Mechanical Properties and Working of Metals and Alloys by Amit Bhaduri download in iPad, ePub, pdf
Some alloys can be mixed with mercury to form a plastic mass which gradually hardens by a chemical reaction followed by crystallization. All pure metals and most simple alloys crystallize easily. Larger crystals of metals can be made by very slow crystal growth, e. The temperature below which work hardening is possible is termed the recrystallization temperature. After crystallization, the grains have approximately the same dimensions in each direction, measured from the central nucleus.
Understand the structure of dislocations and grain boundaries and their role in controlling the mechanical properties of solids. For an applied tensile force the maximum degree of extension is a measure of the ductility of the metal or alloy.
The atoms within each grain are arranged in a regular three-dimensional lattice. Because the crystal grains are now smaller, the amount of grain boundary area has increased, and with it the free energy of the material. When struck many times with a hammer, the copper wire became stiffer, and it is possible to hang a weight from it.
The answer has to do with the motion of dislocations. Gold alloys and alloys containing chromium are used for making crowns, inlays and denture bases whilst dental amalgam, an alloy containing mercury, is the most widely used dental filling material.
The grains are no longer equiaxed but take up a more fibrous structure Fig. The properties of the material are altered, becoming harder and stronger with a higher value of yield stress. In contrast, glassy or amorphous metals can be prepared by very rapid cooling from the melt. Because their glassy structures do not support the movement of dislocations, they are stronger and more wear-resistant than crystalline metals of similar composition. Slow cooling causes relatively few nuclei to be formed which results in a larger grain size as shown in Fig.
Explain the mechanical properties of steel in terms of its phase behavior. Bronzes - originally made as alloys of copper and arsenic, but later as alloys of copper and tin - are harder than either of the constituent metals for the same reason. This is normally a fine grain structure which is achieved by seeding the molten material with an additive metal which forms nuclei for crystallization. The proportional limit is therefore often used as an indication of yield stress. Since a grain boundary is a planar defect, it is much less responsive to stress than a line defect.
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