Diffraction contrast dependence on sample thickness and incident energy in on-axis Transmission Kikuchi Diffraction in SEM.

Automated orientation mapping in SEM, until now relying on EBSD solely, is currently being improved with the development of the TKD technique. As part of the development of TKD, we introduce a new, TEM-like geometric configuration, with a detector "on-axis" relative to the electron beam, while the detector was "off-axis" in its first form. This new technique produces a wide range of diffraction contrast (spots, lines, bands), varying with sample thickness, incident energy, atomic number and scattering angle. Some of the main trends are identified and discussed. In particular, a model based on the plasmon and phonon scattering is proposed to account for the disappearing of diffraction spots with thickness and incident energy. This work should help experimentalists determine which microscope and sample parameters to use in order to obtain a specific contrast. Finally, the strength and weakness of each diffraction feature for orientation mapping are also reviewed.

On-axis versus off-axis Transmission Kikuchi Diffraction technique: application to the characterisation of severe plastic deformation-induced ultrafine-grained microstructures.

A new configuration for Transmission Kikuchi Diffraction (TKD) in a scanning electron microscope is presented; called 'on-axis TKD'. Compared to the usual off-axis configuration, the scintillator is placed perpendicular to the incident beam under the electron-transparent sample, not in vertical position. In this way, the setup benefits from intense forward scattered electrons enabling short acquisition times. At equivalent diffraction pattern quality, the electron dose needed on the sample is estimated to be 20 times lower in comparison to the off-axis configuration. The technique is particularly suited to the characterisation of severe plastic deformation induced ultrafine grained microstructures. The evolution of the microstructure of an Al-Mg alloy deformed by high pressure tube twisting was analysed. It is shown that the grain refinement was in the steady state stage for a shear strain of 24 with a mean grain size of 120 nm.

Orientation mapping by transmission-SEM with an on-axis detector.

Conventional orientation mapping in a scanning electron microscope (SEM) is a valuable technique for characterizing crystalline materials, but its application to ultrafine or nano-grain materials is limited by its spatial resolution. The resolution can be increased by collecting transmission diffraction patterns in SEM. In previous works, such patterns were collected using off-axis detectors in nearly vertical position. To avoid some drawbacks of such arrangement, a new configuration was devised in which the scintillator is located underneath the thin foil on the optical axis of the microscope, and the light is reflected towards the camera by a mirror. This simple configuration gives intense patterns even at very low probe currents, and can be potentially used for collecting maps of relatively high spatial resolution. Example maps reveal details with dimensions of about 5nm. Because of its resolution and geometric simplicity, the proposed configuration will open new opportunities in SEM-based characterization of nanocrystalline materials.

Orientation precision of TEM-based orientation mapping techniques.

Automatic orientation mapping is an important addition to standard capabilities of conventional transmission electron microscopy (TEM) as it facilitates investigation of crystalline materials. A number of different such mapping systems have been implemented. One of their crucial characteristics is the orientation resolution. The precision in determination of orientations and misorientations reached in practice by TEM-based automatic mapping systems is the main subject of the paper. The analysis is focused on two methods: first, using spot diffraction patterns and 'template matching', and second, using Kikuchi patterns and detection of reflections. In simple terms, for typical mapping conditions, their precisions in orientation determination with the confidence of 95% are, respectively, 1.1 ° and 0.3 °. The results are illustrated by example maps of cellular structure in deformed Al, the case for which high orientation sensitivity matters. For more direct comparison, a novel approach to mapping is used: the same patterns are solved by each of the two methods. Proceeding from a classification of the mapping systems, the obtained results may serve as indicators of precisions of other TEM-based orientation mapping methods. The findings are of significance for selection of methods adequate to investigated materials.

Determination of γ-γ' lattice misfit in a single-crystal nickel-based superalloy using convergent beam electron diffraction aided by finite element calculations.

In single-crystal nickel-based superalloys, the lattice mismatch associated with interface coherency between γ matrix and γ' precipitates has a strong influence on mechanical properties. The unconstrained lattice misfit in a single-crystal of the MC2 nickel-based superalloy is determined using convergent beam electron diffraction measurements and finite element calculations. The apparent lattice parameters of both constrained phases are obtained in thin foils, using a new multi-pattern approach, which allows for unambiguous determination of all the lattice parameters considering the real symmetry of the strained crystals. Finite element calculations are used to establish relations between the constrained and unconstrained lattice parameters, with the stress relaxation resulting from the thin foil geometry taken into account.

Determination of lattice parameters from multiple CBED patterns: a statistical approach.

This study deals with the uncertainty of the measurement of lattice parameters by CBED using the kinematic approximation. The analysis of a large number of diffraction patterns acquired on a silicon sample at 93 K with a LaB(6) TEM without energy filter shows the presence of both the systematic and the random parts of errors. It is established that random errors follow the normal statistical distribution and that the precision quantified by the relative standard deviation is about 3-4 x 10(-4) for lattice parameter measurements made from single pattern. The error sources are analyzed, different ways of enhancement are reviewed, and a new approach is proposed. It is shown that both accuracy and precision can be simply improved by taking into account multiple patterns analysis for the determination of the actual voltage, the single lattice parameter "a" or the complete set of lattice parameters. The precision of about 1.5-2 x 10(-4) can be reached using a minimum of three HOLZ line patterns for the single "a" parameter and about 5 x 10(-4) for the complete set of lattice parameters using six diffraction patterns. The use of multiple patterns also allows overcoming the non-uniqueness of solution linked to the CBED studies.

Using EBSD and TEM-Kikuchi patterns to study local crystallography at the domain boundaries of lead zirconate titanate.

Reliable EBSD mapping of 90 degree domains in a tetragonal ferroelectric perovskite has been achieved for the first time, together with reliable automated orientation determination from TEM-Kikuchi patterns. This has been used to determine misorientation angles at 90 degree domain boundaries and thus local c/a ratios. The sources of orientation noise/error and their effects on the misorientation angle data have been thoroughly analyzed and it is found that this gives a cosine distribution of misorientation angles about the mean with a characteristic width related to the width of the orientation noise distribution. In most cases, a good agreement is found between local c/a ratios and global measurements by X-ray diffraction, but some clear discrepancies have also been found suggesting that real local variations are present, perhaps as a consequence of compositional inhomogeneities.

Shear banding in twinned structure of copper deformed at 77 K.

The local crystallography and microstructure within shear bands has been examined in single crystals of {112}<111> orientation of pure copper deformed at 77 K by channel-die compression to true strains of about 1. Setting up a system for making high-resolution orientation maps using transmission electron microscopy has provided advantageous circumstances for the analysis of orientation changes within shear bands. The present work shows that, despite the plane strain deformation mode, the mechanism of lattice rotation within emerging shear bands may lead to Goss {110}< 001> and Brass{110}<112> texture components.

Polycrystal orientation maps from TEM.

Determination of topography of crystallite orientations is an important technique of investigation of polycrystalline materials. A system for creating orientation maps using transmission electron microscope (TEM) Kikuchi patterns and Convergent beam electron diffraction patterns is presented. The orientation maps are obtained using a step-by-step beam scan on a computer-controlled TEM equipped with a CCD camera. At each step, acquired diffraction patterns are indexed and orientations are determined. Although, the approach used is similar to that applied in SEM/electron back scattered diffraction (EBSD) orientation imaging setups, the TEM-based system considerably differs from its SEM counterpart. The main differences appear due to specific features of TEM and SEM diffraction patterns. Also, the resulting maps are not equivalent. On these generated by TEM, the accuracy of orientation determination can be better than 0.1 degrees. The spatial resolution is estimated to be about 10nm. The latter feature makes the TEM orientation mapping system an important tool for studies at fine scale unreachable by SEM/EBSD systems. The automatic orientation mapping is expected to be a useful complement of the conventional TEM contrast images. The new technique will be essential for characterization of fine structure materials. To illustrate that, example maps of an aluminum sample produced by severe plastic deformation are included.