Refining the hierarchical structure of lath martensitic steel by in situ alloying with nickel: morphology, crystallography, and mechanical properties

Lath martensitic high-strength steels are some of the most widely used materials in structural applications, and consequently, refining and controlling the lath martensitic morphology is of significant interest to expand the versatility of the steels and their associated range of applications. It is well known that the steel’s carbon content has a pronounced effect on the lath martensitic morphology, its crystallography, and hierarchical structure, but the effect of other elements like Mn and Ni is still relatively unclear. This study provides a comprehensive investigation on the effect of Ni on the morphology, crystallography, microstructural refinement, internal transformational strain, and mechanical properties of lath martensite using advanced analytical tools at the meso-, nano-, and atomic scales to offer insights that have so far been lacking in the literature. The in situ alloying of Ni with the bulk microstructure triggered a relatively ideal martensitic transformation such that the majority of the austenite (γ) grains transformed into four martensitic packets (with each packet representing one of the four close-packed {111} planes in a given γ-grain) with a relatively homogenous distribution of the twenty-four martensitic variants that can potentially evolve in an γ-grain based on the Kurdjumov–Sachs (K–S) orientation relationship. The addition of Ni lowered the martensite start (M_s) temperature and strengthened the γ-matrix at lower temperatures, which resulted in the refinement of the packets and blocks due to an inherent need by the matrix to self-accommodate the transformational strain, whereby significantly decreasing the internal strain in the microstructure. The results of this study showed that alloying lath martensitic steels with Ni can prove to be an effective method to refine the hierarchical microstructure in a way that can be used to improve the mechanical properties of these steels.