Role of local curvature in grain boundary sliding in a deformed polycrystal

The paper considers experimental and theoretical principles of accounting for curvature in multiscale computer simulation of the behavior of interfaces as independent planar subsystems in a loaded polycrystal. Dynamics of matter and energy flows along grain boundaries is studied using a hybrid discrete-continuous method of excitable cellular automata, which combines synergetic principles of discrete switching and laws of continuum mechanics. It is shown that these flows have a rotational-wave nature and depend on the loading conditions of a grain boundary.     

Cooperative grain boundary sliding in nanocrystalline materials

Superplastic behaviour of microcrystalline materials is now believed to be controlled by cooperative grain boundary sliding (CGBS). An increasing role of grain boundary mediated plasticity with decreasing grain size down to the nanoscale was predicted leading to the prospect of enhanced superplasticity in nanocrystalline materials. Nevertheless, materials with nanosized grains have revealed a significant decrease in plasticity contrary to theoretical prediction. Direct evidence of CGBS in nanocrystalline Ni₃Al alloy from SEM surface analysis and in-situ TEM tensile testing was detected, confirming one similarity in the rheology of deformation processes between micro- and nanomaterials. Thus, differences in deformation behaviour of materials at these two length scales are related to the probability of sliding surface formation, sliding distance and related accommodation mechanisms.     

Formalization of models of a deformable polycrystalline material in terms of mesomechanics

The stage of formalization is necessary for constructing an adequate mathematical model of any phenomenon. By the example of a deformable polycrystalline material, it is shown that the formalization algorithm is based on a closed set of definitions, which represents the material under study as an open nonlinear dynamic structurally stable hierarchical (multilevel) dissipative self-organizing information system. Formalization principles, as applied to models of plastic deformation under static loading, are experimentally verified with polycrystalline fcc materials (MO copper and 08Kh18N10T austenitic steel).     

Anisotropic rheology during grain boundary diffusion creep and its relation to grain rotation,grain boundary sliding and superplasticity

The response of periodic microstructures to deformation can be analysed rigorously and this provides guidance in understanding more complex microstructures. When deforming by diffusion creep accompanied by sliding, irregular hexagons are shown to be anisotropic in their rheology. Analytic solutions are derived in which grain rotation is a key aspect of the deformation. If grain boundaries cannot support shear stress, the polycrystal viscosity is extremely anisotropic. There are two orthogonal directions of zero strength: sliding and rotation cooperate to allow strain parallel to these directions to be accomplished without any dissolution or plating. When a linear velocity/shear stress relationship is introduced for grain boundaries, the anisotropy is less extreme, but two weak directions still exist along which polycrystal strength is controlled only by the grain boundary “viscosity”. Irregular hexagons are characterised by four parameters. A particular subset of hexagons defined by two parameters, which includes regular hexagons as well as some elongate shapes, shows singular behaviour. Grain shapes that are close to that of the subset may exhibit large grain rotation rates and have no well-defined rheology unless there is a finite grain boundary viscosity. This new analysis explains why microstructures based on irregular but near equiaxed grains show high rotation rates during diffusion creep and it provides a framework for understanding strength anisotropy during diffusion creep.     

Effect of solute atoms on grain boundary sliding in magnesium alloys

The effect of solid-solution alloying on grain boundary sliding (GBS) was investigated using pure magnesium and six kinds of Mg–X (X?=?Ag, Al, Li, Pb, Y and Zn) dilute binary solid solutions with an average grain size of 10?µm. A sharp increase in damping capacity caused by GBS was observed above a certain temperature. The temperature at which a sharp increase in damping capacity occurred depended on the alloying element. The addition of Y and Ag markedly increased the onset temperature (more than 100?K) for a sharp increase in damping capacity, whereas the addition of Zn, Al and Li slightly increased the onset temperature (less than 50?K) as compared with that for pure magnesium. Tensile tests at a temperature of 423?K revealed that the higher the onset temperature, the lower the strain rate sensitivity of the flow stress. It is suggested that the former elements (Y and Ag) are more effective in suppressing GBS in magnesium alloys than the latter ones (Zn, Al and Li). The suppression of GBS was associated with low grain boundary energy, and the extent to which the energy is reduced depended on the alloying element. It was suggested that the change in the lattice parameter (the so-called c/a ratio) affects the grain boundary energy, and thus, the occurrence of GBS.     

Influence of grain boundary sliding on fracture toughness of nanocrystalline ceramics

A theoretical model is proposed describing a new physical microscopic mechanism of increased fracture toughness of nanocrystalline ceramics. According to this model, when a ceramic with a microcrack is deformed, intensive grain boundary sliding occurs near the crack tip under certain conditions. This sliding is accompanied by the formation of an array of disclination dipoles (rotational defects) producing elastic stresses. These stresses partially compensate the high local stresses concentrated near the microcrack tip and thereby hamper the microcrack growth. The proposed model is used to theoretically estimate the increase in the critical microcrack length (the length above which the catastrophic growth of microcracks occurs) caused by the formation of disclination dipoles during grain boundary sliding in nanoceramics. The increase in the critical microcrack length is a quantitative characteristic of the increased fracture toughness of nanoceramics.     

Role of helium in the sliding and mechanical properties of a vanadium grain boundary:A first-principles study

The effects of helium （He） on the sliding and mechanical properties of a vanadium （V） E5（310）/[001] grain boundary （GB） have been investigated using a first-principles method. It has been found that He was energetically favorable sitting at the GB region with a segregation energy of -0.27 eV, which was attributed to the special atomic configurations and charge density distributions of the GB. The maximal sliding energy barrier of the He-doped GB was calculated to be 1.73 J/m^2, 35% larger than that of the clean GB. This suggested that the presence of He would hinder the V GB mobility. Based on the thermodynamic criterion, the total energy calculations indicated that the embrittlement of V GB would be enhanced by He segregation.     

Effect of grain boundary sliding on the toughness of ultrafine grain structure steel: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the mechanisms of low-temperature impact toughness of the ultrafine grain structure steel. The simulation results suggest that the sliding of the {001}/{110} type and {110}/{111} type grain boundary can improve the impact toughness. Then, the mechanism of grain boundary sliding is studied and it is found that the motion of dislocations along the grain boundary is the underlying cause of the grain boundary sliding. Finally, the sliding of the grain boundary is analyzed from the standpoint of the energy. We conclude that the measures which can increase the quantity of the {001}/{110} type and {110}/{111} type grain boundary and elongate the free gliding distance of dislocations along these grain boundaries will improve the low-temperature impact toughness of the ultrafine grain structure steel.     

Accommodation mechanisms for grain boundary sliding as inferred from texture evolution during superplastic deformation

To gain insight into accommodation mechanisms for local stress concentrations produced by grain boundary sliding (GBS), we systematically examined texture evolution within a superplastic magnesium alloy undergoing deformation at a relatively low deformation temperature (at which basal slip is known to be the preferred slip system in magnesium). Although we did observe an overall weakening of the initial basal texture during superplastic deformation, we also observed within the interior of the specimen a convergent evolution that depends on loading direction. We attribute this texture evolution within the bulk to the competing effects of (a) orientation divergence due to grain rotation accompanied by GBS and (b) convergent evolution due to slip, which acts primarily as an accommodation mechanism for GBS. In contrast, at the near-surface, we found the initial orientation to be preserved, indicating that slip accommodation is less important near the surface than within the bulk.     

Wetting effects at a grain boundary

We consider a tier of weakened bonds along the center line of a two-dimensional Ising ferromagnet strip as a model of a grain boundary. When an interface traverses such a strip at an angle, whether or not there is a continuous pinning-depinning transition at subcritical temperature depends on this angle and the degree of bond weakening. We also study the relaxation of such a system to its equilibrium state using continuous time Monte Carlo simulation with Kawasaki dynamics; this reveals a matter transport mechanism confined to the grain boundary.