  Short-range order (SRO) alters the mechanical properties of technologically
relevant structural materials such as medium/high entropy alloys and austenitic
stainless steels. In this study, we present a generalized spin cluster
expansion (CE) model and show that magnetism is a primary factor influencing
the level of SRO present in austenitic Fe-Ni-Cr alloys. The spin CE consists of
a chemical cluster expansion combined with an Ising model for Fe-Ni-Cr alloys.
It explicitly accounts for local magnetic exchange interactions, thereby
capturing the effects of finite temperature magnetism on SRO. Model parameters
are obtained by fitting to a first-principles data set comprising both
chemically and magnetically diverse FCC configurations. The magnitude of the
magnetic exchange interactions are found to be comparable to the chemical
interactions. Compared to a conventional implicit magnetism CE built from only
magnetic ground state configurations, the spin CE shows improved performance on
several experimental benchmarks over a broad spectrum of compositions,
particularly at higher temperatures due to the explicit treatment of magnetic
disorder. We find that SRO is strongly influenced by alloy Cr content, since Cr
atoms prefer to align antiferromagnetically with nearest neighbors but become
magnetically frustrated with increasing Cr concentration. We predict that
increasing the Cr concentration in typical austenitic stainless steels promotes
the formation of SRO and increases order-disorder transition temperatures. This
study underscores the significance of considering magnetic interactions
explicitly when exploring the thermodynamic properties of complex transition
metal alloys. It also highlights guidelines for customizing SRO through
adjustments of alloy composition.
