Coarsening Behavior and Coercivity in L1 0 -Ordered Intermetallic Fe-Pd Ferromagnets with Equiaxed Grain Morphology
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Coarsening Behavior and Coercivity in L10-Ordered Intermetallic Fe-Pd Ferromagnets with Equiaxed Grain Morphology Jörg M.K. Wiezorek and Anirudha R. Deshpande Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA;
ABSTRACT Thermo mechanical processing of equiatomic FePd has been used to establish a nearly equiaxed microstructure that consists of mono-variant L10 ordered grains. The technically important hard magnetic properties, e.g. coercivity, of the uniaxial ferromagnetic L10-FePd with equiaxed microstructure depend on the grain size. Thus, here, the evolution of grain size during the later stages of annealing has been studied using SEM and computer assisted image analysis with the goal to determine the activation parameters for the relevant coarsening processes. Using a classical grain growth analysis the activation parameter for coarsening of the 5% largest grains in the populations of L10-ordered grains in FePd has been determined as approximately 30kcal/mol. INTRODUCTION L10-ordered FePd belongs to the class of technologically interesting tetragonal ferromagnetic intermetallics that includes also FePt, CoPt, and MnAl. These intermetallics exhibit uniaxial ferromagnetism with large magnetocrystalline anisotropy and are of significant technological interest for permanent magnet applications [1]. FCC L10 ordering in the equiatomic Fe-Pd system is a first-order phase transformation and associated with considerable transformation strain. The system reduces the transformation strain energy through self-accommodating arrays of {101}-conjugated twins. The resulting characteristic lamellar morphology is known as the polytwin microstructural state [2]. The technologically relevant magnetic properties of the tetragonal L10 ordered γ1-phase in Fe-Pd, e.g. coercivity, are extremely sensitive to microstructure and are quite poor for the polytwinned FePd [3,4]. Previous studies have been able to replace this polytwinned microstructural state by a nearly equiaxed microstructural morphology consisting of mono-variant L10 ordered FePd grains (Fig. 1), which can exhibit significantly enhanced magnetic properties [5]. A thermomechanical processing route that involves cold deformation of the metastable FCC phase followed by annealing at a temperature lower than the critical ordering temperature can alter scale and morphology of the final ordered microstructure. The microstructural transformation after cold-deformation involves a combined reaction (CR) of FCC L10 phase transformation concomitant with annealing of defects introduced during cold deformation of the metastable FCC phase. During CR-annealing at temperature T < Tc (critical ordering temperature, Tc~ 650°C) heterogeneous nucleation and growth of L10 ordered grains occurs at internal strain heterogeneities in the cold-deformed FCC microstructure favoring the formation of equiaxed mono-variant L10-ordered FePd grains. The evolution of the coercivity during order-annealing of CR-processed FePd has been studied previ
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