Interactions of Mechanical Twinning and Dislocation Glide with Polytwin-Interfaces in L1o-Ordered Iron-Palladium Interme
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Interactions of Mechanical Twinning and Dislocation Glide with Polytwin-Interfaces in L1o-Ordered IronPalladium Intermetallics Huiping Xu and Jörg Wiezorek Department of Materials Science and Engineering, University of Pittsburgh, 3700 O’Hara St., 848 Benedum Hall, Pittsburgh PA 15261 USA ABSTRACT The defect structures in polytwinned (PT) FePd have been studied after room temperature deformation by transmission electron microscopy (TEM). Interactions between gliding dislocations and mechanical twins with the {101}-conjugated PT-interfaces have been identified. Based on crystallographic analyses of shear transfer of dislocations and microtwins across the PT-interfaces in FePd boundary reactions have been identified that are consisted with the TEM observations. A model has been proposed, which is suitable to rationalize significant contributions to strain-hardening from these defect-interface interactions in PT-FePd. INTRODUCTION The frequent polytwin (PT) interfaces present in L10-phases, such as CuAu-I, FePt, FePd and TiAl, after appropriate heat treatments strongly influence the properties and behavior of this interesting class of intermetallics [1-4]. Interactions of dislocation glide and mechanical twinning with the PT-interfaces has been studied previously for various cases in (111)-conjugated PT-TiAl [5-8], while published work for {101}-conjugated PT-L10-phases is limited to CuAu-I [1]. For polycrystalline CuAu-I, FePt and FePd the PT-microstructures typically consist of colonies of grains, each of which only contains one of the three possible types of {101}-PT-interface that separates two L10-ordered domains with nearly perpendicular c-axis orientation as shown in figure 1. Hence, the crystallography and the geometry for the transfer of shears across interfaces in hard PT-grains is much simpler for the {101}-conjugated PT-interfaces than for the (111)-conjugated PT-TiAl. Figure 2 depicts the dodecahedral PTinterface with a (01-1)-conjugation plane in terms of a planar representation of the Thompson tetrahedron adapted to the L10-structure by replacing γ, δ and C, D with γ’, δ’ and C’, D’ respectively. Vectors represented by two primed or two unprimed letters (unmixed letters), such as C’D’ (OD with b=1/2[~1~10]) or D’γ’ (TD, b=1/6[112]), correspond to perfect or partial dislocations that do not generate order-violating faults (antiphase boundaries (APB) or complex stacking faults (CSF)). Vectors consisting of mixed letter combinations, i.e. primed and unprimed letters, such as D’B (b=1/2[011]) and D’β (b=1/6[211]), represent superpartials and complex Shockley-type partials that generate APB and CSF respectively. Strain accommodation in L10-phases is commonly accomplished by mechanical twinning on {111} associated with Burgers vectors of the type nb=n/6, where n is an integer, and/or {111}-plane glide of perfect ordinary dislocations (OD) with b=1/2 and superdislocations with b= [1-4]. For polycrystals of PT-TiAl hard and soft grains can be distinguished depending on the relative orientation of the lo
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