Achieving better synergy of strength and ductility by adjusting size and volume fraction of coherent κ’–carbides in a lightweight steel

Abstract

Heterogeneous grain structures with dual-nanoprecipitates have been designed and fabricated in a lightweight steel. The size and the volume fraction of coherent κ’–carbides are increased by aging treatment, while the volume fraction of B2 precipitates and the heterogeneous grain structure are nearly unchanged after aging treatment. The aged samples are found to have a better synergy of strength and ductility, as compared to the unaged samples. The better tensile properties in the aged samples can be owing to the higher hetero-deformation-induced hardening and the higher density of induced geometrically necessary dislocations during tensile deformation. Two distinct precipitation hardening mechanisms are revealed for the dual-nanoprecipitates, i.e., bypassing mechanism for B2 precipitates and shearing mechanism for coherent κ’–carbides. Numerous dislocations can be observed both around and inside coherent κ’–carbides. High density of stacking faults and nanotwins can also be found in the grain interiors of the aged samples, resulting in strong strain hardening by interaction between them and dislocations. The better tensile properties in the aged samples can also be attributed to the stronger shearing precipitation hardening effect by larger size and higher volume fraction of coherent κ’–carbides.