Emmanuel Bouzy

Mechanically driven syntheses of carbides and silcides

Most metal carbides or slicides may be synthesized at room temperature, by ball milling mixtures of elemental powders for some tens of hours with a vibratory mill. Both stable and metastable compounds containing a high density of defects can be obtained. In general, phases stable at low temperatures are synthesized. This observation allows us to confirm recent estimations for the maximum temperature (∼600 K) attained in these powders during mechanical alloying. Exceptions are found for some MSi2 suicides with M = titanium, iron or molybdenum for which both low- and high-temperature phases are formed.

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.

Influence of the nature of milling media on phase transformations induced by grinding in some oxides

Abstract Polymorphic transformations induced by dry ball milling in an argon atmosphere have been investigated in various oxides (TiO2, SnO2, Y2O3, WO3) by X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy (TEM). The transformations have been found to depend on the nature of the milling media, particularly when reduction reactions take place between grinding tools and oxide particles. When ground with steel tools, cubic yttria is transformed into a monoclinic modification as reported in the literature, while tin oxide is reduced. When ground with zirconia tools, cubic yttria with a bixbyite type structure is transformed into cubic yttria with a fluorite type structure, while monoclinic tungsten oxide is transformed into a cubic oxide with an ReO3 type structure. Plausible structural explanations are proposed. In all cases, nanometre-sized domains have been observed by TEM even after grinding times as short as some minutes.

EP – a program for determination of crystallite orientations from TEM Kikuchi and CBED diffraction patterns

Orientations of individual crystallites are of interest in many areas of research on polycrystalline materials (see e.g., Kocks et al., 1998). Analysis of Kikuchi patterns is one of the oldest and best established techniques of orientation determination. Convergent beam electron diffraction (CBED) patterns have the same geometry as Kikuchi patterns and can be used for determining orientations if a suf®cient acquisition solid angle is available. The program EP is a user interface linked with KiKoCh ± an engine for pattern indexing and for determination of orientations. EP allows diffraction patterns to be loaded from bitmap ®les and the crystallite orientation to be obtained by marking lines with a mouse; in good quality patterns lines can be detected automatically. A similar system has been described recently by Zaefferer (2000).

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.

Multiscale γ variant selection in a quaternary near-γ Ti–Al alloy

Classical electron back-scattered diffraction in a scanning electron microscope (EBSD/SEM) technique and a novel approach based on computer-aided collection and indexing of TEM Kikuchi patterns were used to study the γ variant and γ|γ misorientation distributions in lamellar grains of a near-γ TiAl alloy. Coarse and fine lamellar structures obtained by continuous cooling at two different rates were investigated. The two methods were complementary and reveal the presence of one-twin-dominant zones (OTDZ) in all grains of both samples. In agreement with previous works, these OTDZ were proposed to result from a mechanism of self-accommodation through twin-variant selection at the nucleation stage. Moreover, it has been pointed out that one γ -order variant often appears to be dominant in a given OTDZ. As the previous mechanism can hardly explain this experimental evidence, the presence of back-stresses due to grain confinement is proposed as a possible explanation. The statistical analysis of γ|γ misorientation distributions, de-correlated from the effect of the non-random distribution of γ variants, reveals that γ|γ interfaces in true-twin relationship are clearly predominant in the coarse lamellar structure, but not in the fine one. This difference is discussed on the basis of different growth mechanisms recently underlined and whose respective influences depend on the transformation driving force, i.e. on the cooling rate.

Relaxation and kineticsNew metastable carbides produced by crystallization of amorphous CrC alloys

Abstract The crystallization of carbon-rich CrC amorphous alloys allows new metastable chromium carbides to be produced. The structures of these carbides, which are strongly faulted, offer to the carbon atoms prismatic interstices, octahedral interstices, or both. These results suggest that according to the carbon content, more than one type of local order exists in the amorphous state for this metal-metalloid system.

On the reliability of fully automatic indexing of electron diffraction patterns obtained in a transmission electron microscope

Orientation maps similar to electron backscatter diffraction maps can be obtained in a transmission electron microscope. A method of such mapping by automatic indexing of electron diffraction patterns has been proposed recently. The procedure is relatively simple and fast but it does not avoid the 180° ambiguity. Using tests on simulated patterns, it is shown that under current practice automatic indexing may give a considerable fraction of erroneous solutions. Optimization of measurement conditions leading to a reduction of that fraction is considered.

Textures and Anisotropy of Titanium Alloys

The present paper deals with the texture, deformation mechanisms and mechanical properties of α and near α alloys (i.e. with hexagonal symmetry matrix). For items related aspects are reviewed: 1) texture formation during thermomechanical treatments depending on the composition of the alloys 2) determination of active deformation mechanisms 3) mechanical properties depending on the texture and deformation mechanisms 4) modelling and prediction of mechanical properties. For all the alloys, the texture evolution during the whole process must be considered especially during the hot rolling taking into account phase transformations. In the same way, the texture evolutions observed during cold rolling will be explained in terms of the evolution of the microstructures and deformation mechanisms. Finally, a classification of the sheet textures is made as a function of the composition of the alloys. In titanium α or near α alloys, the deformation can be accommodated by both glide and twinning. The selection of the different mechanisms activated during plastic deformation depends of the alloying elements, the grain size, grain orientations and the condition of the test (T,e,e). We present a method to determine different types of glide (prismatic, basal, pyramidal slips) and twinning systems activated as well as their relative critical shear stress ratios. The initial textures of the sheets are generally sharp after thermomechanical treatments and many properties such as yield stress are anisotropic. In addition, yield stress and ductility can be directly correlated with the deformation mechanisms activated in the sample to accommodate the deformation. The present work demonstrates the qualitative correlations between textures, deformation mechanisms and mechanical properties for all the alloys studied. Using different approaches (Taylor or Self - Consistent models), it is possible to model the texture evolution during the deformation taking into account the initial texture and the deformation mechanisms actived, and also to predict the mechanical properties of the studied alloys. Predicted and experimental curves are compared and a discussion of the most important parameters emphasises the great importance of the behavior parameters (i.e. the deformation mechanisms for successful modelling. Insufficient knowledge of the critical resolved shear stresses remains the main obstacle and explains why some results may be questionable. In conclusion the review identifies the next steps which are necessary in order to make progress when establishing relationships between textures -microstructures - mechanical properties.

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

Reliable EBSD mapping of 90° 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° 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.