Transformation from cluster to nano-precipitate in microalloyed ferritic steel

Carbide precipitates are pivotal for strengthening steels and improving their resistance to hydrogen-embrittlement. Combining aberration-corrected high-resolution transmission electron microscopy and atom probe tomography, this work provides an atomic-scale evidence for the precipitation mechanism from embryo cluster to nano-precipitate in ferritic steels. Carbide precipitation begins with disc-shaped embryo clusters of several atom layers thick, which are fully coherent with the ferrite matrix. As the diameter increases, the embryo clusters grow into clusters that are analogous to the Guinier-Preston zone in Aluminium alloys. With further increase in size, particularly in thickness, clusters transform into a new phase with NaCl-structure.