Magnetically induced immiscibility in the ising model of FeCr stainless steel

The iron-chromium alloy system has an unexplained anomaly: although there is a broad miscibility gap it appears to be favorable for chromium to dissolve in iron. This is consistent with ab initio calculation, but no simpler physically intuitive picture has been presented. Here it is shown that the Ising model, based on the bcc lattice with antiferromagnetic and ferromagnetic species, has the potential to exhibit similar behavior, with a skew miscibility gap arising from the solubility of antiferromagnetic species on nonadjacent sites. Essential characteristics of stainless steel (high Cr solubility and surface segregation) are correctly reproduced. Under some conditions, magnetization increases with temperature. The equilibrium miscibility gap due to mixed magnetism and segregation-driven positive dM/dT are fundamental features of the bcc Ising model itself, not just FeCr.