Defects linear

Defects intervening in pieces are listed by official norms. For segmentation needs, we have divided the set of defects in two categories, volumetric and linear defects. A defect is considered as linear if its width is twice inferior to the size of the grain, all the rest are considered as volumetric defects. 

The geometrical measurements previously extracted help the making decision system to decide for example whether the defect is linear or not. This defect discrimination into two categories is considered as a first attempt for defect classification. To this end, we define a linearity ratio (Ri) Rl =Length / width. If Rl is equal or near to "1", the defect is volumic, otherwise it is a linear defect. 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

The introduction to this chapter mentions that crystals often contain extended defects as well as point defects. The simplest linear defect is a dislocation where there is a fault in the arrangement of the atoms in a line through the crystal lattice. There are many different types of planar defects, most of which we are not able to discuss here either for reasons of space or of complexity, such as grain boundaries, which are of more relevance to materials scientists, and chemical twinning, which can contain unit cells mirrored about the twin plane through the crystal. However,... 

Dislocations are linear defects and were first invoked to account for the mechanical properties of solids, particularly the shear strengths. Dislocations play an important role in a variety of solid state phenomena from phase transitions to chemical reactions and the subject has been investigated and reviewed widely (Fine, 1973 Nembach, 1974). The effect of dislocations on the transformations and properties of organic solids has been recognized in recent years (Thomas Williams, 1971 Jones Thomas, 1979). 

The second part of Eqn. (1.23) is obtained from Eqn. (1.22). From the requirement of eleetroneutrality and the definition of a (linearized) defect recombination zone of width R, Eqns. (1.22) and (1.23) yield... 

Slip from the motion of dislocations (linear defects in the crystal structure)... 

In nearly all metal-forming operations, slip is the dominant method of deformation, although twinning can be significant in some materials. Slip occurs when the shear stress is high enough to cause layers of atoms to move relative to one another. The critical resolved shear stress is lowered when the crystalline lattice is not perfect but contains linear defects called dislocations. Slip-induced plasticity was covered in Chapter 9 of the companion to this text (Lalena and Cleary, 2005) and is reviewed here only briefly. The interested reader is advised to consult Lalena and Cleary (2005), Honeycombe (1984), or Dieter (1976). 

The concept of a defect has undergone considerable evolution over the course of the last century. The simplest notion of a defect is a mistake at normal atom site in a solid. These stmcturally simple defects are called point defects. Not long after the recognition of point defects, the concept of linear defects, dislocations, was invoked to explain the mechanical properties of metals. In later years, it became apparent that planar defects, including surfaces, and volume defects such as rods, tubes, or precipitates, also have important roles to play in influencing the physical and chemical properties of the host matrix. More recently, it has become apparent that interactions between point defects are of considerable importance, and the simple model of isolated point defects is often inadequate with... 

During the last decade, numerous reports have appeared of linear defects in GBs in metals observed by TEM. The observed variation in geometry as well as the variation of contrast for various diffracting conditions indicate that a single type of defect cannot account for all the observations. Among the numerous interpretations that have been suggested for the linear defects are (i) absorbed lattice dislocations (ii) steps, at least a few unit cells high, in the boundary plane (iii) structural dislocations in the boundary that accommodate small deviations from the special orien-... 

Figure 2.3(a) Translation by half a unit step in a square lattice creates a linear defect in a two dimensional lattice between the open and filled circles. A defect of this kind may continue to grow sideways, leading to a whole family of new structures. 

Figure 2.3(b) The transformation from a simple square lattice to a centred square lattice through the propagation of a linear defect. 

A possible way to overcome the hurdle of intrinsic defects could be synthesizing strictly linear, defect-free polymers with the use of modern, precisely controlled polymerization techniques and utilizing them as starting materials for dehydrohalogenation. 

Generally speaking, microstrains correspond to atom displacements with respect to their position in crystals which are free of any defects. The presence of dislocations causes this type of displacement, but in this specific case, which we detailed previously, atomic displacements are local, and related to the presence of a linear defect We will now discuss the general case of displacements that are completely independent of one another. 

From tilting experiments it can be deduced that the observed streaks are due to reciprocal (100) and (010) planes of diffuse intensity, intersecting the Ewald sphere, which is supporting the assumption that the scattering is due to linear defects. (See also [7.4, 7.27, 7.28].) This state of order is referred to in... 

The nature of plasticity is rupture and rearrangements of interatomic bonds which in crystalline objects involve peculiar mobile linear defects, referred to as dislocations. Temperature dependence of plasticity may significantly differ from that of Newtonian fluids. Under certain conditions (including the thermal ones) various molecular and ionic crystals, such as NaCl, AgCl, naphthalene, etc., reveal a behavior close to the plastic one. The values of x typically fall into the range between 10s and 109 N m 2. At the same time, plastic behavior is typical for various disperse structures, namely powders and pastes, including dry snow and sand. In this case the mechanism of plastic flow is a combination of acts involving the establishment and rupture of contacts between dispersed particles. Plastic object, in contrast to a liquid, maintains the acquired shape after removal of the stress. It is worth... 

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