Cold-worked crystals

The density of dislocations is usually stated in terms of the number of dislocation lines intersecting unit area in the crystal it ranges from 10 cm for good crystals to 10 cm" in cold-worked metals. Thus, dislocations are separated by 10 -10 A, or every crystal grain larger than about 100 A will have dislocations on its surface one surface atom in a thousand is apt to be near a dislocation. By elastic theory, the increased potential energy of the lattice near... 

At present the iron-based alloys diffusion saturation by nitrogen is widely used in industry for the increase of strength, hardness, corrosion resistance of metal production. Inexhaustible and unrealized potentialities of nitriding are opened when applying it in combination with cold working [1-3], It is connected with one of important factors, which affects diffusion processes and phase formation and determines surface layer structure, mechanical and corrosion properties, like crystal defects and stresses [4, 5], The topical question in this direction is clarification of mechanisms of interstitial atoms diffusion and phase formation in cold worked iron and iron-based alloys under nitriding. 

The effect of dislocations has also been studied by Bloembergen and Rowland 106) in cold-worked copper (Cu and Cu resonances), and also the effect of alloying in Al-Zn alloys (Al resonance) by Rowland 107). Otsuka and Kawamura 108) have studied the NMR of 1" in KI, Na in NaCl-NaBr mixed crystals, and Br in KBr-NaBr mixed crystals and have estimated dislocation densities in these materials. 

ANNEALING. The process of holding a solid material at an elevated temperature for a specified length of time in order that any metastable condition, such as frozen-in stains, dislocations, and vacancies may go into thermodynamic equilibrium. This may result in re-crystallization and polygonization of cold-worked materials. 

All metals are crystalline and nit alteration in the sice nr shape of the grains or crystals takes place in service during or before faligue failure. (Exceptions might be made in Ihe case of lead and other alloys which rccrystallizc when cold worked at room temperature.)... 

As a force is applied to the item through the die, the metal first becomes elastically strained and would return to its initial shape if the force were removed at this point. As the force increases, the metal s elastic limit is exceeded and plastic flow occurs via the motion of dislocations. Many of these dislocations become entangled and trapped within the plastically deformed material thus, plastic deformation produces crystals which are less perfect and contain internal stresses. These crystals are designated as cold-worked and have physical properties which differ from those of the undeformed metal. 

Rhodium was named by Wollaston after its rose colored salts. The hard, silvery, lustrous metal has a speciflc gravity of 12.41 and crystallizes in the face-centered cubic system (a = 3.803 A). Modem work implies that there is no change in stmcture up to the melting point of 1960-1970 °C. Despite this stmcture, it is difficult to cold work and the rate of work... 

On heating, constituents of the crystal within the strained zone of a line dislocation can be displaced relatively readily and the dislocations can thus migrate to surfaces where they are eliminated, or those with opposing Burgers vectors may cancel each other. Such reduction in dislocation concentration is termed annealing. Cold working (distortion or strain imposition) of a crystal increases its dislocation density. Impurities may retard the movement of dislocations through the crystal (e.g. carbon in steel). 

In order to express the effect of this type of defect on the width of the peaks, we will assume that each crystal is an imperfect crystal lattice, which can be described as a three-dimensional repetition of identical cells that have undergone displacements with respect to their original position (see Figure 5.10). These cell displacements are one way of representing the atomic displacements occurring in the crystals in question. These local strains (microstrains) can be significant, even though the mean values of the cell parameters remain the same as those in crystals free of any defects. The typical example of this situation is provided by cold worked metal crystals, but it is also observed in other situations, often in nanocrystals for example, where it corresponds in particular to the fact that the interplanar distances vary between the core and the surface of these crystals. 

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