Alloying variants – A special feature of powder metallurgy

In contrast to classical ingot metallurgy, in which case a chemically homogeneous liquid phase is the initial state, in powder metallurgy solid powders are the starting materials. This enables different alloying routes and thus widely differing degrees of chemical heterogeneity in the powder bulk or compact, according to the alloying variant chosen. This may range from homogeneous systems, as in the case of fully prealloyed powders, to fully heterogeneous ones, e.g. elemental powder mixes. Through appropriate sintering regimes, the process of homogenization to approach chemical equilibrium can be controlled and stopped at desired stages. In addition to metallurgical also chemical heterogeneity has to be considered, which is particularly relevant in reactions involving interstitials such as O, C and N. Transfer reactions, described as “internal gettering”, may have a profound effect on chemistry and properties of the resulting products. These effects have to be carefully assessed for selecting the optimum alloying route for a given material and application.