Cyclic strain induced precipitation in a solution treated aluminium alloy

Cyclic mechanical tests were carried out on specimens of an Al-Mg-Si alloy, 6351, in the solution treated condition and the behaviour compared to that of an Al-Mg alloy, 5182. Both showed marked cyclic hardening typical of non precipitation hardened aluminium alloys. In the 5182 alloy, this hardening has been attributed to dislocation pinning by solute atoms. Differences in mechanical behaviour were observed which suggested that hardening of the 6351 alloy may be due to a cyclic strain induced precipitation hardening effect rather than solute atom pinning. Using a previously described weak beam technique, evidence was found for precipitate formation on dislocations in the cycled 6351 alloy. The evolution of the microstructure during cycling was different in the two alloys. In the 6351 alloy, the dislocation density reached a maximum and then decreased with further cycling even though hardening continued to occur. This was attributed to progressive deactivation of dislocation sources whilst normal annihilation reactions continued to occur among the mobile dislocations.