Elastic diffuse scattering of neutrons in FeNi Invar alloys

Abstract

Elastic diffuse scattering of neutrons was found around various Bragg-peak positions in FeNi Invar alloys. The diffuse scattering intensities depend on the temperature and Ni concentration. The intensities increase with decreasing temperature and decrease with increasing Ni concentration. The distribution of diffuse scattering intensity changes from peak to peak and is well explained by the formation of clusters with a lattice deformation consisting of a shear wave propagating along the ͗1 1 0͘ direction and with the ͗1 −1 0͘ polarization vector. The ranges of temperature and Ni concentration, for which diffuse scattering is observed, coincide with those for which the Invar anomalies are observable. The origin of the clusters together with the lattice deformation and their role with regard to the Invar effects are discussed as well as the possibility of a precursor for the fcc-bcc martensitic transformation observed in FeNi alloys.

Interatomic Interactions in F.C.C.-Ni–Fe Alloys

“…In this case, we should consider the microheterogeneous magneticmoment state effects, itinerant-magnetism contribution, and their interrelations with static local lattice-distortion fields. One of the examples of such microheterogeneous states was mentioned in articles by Ono et al [132] and Tsunoda et al [133]. Here, by means of anomalous X-ray [132] and thermal neutron [133] diffuse-scattering investigations of Fe-Ni Invar, the authors found a few anomalies of intensity behaviour close to the Bragg points in reciprocal space.…”

“…One of the examples of such microheterogeneous states was mentioned in articles by Ono et al [132] and Tsunoda et al [133]. Here, by means of anomalous X-ray [132] and thermal neutron [133] diffuse-scattering investigations of Fe-Ni Invar, the authors found a few anomalies of intensity behaviour close to the Bragg points in reciprocal space. They made conclusion that such anomalies are due to the formation of Fe-rich clusters with a lattice deformation consisting of a shear wave propagating along the 1 1 0 direction that is the appearance of premartensitic embryos with a lattice deformation toward the 'low-temperature' f.c.c.-b.c.c.…”

Effect of magnetism on kinetics of γ–α transformation and pattern formation in iron

“…For the case of elemental iron, this is our prediction which has to be checked experimentally. However, for the alloy Fe-Ni which is frequently used as a model system structural transformations in iron and steel (due to the closeness of the transition temperature to room temperature), the developed short-range order at these spatial scales has been, probably, already observed [23,56]. To our knowledge, all previous discussions of these phenomena [13][14][15] where based on the concept of phonon soft mode which is not directly applied to fcc Fe (see the Introduction).…”

“…Neutron scattering data for the alloy Fe 70 Ni 30 [23] demonstrate that atomic complexes with a strong short-range order of the martensite-phase type emerge in the premartensitic region. Therefore, one can assume that a nonclassical (shear) scheme of the phase transition is realized also for    transformation in iron-based alloys but its mechanism is more complicated than for the Hume-Rothery alloys and cannot be described in terms of individual phonon soft modes.…”

Stress-induced large Curie temperature enhancement inFe64Ni36Invar alloy

“…3,[6][7][8] On the other hand, the origin of the Invar effect, whereby these materials exhibit a low or near zero thermal expansion ͑LTE͒ coefficients below the magnetic ordering temperature has remained an issue of controversial debate for a long time. [9][10][11][12][13][14][15] A microscopic explanation of the Invar effect in iron-nickel alloys has been given considering that the magnetic structure is characterized by a continuous transition from the ferromagnetic state at high volumes to a disordered noncollinear arrangement at low volumes. 10 In simple words, there must be a negative contribution to the thermal expansion, which is related to the magnetic ordering, and which cancels out the ever-present positive contribution coming from the anharmonicity of the lattice vibrations.…”