![]() taking picture of the viewing screen, recording with conventional CCD or CMOS cameras or with pixelated detectors). selected area (SAED), nanobeam (NBD), convergent beam (CBED)) and also on the means of recording (e.g. There is a wealth of information in such 4D data sets, depending both on the type of electron diffraction recorded (e.g. Others call it scanned electron diffraction (SED). This mode of data collection produces a four-dimensional (4D) data set, since at each pixel of a 2D image a 2D-ED is saved. There is also an additional possibility to record a 2D-ED at each position of the beam (each position corresponding to a pixel of the HAADF image). a high angle annular dark-field (HAADF) detector). The beam is scanned line-by-line along a rectangle and an image is formed by a pre-selected electron detector (e.g. Scanning transmission electron microscopes (STEMs) brought the possibility to record such 2D-ED patterns systematically from many locations in a single experiment. This 2D-ED pattern carries structural information about the given location of the sample. Similarly, an electron diffraction (ED) pattern recorded at any locations of the sample is also 2D. Traditional electron microscopic imaging evaluates two-dimensional (2D) images recorded with different contrast mechanisms. Application is exemplified on the (fcc, hcp and bcc) phases in a sample with 4 major components (Co, Cr, Fe, Ni). These phase and orientation maps can complement usual compositional maps collected in the same STEM with energy dispersive x-ray spectrometers (EDS) to give a complete description of the crystalline phases. Many STEMs can collect such a four-dimensional electron diffraction (4D-ED) data sets by proper selection of microscope parameters, even if this fact is not over-emphasized in the operating manuals. The patterns, which are recorded independently from this program by a CCD or CMOS camera or by a pixelated camera are in Tif format, serve as input to DiffMap. The program runs in the Windows operating system on IBM PC compatible computers. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.A free computer program, called DiffMap, is presented for off-line evaluation of both phase maps and orientation maps from a large number of diffraction patterns recorded with a nearly parallel nano-beam scanned line-by-line over a rectangular area in a scanning transmission electron microscope (STEM). We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. Under ultrafast shock wave compression, rather than the transformation from a-Zr to the more disordered hex-3 equilibrium x-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. ![]()
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