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Microstructure and Mechanical Properties of Cast, Homogenized and Aged NiAl Single Crystal Containing Hf

Published online by Cambridge University Press:  01 January 1992

I.E. Locci ,
R. Dickerson ,
R.R. Bowman ,
J.D. Whittenberger ,
M. V. Nathal  and
R. Darolia
I.E. Locci
Affiliation:
NASA-Lewis Research Center, Cleveland, OH 44135
R. Dickerson
Affiliation:
NASA-Lewis Research Center, Cleveland, OH 44135
R.R. Bowman
Affiliation:
NASA-Lewis Research Center, Cleveland, OH 44135
J.D. Whittenberger
Affiliation:
NASA-Lewis Research Center, Cleveland, OH 44135
M. V. Nathal
Affiliation:
NASA-Lewis Research Center, Cleveland, OH 44135
R. Darolia
Affiliation:
GE Aircraft Engines, Cincinnati, OH 45215.
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Abstract

Small additions of Hf to [001] oriented single crystal NiAl are shown to be effective in improving high temperature creep strength. The presence of Hf-rich second phases and/or solid solution strengthening are responsible for the improved behavior observed at high temperatures. In the as-cast condition, large Hf-rich interdendritic regions were found. Homogenization heat-treatments (1590 K for 50 hours) substantially reduced this interdendritic segregation. TEM observations of the as-homogenized microstructure revealed fine G-phase (Ni16Hf6Si7) precipitates, with plate or cuboidal shapes. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G-phase. Aging the homogenized material at 1300 K resulted in the formation of Heusler precipitates (β'-Ni2AlHf), preferentially nucleating at the G-phase sites. These Heusler precipitates were more stable at this or higher temperatures and coarsened at the expense of the less stable G-phase. Post-test analyses of compression tested specimens, conducted at 1200, 1300 and 1400 K, revealed extensive changes in the distribution and size of the second phases. Deformation at 1300 K appears to occur by two distinct mechanisms: at high strain rates the stress exponent is  4 while at slower rates (< 10−6s−1) a much higher exponent (∼ 12) was found. Testing at 1300 K of specimens over-aged at 1400 K reduced the creep resistance of the alloy which suggests a contribution by precipitation strengthening to the overall strength of the alloy.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 288: Symposium L – High-Temperature Ordered Intermetallic Alloys V , 1992 , 685
Copyright
Copyright © Materials Research Society 1995

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References

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