Microstructure and phase stability of single crystal NiAl alloyed with Hf and Zr
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Microstructure and phase stability of single crystal NiAl alloyed with Hf and Zr I. E. Locci Case Western Reserve University, Cleveland, Ohio 44106, and NASA Lewis Research Center, Cleveland, Ohio 44135
R. M. Dickerson NYMA, Inc., Cleveland, Ohio 44124
A. Garg, R. D. Noebe, J. D. Whittenberger, and M. V. Nathal NASA Lewis Research Center, Cleveland, Ohio 44135
R. Darolia General Electric Aircraft Engines, Cincinnati, Ohio 45215 (Received 26 January 1996; accepted 24 July 1996)
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 (Ni2 AlHf), 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, Ni16 X6 Si7 , or as an orthorhombic NiXSi phase, where X is Hf or Zr. Under these conditions the expected Heusler phase (b 0 ) 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.
I. INTRODUCTION
NiAl alloys, regarded as potential candidates for high temperature applications, require improvement in room temperature ductility or toughness as well as high temperature creep resistance to compete with today’s superalloys. NiAl is one of the few intermetallic alloys that possesses excellent oxidation resistance at both intermediate and high temperatures mainly due to the formation of a protective Al2 O3 scale.1,2 While the isothermal oxidation resistance of NiAl is outstanding, the cyclic oxidation resistance of NiAl is equally impressive when trace additions of reactive elements are present. Ternary additions, especially from group IV and V refractory metal elements, have also been shown to provide significant strengthening at high temperatures. In particular, the effect of Hf additions on the high temperature mechanical properties of polycrystalline NiAl was sug
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