SOLIDIFICATION OF UNDERCOOLED FE-CR-NI ALLOYS .1. THERMAL-BEHAVIOR

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pet Fe, with Cr varying from 15 to 19.7 wt pet, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pet) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, al samples solidified with bcc as the primary phase; the bcc then transformed to fee during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fee may occur at bcc/bcc grain boundaries.