Short-range order and precipitation in Fe-rich Fe-Cr alloys: Atomistic off-lattice Monte Carlo simulations

TL;DR

This study uses atomistic off-lattice Monte Carlo simulations to analyze short-range order in Fe-Cr alloys, revealing how Cr precipitates influence ordering and aligning well with experimental observations.

Contribution

It provides a detailed atomistic simulation approach to understand SRO in Fe-Cr alloys and clarifies the physical origin of SRO inversion related to stable Cr precipitates.

Findings

Strong ordering tendency at low Cr concentrations (~< 5%)

SRO parameter minimum at intermediate Cr levels (5-15%)

Inversion of SRO due to stable alpha-prime precipitates

Abstract

Short-range order (SRO) in Fe-rich Fe-Cr alloys is investigated by means of atomistic off-lattice Monte Carlo simulations in the semi-grand canonical ensemble using classical interatomic potentials. The SRO parameter defined by Cowley [Phys. Rev. B 77, 669 (1950)] is used to quantify the degree of ordering. In agreement with experiments a strong ordering tendency in the Cr distribution at low Cr concentrations (~< 5%) is observed, as manifested in negative values of the SRO parameters. For intermediate Cr concentrations (5% ~< c_Cr ~< 15%) the SRO parameter for the alpha-phase goes through a minimum, but at the solubility limit the alpha-phase still displays a rather strong SRO. In thermodynamic equilibrium for concentrations within the two-phase region the SRO parameter measured over the entire sample therefore comprises the contributions from both the alpha and alpha-prime phases. If both of these contributions are taken into account, it is possible to quantitatively reproduce the experimental results and interpret their physical implications. It is thereby shown that the inversion of the SRO observed experimentally is due to the formation of stable (supercritical) alpha-prime precipitates. It is not related to the loss of SRO in the alpha-phase or to the presence of unstable (subcritical) Cr precipitates in the alpha-phase.