Atomic-Scale Grain Boundary Relaxation Modes in Metals and Ceramics

Abstract

The atomic-scale structure of tilt grain boundaries has been studied by high-resolution electron microscopy (HREM) in several ceramics and metals such as NiO, yttria stabilized zirconia, Au, and Al. It is found that when grain boundaries are formed between two crystals a considerable variety of modes of relaxations, i.e., the rearrangements of atoms in and near the grain boundary core are possible, depending on grain boundary geometry and the interatomic interactions. The atomic relaxations within and near the grain boundary core often include relaxations that involve the formation of stacking faults in low-stacking-fault energy fcc materials. Grain boundary dissociations are also observed for several grain boundary geometries. Computer simulations of grain boundary structures in metals have in general been able to reproduce the relaxation features at least on a qualitative level. In contrast, ceramic grain boundaries typically are more complex and not always tractable by simple computer relaxation procedures, since the formation of point defects and partial occupancy of atomic columns may be involved in the relaxation processes. It is found that when a boundary is not allowed to relax to its lowest energy state due to geometrical constraints, a multitude of grain boundary core structures can exist. Such conditions are expected to be characteristic for nanocrystalline materials. Because of its close connection to grain boundary properties, quantification of the volume expansion by HREM techniques is an important goal of grain boundary research. Image simulations of grain boundaries suggest that for most materials of interest the 3-fold astigmatism must be corrected to better than 100 nm to achieve the desired accuracies.

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

“…From the calculation results given in Fig. 9 it is clear that the GB energy is a nearly linear function of the excessive volume, which qualitatively agrees with the data reported by other workers [33]. The calculated proportionality factor is equal to 1600 (mJ/m 2 )/Å, with the uncertainty being 12%.…”

“…There is a well-known example of such behavior of the GB energy, i.e., the excessive free energy of a crystal associated with GBs and calculated per unit area of GB. As a result of numerous theoretical investigations of GBs in fcc-and bcc-metals it was shown that the energy of GBs is a nearly linear function of the excessive volume [33]. Building such relationships allows one to interpolate the simulation results for the case of SMC and NC materials using the experimentally obtained excessive volumes δV in these materials.…”

High-resolution Electron Microscopy of grain Boundary Structures in Yttria-stabilized Cubic Zirconia

“…This is quite surprising and atypical for high-angle grain boundaries in oxides. In metals a similar behavior is found for the connection between (111) planes across the asymmetric (557)(113), [110] tilt GB in Au [3]. In this case the GB dissociates by the incorporation of periodically spaced short lengths of stacking faults.…”

Rapid heating induced ultrahigh stability of nanograined copper

“…Previous experimental and simulation investigations in several fcc metals indicated that generation of twins or stacking faults from GBs, both evolving emission of partial dislocations, may induce GB dissociation (27,28). It provides a general approach to GB relaxation for lowering the energy state, especially in those metals with low stacking fault energy such as Cu.…”