Strain gradient plasticity phenomenon in surface treated plain carbon steel

Surface engineering is essential for materials being used for functional and structural applications. There have been various approaches and mechanisms used for improving, mechanical, tribological and surface properties of materials over the years. One phenomenon that require much attention is indentation size effect (ISE). Mecha-nistic and phenomenological theories have been developed to explain the observed size dependence of me-chanical properties in materials. However, the dislocations microstructure evolution, deformation mechanisms and the contributions of length scale parameters are not fully understood. This paper explores the effects of surface carbo-nitriding on material length scale and dislocation microstructure mechanisms of 1045 steel. The material was carbo-nitrided at 500 degrees C and 900 degrees C, to induce nitrogen and carbon diffusion respectively, prior to, microstructural and mechanical (micro/nano-indentation) characterization. The statistically stored and geometrically necessary dislocation densities increased with increasing carbo-nitriding temperature. A bi-linear relationship was also established based on the indentation depth and hardness measurements. Three regimes of dislocation behaviour (dislocations source-limited/starvation, discrete dislocations, and continuum dislocations) are observed. The observed trends are explained using dislocation densities that were estimated using dislocation theories. The implications of the results are also discussed for the carbonitrided steels.