New approach to in situ observation experiments under irradiation in high voltage electron microscopes

A new method for conducting in situ observations of experiments undergoing irradiation in a high voltage electron microscope (HVEM) is proposed. Intensity profile of a focused electron beam in HVEM introduces an atomic displacement gradient in the vicinity of the beam, which generates distribution of point defect concentration and enhances defect diffusion in matrix. In our experiments, tantalum carbide or yttrium titanate nanometer-scale particles embedded in iron matrices were irradiated at 673K with a focused electron beam at energy ranges from 0.75 to 2.5MeV. The results show that the instabilities of particles undergoing irradiation could be observed as diminishing either in size or contrast. The rate of shrinkage per fluence unit was successfully measured to derive the vacancy diffusion effect, with particles located in the vicinity of the electron beam showing higher rates of shrinkage. This indicates that the diffusion of vacancies enhanced both by irradiation and the concentration gradient is attributable to dissolution of the particle constituents into the matrix.