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Mechanical Behavior of Materials
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
Dislocation mobility is predominantly responsible for the plasticity and ductility of materials, whereas dislocation accumulation and dislocation pileup near grain boundary can contribute toward the strength of materials. Various forms of dislocations involved in the deformation process are edge dislocation, screw dislocation, and mixed dislocation. Dislocation is a complex phenomenon occurring at the atomic level, and dislocation at non-parallel planes leads to a sharp break. The jogs and kinks are breaks of a dislocation line in which jogs lie out of the slip plane and kinks lie in the slip plane. The minimum shear stress needed to move a dislocation through a crystal lattice is known as Peierls stress or Peierls-Nabarro stress. () Peierls-Nabarro stress(τPN)=Ge−(2πW/b)
On Deformability of SiC
Published in C. J. Humphreys, Understanding Materials, 2020
It should be noted that, in reality, the magnitude of Peierls stress depends on the bonding configuration within the core of a dislocation. Thus, a proper calculation of the Peierls stress requires detailed quantum mechanical considerations of the energy changes involved in the (collective) breakage and re-formation of bonds within the dislocation core as it moves, i.e. calculation of the energy and hence the stress required to move the dislocation from one Peierls valley to the next at 0 K. The energy changes in such a process are related to the nature of bonding in the crystal under consideration and enter the Peierls-Nabarro equation through the parameter G, the shear modulus of the crystal. The shear modulus is of course directly determined by the nature of bonding in the material, i.e. whether the material is metallic, ionic, or covalent.
Mechanical Properties of Thermoelectric Materials
Published in Zhifeng Ren, Yucheng Lan, Qinyong Zhang, Advanced Thermoelectrics, 2017
Sonika Gahlawat, Kenneth White, Zhifeng Ren, Yasuo Kogo, Tsutomu Iida
At a microscopic level, plastic deformation and hardness can be explained in terms of dislocation movement. Externally applied loads cause dislocations to move; dislocation movement signifies plastic deformation. The easier the dislocation motion, the lower the yield stress and, hence, the hardness. Indentation causes plastic deformation within a limited volume, termed as plastic zone, underneath the indenter. According to strain gradient plasticity theory,72–74 the applied loads nucleate two kinds of dislocations within the plastic zone: statistically stored dislocations and geometrically necessary dislocations (GNDs). While the former result from shear and depend on the average strain in indentation, the latter arise from strain gradients due to bending.75 In other words, the GNDs depend on strain gradients and address the strain compatibility issues. According to the Taylor model of plasticity,76,77 the nucleated dislocations may interact with one another and form a locked network, known as the Taylor network. A finite externally applied stress, of the order of the Peierls stress,78 can then force these dislocations to move, thereby initiating plastic flow. Metals, especially those with face-centered cubic crystal structure, have low Peierls stresses due to the nondirectional or delocalized bonding, which results in wide dislocations.79–81 This allows for the movement of dislocations and, hence, plastic flow, in metals. In ceramics, however, strong directional bonding exists that causes the dislocations to be narrower and the Peierls stress to be higher than in metals. The high Peierls stress limits dislocation mobility in ceramics, thereby causing them to prefer brittle fracture. Restricted dislocation motion in ceramics results in higher hardness compared with that in metals.
Modelling the rate and temperature-dependent behaviour and texture evolution of the Mg AZ31B alloy TRC sheets
Published in Philosophical Magazine, 2018
G. Ayoub, A. K. Rodrigez, M. Shehadeh, G. Kridli, J. P. Young, H. Zbib
HCP metals exhibit strong plastic anisotropy, leading to large variations in the critical resolved shear stress (CRSS) needed to activate the available slip systems via dislocation glide or twinning. This anisotropic behaviour is mainly attributed to the dislocations’ core structure resulting in low, intermediate and large values of Peierls stress on the basal, prismatic and pyramidal planes, respectively. Furthermore, the CRSS required to activate dislocation slip is found to be sensitive to temperature [3, 4, 7, 10, 41, 44]. Moreover, it is well established that in single crystalline metals, the high-strain-rate or low-temperature deformation is accommodated not only by dislocation slip, but also by deformation twinning [4, 49–55]. However, the CRSS of twin systems is reported to be insensitive to temperature and strain rate [45]. In polycrystalline materials such as Magnesium alloys, additional plasticity mechanisms can be active such as grain boundary sliding (inter-granular deformation mechanisms active at high temperature) [3] and grain fragmentation. The relative activity of inter-granular and intra-granular deformation mechanisms depends on the loading conditions, temperature, texture and grain size. While magnesium alloys at relatively low temperatures exhibit limited plasticity and ductility, which is mainly induced by the activation of crystallographic slip and deformation twinning, the improved ductility of magnesium at elevated temperature may be attributed to the presence of inter-granular deformation mechanisms such as grain boundary sliding (GBS). GBS is one of the most important high-temperature deformation mechanisms when the deformation temperatures exceed one-third of the absolute melting temperature [3, 42, 46]. Furthermore, dynamic recrystallization (DRX) is another important mechanism accommodated by the plastic deformation of Mg alloys. DRX leads to the formation of new grain structures in a deformed material through the development and migration of high-angle grain boundaries. DRX could be associated with twinning, GBS, grain fragmentation and rotation. The literature and data available on the DRX of superplastic Mg alloy sheets are limited [16, 47–49].