Competitive Phase Selection in Fe-Ni Alloy Droplets
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F. GARTNER, A. F. NORMAN, H. ASSADI, A. L. GREER Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K. ABSTRACT In the solidification of Fe-Ni droplets (< 30 at.% Ni), the selection of different microstructures is dominated by the competition between the bcc and ccp phases. In drop-tube experiments ccp is the primary phase in some dilute (up to 7at% Ni) alloys although the bcc phase is favoured by a lower free energy and by a lower interfacial energy with the liquid. Competitive dendrite growth is a possible explanation for the formation of primary ccp. Comprehensive thermodynamic (CALPHAD) and kinetic modelling is undertaken to understand the growth competition. The origin of the observed primary phases is discussed. INTRODUCTION In research on solidification at high undercoolings the Fe-Ni system is of particular interest because of the competition between the bcc c(xFe) or 8(Fe), and the ccp y(Fe, Ni) phases. For Fe-rich compositions, the system shows a peritectic reaction involving the liquid, the bcc 8(Fe) phase and the ccp y(Fe(Ni)) phase. At lower temperatures the y(Fe(Ni)) phase may transform to the bcc cL(Fe) phase. Most research has focused on Fe-Ni alloys containing more than 5 at.% Ni. A comprehensive review of the containerless processing of Fe-Ni alloys can be found in [1]. In droplet levitation experiments, external triggers of bcc structure have been used to promote the formation of the metastable bcc phase in preference to the equilibrium ccp phase [2,3]. In an earlier paper [4] we reported an investigation into the solidification behaviour of a series of Fe-Ni alloys processed by drop-tube. The Fe-Ni alloys were studied by optical metallography and X-ray diffraction. For concentrated alloys, the microstructures appear in the form of primary dendrites and were either bcc (10 at.% Ni) or ccp (30 at.% Ni). For dilute alloys ("pure" Fe and 2 at.% Ni) a Widmanstdtten morphology was observed for all droplet sizes, revealing that the final bec structure had transformed from ccp. In small droplets (< 250 gm) of composition 5 at.% Ni, the structure was ccp and there was no evidence for any transformation. This clearly shows that, unexpectedly, cep is the primary phase (Fig. la). The external cooling rate in these droplets is high enough to prevent solid-state transformations. For larger droplets of composition 5 at.% Ni, a mixture of both phases was observed (Fig. lb); primary ccp dendrites in directions contain some transformed bce platelets on ( 111 ) ccp planes. From these experiments, it has been possible to construct a microstructure-selection map (a simplified version is shown in Fig. 2) for alloy composition and droplet size [5]. For small Ni-concentrations, the thermodynamic driving forces for solidification are quite similar for the ccp and the bee phases. Therefore phase selection should, in the first instance, be governed by differences in the solid-liquid interfacial free energies. Previous research has demonstrated that the interfaci
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