  What determines precipitate morphologies in co-precipitating alloy systems?
We focus on alloys of two precipitating phases, with precipitates of the
fast-precipitating phase acting as heterogeneous nucleation sites for a second
phase manifesting slower kinetics. We study a FeCuMnNiSi alloy using the
combination of atom probe tomography and kinetic Monte Carlo simulations. It is
shown that the interplay between interfacial and ordering energies, plus active
diffusion paths, strongly affect the selection of core-shell verses appendage
morphologies. Specifically, the ordering energy reduction of the MnNiSi phase
heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the
energy penalty of a predominantly Fe/Cu interface, leading to initial
appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around
and through the Cu core towards the ordered phase results in subsequent
appendage growth. We further show that in cases with higher primary precipitate
interface energies and/or suppressed ordering, the core-shell morphology is
favored.
