NEUTRON DIFFRACTION

Because neutrons have no electric charge, they exhibit usually much larger useful penetration depths than X-rays or electrons. However, the low interaction of neutrons with matter makes long counting times necessary to achieve suitable diffraction intensities.

Neutrons allow texture measurement in much larger specimens compared to SXRD, which provides the opportunity for non-destructive and in-situ experiments without the risk of poor grain statistics. Even with coarse-grained materials or with low volume fractions of certain second phases, the statistics can be good enough to build reliable representations of global texture (Wenk and Van Houtte 2004). Environmental stages (heating, cooling, straining) can be combined with neutron sources for in-situ observation of texture changes, e.g. during phase transformations. Several successful experiments have been carried out on dedicated instruments such as the HIPPO (High Pressure Preferred Orientation) diffractometer at LANSCE (Los Alamos Neutron Science Centre) (Matthies et al. 2005), some of them in materials of interest for this project, such as Zircaloy-4 (Wenk et al. 2004) and commercially pure (CP) titanium (Lonardelli et al. 2007, Bhattacharyya et al. 2006).

Texture Evolution during -quenching of a Zirconium Alloy 53 1.2.4 ELECTRON BACKSCATTER DIFFRACTION (EBSD)

EBSD is a technique based on measurement of local crystallographic orientation. EBSD measures the orientation of very small volumes of material at specific points on the surface of a sample by using electron diffraction in an electron microscope. Since this technique provides orientation information at a sub-micron spatial location, it enables to relate the crystal orientation to the microstructure. The fact that the orientation of a single point in a sample can be distinguished, gives EBSD the possibility of becoming a fast and automated orientation analysis method, in which orientation maps can be constructed with increasing spatial resolution, in most modern systems as small as 0.05µm (Wright 2000, Humphreys 2001).

The specimen for EBSD should be positioned in a SEM in such a way that a small angle is formed between the incident beam and the surface of the sample, typically 20. This reduces the path of the electrons that are backscattered in the specimen when the electron beam hits the sample, and increases their chances to be detected. The electron backscatter pattern (or so-called Kikuchi pattern) produced in this way is detected by a phosphor screen, and is the fundamental principle of EBSD. The Kikuchi pattern is unique according to the crystal structure and its orientation with respect to a reference frame. Special algorithms have been developed to solve Kikuchi patterns, which allows determining the crystal structure and the orientation of each point. Figure 1.8 (Randle 2001) illustrates the main components of an EBSD acquisition system.

Figure 1.8 Components of an EBSD acquisition system (Randle 2001)

Modern computer systems allow the scanning of millimetre scale areas and automatic storing of the crystallographic orientation of millions of points. In modern systems each point can be stored at a rate as high as 400s^-1, with angular accuracy of ~1. Speed, spatial resolution and accuracy depend on several factors such as specimen geometry, material, algorithm parameters, microscope voltage/current, and surface preparation (Wright 2000, Randle 2003, Humphreys 2001)

Texture analysis can be divided into two groups: global texture or macrotexture, and local texture or microtexture analysis. Global texture corresponds to the sampling of large areas and/or volumes with multiple orientations, whereas local or microtexture comprises measurements of small areas. The nature of microtexture brings some additional characteristics to the representation of texture. The crystallographic

Texture Evolution during -quenching of a Zirconium Alloy 55 orientation not only can be related to the microstructure but can also be shown with a clear spatial distribution. Analysis of local misorientation is one of the most important advantages of microtexture and EBSD, and was an important part of this work. An important advantage of EBSD is that the orientation information of each point is obtained directly, normally as a set of three Euler angles. This kind of data can be easily represented, interpreted and mathematically handled. The visualisation of orientation maps is one of the novel and interesting features of EBSD, as well as quantification of other orientation aspects such as misorientation, grain boundaries and global texture statistics (Randle and Engler 2000).

Orientation Distribution Functions

In EBSD measurements, statistical distribution of orientation data is often aimed to quantify the orientation distribution within a specific region of the microstructure.

Nevertheless, if the regions mapped are large enough with respect to the grain size, the global texture can be determined, which can be comparable to the results of macrotexture measurements obtained using other methods such as LXRD. However, with EBSD it is important to assess critically the volume of material sampled in order to represent quantitative macrotexture reliably. Some inaccuracies of the conventional pole figure inversion processes discussed above are overcome with EBSD, since grain orientations are measured directly rather than calculated from crystal plane distributions. Provided that enough grains are measured, ODFs obtained from EBSD

measurements produce the true orientation distribution in the sample (Randle and Engler 2000). The question is how many grains should be measured to obtain an acceptable ODF, which is difficult to determine since the answer depends on the texture strength. It has been shown that 100 grains are enough to locate the main features of the ODF, while a minimum of 1000-2000 grains would be required to evaluate the intensity of these main components and reveal secondary features, depending on the sharpness of the texture and the symmetry of the crystal (Bozzolo et al. 2007) .

Misindexing and Pseudosymmetry

Occasionally, points in an EBSD orientation map are wrongly indexed, meaning that they are given a wrong value of orientation, or are identified as an incorrect phase. This can occur if the diffraction pattern is symmetrical and/or the quality of the pattern is not good enough. Indexing errors are a particular problem in crystals for which pseudosymmetric Kikuchi patterns exist. A typical case of pseudosymmetry is seen in hexagonal crystals: patterns of crystals having a misorientation of ±30 about the 



c axis 

are extremely similar. Consequently, when scanning  zirconium it is common to find points indexed wrongly with this 30 rotation. In cubic metals misindexing is very rare, and misindexed points are normally isolated and are highly misoriented with respect to the adjacent points, so they can easily be recognised. This problem can be minimised either by enhancing the quality of the diffraction patterns by means of different sample preparation, increased voltage or probe current, or by improving the indexing

Texture Evolution during -quenching of a Zirconium Alloy 57 parameters such as calibration or number of bands for pattern solution. Misindexed points can often be systematically removed and extrapolated during post-processing of EBSD data (Humphreys 2001).

Sample Preparation

A disadvantage of EBSD is the difficulty to prepare samples of certain materials, since extensive procedures are sometimes required to achieve just acceptable diffraction patterns. Zirconium alloys are soft and ductile metals, which makes them difficult to prepare by standard methods because of the ease with which they form mechanical twins during cutting and grinding. Some compression-mounting pressures are strong enough to produce twinning, which makes cold resins preferred. Sectioning, mounting, grinding and polishing must be carefully controlled (Vander Voort 1999).

Smearing and flow can easily occur in zirconium alloys during grinding and polishing.

Grinding and polishing removal rates must be low. Eliminating all the scratches and deformation can be difficult. Copious water-cooling has to be used with long grinding periods. Only fresh paper should be used. Swab etching between grinding steps is commonly used. If samples are finished mechanically, attack polishing might be necessary (Vander Voort and Van Geertruyden 2006).

Zirconium is quite inert, thus it can be attacked only by hydrofluoric acid (HF) and solutions of nitric (HNO3) and hydrochloric (HCl) acids. For optical microscopy,

however, zirconium offers an advantage: it normally responds well to polarised light as a result of its anisotropy. Chemical and electrolytic polishing procedures are widely used. Chemically polished or electropolished surfaces normally respond well to polarised light. Most of the chemical polishing solutions contain HF, which attacks the surface, and HNO3, which prevents staining. Water, hydrogen peroxide, glycerine or lactic acid can be used as solvents. Most are applied vigorously by swabbing, which also facilitates stain removal (Vander Voort 1999). Specific EBSD sample preparation procedures employed in this project are discussed in sections 3.3 and 3.5.