Loss of Precipitate Coherency by Electron Irradiation in the High Voltage Electron Microscope

Abstract

THE electron microscope contrast associated with almost spherically symmetrical coherent precipitates has been understood for some time¹. The contrast results from displacements in the matrix arising from coherency of the precipitate–matrix interface. (This implies perfect continuity of precipitate and matrix lattice planes.) Mechanisms by which coherency loss can occur have recently been investigated^2,3, and the evidence so far obtained suggests that, unless the constrained mismatch between precipitate and matrix lattice parameters is greater than a certain value (∼ 5 per cent), the precipitates can grow to quite large sizes while retaining full coherency. I have observed that metastable coherent precipitates can lose coherency by interaction with dislocation loops produced directly by electron bombardment in a high voltage electron microscope.