Nanoindentation of silicon carbide and sulfur-induced embrittlement of nickel

This dissertation focuses on the mechanical response, plastic activities, and failure of bulk ceramic and nanocrystalline metal at the atomistic scale. Molecular dynamics simulations have been performed for nanoindentation on the crystalline cubic silicon carbide (3C-SiC) and sulfur-induced embrittlement at nickel grain boundaries.Multimillion-atom molecular dynamics simulations of nanoindentation on 3C-SiC surfaces corresponding to three different crystallographic directions, (110), (001), and (111), have been performed using pyramidal-shaped Vickers indenter with 90° edge angle. Load-displacement (P-h) curves show major and minor pop-in events during loading. Our analysis shows that the first minor discontinuity in the P-h curve of (110) indentation is related to nucleation of dislocations, whereas the subsequent major load-drops are related to the dissipation of accumulated energy by expansion of dislocation loops and changes of slip planes. Detailed quantitative analysis on the stress distribution on slip planes and stress concentration at kinks and dislocation cores has estimated the Peierls stress to be 7.5GPa &ap 3.9x10 -2G, where G is the shear modulus. We have observed anisotropic pileup patterns which all reside on (111) and (1¯1¯1) slip planes after the indenter is unloaded. These patterns are closely related to dislocation activities on the two slip planes. The anisotropy is a consequence of the asymmetry of the 3C-SiC crystal in which only (111) and (1¯1¯1) slip planes are active out of {111} family.Alteration of fracture behaviors of a material by minute segregated impurities at grain boundaries is a fundamental question encompassing chemistry, mechanics and material science. One of the well-known examples is sulfur segregation-induced embrittlement of nickel grain boundaries, where coincidence between the critical sulfur concentration for embrittlement and that for amorphization remains unexplained. We have performed 48 million-atom reactive molecular dynamics simulations and first-principles quantum-mechanical calculations to investigate the role of sulfur. A direct link between sulfur-induced intergranular amorphization and embrittlement is thus established. Analysis shows that an order-of-magnitude reduction of grain-boundary shear strength due to amorphization, combined with tensile-strength reduction, allows the crack tip to always find an easy propagation path. This mechanism explains an experimentally observed crossover from transgranular to intergranular fracture and suppressed plastic activities due to sulfur segregation.