Abstract
Thermal holding at critical temperature to obtain ferrite and martensite dual-phase mi crostructure was adopted to fabricate the ultrafine ferrite grains with a bimodal distribution. Microstructure evolution and mechanical properties by different holding times were analyzed. Results showed that the hierarchical design in the bimodal grains of UFGs via ratio regulation on the dual-phase ferrite/martensite could be achieved by holding time. The tensile strength decreased with the increased holding time at critical temperature, while the elongation in creased with the increase of holding time. The comprehensive mechanical properties were dominated by the volume fraction of introduced ferrite and the final grain size distribution. Furthermore, the elongation was ensured by the better work hardening ability in coarsening grains and plastic strain gradient between ferrite grains with a bimodal distribution.

Key words
ultrafine grain, bimodal grain, ferrite, martensite, tensile strength, ductility

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