Directional solidification and phase equilibria in the Ni-Al system
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I. INTRODUCTION
ALTHOUGH the Ni-rich part of the Ni-Al system has been the topic of numerous publications about phase equilibria and solidification, there is a lack in the understanding of the formation of the various phases and microstructures. Moreover, large uncertainties remain in the phase diagram, especially near the composition of Ni3Al. Several versions[1,2,3] show a peritectic reaction between b-NiAl and g 8 (Figure 1(a)) whereas other versions[4] (Figure 1(b)) including the most recent ones[5–9] (Figure 1(c)) show the peritectic reaction between g and g 8 and the eutectic reaction between g 8 and b separated by only 1 to 3 K. The low velocity solidification results from Lee and Verhoeven[10,11] clearly show that the latter type (Figure 1(c)) is the correct one. They indeed observe lamellar and fibrous eutectic structures with b and g 8 and bands of g and g 8, which are typical of a peritectic reaction.[12] Verhoeven et al. evaluated the eutectic temperature at 75 at. pct Ni in a solidifying sample.[7] This method is somewhat inaccurate and yields a value that is about 15 K above the corresponding values measured by differential thermal analysis by Hilpert et al.[5] or Bremer et al.[6] Apart from this temperature difference, liquidi and solidi of b-NiAl and g-Ni (fcc) phases can be considered as rather well determined (i.e., similar in the different publications (Figure 1(d))). This is due to the fact that both phases are in equilibrium with the liquid over a wide range of compositions of about 25 at. pct. On the contrary, the g 8-Ni3Al phase is in equilibrium with the liquid over less than 1 at pct and several degrees only. This makes any measurement such as solidification interval or liquidus slope very difficult. It explains why the positions of the g 8 liquidus and solidus have been under discussion for more than 50 years[1,4] and are still subject to uncertainties. This situation is unsatisfactory because the Ni-Al phase diagram forms the basis of modern superalloys. O. HUNZIKER, Postdoctoral Research Associate, is with the RollsRoyce University Technology Centre, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. W. KURZ, Professor, is with the Laboratory of Physical Metallurgy, Department of Materials, Swiss Federal Institute of Technology, 1015 Lausanne EPFL, Switzerland. Manuscript submitted January 12, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
Solidification microstructure selection maps for the Nirich part of the binary Ni-Al system have been determined by laser surface resolidification experiments for high growth rates[13] and from Bridgman directional solidification experiments published by Lee and Verhoeven[8,10,11] for low growth rates. It was found that only b-NiAl and g-Ni were formed as primary phases by laser resolidification.[13] Growth of primary g 8-Ni3Al was observed in the Bridgman experiments at velocities below 100 mm/s.[10] More than ten different growth microstructures were actually observed between 1 and 100 mm/s
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