Combined reactions thermomechanical processing applied to ferromagnetic iron-palladium binary alloys

The aim of this thesis was to use a combination of thermomechanical processing and heat treatment, including magnetic field annealing, to tailor ultra-fine microstructures with enhanced properties in ferromagnetic alloys. The combined reactions strategy involves the use of thermomechanical processing to induce two or more solid-state reactions to occur concomitantly, synergistically and / or sequentially during microstructural development, e.g. severe plastic deformation followed by recrystallization in tandem with precipitation, ordering and / or decomposition. The specific thrust is aimed at producing exchange-coupled nanocomposite structures in off-stoichiometric Fe-Pd alloys, which might be expected to form two-phase mixtures of magnetically soft ferrite (alpha) and the magnetically hard L10 intermetallic phase. The severe plastic deformation of the parent phase serves to enhance diffusion kinetics in addition to catalyzing novel reaction paths and microstructural development as the system relaxes toward equilibrium. Vibrating sample magnetometry (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were performed to investigate the microstructure and the magnetic properties of combined reactions transformed hypo-stoichiometric iron-palladium. Two distinct microstructures were observed to develop in these alloys, resulting from two different modes of solid-state transformation. Conventional aging of the solutionized alloys resulted in eutectoid decomposition whereby the ferrite and L10 phases precipitate a cellular product on a 100nm length scale that coarsens as aging progresses. In contrast, aging the deformed alloys produces a nanoscale lamellar composite structure consisting of alternate L10 and metastable FCC phases, on a scale of 10nm. The finer scale product appears to derive from pseudospinodal decomposition, while ferrite is simultaneously observed to precipitate heterogeneously on grain boundaries. The plastic deformation imparts crystallographic texture to the alloys that is substantially retained in the final microstructure, suggesting that the mechanism of transformation preserves the orientation of the parent grains. The soft ferrite is exchange coupled with the aligned L10 phase at length scales exceeding the critical length scale predicted by existing theory. Peak properties were obtained when Fe-35at%Pd was deformed 97% and aged at 425°C for 20 hours resulting in a coercivity of 1kOe and an energy product equal to 2.9MG*Oe. These results have been quantitatively rationalized in terms of a superposition of domain wall pinning by particles and grain boundaries. Nucleation and growth rates of ferrite and L10 FePd increased when the alloys were aged within a 9 Tesla magnetic field.