Structural transformations and improved ductility in ordered FeCo and ZrCo intermetallics
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0980-II02-03
Structural Transformations and Improved Ductility in Ordered FeCo and ZrCo Intermetallics Maja Krcmar1, Chong Long Fu2, and James R. Morris3 1 Physics Department, Grand Valley State University, One Campus Drive, PAD 144, Allendale, MI, 49401-9403 2 Materials Science and Technology Division, Oak Ridge National Laboratory, P.O.Box 2008, Oak Ridge, TN, 37831-6114 3 Materials Science and Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN, 37831-61115
ABSTRACT Using the first-principles calculations, we find that Fe-Co has a tendency for a structural transformation to a lower symmetry sheared L10 phase under the applied shear stresses. This tendency for structural transformation can have a significant influence on the mechanical properties of FeCo, as it might be closely connected with the intrinsic brittleness of Fe-rich and stoichiometric FeCo alloys and with the improved ductility of Co-rich FeCo alloys. We suggest that improved ductility in Co-rich FeCo alloys may originate from transformation toughening due to the B2→L10 structural transformation near the regions of high stress concentration, as the stress energy is fully dissipated by the decrease in the electronic energy due to the structural phase transformation into a lower energy structure. Similarly, in ZrCo, our first-principles calculations find that a B2→B33 martensitic phase transformation can occur under the applied shear stress, which may contribute to the good ductility of this alloy, despite the fact that ZrCo is a strongly ordered line compound. INTRODUCTION There is a growing evidence that common understanding of ductility fails to explain the behavior of many B2 intermetallics: brittleness of Fe-rich and stoichiometric FeCo (and the improved ductility in Co-rich FeCo) remains a puzzle having in mind its symmetry, weak ordering and transformation mode, but the strongly-ordered line compound ZrCo exhibits unusually good ductility. Using first-principles calculations, we investigated structural and electronic structure features of Fe-Co and ZrCo alloys in order to identify the alloy variables that govern their unusual mechanical behavior, and to suggest a possible mechanism potentially contributing to the improved ductility in these alloys. We find that, under the (001) type elastic shear, the B2 FeCo can transform into the low-symmetry L10–like structure. For Fe-rich FeCo and stoichiometric FeCo, the deformed L10 structure is meta-stable, with energy considerably higher than that of the B2 structure. In contrast, for Co-rich FeCo, the L10 structure is stable, with energy lower than that of the B2 structure. For ZrCo, under the periodic [100] shear stress on every alternative (011) atomic plane, we find that the B2 structure is transformed into the lower energy B33 structure. Our calculations suggest that the improved ductility of Co-rich FeCo and ZrCo alloys might originate from the transformation toughening mechanism due to the structural phase transformations into the lower-energy phases. These transforma
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