Free surface effects in TEM imaging of dislocation lines and loops in Fe

I Abstract Understanding of the physical and geometrical characteristics of the radiation induced defects in ferritic steels based on Fe-Cr is essential to optimize their properties as structural materials for nuclear power plants. Their observation in a thin foil by transmission electron microscopy (TEM) in diffraction contrast mode is one of the most used techniques. However, the comparison between the experimental TEM images and those that can be obtained by simulation is needed to give a reliable assessment of the characteristics of these defects, especially for the small radiation induced dislocation loop. Indeed, its black-white contrast is not directly related to its actual characteristics, and even more so when the loop size comes down to nanometres. In addition, it has been already noticed that the relaxation at the free surfaces of the needed thin foil for TEM should be taken into account in the analysis, but it remained a challenge till today. The dislocation loops formed in the Fe-Cr base alloys have a Burgers vector of the type 1⁄2 a0<111> or a0<100>. Experimentally, irradiation by accelerated ions is often used to study the dislocation loop, and the dynamic study of its formation and evolution can be done in situ in a TEM with ions. Ion irradiation presents several advantages over post-irradiation studies conducted on samples irradiated in a nuclear reactor. However, the ratio of 1⁄2 a0<111> to a0<100> loops is modified during in-situ TEM experiments irradiation of ferritic material as the very mobile loops 1⁄2 a0<111> can be lost at the free surfaces of the thin foil. This work is part of a project to validate the simulations with experiments to better understand the mechanism of formation of the dislocation loop in Fe-Cr, particularly in relation to temperature and its characteristics, that is its vector Burgers, habit plane, size and shape. The aim of this work is to implement different models of continuum elasticity to address the problem of surface relaxation of the TEM thin foil, which induces a so-called 'image' force on a dislocation loop or line. Models are implemented in the TEM image simulation code 'CUFOUR', based on Schaeublin-Stadelmann equations, in order to understand the impact on the TEM images. Strengths that characterize this work are: · A robust method for calculating anisotropic image forces has been successfully developed in Fourier space. This new method works for a straight dislocation or a dislocation loop in a semi-infinite space, or a thin foil. · The method of has been successfully implemented in CUFOUR code. · A screw dislocation, 1⁄2 a0<111>, straight and inclined in a thin foil of ultra high purity Fe, was used to validate the simulation of TEM images with experimental TEM. A good agreement between experiment and simulation is found. · Different types of dislocation loop were implemented in CUFOUR. The effect of the diffraction condition , the loop characteristics and properties of the thin foil is systematically studied. The effect of the image forces on the TEM image was particularly studied. It appears that close to the surface they can completely invert its black-white contrast. · The anisotropy of Fe has a strong influence on the images forces, relative to the isotropic case. The anisotropy thus cannot be ignored. In iron, temperature increases this effect. The elastic energy induced by the free surface becomes remarkable when the depth of the dislocation loop beneath the surface is comparable to the characteristic length thereof. Moreover, there is large difference between the 1⁄2 a0[111] loop in a (111) foil and the a0[100] loop in a (100) foil.