Hardening (metallurgy)

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal. The five hardening processes are: The Hall–Petch method, or grain boundary strengthening, is to obtain small grains. Smaller grains increases the likelihood of dislocations running into grain boundaries after shorter distances, which are very strong dislocation barriers. In general, smaller grain size will make the material harder. When the grain size approach sub-micron sizes, some materials may however become softer. This is simply an effect of another deformation mechanism that becomes easier, i.e. grain boundary sliding. At this point, all dislocation related hardening mechanisms become irrelevant. In work hardening (also referred to as strain hardening) the material is strained past its yield point, e.g. by cold working. Ductile metal becomes harder and stronger as it's physically deformed. The plastic straining generates new dislocations. As the dislocation density increases, further dislocation movement becomes more difficult since they hinder each other, which means the material hardness increases. In solid solution strengthening, a soluble alloying element is added to the material desired to be strengthened, and together they form a “solid solution”. A solid solution can be thought of just as a "normal" liquid solution, e.g. salt in water, except it is solid. Depending on the size of the dissolved alloying element's ion compared to that of the matrix-metal, it is dissolved either substitutionally (large alloying element substituting for an atom in the crystal) or interstitially (small alloying element taking a place between atoms in the crystal lattice). In both cases, the size difference of the foreign elements make them act as sand grains in sandpaper, resisting dislocations that try to slip by, resulting in higher material strength.