COURSE: The Structure of Metals Grain and Grain Boundaries
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Grains and Grain Boundaries When a mass of molten metal begins to solidify, crystals begin to form independently of each other at various locations within the liquid mass; they have random and unrelated orientations. Each of these crystals then grows into a crystalline structure, or grain. Each grain consists of either a single crystal (for pure metals) or a polycrystalline aggregate (for alloys) The number and size of the grains developed in a unit volume of the metal depends on the rate at which nucleation (the initial stage of crystal formati n) takes place. The median size of the grains developed depends on the number of different sites at which individual crystals begin to form and the rate at which these crystals grow. lf the nucleation rate is high, the number of grains in a unit volume of metal will be large, and thus grain size will be small. Conversely, if the rate of growth of the crystals is high (compared with their nucleation rate), there will be fewer grains per unit volume, and thus grain size will be larger. Generally, rapid cooling produces smaller grains, whereas slow cooling produces larger grains
Grains and Grain Boundaries The growing grains eventually interfere with and impinge upon one another. The surfaces that separate these individual grains are called grain boundaries. Note also that the crystallographic orientation changes abruptly from one grain to the next across the grain boundaries. Thus, because its many grains have random crystallographic orientations, the behavior of a piece of polycrystalline metal is essentially isotropic; that is, its properties do not vary with the direction of testing. (a) Nucleation of crystals at random site in the molten metal; note that the crystallographic orientation of each site is different. (b)and (c) Growth of crystals as solidification continues. (d) Solidified metal
Grain Size Grain size has a major influence on the mechanical properties of metals. At room temperature, for example, a large grain size is generally associated with low strength, low hardness, and low ductility. Grains can be so large as to be visible with the naked eye; zinc grains on the surface of galvanized sheet steels are an example. Large grains also cause a rough surface appearance after the material has been plastically deformed, particularly in the stretching of sheet metals Grain size is usually measured by counting the number of grains in a given area, or by counting the number of grains that intersect a length of a line randomly drawn on an enlarged photograph of the grains (taken under a microscope on a polished and etched specimen)
Influence of Grain Boundaries Grain boundaries have an important influence on the strength and ductility of metals, and because they interfere with the movement of dislocations. Grain boundaries also influence strain hardening. These effects depend on temperature, deformation rate, and the type and amount of impurities present along the grain boundaries. Because the atoms along the grain boundaries are packed less efficiently and are more disordered, grain boundaries are more reactive than the grains themselves. As a result, the boundaries have lower energy than the atoms in the orderly lattice within the grains, and thus they can be more easily removed or chemically bonded to another atom. For example, a metal surface becomes rougher when etched or subjected to corrosive environments.
Plastic Deformation of Polycrystalline Metals When a polycrystalline metal with uniform equiaxed grains (grains having equal dimensions in all directions) is subjected to plastic deformation at room temperature (a process known as cold working), the grains become deformed and elongated. Deformation may be carried out, for example, by compressing the metal piece, as is done in a forging operation to make a turbine disk or by subjecting it to tension, as is done in stretch forming of sheet metal to make an automobile body. During plastic deformation, the grain boundaries remain intact and mass continuity is maintained. The deformed metal exhibits higher strength, because of the entanglement of dislocations with grain boundaries and with each other.
Plastic Deformation of Polycrystalline Metals The increase in strength depends on the degree of deformation (strain) to which the metal is subjected; the higher the deformation, the stronger the metal becomes. The strength is higher for metals with smaller grains, because they have a larger grain-boundary surface area per unit volume of metal and hence more entanglement of dislocations. a) b) a) Before deformation b) After Deformation
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