Biomedical subjects

AJ Wilkinson

Publications and source records attributed to AJ Wilkinson.

4 recordsLinked to original sources

PubMed1999

Decoherence in electron backscattering by kinked dislocations.

A model is proposed that explains the origin of the bright contrast of dislocation walls consisting of edge dislocation dipoles in electron channelling contrast images (ECCI) of fatigued crystals, when the incident beam is parallel to the edge dislocations. The model is based on the assumption that the contrast arises from the dislocation segments terminating the dipoles. These are modelled as screw-type kinks which scatter electrons. Scattering by randomly distributed kinks leads to the randomization of phase of transmitted and diffracted beams and suppresses the anomalous transmission of electrons. The predicted behaviour of electron-channeling contrast images agrees well with experimental observations.With apologies for using two beams instead of three!

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PubMed1999

Influence of grain orientations on the initiation of fatigue damage in an Al-Li alloy.

The variation in microstructure and texture in a rectangular bar extruded from a billet of spray-cast 8090 Al-Li alloy has been examined. The fine grain size of the as sprayed billet and the moderate extrusion ratio ( approximately 25 : 1) were seen to cause geometric dynamic recrystallization (GDR) in regions of higher strain towards the edge of the bar. The grain morphology varied from the expected elongated grains at the centre of the bar to equiaxed grains where GDR occurred at the bar edges. A <111> + <100> double fibre texture, significantly distorted towards rolling components and varying through the bar thickness, was found using electron backscatter diffraction. Fatigue resulted in a high density of short secondary cracks, many of which had arrested at grain boundaries. The cracks preferentially nucleated in grains from the <100> fibre texture corresponding to high Schmid factors.

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PubMed1999

Study of dislocation structures near fatigue cracks using electron channelling contrast imaging technique (ECCI).

The fatigue of copper single crystals, orientated for single slip, has been studied using electron channelling contrast imaging in a scanning electron microscope. With the incident beam set at the Bragg condition, changes in the backscattered electron intensity occur as the beam is scanned over dislocations that cause a local tilting of the diffraction planes. This technique allows the evolution of dislocation structures over large areas to be followed through different stages of the fatigue life. Furthermore, it enables direct imaging of dislocation configurations at crack tips. The technique is compared with transmission electron microscopy and electron backscatter diffraction in its application to fatigue studies.

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PubMed1999

Introduction.

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