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Microstructure and phase stability of single crystal NiAl alloyed with Hf and Zr

Published online by Cambridge University Press:  31 January 2011

I. E. Locci ,
R.M. Dickerson ,
A. Garg ,
R. D. Noebe ,
J.D. Whittenberger ,
M. V. Nathal  and
R. Darolia
I. E. Locci
Affiliation:
Case Western Reserve University, Cleveland, Ohio 44106, and NASA Lewis Research Center, Cleveland, Ohio 44135
R.M. Dickerson
Affiliation:
NYMA, Inc., Cleveland, Ohio 44124
A. Garg
Affiliation:
NASA Lewis Research Center, Cleveland, Ohio 44135
R. D. Noebe
Affiliation:
NASA Lewis Research Center, Cleveland, Ohio 44135
J.D. Whittenberger
Affiliation:
NASA Lewis Research Center, Cleveland, Ohio 44135
M. V. Nathal
Affiliation:
NASA Lewis Research Center, Cleveland, Ohio 44135
R. Darolia
Affiliation:
General Electric Aircraft Engines, Cincinnati, Ohio 45215
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Abstract

Six near stoichiometric, NiAl single-crystal alloys, with 0.05−1.5 at.% of Hf and Zr additions plus Si impurities, were microstructurally analyzed in the as-cast, homogenized, and aged conditions. Hafnium-rich interdendritic regions, containing the Heusler phase (Ni2AlHf), were found in all the as-cast alloys containing Hf. Homogenization heat treatments partially reduced these interdendritic segregated regions. Transmission electron microscopy (TEM) observations of the as-cast and homogenized microstructures revealed the presence of a high density of fine Hf (or Zr) and Si-rich precipitates. These were identified as G-phase, Ni16X6Si7, or as an orthorhombic NiXSi phase, where X is Hf or Zr. Under these conditions the expected Heusler phase (β′) was almost completely absent. The Si responsible for the formation of the G and NiHfSi phases is the result of molten metal reacting with the Si-containing crucible used during the casting process. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G and NiHfSi phases. In some of the alloys studied, long-term aging heat treatments resulted in the formation of Heusler precipitates, which were more stable at the aging temperature and coarsened at the expense of the G-phase. In other alloys, long-term aging resulted in the formation of the NiXSi phase. The stability of the Heusler or NiXSi phases can be traced to the reactive element (Hf or Zr) to silicon ratio. If the ratio is high, then the Heusler phase appears stable after long time aging. If the ratio is low, then the NiHfSi phase appears to be the stable phase.

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Articles
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Journal of Materials Research , Volume 11 , Issue 12 , December 1996 , pp. 3024 - 3038
Copyright
Copyright © Materials Research Society 1996

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