SLIP HOMOGENIZATION IN AL-LI ALLOYS BY THERMOMECHANICAL TREATMENT AND ITS EFFECT ON MECHANICAL-BEHAVIOR

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-degrees-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-degrees-C the S' precipitates were thicker, leading to a change in deformation behaviour.