The aluminium alloy with 4wt% copper has been age hardened using the following heat treatment.
Heat Treatable alloys such as this are commonly used for aerospace applications, including struts and panels, making use of the high specific strength.
Heat Treatment and the Mechanisms of Age-hardening.
The aim of age-hardening is to produce a large number of fine precipitates in the aluminium grains. These interfere with the movement of dislocations when the metal yields. This has the effect of increasing the strength of the alloy. The heat treatment used to produce the precipitates involves a high temperature solution treatment, quenching and then ageing. This is described in more detail below.
Solution Heat Treatment.
Hardening is achieved by the controlled rejection of copper from a supersaturated solid solution. There is some additional hardening from precipitates such as MgSi2, though this will not be considered in this experiment. From the phase diagram for the pure aluminium-copper binary system, it can be seen that the solubility of copper in aluminium increases with increasing temperature up to the eutectic temperature of about 550°C. The equilibrium microstructure below the eutectic temperature is a two-phase mixture of aluminium and the Al2Cu intermetallic phase (also known as theta phase). The initial solution heat treatment aims to obtain the maximum possible concentration of copper in solution.
Rapid quenching from the solution temperature prevents the kinetically slow precipitation, forming a highly supersaturated solid solution of copper. Rapid quenching also preserves the large number density of vacancies in the aluminium lattice from the high solution temperature. This increases copper diffusion rates at low temperature and accelerates ageing. Care must be taken with commercial alloys where the additional alloying elements reduce the eutectic tempereature. This reduces the maximum solution heat treatment temperature since heating above the eutectic temperature causes the growth of a brittle intergranular eutectic.
The Effect of Ageing on Microstructure and Strength.
Ageing may occur at room temperature (Natural Ageing) or above (Artificial Ageing), and takes place via a sequence of precipitation reactions. At low temperatures, the sequence may not be completed even after very long times, whereas at high temperatures, where diffusion is rapid, the early stages may not be observed. The precipitation sequence is driven by the supersaturation of copper in solution, and is briefly summarised below.
(a) At room temperature, copper atoms segregate by diffusion to form copper-rich zones called Guinier-Preston or GP(1) zones. These are only one atom thick and around 5 nm to 10 nm in diameter, lying parallel to the {001} planes in the aluminium. They are ordered in 2 dimensions and are coherent with the lattice. The difference in atomic size of copper and aluminium strains the lattice. Hardening is therefore due to the increased work required to move dislocations through the strained lattice (coherency stress) and work required for dislocations to pass though the GP(1) zones (cutting stress). This more than counteracts the decrease in solid solution strengthening as the copper concentration in the aluminium decreases.
(b) Above around 100°C, further copper segregation produces GP(2) zones, sometimes known as theta" phase. These lie parallel to {001), and are a few atoms thick and up to 15 nm in diameter. They are ordered in 3 dimensions and are also fully coherent with the lattice. Hardening occurs by the same machanisms as the GP(1) zones, though GP(2) zones have a stronger effect. A high number density of GP(2) zones or theta" gives the maximum strengthening effect.
(c) Further growth of the GP(2) zones leads to the formation of the theta' phase, which is not fully coherent with the lattice. Theta" and theta' co-exist at the peak hardness, where strengthing is due to a combination of lattice coherency strain (coherency stress), precipitate cutting by dislocations (cutting stress) and dislocation bowing between precipitates (bowing stress). With further ageing, the precipitate distribution changes as large, widely dispersed precipitates grow at the expense of finely dispersed small precipitates. This process is known as "Ostwald ripening". Dislocation bowing between precipitates becomes easy and the strengthening contribution from coherency strain and precipitate cutting is lost.
(d) Over-aging develops the equilibrium tetragonal phase (Al2Cu), which is fully incoherent and does not have a strong strengthening effect. Compare this microstructure with an overaged aluminium-4wt% copper alloy.
The microstructure in this sample shows theta' phase at high magnification. The crystallographic relationship between the precipitates and the aluminium lattice is apparent from the alignment of the precipitates.