SummaryThe spatial resolution of electron diffraction within the scanning electron microscope (SEM) has progressed from channelling methods capable of measuring crystallographic characteristics from 10 μm regions to electron backscatter diffraction (EBSD) methods capable of measuring 120 nm particles. Here, we report a new form of low-energy transmission Kikuchi diffraction, performed in the SEM. Transmission-EBSD (t-EBSD) makes use of an EBSD detector and software to capture and analyse the angular intensity variation in large-angle forward scattering of electrons in transmission, without postspecimen coils. We collected t-EBSD patterns from Fe-Co nanoparticles of diameter 10 nm and from 40 nm-thick Ni films with in-plane grain size 15 nm. The patterns exhibited contrast similar to that seen in EBSD, but are formed in transmission. Monte Carlo scattering simulations showed that in addition to the order of magnitude improvement in spatial resolution from isolated particles, the energy width of the scattered electrons in t-EBSD is nearly two orders of magnitude narrower than that of conventional EBSD. This new low-energy transmission diffraction approach builds upon recent progress in achieving unprecedented levels of imaging resolution for materials characterization in the SEM by adding high-spatial-resolution analytical capabilities.

SummaryWe report the effects of varying specimen thickness on the generation of transmission Kikuchi patterns in the scanning electron microscope. Diffraction patterns sufficient for automated indexing were observed from films spanning nearly three orders of magnitude in thickness in several materials, from 5 nm of hafnium dioxide to 3 μm of aluminum, corresponding to a mass-thickness range of ß5 to 810 μg cm -2 . The scattering events that are most likely to be detected in transmission are shown to be very near the exit surface of the films. The energies, spatial distribution and trajectories of the electrons that are transmitted through the film and are collected by the detector are predicted using Monte Carlo simulations.

The stability of the pool-riffle sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of poolriffle sequence bars during high flows, offsetting gradual scour of riffles and deposition in pools during low flows. Despite several attempts, reversal has rarely been recorded in field measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase-shifted with respect to the pool-riffle sequence bedform profile, so that maximum shear stress occurs upstream of riffle crests at high flow, and downstream at low flow. Such phase-shifts produce gradients of shear stress that explain riffle deposition, and pool scour, at high flow, in accord with sediment continuity. The proposal is supported by results of a one-dimensional hydraulic model applied to the surveyed bathymetry of a pool-riffle sequence in a straight reach of a gravel-bed river. In the sequence studied, the upstream phase-shift in shear stress at high flow was associated with variations in channel width, with width minima occurring upstream of riffle crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of riffle crests. Assuming that the width variation is itself the result of flow deflection by riffle crests at low flow, and associated bank-toe scour downstream, low and high flow can be seen to have complementary roles in maintaining alluvial pool-riffle sequences.

An experimental method is described for performing thermal fatigue testing of thin films and lines on substrates. The method uses Joule heating from alternating currents to generate temperature, strain, and stress cycles in the metal structures. The apparatus has been installed in a scanning electron microscope and allows in situ observations of the fatigue damage evolution. First observations on Cu films reveal that fatigue damage forms in submicrometer thick films and is strongly affected by the film thickness and grain size. In addition, results from a special test structure confirm that the damage is caused by fatigue and not by electromigration.

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