Plastic deformation behavior of functionally graded 9Cr18 steel after thixoforging and heat treatment

The strain hardening behavior and evolution of deformation structure for functionally graded 9Cr18 steel were investigated in the article. Meta-austenite was retained after semi-solid forming, while the liquid was extruded outside to form fine dendrites. Thus the material could demonstrate functionally graded property. Solid austenite grains inside the solid/liquid boundary were connected with each other and would support the high strength with compressive strength of 4680 MPa at compression ratio of 53.2%. The material exhibited obvious four-stage strain hardening characteristics with a long continuous strain hardening stage. The stacking fault energy (SFE) for inner austenite was calculated to be ~24.2 mJ/m2 and the value fell into intermediate range. The deformation behavior was dominated by planar dislocation, suppression of cross slip and formation of secondary deformation structures (mechanical twinning and ϵ-martensite). The formation of multiple variants of mechanical twinning or ϵ-martensite played a dynamic Hall-Petch effect and the tendency of synchronous improvement of strength and plasticity (SISP) was obtained. The functionally graded property avoided crack formation from brittle eutectic and the material demonstrated a unique ductile fracture characterization.