Microstructural dependence of Fe-high Mn tensile behavior

The tensile properties of Fe-high Mn (16 to 36 wt pct Mn) binary alloys were examined in detail at temperatures from 77 to 553 K. The Mn content dependence of the deformation and fracture behavior in this alloy system has been clarified by placing special emphasis on the starting microstructure and its change during deformation. In general, the intrusion of hcp epsilon martensite (ε) into austenite (γ) significantly increases the work hardening rate in these alloys by creating strong barriers to further plastic flow. Due to the resulting high work hardening rates, large amounts of e lead to high flow stresses and low ductility. Alloys of 16 to 20 wt pct Mn are of particular interest. While these alloys are thermally stable with respect to bcc α’ martensite formation, 16 to 20 wt pct Mn alloys undergo a deformation induced ε →α’ transformation. The martensitic transformation plays two contrasting roles. The stress-induced ε→ α’ transformation decreases the initial work hardening rate by reducing locally high internal stress. However, the work hardening rate increases as the accumulated α’ laths become obstacles against succeeding plastic flow. These rather complicated microstructural effects result in a stress-strain curve of anomolous shape. Since both the M_s and M_d temperatures for both the ε and α’-martensite transformations are strongly dependent on the Mn content, characteristic relationships between the tensile behavior and the Mn content of each alloy are observed.