Dislocations
Dislocations in materials science refer to line defects in the crystal structure of a solid. They can significantly affect the mechanical properties of materials, such as strength and ductility. Dislocations can move through the crystal lattice, leading to plastic deformation in metals and other materials. Understanding and controlling dislocations is crucial for designing and engineering materials with desired properties.
6 Key excerpts on "Dislocations"
eBook - ePub- R. E. Smallman, A.H.W. Ngan(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
...Chapter 4 Introduction to Dislocations Dislocations are an important form of crystal imperfections responsible for plastic deformation as well as material strengthening via work hardening. The occurrence of Dislocations as agencies for plastic deformation greatly reduces the critical stress needed for the deformation. This chapter discusses the configurations, stress fields and interactions of Dislocations. The ability of Dislocations to dissociate into partial Dislocations is a consequence of energy reduction for materials with low stacking fault energies, and the nature of such dissociation varies in different crystal structures. Dislocation properties in specific crystal structures including fcc, bcc, hcp and ordered lattices are discussed. Keywords Burgers vector; edge Dislocations; screw Dislocations; stress field; dislocation dissociation; Thompson tetrahedron 4.1 Concept of a dislocation All crystalline materials usually contain lines of structural discontinuities running throughout each crystal or grain. These line discontinuities are termed Dislocations and there is usually about 10 10 –10 12 m of dislocation line in a metre cube of material. This is usually expressed as the density of Dislocations ρ =10 10 –10 12 m −2. Dislocations enable materials to deform without destroying the basic crystal structure at stresses below that at which the material would break or fracture if they were not present. A crystal changes its shape during deformation by the slipping of atomic layers over one another. The theoretical shear strength of perfect crystals was first calculated by Frenkel for the simple rectangular-type lattice shown in Figure 4.1 with spacing a between the planes. The shearing force required to move a plane of atoms over the plane below will be periodic, since for displacements x < b /2, where b is the spacing of atoms in the shear direction, the lattice resists the applied stress but for x > b /2 the lattice forces assist the applied stress...
eBook - ePubAdditive and Traditionally Manufactured Components
A Comparative Analysis of Mechanical Properties
- Joshua Pelleg(Author)
- 2020(Publication Date)
- Elsevier(Publisher)
...Chapter Four Dislocations in AM and traditional manufacturing: A comparison Abstract Mechanical properties of materials are determined by defects, in particular of Dislocations. Actual existence of Dislocations has been observed by TEM, Field Ion Microscopy (FIM), and atom probe techniques. Atom probe technique permits direct observation of Dislocations on an atomic scale. Dislocations observed in Ti-6Al-4V, Al 6061, steel 304L, and alumina are discussed. Dislocation motion during deformation, observed by TEM at room temperature, is illustrated, for example, in Ti-6Al-4V. In addition to the usual glide of Dislocations, cross-slip occurs frequently. Their motion and propagation can be inhibited by pinning techniques, with consequent strengthening of the material. Low-angle grain boundaries which are an array of Dislocations are also discussed in this chapter. Keywords Motion of Dislocations; Glide; Cross-slip; Pinning of Dislocations; Low-angle grain boundaries; AM; CP 4.1 Introduction A detailed discussion on Dislocations can be found in Mechanical Properties of Materials (Pelleg, 2012) and Mechanical Properties of Ceramics (Pelleg, 2014). Briefly, mechanical properties of materials are determined by defects, particularly those known as Dislocations. The postulate for the existence of such defects by the fathers of the dislocation theory, namely, Taylor, Orowan, and Polanyi, has been confirmed by numerous research studies. The actual existence of Dislocations has been observed by TEM, Field Ion Microscopy (FIM), and atom probe techniques (permits direct observation of Dislocations on an atomic scale). An etch pit is an indirect technique to detect the presence of Dislocations in solids. Therefore, in this chapter—essential to understand plastic deformation to be discussed in the next chapter—the emphasis is on the comparison of the dislocation structures in selected alloys and ceramics...
eBook - ePubAn Introduction to Nuclear Materials
Fundamentals and Applications
- K. Linga Murty, Indrajit Charit(Authors)
- 2013(Publication Date)
- Wiley-VCH(Publisher)
...4 Dislocation Theory “Science is facts; just as houses are made of stones, so is science made of facts; but a pile of stones is not a house and a collection of facts is not necessarily science.” — Henri Poincare The dislocation concept has already been introduced in Section 2.2 dealing with crystal defects. Now we need to develop the concept further. The importance of Dislocations in plastic deformation (i.e., permanent deformation) is well documented. But the question arises as to why we should be concerned about them in a textbook on nuclear materials. We will see in a later chapter how dislocation loops can form from the primary radiation damage; the dislocation loops can either stay as loops or join the overall dislocation networks in the irradiated materials. Indeed, Dislocations are the major microscopic defects that are created during irradiation. Hence, this chapter serves as a prelude to understanding these different aspects of Dislocations and their significance. 4.1 Deformation by Slip in Single Crystals We have already gained some basic idea about slip from the previous chapters. Slip is nothing but the movement of one crystal part over another causing plastic deformation. Slip occurs only when the shear stress on the slip plane along the slip direction attains a critical value (known as critical resolved shear stress (CRSS)). Generally, the slip planes are the crystallographic planes with the highest atomic density (closest-packed planes (CPPs)) in that particular crystal structure, and the slip directions are the closest--packed directions (CPDs) in the respective crystal structures. 1 A combination of slip plane and slip direction is called a slip system. Due to the slip, steps are formed on the prepolished surface of a material that has been plastically deformed. Due to the height variations in the different slip steps, they are observable on the sample surface as lines, and hence known as slip lines...
eBook - ePubCrystal Plasticity Finite Element Methods
in Materials Science and Engineering
- Franz Roters, Philip Eisenlohr, Thomas R. Bieler, Dierk Raabe(Authors)
- 2011(Publication Date)
- Wiley-VCH(Publisher)
...Part One Fundamentals 2 Metallurgical Fundamentals of Plastic Deformation 2.1 Introduction One of the most essential aspects of microstructures is that although their evolution direction is prescribed by thermodynamic potentials and their gradients, the selection of the actual evolution path is strongly determined by kinetics. This means that microstructures form on thermodynamic transients and as a rule not in full thermodynamic equilibrium. It is this strong influence of thermodynamic nonequilibrium mechanisms that entails the large variety and complexity of microstructures typically encountered in engineering materials. Frequently, microstructures that correspond to a highly nonequilibrium state provide particularly advantageous material property profiles (see Figure 2.1). Figure 2.1 Some of the important scales and lattice defects in metallurgical engineering. This book is concerned with those microstructural defects that contribute to the elastic-plastic deformation of metals. Plastic deformation at ambient temperature occurs in crystalline metals mainly through Dislocations, martensite formation, and mechanical twinning. In this context Dislocations are the most important lattice defects and they are usually the main carriers of plastic deformation (see Figure 2.2). Figure 2.2 Edge dislocation and its effect on crystallographic shear. Mechanical twinning and martensite formation are referred to as displacive deformation mechanisms and they typically contribute less to the plastic shape changes than do Dislocations but have a large impact on hardening and flow stress. 2.2 Lattice Dislocations Dislocations are linear crystallographic defects within the otherwise regular crystal structure. They are geometrically described by the line tangential vector and the Burgers vector (shear vector)...
- Shashanka Rajendrachari, Orhan Uzun(Authors)
- 2021(Publication Date)
- Bentham Science Publishers(Publisher)
...Fig. (10) represents the stacking fault defect. Fig. (10)) Stacking fault defect. 6. Volume Defects Volume defects occur on a larger scale than the other types of crystal defects. However, for the sake of completeness and since they do affect the movement of Dislocations, a few of the more common bulk defects are mentioned. Voids are regions where a large number of atoms missing from the lattice. Generally, voids occur due to the trapping of air bubbles during the solidification of materials, and it is commonly called porosity. When a void occurs due to the shrinkage of material during solidification is called cavitation [ 12 ]. The bulk defect can also occur when impurity atoms clusters together to form a small region of a different phase called precipitates; which are physically homogeneous [ 12 ]. 7. STRENGTHENING OF MATERIALS The ability of a metal or an alloy to plastically deform depends on the movement of Dislocations. Strength is a measure of how easily a metal can undergo plastic deformation. Therefore, if we restrict the movement of dislocation; we can increase the strength. On the other hand, if the dislocation motion is easy, then the strength of a material decreases; materials will become soft and undergo deformation very easily. There are different ways one can use to improve the strength of the materials. They are strain hardening, grain boundary strengthening, solid solution strengthening, precipitation hardening, dispersion strengthening. 7.1. Strain Hardening Strain hardening is also called work hardening and it is defined as a phenomenon where ductile metals become stronger and harder when they undergo plastic deformation at a temperature well below its melting point. The rate of strain hardening decreases with increasing temperature. Therefore, the materials are strain hardened at low temperatures called cold working. Strain hardening takes place when the dislocation density increases with plastic deformation (cold work) due to multiplication...
eBook - ePub- W. Bolton, R.A. Higgins(Authors)
- 2014(Publication Date)
- Routledge(Publisher)
...It is therefore possible to calculate the theoretical strength of a metal, i.e. the force needed to overcome the sum total of all the net forces of attraction of the metallic bond across a given plane in a metal. Under controlled laboratory conditions, a single crystal of a metal can be grown and if this is subject to a tensile test its true strength is found to be only about one-thousandth of the theoretical when the latter is calculated as outlined earlier. It is now realised that instead of slip taking place simultaneously by one block of atoms sliding wholesale over another block across a complete crystal plane, it occurs step-by-step by the movement of faults of discontinuities within the crystallographic planes. These faults, where in effect half planes of atoms are missing within a crystal (Figure 6.3), are known as Dislocations. When the crystal is stressed to its yield point, these Dislocations will move step-by-step along the crystallographic plane until they are halted or 'pegged' by some obstruction such as the atom of some alloy metal which is larger (or smaller) than those of the parent metal. Alternatively, the dislocation will move along until it is stopped by a crystal boundary of another dislocation moving across its path. Figure 6.3 An 'edge' dislocation. This is the most simple form of dislocation and can actually be seen in some materials when they are examined under very high magnifications in electron microscopy. Figure 6.4 Movement of an edge dislocation under the action of stress S. Dislocations are denoted by If the surface of the metal cuts through the slip plane a minute 'step' will be formed there (Figure 6.4)...
