Imperfections in crystal
Metals are crystalline in solid state. In a piece of real metal, the vast majority of atoms line up evenly and follow the perfect arrangement in the ideal crystalline lattice. However, a very tiny fraction of the atom sites, often less than one in a million, are not perfect. Defects in crystals may be points (a vacant site), lines (a dislocation) or surfaces (a grain boundary).
Even though crystal imperfections account for few of the atom sites, they usually dominate the properties of the microscopic specimen. Vacancies enable diffusion in solid metals, and dislocations produce plastic deformation. A materials engineer must know how to introduce defects, control their number and arrangement so as to produce a metal with desirable combinations of properties. This is usually achieved through composition, fabrication processing and heat treatment.
An early theory to explain plastic deformation was the “block slip” theory. Large blocks of atoms were thought to slide over one another across the slip planes under the action of a shear force. However, it was found that the yield stress calculated from the block slip was far much higher than the actually observed yield stress. It means some other mechanisms involving much lower energy occurs first. There is still something correct about block slip. Plastic deformation occurs through slip across slip planes and along slip directions. It is because this requires the least energy for sliding of a layer of atoms over another.
Dislocations are line imperfections where the three-dimensional lattice is offset. One common type is edge dislocation. An edge dislocation can be most easily visualized as the insertion of an extra plane of atoms into the lattice, so that where it ends, the orderly rows of atoms are offset. The edge of the half-plane of atoms is called the dislocation line. Atoms near the dislocation are not in their regular alignment and can move relatively easily from one side of the dislocation to the other.
Slip by dislocation movement
The presence of dislocations enables slip to occur in a progressive manner instead of the block slip manner. Not all the metallic bonds involved need to be broken at the same time (which requires a very high level of stress). Now bonds are broken and formed again between atoms along the dislocation line only. This can occur at a much lower stress level.
Effectively, a dislocation allows atom movement and thus deformation. Atoms near the dislocation are not in their regular alignment and can move relatively easily from one side of the dislocation to the other. This represents the displacement of the entire plane of atoms, and effectively shifts the dislocation. Repeating the process until the dislocation reaches the surface produces a deformation step of one atom spacing.
Slip still occurs under a shear force. The direction of dislocation movement is still preferably the slip direction. Slip occurs most easily on slip planes. In most real metals, there are many grains with different orientations, so there are many slip planes. Some of them will be inclined to the direction of the applied force and slip can always occur when the load is high enough to star dislocation movement. Slip is responsible for the necking of specimens pulled in tension.
A point defect is an imperfection at an atom site. Examples are vacancies, interstitial atoms, or substitutional atoms (large or smaller than the parent atoms). When an atom is missing from the lattice, there is a vacancy. If another atom manages to squeeze in between the normal sites, it is called an interstitial atom. Other atoms may also take the place of the regular atom.
The properties of a metal may be controlled by intentionally creating and controlling point defects through heat treatment, strain hardening, solid solution strengthening, and other related processes. Point defects hamper the movement of dislocations by disrupting the atom positions and making the slip planes bent or bumpy, and therefore strengthen the material. Either larger or smaller substitution atoms produce strengthening, which depend only on the amount of the size difference. Steel is a good example of how interstitial atoms strengthen a metal. Carbon atoms are quite small (a radius of 0.071nm) and can occupy interstitial positions in the lattice of iron.
The presence of vacancies enables diffusion in solid metals. Diffusion means the migration of atoms from one atomic site to another in the crystal lattice. Effectively, the diffusing atom breaks the bonds with the originally neighboring atoms and causes some lattice distortion on its migration to the next site when new bonds are formed with the new neighbors. High energy is required and this is usually vibrational in nature (at elevated temperature). The presence of a vacancy lowers this energy and makes diffusion easy to occur.
Surface defects and grain boundaries
Most metals consist of many small crystalline regions, called grains, that form independently with different orientations. The surfaces of these grains are the boundaries that form when the growing crystals run into each other. The atomic arrangement is imperfect along these internal surfaces. A large angle boundary may have local stresses because of the atom mismatch. In any case, it might seem that such boundaries would weaken material, but in fact the opposite is true. In a material containing dislocations, the grain boundaries represent places that the dislocations can begin or end, and can be consumed or created. A small grain size produces many boundaries. Reducing the grain size normally produces an increased yield strength.
Strengthening mechanisms in mechanisms in metals
If dislocation movements are made more difficult, a high stress is required to move the dislocations and produce plastic deformation. The yield stress is thus increased and the metal becomes “stronger” or harder. Strengthening a material can be accomplished by placing obstacles in the path of the moving dislocation. Three commonest mechanisms are:
Grain size strengthening
Solid solution strengthening
Work hardening, also known as strain hardening, cold work strengthening
Refining the grain sizes is a mechanism of strengthening because grain boundaries are barriers to dislocation movements. Interstitial or substitutional atoms hinder the movement of dislocations by disrupting the atom positions and making the slip planes bent or bumpy, and therefore strengthen the material. Other dislocations present in the material can also act to hinder dislocation movement due to the interactions among dislocations. This is the basis of “work hardening” in cold work.
Iron is an allotropic metal, meaning that its crystal structure changes with temperature. Above910℃, the structure is FCC. The largest site between the iron atoms has a radius of 0.052nm, somewhat smaller than the carbon atom. A maximum of about 2.1% carbon by weight can dissolve in FCC iron, limited by distortion of the lattice due to the presence of the interstitial atoms. Below 910℃ the iron structure changes to BCC. Even though this is a less close-packed structure, the largest available interstitial site is smaller than in FCC, only about 0.026nm. Because of the greater lattice distortion the atoms cause, less carbon can be dissolved in the BCC iron (a maximum of about 0.02%). Heat treating steels often involves heating the alloy so that it transforms to FCC and dissolves all the carbon, then cooling it so that the carbon atoms are either trapped in the BCC structure (causing distortion) or forcing the atoms out of solution to form iron carbides.
Plastic deformation of metals takes place most commonly by the slip process. Slip usually takes place on the closest-packed planes and in the closest-packed directions. A slip plane and a slip direction make a slip system. In general, metals with a high number of slip systems are more ductile.
The “block slip” theory assumes sliding of one plane of atoms over an adjacent plane at the same time. The stress required is over one order of magnitude larger than the observed stress.
Plastic deformation actually occurs through the slip process which takes place progressively with the movement of a dislocation.
The easier dislocations can move, the easier the metal can deform plastically. On the other hand, the metal can become stronger and harder if dislocation movement is hindered.
Grain boundaries (at lower temperatures) usually strengthen metals by providing barriers to dislocation movement.
Some strengthening (hardening) processes are work hardening, solution hardening and precipitation hardening.
Solute atoms in a solid solution can serve as obstacles to dislocation movement. The alloy becomes stronger and harder by this strengthening process known as “solution hardening”
“Cold working”, “work hardening” or “strain hardening” is a common strengthening process for metals ad alloys. Plastic deformation is imposed on the metal by cold work. During cold working, the number of dislocations increases. As the dislocation density increases with deformation, it becomes more and more difficult for the dislocations to move through the existing “forest of dislocations”. (A dislocation hinders the movement of another dislocation.) Thus, the metal strain hardens with increased cold work.
Cold worked metals can be made soft again by a process known as “annealing”. Annealing is a reheating treatment and the temperature at which atomic mobility is sufficient to affect mechanical properties is about 1/3 to 1/2 the absolute melting point.
During annealing, the metal will go through a series of changes called
Alloys are slid solutions of pure metals (and some other elements). Solid solutions can be substitutional or interstitial.
In a substitutional solid solution, solute atoms substitute for some parent solvent atoms in the crystal lattice.
In an interstitial solution, solute atoms fit into the spaces or holes in the solvent-atom lattice.
Solid solutions can also be formed by the diffusion process.
Thursday, 15 January 2009
Imperfections in crystal