Formation mechanism of κ -carbides and deformation behavior in Si-alloyed FeMnAlC lightweight steels

The formation of κ-carbides in austenite Fe-30Mn-9Al-1.2C (wt. %) lightweight steels is tuned via alloying of Si (0, 1, 2 wt. %), an element that can remarkably raise the activities of Al and C based on thermodynamic calculations. Ordered L1 ₂ nano-domains (with a size <1 nm), lacking elemental partition, were observed in the solution-treated steel without Si alloying, while with the increase of Si to 2 wt. %, cuboidal L′1 ₂ intragranular κ-carbides were well developed with an average size of 11.5 nm and a volume fraction of 25.9 %. These κ-carbides found in the solution-treated steel with 2 wt. % Si follow a different precipitation route from previous pathways that require aging. Also, particle-shaped L′1 ₂ intergranular κ ₀-carbides and DO ₃ phase were formed at austenite grain boundaries in the steel with 2 wt. % Si. The precipitation of κ -carbides in grain interiors leads to an improvement of the yield strength from ~450 MPa to ~950 MPa as the Si content increases from 0 to 2 wt. %. The primary deformation mechanism is the formation of slip bands in all three steels, which involves the shear of ordered nano-domains or κ -carbides. The uniform distribution of the slip bands is essential for the high strain hardening, provided by the dynamic slip band refinement in the steel without Si. Lower strain hardening is seen in the steel with 2 wt. % Si due to the formation of localized coarse slip bands. These findings offer valuable insights into the design of high-performance lightweight steels.