Directional solidification and solid state transformation in an Fe-Cr alloy

Allotropic materials, such as steels, show several successive transformations during cooling. For example, in Fe-Cr alloys with composition lower than 12 wt.% Cr, two successive solid state transformations occur after solidification of the ferrite: alpha-ferrite changes to gamma-austenite and, at lower temperature, gamma-austenite transforms to alpha-ferrite. Similar transformations take place in steels with many other alloying elements such as Ni and C. The primary solidification microstructure and segregation influence the above transformations. Therefore, it is of interest to get more insight into the growth kinetics of the various transformation products and their mutual influences. This paper presents results of the Bridgman directional growth experiments in an Fe-Cr alloy under a constant temperature gradient. The observations are compared with microstructural models. For instance, the massive transformation can be explained by the plane front growth due to absolute stability rather than due to solute trapping.