  The Fe-Cu system has attracted much attention over the last several decades
due to its technological importance as a model alloy for Cu steels. In spite of
these efforts several aspects of its phase diagram remain unexplained. Here we
use atomistic simulations to characterize the polymorphic phase diagram of Cu
precipitates in body-centered cubic (BCC) Fe and establish a consistent link
between their thermodynamic and mechanical properties in terms of thermal
stability, shape, and strength. The size at which Cu precipitates transform
from BCC to a close-packed 9R structure is found to be strongly temperature
dependent, ranging from approximately 4 nm in diameter (~2,700 atoms) at 200 K
to about 8 nm (~22,800 atoms) at 700 K. These numbers are in very good
agreement with the interpretation of experimental data given Monzen et al.
[Phil. Mag. A 80, 711 (2000)]. The strong temperature dependence originates
from the entropic stabilization of BCC Cu, which is mechanically unstable as a
bulk phase. While at high temperatures the transition exhibits first-order
characteristics, the hysteresis, and thus the nucleation barrier, vanish at
temperatures below approximately 300\,K. This behavior is explained in terms of
the mutual cancellation of the energy differences between core and shell
(wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The
proposed mechanism is not specific for the Fe--Cu system but could generally be
observed in immiscible systems, whenever the minority component is unstable in
the lattice structure of the host matrix. Finally, we also study the
interaction of precipitates with screw dislocations as a function of both
structure and orientation. The results provide a coherent picture of
precipitate strength that unifies previous calculations and experimental
observations.
