The history of precipitation hardening of aluminium alloys goes back to 1906 when A. Wilm discovered that a rapidly cooled (quenched from a high temperature ~ 550°C into cold water) Al - Cu - Mg alloy initially increased in hardness as it was left at room temperature; the alloy hardened with age, which led to the phenomenon being called age hardening. Wilm examined his samples in an optical microscope, but was unable to detect any structural change as the hardness increased. It was not until 1919 and the work of Mercia, Waltenberg and Scott that the beginning of an explanation was proposed. In their study of an Al - Cu alloy, they also observed a hardness increase after quenching. They demonstrated that the solid solubility of copper in aluminium decreases with decrease in temperature and this led them to propose that the hardening with age after quenching was due to copper atoms

precipitating out as particles from supersaturated solid solution (in other words, the alloy matrix after quenching contains more than the equilibrium concentration of solute atoms). In a review paper published in 1932, Mercia suggested that age hardening in Al - Cu alloys resulted from the assembly of copper atoms into a random array of small clusters (“knots”) which interfere with slip when grains are deformed; for example, when the hardening associated with ageing is measured by indentation (deformation) with a diamond indenter. The first direct evidence of Mercia’s “knots” was provided in 1938 by the historic work of Guinier and Preston who, independently, interpreted features in diffuse x-ray scattering from aged aluminium alloys as evidence for clustering of atoms into very small zones; since termed Guinier

- Preston zones , or GP zones for short.Now I have to know why the precipitates in Al-Cu alloy called theta?