Abstract
Recent improvements to the atom location by channeling-enhanced microanalysis (ALCHEMI) technique now allow reliable measurements of site distributions in ordered alloys. Early zone-axis ALCHEMI of Ll2-ordered Ni3Al-based alloys was plagued by ionization delocalization; for Ni76Al21Hf3at <001> the fraction of Hf on the Al sublattice, pHf ‘Al’, = −127%. Reanalysis by statistical ALCHEMI was no better, giving pHf ‘AT’ = −220 ± 190%, contrary to the claims that delocalization effects should be minimized. The same data set analyzed with the reformulated ALCHEMI method yields pHf ‘Al’, = 80 ± 25%.Further experiments have been performed at ∼140K in a Philips CM12 operated at 120 kV and equipped with an EDAX 9900 spectrometer and superUTW Si(Li) detector. The diffracting conditions were carefully adjusted with the beam (not specimen) tilt, a probe size of ∼200 nm and beam divergence of ∼3 mrad were used, and the current in the probe was monitored and held constant at ∼1 nA.
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