Impact of solutes on the lattice parameters and elastic stiffness coefficients of hcp Fe from first-principles calculations

Abstract

The hexagonal close-packed (hcp) $\epsilon$-martensite phase in steels nucleates from the austenite parent phase during plastic straining and can be stabilized by solute additions. We compute the lattice parameters and elastic stiffness coefficients $C_{ij}$ of single-crystal hcp Fe as functions of solute concentration in the dilute limit for the substitutional solutes Al, B, Cu, Mn, and Si, and the octahedral interstitial solutes C and N. Solute strain misfit tensors determine the solute dependence of the lattice parameters, as well as the strain contributions to the solute-induced changes in the $C_{ij}$. We also compute chemical contributions to the changes in the $C_{ij}$ for each solute, and show that the sum of the strain and chemical contributions agrees with more computationally expensive direct calculations that simultaneously incorporate both effects. The computed data can be used to estimate solute-induced changes in polycrystalline elastic moduli and changes in mechanical properties such as strength and ductility, and can be directly incorporated into mesoscale simulations of multiphase steels to model solute effects on the $\epsilon$-martensite phase.