Precipitation Hardening ‐ The Oldest Nanotechnology

Precipitation hardening of aluminum was discovered about 100 years ago by Dr. Alfred Wilm [1,2]. Using aluminum alloys as example a survey is given on mechanism and limits of precipitation hardening. It is discussed how hard, nanometer-size particles (nanos, greek, the dwarf) can form as an ultra fine dispersoid. A simple example for optimum conditions is provided by diamond cubic particles (Si, Ge) in the f.c.c. Al-Matrix. The role of a sequence from more to less metastable phases is discussed, as well as the effects of additional (and trace) alloying elements. The amount of precipitation hardening is limited, besides by the volume fraction of particles, by their strength. This in turn determines the critical diameter above which the transition from passing to by-passing by dislocations takes place. Simple models for the calculation of this microstructural parameter are discussed. From combinations of precipitation hardening with other hardening mechanisms the limits for ultra high strengths are defined.