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

Journal Article · Sun Dec 01 04:00:00 EST 1996 · Journal of Materials Research

DOI:https://doi.org/10.1557/JMR.1996.0384· OSTI ID:432499

Locci, I E ^[1]; Dickerson, R M ^[2]; Garg, A; Noebe, R D; Whittenberger, J D; Nathal, M V ^[3]; Darolia, R ^[4]

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1. Case Western Reserve University, Cleveland, Ohio 44106 (United States)
2. NYMA, Inc., Cleveland, Ohio 44124 (United States)
3. NASA Lewis Research Center, Cleveland, Ohio 44135 (United States)
4. General Electric Aircraft Engines, Cincinnati, Ohio 45215 (United States)

Six near stoichiometric, NiAl single-crystal alloys, with 0.05{endash}1.5 at.{percent} 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 (Ni{sub 2}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, Ni{sub 16}X{sub 6}Si{sub 7}, or as an orthorhombic NiXSi phase, where X is Hf or Zr. Under these conditions the expected Heusler phase ({beta}{prime}) 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. {copyright} {ital 1996 Materials Research Society.}

• OSTI ID: 432499
• Journal Information: Journal of Materials Research, Journal Name: Journal of Materials Research Journal Issue: 12 Vol. 11; ISSN JMREEE; ISSN 0884-2914
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM ALLOYS
ANNEALING
HAFNIUM ALLOYS
MICROSTRUCTURE
MONOCRYSTALS
NICKEL ALLOYS
PHASE STUDIES
PRECIPITATION
SILICON ALLOYS
STABILITY
ZIRCONIUM ALLOYS