The heat treatment of an age-hardenable aluminium alloy, having alloying elements in solid solution includes the stages of holding the alloy for a relatively short time at an elevated temperature TA appropriate for ageing the alloy; cooling the alloy from the temperature TA at a sufficiently rapid rate and to a lower temperature so that primary precipitation of solute elements is substantially arrested; holding the alloy at a temperature TB for a time sufficient to achieve a suitable level of secondary nucleation or continuing precipitation of solute elements; and heating the alloy to a temperature which is at, sufficiently close to, or higher than temperature TA and holding for a further sufficient period of time at temperature TC for achieving substantially maximum strength. |