(CHAPTER 17) KEY TERMS l WROUGHT METALS

Annealing

Process of controlled heating and cooling that is designed to produce desired properties in a metal. Typically, this process is intended to soften metals, increase their ductility, stabilize shape, and increase machinability. In the case of gold foil, the term refers to removal of contaminants from the surface of the foil.

Cold welding

Process of joining metals by metallic bonding. The metal joining process does not occur by heating or fusion but by pressure applied to the interface between the two parts to be joined; no liquid or molten phase is produced within the interface joint.

Cold working

Process of plastically deforming metal at room temperature.

Compaction (condensation)

Process of increasing the density of metal foil, pellets, or powder through compressive pressure.

Degassing

Process of removing gases (or other impurities) from the surface of a solid or liquid, usually by heating.

Desorption

Process of removing molecules that have attached to the surface of a solid by physical or chemical action; it is a broader term than degassing.

Dislocation

Imperfection in the crystalline arrangement of atoms consisting of either an extra partial plane of atoms (edge dislocation), a spiral distortion of normally parallel atom planes (screw dislocation), or a combination of the two types.

Ductility

Ability of a solid to be elongated or thinned plastically without fracturing.

Ductile fracture

Rupture of a solid structure resulting in measurable plastic deformation.

Grain growth

Increase in the mean crystal size of a polycrystalline metal produced by a heat-treatment process.

Grain refinement

Decrease in the crystal (grain) size in a solid metal through the action of a heat-treatment process.

Interstitial atom

Imperfection in a crystal lattice consisting of an extra atom located between adjacent atoms in normal lattice sites.

Malleability

Ability of a metal to be hammered into thin sheets without fracturing.

Point defect

Lattice imperfection of atomic size in three dimensions, such as a vacancy, divacancy, trivacancy, or interstitial atom.

Precipitation hardening

Process of strengthening and hardening a metal by precipitating a phase or constituent from a saturated solid solution.

Recovery

Stage of heat treatment that results in the partial restoration of properties of a work-hardened metal without a change in the grain structure.

Recrystallization

Process of forming new stress-free crystals in a work-hardened metal through a controlled heat-treatment process.

Springback

Amount of elastic strain that a metal can recover when loaded to and unloaded from its yield strength; an important property of orthodontic wires.

Strain hardening

Increase in strength and hardness and decrease in ductility of a metal caused by plastic deformation below its recrystallization temperature; also called work hardening.

Stress relief

Reduction of residual stress by heat treatment.

Superelasticity

Ability of certain nickel-titanium alloys to undergo extensive deformation resulting from a stress-assisted phase transformation, with the reverse transformation occurring on unloading; sometimes called pseudoelasticity.

Vacancy

Imperfection in a crystal lattice consisting of an unoccupied atom site.

Work hardening

See strain hardening.

Working range

Maximum amount of elastic strain that an orthodontic wire can sustain before it plastically deforms.

Wrought metal

Metal that has been plastically deformed to alter the shape of the structure and certain mechanical properties, such as strength, hardness, and ductility.