Abstract
In this work a high voltage electron microscope has been used to produce and image voids in nickel. From these experiments the dislocation bias has been estimated to be 6. 5% and it has been shown that voids also have a bias of about 0. 5% for interstitials over vacancies. A computer program has been written which allows investigation of the strain field existing in the matrix material surrounding voids and bubbles. This program simulates the electron microscope contrast of defects using the two-beam and column approximations and can incorporate realistic pore shapes and strain fields. Results are presented both making the assumption that the strain fields are spherically symmetric and not assuming this constraint. In the latter case a Green's function method has been applied to calculate the defect strain field. This program has been used to study the effects on the image of varying the parameters involved such as defect depth, size and shape, foil thickness and orientation, diffraction vector and deviation from the Bragg position. Images are displayed at the in focus and defocussed image planes and are compared with experimental micrographs of helium bubbles in vanadium and niobium/1% zirconium. . The simulated images agree well with the experimental images and have shown that the extent of the strain contrast is very limited even at large g and with high gas pressure in the pore. Experimental images are often taken with the microscope defocussed in order to enhance the pore edges. It is demonstrated that the small strain contrast will be reduced and may even be Lost completely when the microscope is operated in this condition.