Microstructure and mechanical properties of steel/TiC nano-composite surface layer produced by friction stir processing

A steel/TiC nano-composite surface layer with ultra fine grains of less than 600 nm was fabricated on mild steel substrate by introduction of nano-sized TiC powder into the stir zone employing four passes of friction stir processing. TiC clusters were formed after the first pass. Sequential break-up of clusters and refinement of matrix grains were caused by subsequent FSP passes. A near uniform dispersion of nano-sized TiC particles was achieved after the fourth pass. The fabricated nano-composite layer exhibited a maximum micro hardness value of ~ 450HV; this is much greater than 185 and 130HV of the friction stir processed layer without introduction of TiC powder and as-received substrates, respectively. Moreover, a significant improvement in wear resistance of the nano-composite layer was observed as compared with that of the as-received substrate. The enhanced properties are attributed to the uniform dispersion of hard nano-sized TiC reinforcements in a matrix of ultra fine dynamically recrystallized grains.