Correlation between the microstructure and deformation behavior of a PM fine-grained duplex steel

A fine-grained duplex steel with a composition of Fe-5Mn-2Al-0.6C (wt.%) was fabricated using a powder metallurgy (PM) technology of thermomechanical consolidation by hot extrusion of powder compact. The microstructure consisted of alpha '-martensite, residual austenite (RA), and a small amount of Al2O3 particles. Subsequent heat treatment was performed to adjust the microstructure and mechanical properties. This study focuses on the pre-and post-heat treatment microstructure of the material, elucidates the reasons for its formation, and explains how microstructural changes influence tensile deformation behavior. The results show that the heat treatment caused the precipitation of nanoscale carbide within the martensite, which significantly increased the yield strength from 792 MPa to 1078 MPa. While heat treatment altered the size and distribution of RA regions in the microstructure, the amount of RA transformed during deformation remained proportional to the flow stress in both materials, indicating that the flow stress dictates the amount of phase transformation of ultrafine grained RA. Therefore, in the heat-treated material with a higher yield strength, a larger amount of RA transformed during the initial stage of plastic deformation, leaving less RA to sustain a work-hardening rate in subsequent deformation, resulting in a decrease in ductility from 5.5 % to 3.6 % and fracture stress from 1674 to 1453 MPa. In addition, the in-situ formed Al2O3 particles facilitated grain refinement of the extruded rod, but had less effect on the microstructure and properties of the heat-treated sample. This study serves as a valuable reference for comprehending the influence of microstructural changes in PM manganese steel on tensile deformation behavior.