The interaction of creep and cold work with ageing of some aluminium alloys.

The effect of cold work and creep strain on the ageing of three aluminium alloys has been investigated by means of mechanical testing and electron microscopy. Cold work prior to ageing was found to accelerate the rate of zone formation in all alloys used, but the increase in peak strength,and rate of overageing, was dependent upon the type of intermediate precipitate and composition of the alloy. The effect of creep upon ageing is complex and depends critically on the ageing temperature and straining rate of the alloy. If the straining rate is high and creep temperature low,the dislocations introduced by creep deformation become entangled and form subgrains. On further ageing heterogeneous precipitation occurs in these regions and they overage at a very high rate. At lower strain rates, and elevated temperatures,the interaction of dislocations with solute atoms and vacancies would reduce the diffusion of the solute atoms through the matrix. The effect of creep deformation on the ageing of the already aged material is dependent also upon the previous thermal history of the material. Creep deformation increased the rate of overageing by providing plenty of dislocations to accommodate strain caused by the coherency of the precipitate with the matrix. However,at early stages of ageing i. e. when the size of the precipitate is so small that it cannot support a dislocation loop at its interface and whilst dislocations may still provide nucleation sites for precipitates,creep deformation reduced the rate of ageing