Evolution of microstructures resulting from dendrite growth in undercooled Fe-Cr-Ni melts

Experiments have been carried out For two alloys Fe69Cr1Ni30 and Fe69Cr7Ni24 to investigate the evolution of microstructures of steels grown from undercooled melts. The microstructures undergo several critical transitions as the undercooling at which solidification starts is increased. Firstly, grain size can change by up to an order of magnitude from small to large and back to small at critical undercoolings. Secondly, the segregation pattern changes from dendrilic/equiaxed to dendritic and finally to equiaxed with increasing undercooling. Measurements from the segregation patterns indicate that the primary dendrites had undergone considerable Ostwald ripening depending on the processing conditions (heat-extraction rate). Relating the measured trunk sizes to an original trunk size through ripening and then applying this value to a recent model for grain refinement, which is based on dendrite fragmentation, yields a rough approximation to the critical transitions of the grain size. (C) 1998 Acta Metallurgica Inc.