Experiments in high voltage electron microscopy. Progress report, October 31, 1976--August 1977

High voltage electron microscopy (HVEM) is being used to study the effects of irradiation on a variety of materials. The vacancies and interstitials produced by displacement can aggregate to form dislocation loops and voids, annihilate at sinks, or enhance various diffusion processes such as precipitation and recrystallization. Threshold displacement energies, E/sub d/, have been determined for a number of fcc, bcc and hcp metals and for various oxides. In MgO, E/sub d/ is less along (100) than (110); also, E/sub d/ decreases with increasing temperature, possibly due to thermally activated escape of interstitials from recombination volumes or softening of saddle points. The effects of electron irradiation on precipitation in Al--Cu, Al--Si and Ni--Al alloys have been investigated. Precipitation respectively of theta', Si and γ' is enhanced and growth rates are explicable in terms of theories of radiation-enhanced diffusion, with D/sub rad/ approx. 10-15 cm2 sec-1. In oxides, damage gives rise to interstitial dislocation loop nucleation and growth in all cases, perfect [110] loops in MgO faulted basal and prismatic loops in Al2O3. Quantitative analysis of loop growth rates in MgO gives a migration energy of 3.3 eV for anion vacancies. Other radiation effects include sublimation of MgO and decomposition of MgAl2O4 and Mg2SiO4 into MgO plus other phases