First principle study of cobalt impurity in bcc Fe with Cu.pdfVIP

J Mater Sci (2013) 48:1377–1386 DOI 10.1007/s10853-012-6884-2 First principle study of cobalt impurity in bcc Fe with Cu precipitates N. I. Medvedeva • A. S. Murthy • V. L. Richards • D. C. Van Aken • J. E. Medvedeva Received: 30 May 2012 / Accepted: 7 September 2012 / Published online: 21 September 2012 Springer Science+Business Media, LLC 2012 Abstract The addition of cobalt was experimentally strengthening effect is complicated and has been inten- observed to increase the strength and impact toughness of Cu sively studied [1–7]. Atomistic simulations were employed precipitation hardened steel. In order to understand the to investigate the effect of size, shape, composition, and mechanism of this strengthening, we studied the effect of structural instability of Cu particles on strengthening [2, 3, cobalt in the bulks and surfaces of bcc Fe and bcc Cu, as well 7– 15]. According to [2–7], precipitation strengthening was as at the Fe/Cu interface by ab initio density-functional related to the dislocation pinning caused by the transfor- approach. We investigated the cobalt distribution between mation of the unstable bcc structure to more stable Cu the Fe matrix and Cu precipitate, and found that cobalt is structures under shearing of Cu precipitates by dislocations rejected from the core of the Cu particle. The calculated (structural-transformation mechanism). elastic constants and stacking fault energies show that cobalt Recently, it was observed that cobalt addition improves does not produce any solid solution softening or hardening in the room temperature strength as well as the toughness of bcc Fe. However, cobalt segregated in the interfacial region precipitation hardened steel; however, the mechanism of increases the cleavage fr