Annealing (metallurgy)

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Annealing, in metallurgy and materials science, is a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces equilibrium conditions by heating and maintaining at a suitable temperature, and then cooling very slowly. It is used to induce softness, relieve internal stresses, refine the structure and improve cold working properties. There are several phases in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects and the internal stresses which they cause. The second phase is recrystallization, where new grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.

In the semiconductor industry, silicon wafers are annealed, so that dopant atoms, usually boron, phosphorus or arsenic, can be incorporated into substitutional positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material.

In the cases of copper, steel, and brass this process is performed by substantially heating the material (generally until glowing) for an extended period of time and allowing it to cool slowly. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming.

[edit] The Thermodynamics of Annealing

Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break and form new bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent)in ceramics . This alteration in dislocations allows metals to deform more easily, so increases their ductility.