Excellent mechanical properties from the synergy of carbon partitioning, L1₂-nano-precipitation and TRIP effects in Fe-Ni-Al-Ti-C steel

Multiple strategies and technological pathways exist in developing new advanced high strength steels (AHSSs). For plain carbon steels, carbon partitioning has been utilized to generate a mixture of ferrite/martensite and retained austenite, whereas higher carbon content will stabilize austenite phase. The austenite can be metastable, which can trigger phase transition under stress, so called phase-transformation-induced plasticity (TRIP). For highly alloyed steels with Ni, Al, Ti or other elements, precipitates of the body-centered cubic (BCC), hexagonal close-packed (HCP), L2₁, L1₂ types can form during aging/partitioning. L12 phase shows exceptional deformation capability because itself can sustain significant plastic deformation. Motivated by these two design strategies, this work started from a Fe-Ni alloy by added with appropriate amounts of Al, Ti, and C to obtain a series of Fe-Ni-Al-Ti-C steels by melting, cold rolling and a simple heat treatment (recrystallization and aging/partitioning) history. Microstructural observation and mechanical property testing reveal that the Fe-Ni-Al-Ti-C steels successfully achieves: (1) nanosized and densely populated L1₂ precipitates in both ferrite and austenite phases, (2) enhanced stability of austenitic phase with TRIP capability, (3) ultrafine-grained microstructure due to precipitate-retarded ferrite grain growth, and (4) extra dislocation storage of precipitate-cutting dislocation loops. The synergy of all these factors results into tensile strengths of 1.2-1.8 GPa and uniform ductility of 10-30%, which is comparable to twining-induced plasticity steels.