Phase formation and martensitic transformation in mechanically alloyed nanocrystalline Fe—Ni

Abstract

Fe—Ni alloys were prepared by mechanical attrition of elemental powders in a planetary ball mill in argon atmosphere. A nonequilibrium phase diagram of Fe100−xNix, mechanically alloyed at various milling intensities, is presented. Disordered body-centered-cubic and/or face-centered-cubic supersaturated solid solutions with a two-phase region are found. The maximum solubilities are 30 at. % Ni in Fe and 40 at. % Fe in Ni and decrease slightly with increasing milling intensity, i.e., processing temperature. Mechanically alloyed Fe—Ni is in a nanocrystalline state with an average grain size of 25–35 nm. The grain refinement is accompanied by an increase in atomic-level strain up to about 1% root-mean-square. This high internal strain affects predominantly the coercivity which is at least 4 A/cm even for single-phase alloys. The kinetics of the martensitic γ→α′ transformation in mechanically alloyed nanocrystalline Fe—Ni are significantly modified in comparison to coarse-grained as-cast alloys. The nonequilibrium microstructure is assumed to hinder the growth of martensite which is confirmed by the investigation of the magnetic field-induced martensitic transformation.