Formation of Defect Structures during Annealing of Cold-deformed L1 0 -ordered equiatomic FePd Intermetallics
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Formation of Defect Structures during Annealing of Cold-deformed L10-ordered equiatomic FePd Intermetallics Anirudha R. Deshpande and Jörg M.K. Wiezorek Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA; ABSTRACT Planar defects produced in L10-ordered FePd during annealing after cold-deformation in the disordered cubic state have been characterized by transmission electron microscopy (TEM). The defects evolving during annealing include arrays of overlapping stacking faults (SF's), {111}conjugated microtwins (µT's) and thermal antiphase boundaries (APB's). The defect formation mechanisms proposed here are similar to twinning mechanism reported for FCC-metals during annealing. Thus, SF arrays and faulted µT's in the L10-ordered FePd appear to form during the early stages of annealing by atomic attachment faulting on {111}-facets of the transformation interfaces. During later stages of annealing the reduced amount and the change in nature of the driving forces for the microstructural rearrangement result in changes in the predominant defect formation mechanism. The features of the defect genesis in L10-FePd are discussed with respect to solid-state transformations during processing of these ferromagnetic intermetallics. INTRODUCTION The tetragonal L10-ordered phase γ1-FePd exhibits strongly anisotropic, uniaxial ferromagnetic properties, which are very attractive for applications in magnetic memory devices and other ferromagnetic applications [1]. The γ1-FePd forms below the ordering temperature of about 925K for the equiatomic composition from a concentrated solid-solution with disordered face-centered cubic (FCC) structure via a thermodynamically first-order type phase transformation [2]. Continuous cooling from the fcc-phase field or isothermal annealing of the quenched metastable FCC-phase at temperatures below about 925K results in morphologically lamellar microstructures of the L10-ordered product that are characteristically twinned on dodecahedral, {101}, planes, [2,3]. The conventional mode of ordering involves the nucleation of coherent precipitates of three possible orientation variants of the tetragonal product on {101} in the FCC-parent phase. Formation of the {101}-conjugated polytwins during strain affected growth of the L10-precipitates facilitates self-accommodation of the considerable transformation strains [3]. The polytwinned (PT) microstructural state of the L10-FePd formed by this conventional ordering mode is associated with disappointing technical magnetic properties, low failure strain and brittleness in tensile conditions [3-5]. Room temperature deformation of the FCC-phase prior to order annealing at T
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