In situ observations of precipitation and recrystallization with a 1 MeV electron microscope

Abstract

SUMMARYWhen fitted with a heating stage and suitable equipment for continuous image recording, the high voltage electron microscope is a powerful tool for the direct observation of recrystallization and precipitation in thin metal foils. However, the behaviour of thin specimens is sometimes not even qualitatively typical of bulk material. For example, true precipitation from supersaturated solid solution can only be observed if vacancies play no significant part in the process as in iron‐nitrogen alloys. In this case, the formation of the intermediate precipitates, α“—Fe8N, and the equilibrium phase, γ—Fe4N, is qualitatively and in some respects quantitatively typical of bulk material even in very thin foils. However, extensive electron displacement damage should be avoided because it seriously retards precipitate growth.It is shown elsewhere (Roberts & Lehtinen, 1972) that true bulk recrystallization is only possible in thick areas of an electron microscope specimen. This is confirmed for several metals in the present work. The critical specimen thickness for bulk behaviour is generally between 3–800 nm depending the degree of cold work etc.; this concurs with earlier heating‐stage experiments at 100 kV in which true recrystallization was not observed.