Diffraction Contrast from Dissociated Frank Dislocations. I. Computed Electron Micrographs

Abstract

SummaryThe influence of degree of dissociation on the diffraction contrast from a Frank dislocation for lll, 220, and 020 reflections has been investigated using the technique devised by Head and Humble for computing electron microscope images.The diffraction contrast from a dissociated Frank dislocation differs in several ways from that of an undissociated dislocation. For 111 reflections, strong contrast occurs when g. u oF O. For 220 reflections, images are single and continuous with strong contrast on one side only. The strong contrast does not invert from side to side for +g and -g. For 020 reflections the extent to which reversal of contrast occurs in ± g is a function of g, the beam direction, and the separation of the Shockley and stair-rod dislocations.The computations for an undissociated Frank dislocation are in general agreement with previous computations, but indicate that a Frank dislocation may show strong contrast for certain beam directions when g. b = ±l.

On the mechanism of twin formation in Fe–Mn–C TWIP steels

“…14. According to the results of Clarebrough and Morton [34], the contrast of the sessile dislocation in Fig. 14 corresponds to the a 3 ½111 Frank dissociated dislocations. It can be thus concluded that, since the reaction of Eq.…”

“…(15) is energetically favourable, the Frank dislocations were relaxed after unloading. Clarebrough and Morton [34] reported that, in many cases, the separation distance between the two resulting dislocations of Eq. (15) does not exceed a few angstroms, which makes the observation of such dissociation impossible in conventional TEM.…”

“…Clarebrough and Morton [34] studied such dissociation by comparing the experimental and the simulated TEM contrasts of Frank dislocations. The authors observed that dissociated Frank dislocations exhibit a characteristic contrast with a light band separating the dislocation contrast and the stacking fault fringes in micrographs obtained with g ¼ 0 0 2, W = 0 and g Á u = 0 (u corresponds to the orientation of the dislocation line).…”

Precipitate assemblies formed on dislocation loops in aluminium-silver alloys

“…Hardness measurements Hardness curves obtained from Vickers hardness tests on the binary alloys are shown in Figure 1 , however there is no reversal of the dislocation line strain contrast (indicated with hollow arrows). The latter feature is characteristic of a dissociated Frank loop [12,13]. The presence of the fine-scale silver-rich spheroidal GP zones is shown in the enlarged inset in (a).…”

A review of the determination of dislocation parameters using strong‐ and weak‐beam electron microscopy

“…In subsequent papers (Clarebrough & Morton, 1969;Clarebrough, 1971) these empirical rules were somewhat modified. They remained valid for Shockley dislocations with b, = a/6(112) under conditions of gb, = f 1/3.…”