“In-plane” site-specific FIB lamella extraction from deformed magnetite and the investigation of low angle grain boundaries under TEM

Abstract

In this study a <em>modus operandi</em> to investigate site-specific nanostructures in <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/thin-films" target="_blank" rel="nofollow noopener noreferrer">thin films</a> (lamellae) excavated “<em>in-plane</em>” across (sub)grain boundaries is presented. This is done by discussing the case of a magnetite grain hosted in a thin section of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/banded-iron-formation" target="_blank" rel="nofollow noopener noreferrer">banded iron formation</a> (Norway) that is prepared parallel to the kinematic reference frame (XZ section of the strain ellipsoid). SEM-EBSD analysis reveal that the magnetite grains do not develop a strong crystallographic preferred orientation, although individual grains are strained and show evidence of intracrystalline deformation in form of low angle grain boundaries (LAGB's). Two “<em>in-plane</em>” lamellae using focused <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/ion-beam" target="_blank" rel="nofollow noopener noreferrer">ion beam</a> (FIB) technique are excavated from a magnetite grain in the kinematic reference frame, and nanostructures are studied along three LAGB's using high resolution <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/transmission-electron-microscopy" target="_blank" rel="nofollow noopener noreferrer">transmission electron microscopy</a> imaging followed by Fourier transformation (FT), inverse FT and estimation of dislocation densities. Our data establish an empirical relationship for the studied LAGBs, namely, the smaller the angle between LAGB and X-direction, the larger are the shear strain and dislocation density. This relationship is validated from numerical simulations of viscoplastic deformation and dynamic recrystallisation of polycrystalline aggregates of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/halite" target="_blank" rel="nofollow noopener noreferrer">halite</a>, which is also a cubic mineral analogous to magnetite. In addition to the site-specific “<em>in-plane</em>” FIB lamella information, this study also shows that in a deformed mineral the different orientations of the LAGB compared to the principal strain axes show a different dislocation density. This approach of full tracking of the extension direction (X) from the macroscopic to the nano-scale could play an important role in forward modelling of microstructure evolution in future studies.