Overlapping solid solubility in mechanically alloyed Fe-Ni and Fe-Cu

Abstract

Solid solutions formed by mechanical alloying in the negative heat of mixing Fe-Ni system and the positive heat of mixing Fe-Cu system were studied. X-ray absorption near edge structure (XANES) spectroscopy data were analyzed to obtain the phase fractions and phase compositions for various overall compositions across the bcc/fcc two-phase region. For both systems, at each nominal composition of the powder mixture, the two solutions formed both have the same composition as the overall alloy. It is concluded that the two-phase coexistence represents an extended region of overlapping solubility for the two phases, rather than the usual two-phase region or a clear switchover at the concentration where the two phases have the same free energy. In terms of these features, the two binary systems, one with negative heat of mixing and one with positive heat of mixing, behave the same way. The external forcing action of mechanical alloying at low temperatures drives the system toward composition homogenization. There is no stable or metastable equilibrium, and the common tangent construction and lever rule are not applicable. What results is a novel two-phase coexistence which is defined by a region of overlapping solubility, rather than the normal solubility gap. That there can be two polymorphs at each composition even after the long ball-milling treatment represents a new type of steady-state two-phase coexistence that is completely different from the normal two-phase region dictated by thermodynamics.