Effects of Thermomechanical Processing on the Microstructure and Mechanical Properties of Fe-Based Alloys

This study focuses on the unusual evolution of microstructure and mechanical properties caused by thermomechanical processing of FeMnAlNi alloys. For Fe43.5Mn34Al15Ni7.5, which is an eligible candidate for superelastic applications, the effects of substituting ± 1.5 at.% Al with Ni were analyzed. The ingots were subjected to five thermomechanical processing steps: (1) hot rolling; (2) annealing; (3) cold rolling; (4) cyclic heat treatment under protective Ar atmosphere; and (5) solution treatment and aging. The microstructure has been biphasic for all chemical compositions and processing steps, α-body-centered cubic (bcc) phase being stabilized by higher Al amount and γ-face-centered cubic (fcc) phase by higher Ni amount and slow cooling. Oxygen contamination due to heating in unprotected atmosphere was completely removed by mechanical grinding. At Fe43.5Mn34Al15Ni7.5 specimen, cold rolling caused a fibering structure, the spacing of which decreased with the augmentation of deformation degree. Cyclic heat treatment under continuous argon flow caused the formation of globular γ-fcc which did not dissolve at 1200 °C and impeded abnormal grain growth. This unusual globular γ phase was strongly stabilized, in such a way that it could hardly be dissolved by an additional solution treatment at 1200 °C. The tensile failure tests revealed that Al substitution with Ni caused the decrease in the ultimate stress in cold-rolled condition and ultimate strain in hot-rolled and annealed conditions.