Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at, pet were undercool ed to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fee phase in the Fe-10 a t. pet Co alloy and the metastable bcc phase in the Fe-30 at. pet Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both allo ys. No metastable bce phase can be observed in the Fe-60 at, pet Co alloy, even when solidified at the maximum undercooling of Delta T = 312 K. Micros tructural investigation shows that the grain size in Fe-10 and Fe-30 at, pe t Co alloys increases with undercoolings when the undercoolings of the melt s exceed the critical undercoolings. The grain size of the Fe-60 at. pet Co alloy solidified in the undercooling range of 30 to 312 K, in which no met astable phase can be produced, is much finer than those of the Fe-10 and Fe -30 at. pet Co alloys after the formation of metastable phases. The model f or breakage of the primary metastable dendrite at the solid-liquid interfac e during recalescence and remelting of dendrite cores is suggested on the b asis of microstructures observed in the Fe-10 and Fe-30 at. pet Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystall ization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.