Methods for Mesoscale Structural Characterization

Abstract

As a motivation for introducing the 3DXRD method, this chapter contains a survey of the experimental methods conventionally used for structural characterization. The options for and limitations of studies on the mesoscale are discussed. The list of methods is not complete, but is intended to reflect the methods commonly used in materials science. This survey is followed by an account of the properties of hard x-raysdefined as x-rays with an energy of or above 50 keV -and the utilization of these at hard x-ray synchrotron beamlines. To put the development of the 3DXRD microscope into perspective, a brief account is given of the historical development of the use of such beamlines, with the prime focus initially on macroscale problems in condensed-matter physics. Electron and Optical MicroscopyThe predominant set of tools for structural characterization today belong to the family of electron microscopy (EM). Thanks to its atomic-scale resolution, transmission electron microscopy (TEM) is an indispensable tool for studies of dislocation structures, precipitates, interfaces and nanoscale structures in general [1]. Scanning electron microscopy (SEM) , on the other hand, offers relatively easy sample preparation, larger inspection areas and very userfriendly measurements, but at the cost of a spatial resolution of 1-50 nm [2].Within the last decade, SEM studies in combination with the automatic electron back-scattering pattern (EBSP) method [3,4,5,6] have become the workhorse for many metallurgical studies. The EBSP method, also known as EBSD, enables determination of the crystallographic orientation of selected local areas and reveals the microstructure by orientation contrast. When it is used in conventional SEMs, the spatial resolution is about 0.5-1 µm, whereas field-emission-gun SEMs may give resolutions as high as 50 nm [7]. A limitation for certain studies may be the angular resolution of the orientation determination, which presently is 0.5 0 -1 0 . Despite the enormous impact of EM, it is well recognized that using electrons as a probe is associated with a set of inherent disadvantages. Firstly, probing the elastic and plastic strain by EM is notoriously difficult. Secondly, the dynamical scattering processes involved complicate the process of structural refinement on the basis of diffraction data. Thirdly, the electrons penetrate ≤ 1 µm into materials, such that only the surface region is probed. This fact implies a number of limitations, which are central to this book. They are discussed in detail below.1. 2D characterization. The structural objects of interest, i.e. grains, dislocation structures, cracks, etc., are in general truly three-dimensional in nature. On the basis of observations from a single section, it is only possible to provide a 3D description by subjecting the objects to a statistical treatment. The discipline of stereology [8] was invented to facilitate such a description. However, it turns out that a number of parameters of interest simply cannot be inferred from a single 2D section without...