The resistance which a crystal lattice offers to a gliding dislocation in the absence of other imperfections and impurities depends on the width of the dislocation. In a positive edge dislocation, the atoms above and below the slip plane, in the vicinity of the dislocation lines are displaced from their equilibrium positions of the perfect lattice. The extent of the [...]
in this article we will discuss about:- 1. Geometry of the Edge Dislocations in Crystals 2. Motion of an Edge Dislocation of Crystals 3. Screw Dislocation in Crystals. Geometry of the Edge Dislocations in Crystals: Suppose, we have a simple cubic perfect crystal (we are taking this crystal structure for better clarity, though Polonium is the only metal having this [...]
Various types of point defects are: 1. Vacancy 2. Interstitialcy 3. Frenkel Defect 4. Impurity Atoms. A point defect is a very localised disruption in the regularity of a lattice. It is a defect of dimensions just like a point (zero dimensions). The size of the defect could be one atom, or two atomic diameters, which is just like a [...]
Vacancies have been seen to be thermodynamically stable in crystalline solids above 0°K, and that their number increase exponentially with the increase of temperature, as illustrated in Fig. 4.8 for aluminium. To understand this, the usual concept of Gibbs free energy for metal-system is used. In a perfect crystal, all lattice sites are occupied by atoms so that no vacancies [...]
The following points highlight the three main surface defects in crystals. The types are: 1. Free Surfaces 2. Grain Boundaries 3. Stacking Faults. Surface defects are defects of two dimensional nature in the mathematical sense (that is, they have two dimensions). These are areas of distortions that lie about a surface, having a thickness of a few atomic diameters (that [...]
In this article we will discus about:- 1. Introduction to Polycrystal 2. Dislocation Motion in Polycrystals 3. Ductility. Introduction to Polycrystal: The study of the basic mechanisms of the plastic deformations is facilitated by the study of plastic behaviour of single crystals. Most of the engineering materials are, however, polycrystalline, which contain a large number of grains with varying orientations. [...]
When a single crystal is continued to be deformed in the plastic range beyond the yield point, the shear stress required to produce further deformation continues to increase. The increase in the stress required to continue the deformation because of the previous plastic deformation is called as strain-hardening, or work-hardening. As the dislocation density also increases simultaneously with strain (dislocation [...]
The perfectly regular crystal structures that have been considered upto now are called ideal crystals in which atoms are arranged in a regular way. In actual crystals, however, imperfections or defects are always present and their nature and effects are very important in understanding the properties of crystals. These imperfections affect the properties of crystals such as mechanical strength, chemical [...]
In this article we will discuss about:- 1. Concept of Miller Indices 2. Important Features of Miller Indices 3. Spacing of Planes 4. Relation between Interplanar Spacing 'd' and Cube Edge ‘a’. Concept of Miller Indices: Miller indices is a system of notation of planes within a crystal of space lattice. They are based on the intercepts of plane with [...]