Here we report about our novel design approach for precipitation hardened ductile high strength martensitic and austenitic-martensitic steels (up to 1.5 GPa strength). The alloys are characterized by a low carbon content (0.01 wt.% C), 9-15 wt.% Mn to obtain different levels of austenite stability, and minor additions of Ni, Ti, and Mo (1-2 wt.%). The latter are required for creating precipitates during aging heat treatment.
Hardening in these materials is realized by combining the TRIP effect with a maraging treatment (TRIP: transformation-induced plasticity; maraging: martensite aging through thermally stimulated precipitation of particles). The TRIP mechanism is based on the deformation-stimulated athermal transformation of metastable austenite (face centered cubic Fe-Mn phase) into martensite (metastable body centered cubic or orthorhombic Fe-Mn phase) and the resulting matrix and martensite plasticity required to accommodate the transformation misfit. The maraging treatment is based on hardening the heavily strained martensite through the formation of small intermetallic precipitates (of the order of several nanometers). These particles act as highly efficient obstacles against dislocation motion through the Orowan and Fine-Kelly mechanisms enhancing the strength of the material.
While both types of alloys, i.e. TRIP steels and maraging steels have been well investigated, the combination of the two mechanisms in the form of a set of simple Fe-Mn alloys as suggested in this work, namely, the precipitation hardening of transformation-induced martensite by intermetallic nanoparticles, opens a novel and lean alloy path to the development of ultrahigh strength steels that has not been much explored in the past.