Papers by Author: Edgar F. Rauch

Abstract: EBSD is a well known technique that allows orientation and phase mapping using an SEM. Although the technique is very powerful, has serious limitations related with a) special resolution limited to 50 nm (SEM-FEG) and b) specimen preparation issues as is not possible to obtain EBSD signal from rough surfaces or strained materials , nanoparticles etc.. To address those difficulties , a novel technique has been developed recently (EBSD-TEM like) allowing automatic orientation and phase mapping using template matching analysis of acquired diffraction patterns in TEM. Electron beam is scanned through the sample area of interest ; the acquired electron diffraction patterns from several sample locations are compared via cross-correlation matching techniques with pre-calculted simulated templates to reveal local crystal orientation and phases. The dedicated device (ASTAR) allows orientation and phase identification of crystallographic orientation in a region of interest up to 10µm2, with a step size ranging from 1nm to 20nm depending on the transmission electron microscope setting (FEG or LaB6).

Abstract: Precession electron diffraction (PED) is a new promising technique for electron diffraction pattern collection under quasi-kinematical conditions (as in X-ray Diffraction), which enables “ab-initio” solving of crystalline structures of nanocrystals. The PED technique may be used in TEM instruments of voltages 100 to 400 kV and is an effective upgrade of the TEM instrument to a true electron diffractometer. The PED technique, when combined with fast electron diffraction acquisition and pattern matching software techniques, may also be used for the high magnification ultra-fast mapping of variable crystal orientations and phases, similarly to what is achieved with the Electron Backscattered Diffraction (EBSD) technique in Scanning Electron Microscopes (SEM) at lower magnifications and longer acquisition times.

Abstract: Equal-channel angular pressing (ECAP) at 443 K was used to introduce an ultra-fine grained (UFG) microstructure to a Zr and Sc modified 7075 aluminum alloy. Using the methods of TEM and EBSD, an average grain size of 0.6 1m was recorded after the pressing. The UFG microstructure remained very stable up to the temperature of 723 K, where the material exhibited high strain rate superplasticity (HSRSP) with elongations to failure of 610 % and 410 % at initial strain rates of 6.4 x 10-2 s-1 and 1 x 10-1 s-1, respectively. A strain rate sensitivity parameter m in the vicinity of 0.45 was observed at temperatures as high as 773 K. At this temperature, the material still reached an elongation to failure of 430 % at 2 x 10-2 s-1. These results confirm the stabilizing effect of the Zr and Sc additions on the UFG microstructure in a 7XXX series aluminum alloy produced by severe plastic deformation.

Abstract: The dislocation substructure that appears in deformed metals and alloys have been extensively investigated in the past by transmission electron microscopy (TEM). They are known to form a broad variety of microstructures. These substructures are characterized by three main parameters, namely the density of the dislocations that are trapped in the tangles, their degree of patterning and the misorientation between the cells. The aim of the present work is to investigate the relationship between these features and the mechanical properties of the material.

Abstract: An automatic crystallographic orientation indexing procedure is developed for transmission electron microscopes. The numerical identification is performed by mapping the spot diffraction patterns with pre-calculated templates. The diffraction patterns are acquired thanks to an external CCD camera that points to the fluorescent screen through the TEM window. Orientation maps with spatial resolution better than 10 nm were obtained with this low cost equipment.

Abstract: A two parameters rupture criterion for Ti-6Al-4V is proposed, based on cryogenic rupture tests. microstructural observations and FEM analysis,. The damage mechanisms including the cavities nucleation process are analyzed. Moreover, the final steps of rupture are discussed with the help of the thermo-mechanical coupling mechanisms that are expected to operate at low temperatures.