Effect of electron beam irradiation in TEM on the microstructure and composition of nanoprecipitates in Al-Mg-Si alloys

The evolution of nanoprecipitates β″ to disordered precipitates under electron beam irradiation in transmission electron microscope (TEM) has been studied. High-resolution TEM images show that β″ precipitates became disordered and diminished gradually under the 200 keV or 300 keV electron beam but displayed no significant changes under 80 keV. Under the same energy, the electron beam irradiation damage to the β″ was found as a function of electron dose. Knock-on damage and diffusion rather than radiolysis or heating effect are the main reasons for the disordering of β″. The vacancies caused by knock-on damage facilitated outward diffusion of solutes which lowered their solute concentration inside the nanoprecipitates. The occasional growth of β″ precipitates was induced by the outward diffusion of solute atoms but was dissolved soon under the 200 keV electron beam irradiation. 80 keV electron beam can evaporate hydrocarbon from the specimen and compensate for the resolution loss caused by the reduction of acceleration voltage. Our work validates that electron beam irradiation is one of the reasons for disordering in nanoprecipitates in Al alloys, and suggests that a careful check at low energies down to 80 keV should be performed during TEM studies.