Metastable phase formation in undercooled Fe-Cr-Ni alloy melts

Kurzfassung

The solidification behaviour of undercooled Fe69Cr31-xNix melts, which represents the basis for the technically important stainless steels, was investigated in a wide composition range (7 at.% < x < 22 at.%) with respect to the competitive formation of ferrite (bcc) and austenite (fcc). The electromagnetic levitation technique was used for undercooling of bulk samples. The primary solidifying phase was identified by the analysis of time-resolved recalescence profiles that were detected by a fast responding photo diode at a rate of 1 MHz. Under equilibrium conditions the solidification mode changes from ferrite to austenite if the atomic fraction ratio of Ni/Cr = 0.5 is exceeded. It is shown that crystallization of bcc phase is preferred even at compositions where bcc is metastable. With rising undercooling a transition from primary fcc to primary bcc solidification occurs at a critical undercooling. The experimental results are compared to the predictions of the classical nucleation theory and an improved theory taking into account a finite thickness of the interface between the nucleus and the undercooled melt. The financial support from the Deutsche Forschungsgemeinschaft under contract no. He 1601/3 is gratefully acknowledged.