In the present study we give an account of the stabilities of the structural phases of Fe and its alloys and discuss how the stability of a particular phase is related to the magnetic properties. We discuss the pressure-temperature phase diagram of Fe and the differences between the Gibbs free energies of its various structural phases. We extend the discussion to fee Fe alloys and present results on polarized paramagnetic neutron scattering experiments in Fe-rich Fe-Ni. In particular we show evidence for the presence of transitions from low-spin to high-spin state and the reversal of the sign of the energy difference between these states as the fee phase gains stability. The latter effect becomes a crucial entity in determining the phase stability of Fe alloys.

The ferromagnetic invar effect in f.c.c. 3d-transition metal alloys spans an (s+d) valence electron concentration range of 8.6 less than or equal to e/a less than or equal to 9.5. The maximum spontaneous magnetostriction associated with the invar effect occurs at ela approximate to 8.7. For e/a less than or equal to 8.6, the anti-invar effect develops, for which the associated volume enhancement increases as the pure f.c.c. Fe composition of e/a = 8 is approached. The progressive change from anti-invar to invar in Fe1-xNix alloys takes place in the range of 0 less than or equal to x less than or equal to 0.6 (8 less than or equal to e/a less than or equal to 9.2). The electron concentration of Fe1-xCox alloys falls within this range with 8 less than or equal to e/a less than or equal to 9, implying that Co should be an element having invar properties. To verify this assessment we examine, by thermal expansion measurements in the temperature range of 4 K less than or equal to T less than or equal to 1500 K, the volumetric properties of FexCo1-x alloys with 0 less than or equal to x less than or equal to 1. Indeed, we find invar behavior in the f.c.c. phase of the alloys with concentrations ranging from x = 0.6 to pure Co. Aside from the invar property of pure f.c.c. Co, we observe features associated with the order-disorder transition (B2 <--> b.c.c.) and a large magnetostriction in the b.c.c. phase related to magnetic ordering.

Based on experimental results of high temperature thermal expansion, paramagnetic neutron scattering, considerations of the relative stability of the various phases of elemental Fe and total energy calculations, we discuss the interrelated aspects of Invar, anti-Invar and martensitic phase transformations in Fe-rich alloys. The physical background to the discussions is provided by ab initio calculations of the magnetic and structural binding surfaces of pure Fe and Fe- based alloys. Decisive are charge transfers between electronic levels with different symmetry (eg and t2g) and their non-bonding and anti-bonding character. The variation with composition of the size of the Invar and anti-Invar effects is a question of the position of the Fermi energy relative to these levels. How an effect manifests itself as a function of temperature is a question of the energetic difference between these levels.

We measured the thermal expansion and the specific heat of TixFe100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn2) like TiFe2, and determine the temperature dependence of the magnetic contributions to the thermal expansion alpha(mag) and the specific heat c(mag). For fixed composition alpha(mag)(T) and c(mag)(T) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe2 reveals.

We report about X-ray and magnetic investigations on polycrystalline alloys with hexagonal C14 (MgZn2) structure of the system TixFe100−x in the concentration range 30.5⩽x⩽36.5 around the Laves-phase composition TiFe2 (Ti33.3Fe66.6). Neighboring compositions Fe50Ti50 (with B2 structure) and Ti15Fe85 and Ti20Fe80 (with BCC structure) have also been investigated. From the data we establish a magnetic phase diagram for the C14 range of the system, showing a rather sharp transition from mainly ferromagnetic ordering in the range x<32 to a mainly antiferromagnetic ordering in the range x>32. The results can be made plausible microscopically by taking the site dependence of the moments into account. A revised structural equilibrium phase diagram is also given.

Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations in magnetic transition metal alloys. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics simulations can be used to trace out the dependence of the transformation on temperature and concentration. We use the semi-empiric embedded-atom potential in the molecular-dynamics simulations. The simulations allow to describe supercooling and superheating associated with the change of structure as a function of temperature and concentration.

We measured the thermal expansion and the specific heat of TixFe100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn2) like TiFe2, and determine the temperature dependence of the magnetic contributions to the thermal expansion \(\) and the specific heat cmag. For fixed composition \(\) and cmag(T) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe2 reveals.

Polycrystalline TixFe100−x alloys (30≤x≤35 at%) show Invar-like behavior in the thermal expansion, originating from the volume instability of the Fe-moments. This is explained on the basis of band calculations similar to those for FeNi Invar.

Anti-Invar behavior in a material can be characterized by an anomalously large and a weak temperature-dependent thermal-expansion coefficient, when compared to the respective Grüneisen lattice expansion. It is just the opposite of the Invar effect, which is characterized by an anomalously small thermal-expansion coefficient. Common to Invar and anti-Invar is the fact that both posess moment-volume instabilities. Anti-Invar is observed in the paramagnetic state. It occurs in γ-Fe and in a number of 3d fcc binary and ternary alloys. In FexNi100−x alloys it is observed in the concentration range 70≤x≤100 at. % within the fcc stability range. The effect vanishes as the Invar concentration, x=65 at. %, is approached. To examine the valence electron concentration dependence of the anti-Invar effect the thermal expansion has been measured in the fcc state of FexNi100−x for 63≤x≤100 at. %. Using a model based on moment-volume instabilities in conjunction with a thermal activation process the size of the anti-Invar effect in these alloys has been determined. The volume enhancement is found to decrease with increasing Ni concentration from 2.8% in γ-Fe to 0% at x=65 at. %.

UHV IR emission spectroscopy has been carried out at energies 9–33 mRy (wavelength 2.8–9.8 μm) in the temperature range 430–800 K on single crystals of FeNi and FePt Invar, with Curie temperatures in the respective range. The results show a drastic (up to 30%) decrease of the IR absorptivity around Tc in the energy range 23–33 mRy (2.8–4 μm), while at small energies around 9 mRy (10 μm) the absorptivity is temperature independent, as expected for 3d metals, and found on pure Ni. The decrease in absorptivity on the Invar alloys cannot be understood by the common temperature dependence of interband transitions; rather, it calls for the existence of moment-volume instabilities, characterized by transitions between a high-spin ground state and a temperature-induced low-spin state. Qualitative agreement of the absorptivity with respective total density-of-states calculations on Fe3Ni is demonstrated.