Slip homogenization in Al-Li alloys by thermomechanical treatment and its effect on mechanical behaviour

The affects of a pre-ageing stretch and of duplex ageing on slip distribution have been examined in the Al-Li-Cu-Mg-Zr alloy 8090 peak aged at 170 °C. Stretching prior to ageing and duplex ageing were found to effectively homogenize the distribution of S in these alloys. In contrast, in unstretched materials that were not duplex aged, precipitation of S was intense on the grain and subgrain boundaries but scarce elsewhere.Tensile, cyclic stress-strain, long fatigue crack growth and small fatigue crack growth data were gathered. These data showed that slip was planar in the unstretched and duplex-aged materials as compared to materials that were stretched prior to ageing. A model is developed to calculate the diameter where Orowan looping of the S precipitates was likely to give way to shearing of those precipitates. Based on this model, it was concluded that, although the distribution of S precipitates was homogeneous in the duplex-aged materials, the precipitates were too fine to effectively homogenize slip. It was also shown that if the artificial ageing temperature was increased to 190 °C the S precipitates were thicker, leading to a change in deformation behaviour.