Formation and transformation of the metastable bcc phase in undercooled Fe-Co and Fe-Ni alloy melts

Abstract

Solidification of metastable bcc phase and subsequent transformation into the stable fcc phase in undercooled Fe-Co and Fe-Ni alloy melts have been investigated on electromagnetically levitated samples. The effect of fluid flow on growth kinetics and transformation behavior has been analyzed by comparative experiments on ground and under microgravity utilizing the electromagnetic levitator ISS-EML onboard the International Space Station. Rapid solidification is monitored by a high-speed video camera, which enables the direct observation of propagation of the solidification front. While electromagnetic levitation on earth is accompanied with strong, turbulent melt convection the tests using ISS-EML allow the control of fluid flow by applying different levels of electromagnetic stirring during solidification. Metastable phase formation is revealed by a two-step recalescence event showing primary solidification of metastable bcc phase and subsequent formation of stable fcc phase within the mushy-zone of primary bcc phase. Besides the growth velocities of competing bcc and fcc phase a decisive parameter describing the kinetics of phase transformation is the delay time between nucleation of the metastable and the stable phase. The delay obtained from high-speed video recordings strongly decreases with rising undercooling prior to metastable bcc phase solidification and is raised if electromagnetic stirring is increased. The experimental results on transformation kinetics are discussed in terms of current models for dendritic growth and nucleation of the stable phase in a metastable mushy-zone. This research work is supported by the European Space Agency (ESA) in the frame of the projects MAGNEPHAS and PARSEC.