Electron microscopy of defect clusters produced by radiation damage
The methods of characterization of different types of radiation-induced defect clusters bytem have been reviewed. Point defects produced in irradiated materials agglomerate in two or three dimensional clusters to reduce the strain energy associated with them. Two-dimensional clusters assume the configuration of vacancy or interstitial type dislocation loops which can be resolved if the size of the loops is large compared to the extinction distance associated with the imaging reflection. The small loops give rise to a black dot contrast under the kinematical and a black-white contrast under the dynamical imaging conditions. The method of characterization of dislocation loops which include the determination of the nature of the loop, the Burgers vector and the loop plane normal is discussed taking examples from the work done on the ion irradiated Ni4Mo samples. A summary of available experimental results on the characterization of dislocation loops in different metals and alloys having fcc, bcc and hcp structures is presented. The contrast from stacking fault tetrahedra which form in some fcc metals and alloys after a certain degree of annealing is also discussed. The optimum conditions for imaging three-dimensional clusters or voids are derived on the basis of the contrast theory proposed for such defects. Special reference is made to the usefulness of “through focus analysis” in the imaging of very small cavities (with diameters as small as about 10 Å).
It is shown that the formation of disordered zones resulting from displacement cascades in the ordered matrix can be utilized in determining the shape and the volume of cascades in the virgin state. The importance of different contributing factors like the strain contrast and the structure factor contrast in producing the overall contrast from the disordered zones is discussed. Detailed observations on the shape of the disordered zones are shown to be important to establish the occurrence of the replacement collision sequence and the formation of sub-cascades.
Volume 42 | Issue 6
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