LW TEMPERATURE PHASE TRANSFORMATIONS IN WE METAlLIC PHASES OF METEORITES

The metallic phases of meteorites are predominantly composed of Fe with B to 50 wtR Mi. During slow cooling (10-1 to 103°C per million years) be low 500°C three types of phase transformations take place: 1) Fe—Mi mar tensite (m2) decomposition to c2(ferrite) and y(austenite); 2) Decomposition of austenite to a so—called “cloudy border” of a and ; and 3) Partial ordering of 48 to 55 wtR Mi austenite. These transformations have been studied by TEM, SEN and analytical electron microscopy (AME). Orientation relationships between a and ‘ were obtained by microdiffraction and the composition of the a2, a and ‘ phases were determined by x-ray microanalysis in the AEM. Using this combination of techniques insight can be gained into the mechanism of some of the above phase transformations that only occur over geological time and cannot be produced in the laboratory. Conversely, because of the high resolution of AEM, those reactions which can be repro duced in the laboratory within a reasonable time can be studied from their earliest stages.