Dresden 2006 – scientific programme

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MM: Metall- und Materialphysik

MM 5: Phase transitions II

MM 5.4: Talk

Monday, March 27, 2006, 12:30–12:45, IFW D

Experimental and Theoretical Studies of Solidification in Undercooled Fe-Ni and Fe-Co Droplets — •T.G. Woodcock, H.-G. Lindenkreuz, R. Hermann, and W. Loeser — IFW Dresden, PO Box 270116, D-01171 Dresden, Germany

Fe-Ni and Fe-Co based alloys have important applications as soft magnetic materials. The solidification of such alloys is therefore of interest. Droplets of Fe-Ni and Fe-Co alloys were electromagnetically levitated enabling very high melt undercooling (maximum approx. 300 K). The temperature of the samples was monitored during the solidification process using a two-colour pyrometer and a Si diode with a very fast reaction rate. The recalescence during solidification showed a transition from single step to double step at a critical value of undercooling. The double step recalescence was caused by formation of a metastable bcc phase and subsequent transformation to the equilibrium fcc phase in both Fe-Ni and Fe-Co alloys. A thermodynamic database was used to generate values of the enthalpy of formation for the fcc and bcc phases in three different Fe-Co alloys. Based on classical nucleation theory, the free energy of formation of a spherical nucleus was calculated. Calculation of the nucleation rate of these two phases showed that below a critical undercooling, the metastable bcc phase can have a higher nucleation rate than the equilibrium fcc phase.