Microscopic Investigation of Shearing and By-passing of Coherent Ordered δ’ Precipitates in Al-Li Alloys

The interaction between dislocation and shearable ordered precipitates is studied on both sides of the peak-ageing distribution. It is shown that small precipitates are sheared as expected, but can be dissolved at large local strains. Large precipitates are by-passed by Orowan looping, but can be sheared by their loops if the stress exceeds a critical value. The model holds for all kinds of ordered coherent precipitates, and is illustrated in the case of Al-Li alloys, containing δ′ ordered precipitates of L12 structure, and studied during and after straining by in situ TEM straining, TEM observations, STEM-EELS microanalysis and filtered imaging, and high resolution electron microscopy.