EFFECTS OF DEBYE-WALLER FACTORS AND COMPOSITIONAL UNCERTAINTIES ON THE 200 STRUCTURE FACTOR REFINEMENT IN GAMMA-TIAL

The effects of uncertainties in Debye-Waller factors and chemical comp osition on 200 structure factor refinement in gamma-TiAl have been inv estigated. Energy-filtered convergent-beam electron diffraction has be en used to record 200 rocking curves from two different thicknesses of a thin foil taken from a Ti-51.3 +/- 1 Al alloy (all compositions in atomic per cent). The structure factor has been refined by matching ro cking curves determined by experiment to those theoretically calculate d using the Bloch wave formalism of the dynamical theory of electron d iffraction. The influence of uncertainties in the Debye-Waller factors and sample composition have been assessed: for samples deviating from stoichiometry, the need to use accurate site-specific Debye-Waller fa ctors has been demonstrated. Also, to extract accurate structure facto rs, it is necessary to make use of homogeneous samples, the compositio ns of which must be known to better than +/-0.2%, a rather restrictive limitation.