The Al-Mg-Si-(Cu) alloys from the 6016 series are used in automobile industry asclosure panel sheets. They show a strong paint bake hardening behaviour (30 minutesat 185°C, typically). This behaviour comes from structural hardening : the diffusion ofthe solute elements in the supersaturated solid solution is activated by the heat treatmentand metastable phases precipitates form. These phases are coherent with the aluminiummatrix and slow the motion of the dislocations.The dimensions of these metastable phases are sometimes below 1 nm and makesthem difficult to analyse otherwise than with a tomographic atom probe. New techniquesof data interpretation and processing have been developed in this field in order to push theinstrument beyond its limits.With the help of the atom probe, sometimes associated with the transmission electronmicroscope, the beta" precipitation mechanisms have been elucidated. In particular, we haveshown that a preageing treatment (90°C) makes the ageing treatment more efficient. Thisis due to the formation of Mg- and Si-rich coherent clusters that increase the numberdensity of hardening objects (clusters + beta"). The presence of aluminium inside the b00precipitates has been demonstrated.The effect of the preageing treatment on the short range correlation between the soluteatoms during the very early stages of decomposition has been directly studied bythe use of partial pair correlation functions. By this method, we have also been able tosuggest a scheme for the natural ageing behaviour of the alloy : it could be explained bythe formation of solute atoms atmosphere in the vicinity of the dislocations. The interactionbetween dislocations and atmospheres could be responsible for the natural ageinghardening.
