Microstructure evolution in directionally solidified Fe-18Cr stainless steels

Directional solidification experiments were conducted in commercial Fe-18Cr stainless steels with 0, 3, 5 wt.% Al contents. The unique feature of this alloy system is the narrow freezing range that causes microstructure transitions to occur at relatively low solidification rate which can be achieved by using the directional solidification technique. The velocity range over which dendrites form was found to decrease, whereas the velocity range over which high-velocity cells were present increased significantly, as the aluminum content of the alloy was increased. The transition conditions for dendrite to high-velocity cells were investigated. Cellular and dendritic spacing variation with velocity was also characterized, and the cell spacing was found to vary continuously with growth rate for the low and high-velocity cells. Based on these results, theoretical models are discussed to characterize microstructure transitions from planar to cellular and from dendritic to high-velocity cellular microstructures.