Microtexture Determination and its Applications 2nd Edition PDF

Introduction

A cornerstone in the study of both natural and technological materials is characterisation of microstructure. In the widest sense this topic encompasses,for all phases present:

• Morphology, including size and shape distributions
• Chemical composition
• Crystallographic parameters, including orientation and orientation relationships

Until the last part of the twentieth century these aspects of microstructural characterisation were measured separately, as dictated by available technology. A landmark advance for the materials community occurred with the genesis of 'microtexture', which for the first time provided integration of crystallographic parameters and other aspects of the microstructure. A definition of microtexture is:'a population of crystallographic orientations whose individual components are linked to their location within the microstructure.'

The term microtexture also describes any experimental technique used to determine this information (Randle and Engler, 2000). Essentially,a stationary beam of electrons is diffracted by atomic planes in the sampled volume of specimen. Analysis of the resulting diffraction pattern provides crystallographic information which can be related back to its position of origin. An estimated 95% of microtexture determination is by 'electron backscatter diffraction' (EBSD) in a scanning electron microscope (SEM),with the remaining 5% contributed mainly by transmission electron microscopy (TEM) counterparts to EBSD.

Evaluation (indexing) of EBSD diffraction patterns and output of data in a variety offormats is in most casesfully automated.The most exciting EBSD output is an 'orientation map', which is a quantitative depiction of the microstructure in terms of its orientation constituents. Figure 1.1 shows EBSD maps from a material containing five phases: alumina, titanium aluminate, erbium oxide, zirconium oxide (tetragonal) and zirconium oxide (monoclinic). This is a complex example, selected to illustrate compactly the capabilities of microtexture determination and EBSD mapping, which are the subject of this book. The orientation map in Fig. l.la shows the morphology of the specimen; dark grey grain boundaries or phase boundaries are shown on alighter grey background. Figure 1.1b shows the chemical composition for each phase via colour coding. The phases have been identified by a combined application of EBSD and energy dispersive spectroscopy (EDS) mapping. Finally Fig. 1.1c depicts the crystallographic orientation distribution of the titanium aluminate phase (the blue component in Fig. 1.1b), accompanied by an orientation key in Fig. 1.1d. All the microstructural elements of this specimen are determined: morphology, chemistry and orientation.

Microtexture determination is now firmly established asthe most comprehensive experimental tool for quantitative characterisation and analysis of microstructure, and is used extensively in both research and industry. For the first time a wealth of information on topics relating to, for example, orientation distribution (texture), processing history, microstructure evolution, interfaces and structure/property links is now routinely accessible.Microtexture analysiscan in principle be applied to any crystalline material, subject to suitable specimen preparation. For most casesEBSD is the most appropriate technique; however, for very fine-scale analysis,such asthat associated with heavily deformed materials, TEM-based techniques are used.

Edited by: V. Randle