Effects of Grain Boundary Disorder on Yield Strength

It was recently reported that segregation of Zr to grain boundaries (GB) in nanocrystalline Cu can lead to the formation of disordered intergranular films [1,2]. In this study we employ atomistic computer simulations to study how the formation of these films affects the dislocation nucleation from the GBs. We found that full disorder of the grain boundary structure leads to the suppression of dislocation emission and significant increase of the yield stress. Depending on the solute concentration and heat-treatment, however, a partial disorder may also occur and this aids dislocation nucleation rather than suppressing it, resulting in elimination of the strengthening effect

Effects of solutes on migration of incoherent twin boundary in FCC metals

Nanoscale twins form in many metallic materials, especially those with low stacking fault energy. Their presence can significantly enhance the strength of a material. However, Nanotwins are unstable and can be annihilated, e.g., by thermal annealing. We present the results of molecular dynamics (MD) simulations that demonstrate that additions of solutes can significantly stabilize nanotwinned structures. The MD simulations reveal that the mechanism of the solute drag on the twin boundary is associated not with the solute segregation on the incoherent twin boundary (ITB), but rather with changing of the ITB shape to accommodate as many solutes as possible

Step bunching of vicinal 6H-SiC{0001} surfaces

We use kinetic Monte Carlo simulations to understand growth- and etching-induced step bunching of 6H-SiC{0001} vicinal surfaces oriented towards [1-100] and [11-20]. By taking account of the different rates of surface diffusion on three inequivalent terraces, we reproduce the experimentally observed tendency for single bilayer height steps to bunch into half unit cell height steps. By taking account of the different mobilities of steps with different structures, we reproduce the experimentally observed tendency for adjacent pairs of half unit cell height steps to bunch into full unit cell height steps. A prediction of our simulations is that growth-induced and etching-induced step bunching lead to different surface terminations for the exposed terraces when full unit cell height steps are present.Comment: 10 pages, 12 figure

Solute effects on interfacial dislocation emission in nanomaterials: nucleation site competition and neutralization

Interfacial nucleation is the dominant process of dislocation generation during the plastic deformation of nano-crystalline materials. Solute additions intended to stabilize nano-crystalline metals against grain growth, may segregate to the grain boundaries and triple junctions where they can affect the process of the dislocation emission. In this Letter we demonstrate that the effect of solute addition in a nano-crystalline material containing competing solute segregation sites and dislocation sources can be very complex due to different rates of segregation at different interfaces. Moreover, at large concentrations, when the solutes form clusters near the grain boundaries or triple junctions, the interfaces between these clusters and the matrix can introduce new dislocation emission sources, which can be activated under lower applied stress. Thus, the strength maximum can occur at a certain solute concentration: adding solutes beyond this optimal solute concentration can reduce the strength of the material.Comment: 12 pages, 7 figure

Effect of Stacking Fault Energy on Mechanism of Plastic Deformation in Nanotwinned FCC Metals

Starting from a semi-empirical potential designed for Cu, we developed a series of potentials that provide essentially constant values of all significant (calculated) materials properties except for the intrinsic stacking fault energy, which varies over a range that encompasses the lowest and highest values observed in nature. These potentials were employed in molecular dynamics (MD) simulations to investigate how stacking fault energy affects the mechanical behavior of nanotwinned face-centered cubic (fcc) materials. The results indicate that properties such as yield strength and microstructural stability do not vary systematically with stacking fault energy, but rather fall into two distinct regimes corresponding to low and high stacking fault energies