Thermodynamics and kinetics of core-shell versus appendage co-precipitation morphologies: An example in the Fe-Cu-Mn-Ni-Si system

TL;DR

This study investigates how interfacial energies, ordering energies, and diffusion pathways influence the formation of core-shell versus appendage precipitate morphologies in Fe-Cu-Mn-Ni-Si alloys, combining experiments and simulations.

Contribution

It provides a detailed analysis of the thermodynamic and kinetic factors determining precipitate morphologies in multi-phase alloys, supported by atom probe tomography and kinetic Monte Carlo simulations.

Findings

Ordering energy reduction favors appendage formation.

Higher interface energies promote core-shell morphology.

Diffusion pathways influence subsequent growth of precipitates.

Abstract

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.