Plastic deformation of metal
Plastic deformation of metal
The permanent change in shape of a metallic body as the result of forces acting on its surface. The plasticity of a metal permits it to be shaped into various useful forms that are retained after the forming pressures have been removed. Complete comprehension of plastic deformation of metals requires an understanding of three areas: (1) the mechanisms by which plastic deformation occurs in metals; (2) the way in which different metals respond to a variety of imposed external or environmental conditions; and (3) the relation between the internal structure of a metal and its ability to plastically deform under a given set of conditions.
Pure metals are crystalline solids, or mixtures of crystalline solids in the case of some alloys. Most metals and alloys that can undergo significant amounts of plastic deformation have their atoms orderly packed in one of three types of crystal structure: hexagonal close-packed, face-centered cubic, or body-centered cubic, or slight variations thereof. See Alloy
For any type of atomic packing, as the crystal is viewed from different directions, the atoms can be visualized as lying on differently oriented planes in space. Within each plane the atoms are in a regular array, and certain directions are equivalent with respect to the distance between atoms and the location of their neighbors. The primary step in the plastic deformation of a metal crystal is the translation, or slip, of one part of the crystal with respect to the other across one of a set of crystallographically equivalent planes and in one of several possible crystallographically equivalent directions. These are known as the slip plane and slip direction, respectively. The particular direction and plane orientation differ from one metal to another, depending principally on the type of atom packing and the temperature of plastic deformation. Metals with equivalent crystal structures tend to exhibit a similar plastic response to stresses even though the actual strength and temperature range of such a like response will differ from metal to metal.
When a metal consists of a single crystal, it deforms anisotropically when stressed, depending on the orientation of the operative slip system. These translations leave linear traces on the surface called slip lines which are observable under a light microscope. As normally produced, however, metals are polycrystalline; that is, they are composed of a multitude of tiny crystals or grains, all with identical packing but with each crystal having its principal slip planes or directions oriented differently from its neighbors. On a gross scale this permits a metal when stressed to act as an isotropic body even though each grain, if isolated, would behave in an anisotropic manner that would depend on both its orientation with respect to the stress imposed on it and the particular crystal structure of the metal of which it is a part. One structural factor that the metallurgist can control to alter the properties of a metal is grain size and shape.
Most substances are weak relative to the strength that is theoretically calculated for them on the basis of the strength of the bonds between atoms in the crystal and the interatomic spacing. This strength is estimated to be in the neighborhood of one-tenth of the elastic modulus of the particular metal. The observed maximum strengths of metals, moreover, are more like one-tenth of this calculated strength, and the stress under which plastic deformation begins is often several times lower than the observed maximum strength. The reason for this discrepancy between the predicted and observed strengths of metal has been explained to be caused by submicroscopic defects called dislocations. These defects permit metals to be plastically deformed even though their presence also reduces the maximum attainable strength of the metals to the observed value. Understanding the nature and behavior of individual dislocations and their interactions forms the modern basis for understanding the various phenomena associated with plastic deformation in metals.
The phenomenology of metal behavior has been explored and documented by metalworkers and metallurgical engineers for centuries. This information has been vital to the design and manufacture or construction of metal objects from tin cans to complex gas turbines. The properties of metals that are associated with plastic deformation are ductility (the ability of a metal to be deformed considerably before breaking), behavior in creep (the time-dependent deformation of metal under stress), and the response to fatigue (conditions where the stresses are applied in a cyclic fashion rather than steadily). See Creep (materials), Metal, Metal, mechanical properties of, Metal forming, Metallography