Discovery of metastable tetragonal disordered phase upon phase transitions in the equiatomic nanostructured FePd alloy

Abstract

Specific features of the phase transformation A1 ! L1 0 (space groups Fm3m and P4/mmm, respectively) in single crystals of the equiatomic alloy FePd subjected to annealings both in the absence and in the presence of external uniaxial load, as well as in polycrystalline samples that had undergone severe plastic deformation via high-pressure torsion and subsequent annealing, have been studied. An investigation of the single crystals in a nanostructured state formed at different stages of ordering annealing was performed using optical polarization, thermomagnetic and transmission electron microscopy (TEM) methods. The nanostructured state of the polycrystalline samples FePd after deformation of both disordered and ordered FePd alloys and subsequent annealing was examined with the help of TEM and X-ray techniques. The results obtained were analyzed based on the known concepts of the symmetry theory of phase transitions. It was concluded that the atomic ordering in the FePd alloys is preceded by the formation of a ferroelastic disordered bodycentered tetragonal phase with the structural type A6 and the space group I4/mmm. Experimental data that evidence the existence of a tetragonal disordered phase both in the single crystals and in the polycrystalline samples of FePd after severe plastic deformation and subsequent annealing are reported. Thus, the A1 M A6 M L1 0 phase transformation represents a combination of different types of phase transitions such as cooperative displacement A1 M A6 and ordering A6 M L1 0 of atoms.

Discovery of process-induced tetragonality in equiatomic ferromagnetic FeNi

“…The diffuse scattering took the form of a low-intensity, semi-regular waveform concentrated at low dspacings, consistent with that produced by type-1 Krivoglaz defects [45] such as coherent precipitates, and was most clearly observable in the deformed post-anneal Fe 62 Ni sample. This diffuse scattering signal is hypothesized to be associated with the formation of a pretransition state, in accordance with analogous observations of Vlasova et al of electron diffraction patterns obtained from equiatomic FePd single crystals subjected to annealing and quenching processes [46]. It is preliminarily concluded that the diffuse neutron scattering signal observed from the processed FeNi samples is consistent with local correlations that are related to a very short-range chemical order phenomena.…”

“…Vlasova et al [46] has recently reported that the A6 structure exists as a symmetry-permitted transitional tetragonal phase in the FePd system, bridging the disordered cubic A1 phase and the chemically ordered tetragonal L1 0 FePd phase. She predicted that this A6 structure likely exists in other L1 0 -forming alloys as well.…”

Designing magnetic anisotropy in ferrous alloys

“…Additionally, inspired by previous work performed on the FePd alloy system, a custom-built annealing furnace is employed to study the effects of extrinsic factors (magnetic field, mechanical stress) on phase selection in L1 0 -phase forming compounds [27], [29].…”

“…For these reasons, L1 0 FeNi has only been found naturally in stony, stony-iron, or iron meteorites that have cooled extremely slowly over billions of years [31]- [34]. Due to these sluggish transformation kinetics it has been decidedly difficult to study details of the transformation process and the extrinsic factors that impact this high magnetocrystalline anisotropy [27]- [30], [35]. Furthermore, identification of the L1 0 phase can be a challenge, particularly for L1 0 FeNi, due to the similarity of the X-ray scattering cross sections of Fe and Ni as well as the very small tetragonal distortion (c/a ~ 1.0036) that accompanies the transformation to the L1 0 phase.…”

“…In this work, the A1 -L1 0 transformation process and characteristics of the high magnetocrystalline anisotropy arising from the chemically ordered L1 0 phase are studied in the context of the FeNi and FePd material systems. The analogous L1 0 FePd has been shown to exhibit characteristic similarities to the L1 0 FeNi phase, however it is far easier to identify through conventional means due to a larger tetragonal distortion (c/a = 0.96) as well as more apparent superstructure peaks [27]- [30]. FeNi and L1 0 FePd, providing insight into the A1-L1 0 phase transformation and characteristic signatures of this process.…”