Effects of indentation depth and grain size on scratching behavior of nanograin FCC Fe polycrystalline substrate

Abstract

Owing to their high grain boundary (GB) density, nanograin materials have excellent mechanical and physicochemical properties. The variation of GB density can dominate their plastic deformation mechanism. This work investigated the effects of indentation depth and mean grain size (MGS) on the scratching plastic deformation of FCC Fe nanograin substrates by molecular dynamics method. The contact strengths of the models with MGSs of 2.05 and 8.71 nm were the smallest (9.26 GPa) and largest (11.67 GPa), respectively. When the MGS was refined smaller than the critical size (5.07 nm), the Hall–Petch breakdown occurred in the nanograin polycrystalline substrate. With the increase in the indentation depth, plastic deformation became dominant, and abrasion volume first increased and then decreased with a decrease in the MGS. The lateral friction force and normal forces (including their increments) increased with the indentation depth. Models with moderate grain sizes could maintain larger "acceleration" of friction and normal force increments. After grain refinement, dislocations increased during the scratching process, and all of them were short dislocations limited by grain boundaries; in particular, there was no long straight stair-rod dislocation. The shear strain in ultrafine grain model was distributed along GBs. Grain lattice rotation extends throughout small grains with the MGS decrease to 2.05 nm. The deformation mechanism changed from dislocation-mediated mechanism to that involving grain lattice rotation and GBs sliding. This work can provide a theoretical basis for the design, machining, and performance prediction of nanograin polycrystalline materials.