Effects of annealing and thermo-mechanical treatment on the microstructures and mechanical properties of a carbon-doped FeNiMnAl multi-component alloy

The microstructures and mechanical properties of both undoped and carbon-doped (1.26 at%) f.c.c./B2 Fe36Ni18Mn33Al13 multi-component alloys have been investigated in the as-cast, annealed and recrystallized states. A lamellar structure is present in the undoped alloy, while a tetragonal martensite with irregular shape is present in the carbon-doped alloy. B2-structured precipitates form upon annealing in both alloys, whose size and volume fraction increases with increasing annealing time. Lamellar coarsening also occurs during annealing of the undoped alloy. Two different thermo-mechanical treatments were applied to the carbon-doped alloy in order to decrease the grain size and disperse the martensite, which produced a significant increase in strength. The changes in yield strength are discussed in terms of the underlying strengthening mechanisms, i.e., phase boundary strengthening, grain boundary strengthening, interstitial strengthening, and precipitation strengthening. The carbon addition results in a sharp increase in ductility of the as-cast alloy, a feature ascribed to microband formation during the tensile test arising from the increase of lattice friction stress. Similar to some single-phase f.c.c. alloys, microband-induced plasticity (MBIP) effect is found to present in a two-phase multicomponent alloy in this study.