INSITU PHASE-TRANSFORMATION IN THE FIELD-ION MICROSCOPE

Many materials undergo an athermal martensitic transformation when cooled. This transformation has been observed in the Tishomingo meteorite during cooling to cryogenic temperatures. The meteorite is unstable when cooled to the cryogenic temperatures (40-85 K) suitable for field ion imaging since the martensite start (M(s)) temperature of this material (Fe-32.5wt%Ni) is approximately 235 K. In many cases, the in-situ martensitic transformation destroyed the field ion specimen prior to imaging. It was also found that the martensite to taenite transformation could be repeatedly induced at cryogenic temperatures by cycling the voltage and hence applying stress to the specimen.