and -phases. Microstructural Transformations After casting, the material is usually oversaturated - Mixed crystal, which also contains more copper at room temperature than could actually be dissolved at this temperature. After that, GP zones (GP(I) zones) form at temperatures below 80 °C, in which increased concentrations of copper are present, but which do not yet have a structure or form their own phases. At somewhat higher temperatures of up to 250 °C, this forms -phase (also called GP(II) zones), which increases strength. At even higher temperatures, the partially coherent forms -Phase and at still higher temperatures of about 300 °C the incoherent one forms -phase in which the strength decreases again. The individual temperature ranges overlap: Even at low temperatures, there is formation of - or phases, but these form much more slowly than the GP(I/II) zones. Each of the phases forms faster the higher the temperature. GP(I) zones The formation of GP(I) zones is referred to as natural hardening and occurs at temperatures up to 80 °C. They are tiny disc-shaped layers just one atom thick and 2 to 5 nanometers in diameter. With time, the number of zones increases and the copper concentration in them increases, but not their diameter. They are coherent with the aluminum lattice and form on the {100} planes. GP(II) zones The GP(II) zones (-phases) are largely responsible for the increase in strength of the AlCu alloys. They are coherent with the aluminium crystal and consist of alternating layers of aluminum and copper with layer thicknesses of about 10 nanometers and dimensions of up to 150 nanometers. In contrast to the GP(I) zones, these are three-dimensional precipitations. Their layers are parallel to the aluminum {100} plane. From the -phase forms the -phases, but there are overlaps. The GP(II) zones need vacancies for growth, which is why a lack of these (e.g. due to magnesium) leads to delayed growth. Partially coherent phases The -phase is only partially coherent with the aluminum lattice and forms at temperatures from 150 °C to 300 °C. It has the form of platelets and can arise from the GP(II) zones. However, it can also arise directly as a precipitation from the mixed crystal. In the first case, the increasing interfacial energy is reduced by dislocations, in the second case, the precipitates form preferentially at dislocations. Incoherent phases The -phase is incoherent with the lattice of the mixed crystal. It forms