E. L. Hall

On producing high-spatial-resolution composition profiles via scanning transmission electron microscopy

Abstract Measurement of the concentration variation of a grain boundary segregant near a grain boundary has been chosen as a system in which to investigate the spatial resolution attainable by X-ray microanalysis in the scanning transmission electron microscope (STEM). In this paper extensive experimental work on Fe-doped MgO is compared with a theoretical model which examines the effect of incident probe size and electron beam broadening in the sample on concentration profiles measured using standard analysis of X-ray data. It is shown that the spatial extent of segregation can be determined to a resolution dependent on the incident probe size. The magnitude of the peak concentrations determined at the boundary are, however, strongly dependent on beam broadening and hence foil thickness. Comparing experimental and calculated results suggests that the extent of beam broadening may not be as great as current theoretical estimates would predict.

On the nature of extended dislocations in deformed cadmium telluride

Abstract Glide dislocations in specimens of single-crystal CdTe, which had been deformed at 200°C and 300°C, are shown by weak-beam, dark-field transmission electron microscopy to be dissociated into two Shockley partials. The partial separation is calculated to correspond to a stacking-fault energy of 10·1 ± 1·4 erg/cm2. The presence of dissociated dislocations strongly suggests that the dislocations in CdTe are of the aB type (glide set). Constrictions are observed to occur at random intervals along the extended edge and near-edge dislocations in the samples, and the possible origins and nature of these constrictions are discussed.

Grain boundary segregation of iron in polycrystalline magnesium oxide observed by STEM

Abstract Utilizing X-ray microanalysis in samples studied with scanning transmission electron microscopy, substantial grain boundary segregation of Fe is found in grain boundaries in polycrystalline MgO. Samples studied contained between 540 and 5400 cation ppm Fe. Grain boundary segregation in the region within 200 A of the boundary was found in all cases, with the amount of segregation increasing with increasing Fe content. Samples slow-cooled from 1500°C showed an increased amount of segregation attributed to continued diffusion during cooling. Samples quenched from 1100°C showed greater segregation than those quenched from 1500°C, consistent with space charge theory. In some samples precipitates were observed which are correlated with a low-temperature phase diagram for the solidus in the iron oxide-magnesium oxide system. In addition to Fe, grain boundary segregation of Ca and Si impurities in the samples was also observed.

On projected widths of stacking faults used for foil thickness determinations

Abstract Transmission electron microscopy studies of stacking faults in a Co-based alloy show that under some microscopy conditions the width of the bright-field image of a fault is significantly less than the width of the corresponding weak-beam dark-field image, the latter providing a good indication of the true fault width. Further microscopy studies and examination of the equations describing the contrast caused by a stacking fault in a perfect crystal demonstrate that the discrepancy in observed width exists for faults with a negative phase angle α, when the deviation of the foil from the perfect Bragg reflecting position is greater than zero. An additional condition is that the foil thickness must be in the range ξ g ≤t≤2ξ g , where t is the thickness and ξ g the extinction distance. This effect can lead to serious errors in the calculation of foil thickness in materials of low stacking fault energy, where a common method of thickness determination is based upon the projected width of stacking fault...

Defects formed during quenching in Al-4 pct Cu

Thin foils of Al-4 pct Cu alloys were examined by transmission electron microscopy in the as-quenched and the aged states in an attempt to clarify the mechanism of the prolonged low-temperature aging phenomenon in these alloys known as the slow reaction. The asquenched microstructure is found to consist of bands of vacancy clusters or small dislocation loops with helical dislocations sometimes associated with them. It is postulated that these defects are formed by the interaction of vacancy agglomerates and dislocations during the quench. The contrast images associated with the defects are found to vary in radius from approximately 15 to 85 Â, and in density roughly from 1 to 5 x 1015/cm3. These results are consistent with the calculations of Okamoto and Kimura, who showed that the slow reaction could best be explained as being caused by supersaturated vacancies in equilibrium with the surface tension of defects of this size. As the slow reaction proceeds, these vacancies are annihilated at permanent sinks, and this disturbance of equilibrium causes the defects to emit vacancies and, hence, shrink and disappear completely in the aged state.