China
Africa
Explore chapters and articles related to this topic
Crystal Structure
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
A plane defect is a disruption of the long-range stacking sequence of atomic planes. Plane defects are generally manifested as stacking faults or twin regions. This type of error is only applicable for structures that can be close packed, for example, HCP and FCC structures. BCC structures have no close packed planes and therefore do not have a stacking sequence and stacking faults. A stacking fault is a change in the stacking sequence over a few atomic spacings. As such, it is a simple two-dimensional defect. The normal stacking sequence in an HCP crystal is AB AB AB, etc., whilst in an FCC crystal it is ABC ABC ABC, etc. Here, A B and C represent the three distinct crystallographic planes that allow the crystal to be most efficiently packed or stacked together, whilst replicating and preserving the basic unit cell structure. This is illustrated in Fig. 3.18 for both HCP and FCC structures. Whilst the closed packed structure for the hexagonal lattice visually resembles that of its Bravais lattice (i.e., the sequence BAB), that of the FCC is not so obvious and is in fact formed of hexagonal planes along the diagonal of the corresponding Bravais lattice. For HCP crystals, the normal stacking sequence is AB AB AB … etc. If the structure now switches to AB AB AB C AB AB, there is a stacking fault present at C. For FCC crystals, two main kinds of stacking fault can occur. The first type is where one plane is missing, that is, the stacking sequence is ABC A_C ABC …. and is known as an intrinsic stacking fault (the fault is indicted by _). Near the fault, the packing sequence is similar to an HCP lattice. Alternately, if an additional plane is present (i.e., ABC ABAC ABC), the defect is called an extrinsic stacking fault. An extended stacking fault in which the order of stacking is reversed is called a twin lamella (e.g., ABC ABC BAC BABC ABC).
Defects and Nonstoichiometry
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
A stacking fault occurs in a crystal when there are either one or two extra planes or missing planes of atoms. This can occur quite easily in close-packed structures. Think of a ccp structure ABCABCABC…; if one of the planes is missing, an intrinsic fault, we get a faulty stacking sequence such as ABCACABCABC…. Similarly, in a ccp structure if an extra layer interposes itself, we could get the sequence ABCABACABCABCA…, now called an extrinsic fault.
TEM observations of silicon deformed under an hydrostatic pressure
Published in A G Cullis, P D Augustus, Microscopy of Semiconducting Materials, 1987, 2021
J L Demenet, J Rabier, H Garem
The deformation mechanism at T = 425°C, ∈˙=2×10−6 s−1 and P = 1500 MPa is thus controlled by perfect dislocations whatever the strain is, provided the compression axis is <213>. In contrast, with a <100> compression axis, and for the same stress level, twins are the dominant microscopic features observed at the lower yield point (Demenet to be published). For a <213> compression axis, the larger force is acting on the trailing partial compared with that acting on the leading one. In order for the intrinsic stacking fault to widen, the trailing partial should be pinned by obstacle or have a lower mobility than the leading one. If the trailing partial is pinned by obstacle, then the leading partial needs a stress to overcome the stacking fault τc ≈ 370 MPa (see Alexander et al 1980). In the case of a <100> compression axis the force on the leading partial is the larger one and when the trailing partial is pinned, τc ≈ 230 MPa which is smaller than τc (213). But it is not necessary that the trailing partial must be pinned: if one assumes that the two partials have the same mobility, a stress τc ≈ 900 MPa is required to separate the two partials. This explains why the formation of extended intrinsic stacking fault is easier for the <100> than <213> compression axis. These extended intrinsic stacking faults could be nuclei for twin formation. However, the occurrence of several slip planes leading to specific dislocation junctions can be an alternative mechanism to promote twin nucleation (Vergnol and Grilhé 1984).
Investigation of the adhesive contact between a diamond indenter and single-Crystal copper substrate at low temperatures
Published in The Journal of Adhesion, 2022
Qiyin Lin, Yuhan Zhang, Ting Yue, Shaoke Wan, Jun Hong
To analyse the causes of fluctuations of the mechanical properties of single-crystal copper substrates at different temperatures, dislocation extraction technology[21] (DXA) is used to extract the distribution of dislocations within the substrate at the maximum depth at different temperatures. We can clearly see the location, type, quantity and length of the dislocations, as shown in Figure 7. The grey atoms in the figure represent defects, including surface atoms, dislocation atoms, and vacancies, the red atoms represent atoms in the HCP structure, and the blue atoms represent atoms in the BCC structure. According to crystallography principles, a stacking fault in an FCC structure is equivalent to the formation of a thin layer of an HCP crystal structure. It can be seen in Figure 7 that at different temperatures, there are many stacking faults inside the substrate, which are distributed along the slip system of the FCC crystal. The boundary between a stacking fault and a complete FCC crystal is a Shockley dislocation, which slips along the slip plane, causing the stacking fault to expand or shrink. From the Thompson tetrahedron, it can be seen that the main types of dislocations in an FCC crystal are perfect dislocations, Shockley dislocations, Frank dislocations, and stair-rod dislocations. Table 4 shows the number of main types of dislocations at a maximum depth under different temperatures.
Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals
Published in Philosophical Magazine, 2019
Lin Yuan, Peng Jing, Rajiv Shivpuri, Chuanlong Xu, Zhenhai Xu, Debin Shan, Bin Guo
In equiaxed grains, dislocations annihilate at grain boundaries surrounding the grain interiors homogeneously, as shown in Figure 7. In contrast, in layered grains, dislocations annihilate almost at the grain boundaries perpendicular to the normal of the grains, as seen from Figure 11(b), and seldom dislocations annihilate at the grain boundaries at both sides, i.e. the grain boundaries parallel to the normal of the grains. As to the effect of stacking fault energy on dislocation interaction with grain boundary of layer-grained models, as shown in Figure 13, the higher the stacking fault energy is, the more dislocations will annihilate at grain boundaries. Although, some dislocation will be obstructed by the stacking faults left by dislocation slip. Stacking fault energy affects the dislocation interaction with grain boundaries by affecting the stacking faults and dislocation contents. Grain morphology and stacking fault energy have an effect on the dislocation interaction with grain boundary, which have reference significance on nanocrystalline synthesis and processing.
The effect of phason defects on the radiation-induced swelling of quasicrystalline materials
Published in Radiation Effects and Defects in Solids, 2018
Galina N. Lavrova, Anatoliy A. Turkin, Alexander S. Bakai
Quasicrystals are some of the most intriguing materials discovered 30 years ago. Unlike crystals with periodic structures, quasicrystals lack translational symmetry and possess non-crystallographic long-range rotational invariance. The exact structure of quasicrystals is still a subject of debates despite extensive research in this area. Similar to crystalline materials, real quasicrystals have defects such as PD (vacancies and interstitials), dislocations, grain boundaries, etc. There are also defects specific for quasicrystals which are called phasons (1). They were described in the density-wave picture as additional hydrodynamic (long-wavelength) modes besides phonon modes present in conventional crystals (2). Phasons appear in a variety of physical forms related to different atomic displacements, such as phason mode, phason-shift, phason-strain, phason-hop, phason-flips, phason-fluctuations, etc. (3). Such localized atomic rearrangements can be represented as linear superpositions of phason modes of different wavevectors, similar to the representation of an atomic vibration as a superposition of phonons. Mompiou (4) and Feuerbacher (5) noted that the formation and movement of a dislocation in a quasicrystal is accompanied by a so-called phason fault, i.e. a planar agglomeration of individual phasons, chemical and structural PD. Likewise, the dislocation moving in a conventional crystal induces the formation of a stacking fault. We will be following this notion of a phason to describe the kinetics of defects in a quasicrystal.