Structure Of Low-angle Boundaries Research Articles

Thermally stable and corrosion resistant nanolaminated Al-Mn alloy with low angle boundary structures

Metals with nanometer-sized grains are strong but often suffer from poor thermal stability. Herein, nanolaminated (NL) structure with an average thickness of 110 nm and a low angle grain boundaries (LAGBs) fraction of 80% was obtained in an Al-1Mn (in wt%) alloy produced by high strain rate deformation at cryogenic temperature. The NL structures are thermally stable up to 523 K, which can effectively suppress GB precipitation during aging, thus improving the pitting corrosion resistance in 0.6 M NaCl solution. The high thermal stability of NL structures can be mainly attributed to the formation of a large fraction (80%) of LAGBs.

Low-angle boundaries in ZnGeP2single crystals

The structure of low-angle boundaries in ZnGeP2crystals grown by the vertical Bridgman technique was studied using Borrmann X-ray topography. The slip systems of the dislocations in the boundaries were identified by studying the contrast rosettes generated by the Borrmann effect, in the region near the dislocation core. It was shown that the boundaries are of two types: type I consists of edge dislocations of the {1\overline{1}0}〈110〉 slip system, and type II of edge and mixed dislocations of the {010}〈100〉 slip system. The boundaries of both types, consisting of pure edge dislocations with lines along [001], are symmetrical tilt boundaries with [001] rotation axes. The misorientations generated by the boundaries were estimated to range between 2–20 and 1–40′′, respectively. Low-angle boundaries are thought to be formed by polygonization of dislocations, caused by thermoelastic stresses.

Microstructural evolution and grain subdivision mechanisms during severe plastic deformation of aluminum particles by ball milling

Electron backscattered diffraction technique was used to investigate the microstructure of aluminum particles deformed by high-energy ball milling. The lengths of different types of boundaries per area were calculated for different samples. The results show that the deformation mechanism and the rate of grain subdivision changed considerably as milling time increased. At the beginning of the milling, deformation banding subdivided grains and dynamic recovery formed a cellular structure of low angle boundaries. After further milling, particles were flattened; an increase in the aspect ratio of the original grains together with cold welding of the particles contributed to the formation of high angle grain boundaries (HAGBs). Lattice rotation progressively increased the misorientation of low and medium angle boundaries and transformed them to HAGBs, which resulted in formation of new small equiaxed grains by continuous dynamic recrystallization. This research shows subgrain rotation was the main mechanism for formation of new HAGBs.

High-resolution z-contrast imaging of YBa2Cu3O7-δ grain boundaries

A rapid reduction of the critical current density across grain boundaries with increasing tilt angle up to ∽10° has been observed in YBa2Cu3O7−δ superconductors. These results fit nicely with an investigation of the structure of low-angle tilt boundaries in these materials. The boundaries are observed to be composed of an asymmetric array of dislocations with a Burgers vector of 1.17 nm for [100] tilt boundaries (Fig. 1) and 0.389 nm for [001] tilt boundaries. It is observed that at 7.5° tilts, the dislocation cores begin to overlap (Fig. 2). Increasing the tilt angle beyond this value is not expected to significantly change the boundary structure which corresponds closely to the point where tilt angle had little further effect on critical current. A complication which was not completely addressed in the study of the structure of these low-angle boundaries is that ceramic materials have a strong tendency to form thin intergranular glass phases and to exhibit enhanced segregation of impurities to the grain boundaries. Conventional transmission electron microscopy and x-ray microanalysis with emitted photons using an energy dispersive spectrometer is of questionable use for this application. A new method for forming high resolution images with strong chemical sensitivity using large-angle elastically scattered electrons in a scanning transmission electron microscope has been used in this study to directly address the grain boundary segregation question.

Structure Determination of Tilt Boundaries in Gold by High Resolution Electron Microscopy

ABSTRACTA number of tilt grain boundaries prepared from evaporated gold thin films have been investigated by high resolution transmission microscopy. When the grain boundary is parallel to the electron beam and the beam is parallel to a low index rotation axis such as [110] or [001]it is possible to identify atomic positions at the cores of these boundaries as demonstrated here by a Σ = 3 70.5°/[110], (112) growth twin. In many cases it is not possible to make a full atomistic structure determination because the specimen does not have translational periodicity in the beam direction. This may be because the boundary plane is not parallel to the beam or the specimen contains dislocations which have a component of Burgers vector parallel to the beam. Examples are given of various low angle boundary structures in goldwhere there are complications because of the three dimensional nature of their structure.

Grain boundary-dislocation interactions

Our current understanding of the structure of grain boundaries will be described first. The structure of low angle boundaries can be rigorously described in terms of arrays of dislocations. The structure of high angle boundaries continues to defy a complete and rigorous description. A model has been developed based on coincidence site lattices. This model postulates the presence of grain boundary dislocations even at high angles of misorientation to accommodate the deviation from exact coincidence conditions. The Burgers vectors of such grain boundary dislocations can be found by the translation vectors of thedsc lattice. An interesting point is that the Burgers vectors are not lattice translations. Hence the dislocations are confined to the surface of the boundary and cannot move into the grain. Alternative descriptions of the structure of grain boundaries make appeal to the Bernal type of polyhedral voids that occur in metallic glasses. A brief discussion of the strength of this approach will be outlined. Dislocations at grain boundaries can affect both grain boundary migration and sliding. The possible mechanisms for these phenomena will be described. The importance of understanding these mechanisms to explain deformation of metals at high temperatures will be stressed.