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

Abstract Multiple strategies and technological pathways exist in developing new advanced high strength steels. 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, hexagonal close-packed, L2 1 , L1 2 types can form during aging/partitioning. L1 2 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 L1 2 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.