The Fe-Cu system has attracted much attention over the last several decadesdue to its technological importance as a model alloy for Cu steels. In spite ofthese efforts several aspects of its phase diagram remain unexplained. Here weuse atomistic simulations to characterize the polymorphic phase diagram of Cuprecipitates in body-centered cubic (BCC) Fe and establish a consistent linkbetween their thermodynamic and mechanical properties in terms of thermalstability, shape, and strength. The size at which Cu precipitates transformfrom BCC to a close-packed 9R structure is found to be strongly temperaturedependent, ranging from approximately 4 nm in diameter (~2,700 atoms) at 200 Kto about 8 nm (~22,800 atoms) at 700 K. These numbers are in very goodagreement with the interpretation of experimental data given Monzen et al.[Phil. Mag. A 80, 711 (2000)]. The strong temperature dependence originatesfrom the entropic stabilization of BCC Cu, which is mechanically unstable as abulk phase. While at high temperatures the transition exhibits first-ordercharacteristics, the hysteresis, and thus the nucleation barrier, vanish attemperatures below approximately 300\,K. This behavior is explained in terms ofthe mutual cancellation of the energy differences between core and shell(wetting layer) regions of BCC and 9R nanoprecipitates, respectively. Theproposed mechanism is not specific for the Fe--Cu system but could generally beobserved in immiscible systems, whenever the minority component is unstable inthe lattice structure of the host matrix. Finally, we also study theinteraction of precipitates with screw dislocations as a function of bothstructure and orientation. The results provide a coherent picture ofprecipitate strength that unifies previous calculations and experimentalobservations.
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