Investigation of the Coherent Strain Evolution of the η' phase in Al–Zn–Mg–Cu alloys via scanning transmission electron microscopy

Abstract The coherent strain caused by the different sizes of the nanoscale η' phases in Al–Zn–Mg–Cu alloys was investigated via scanning transmission electron microscopy and geometric phase analysis. It is found that after the η' phase precipitates, a coherent strain field is generated in the matrix and inside the precipitated phase. Such coherent strain field changes with the thickness of the η' phase. The tensile strain is mainly distributed in the 7-layers-thick η' phase, whereas the compressive strain mainly exists in the 11-layers-thick η' phase. The asymmetrically coherent compressive strains are present in the matrices around the 7-layers-thick and 11-layers-thick η' phases, and the corresponding maximum values of the compressive strains are 11.2% and 14.4%, respectively. The quantitative characterization of the coherent strain field induced by the nanoscale η' phase provides a valuable reference for the optimization of the mechanical properties of the Al–Zn–Mg–Cu alloy.