Strain-induced refinement in a steel with spheroidal cementite subjected to surface mechanical attrition treatment

Steel with spheroidal cementite particles dispersed in a ferrite matrix was subjected to surface mechanical attrition treatment. Strain-induced microstructure evolution, including grain refinement of ferrite and cementite as well as dissolution of cementite, was examined using transmission electron microscopy. Upon straining, dense dislocation walls were developed in ferrite grains, which evolved gradually into sub-boundaries and highly misoriented grain boundaries (GB) at increasing strains, leading to grain refinement of the ferrite. The ferrite refinement process is greatly facilitated by the presence of dispersed cementite particles, as the cementite/ferrite interfaces are effective nucleation sites for dislocations and also barriers for dislocation motions. When ferrite grains are refined to sizes smaller than that of cementite particles, plastic deformation occurs in cementite particles, characterized by edge dislocation gliding along the {1 1 0} and {1 0 0} planes. Dislocation gliding in cementite is initiated at intersections of the cementite/ferrite interfaces and ferrite GB. Accumulated multiple gliding progressively refines cementite into nano-sized cementite particles mixed within ferrite nano-grains in the top surface layer. A decreasing volume fraction of cementite is observed with decreasing depth in the top deformed layer, indicative of dissolution of cementite induced by increased plastic straining.