What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys
of two precipitating phases, with the fast-precipitating phase acting as heterogeneous nucleation
sites for a second phase manifesting slower kinetics. Kinetic lattice Monte Carlo simulations show
that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly
affect the selection of core-shell verses appendage morphologies. We study a FeCuMnNiSi alloy
using the combination of atom probe tomography and simulations, and show that 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 coreshell morphology is favored.