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Development of Creep-Resistant NiAl(Ti,Hf)
Single-Crystal Alloys
Nickel-base superalloys are the current choice for
high-temperature jet engine applications such as turbine blades
and vanes. However, after more than five decades of use, nickel-base
superalloys have reached their limit, since the operating temperatures
in gas turbine engines are now approaching the melting temperature
of these alloys. Thus alternative materials, such as lightweight
NiAl intermetallic alloys with superior properties, (e.g., high
melting temperature, high thermal conductivity, and excellent
chemical stability and oxidation resistance) are required for
the next generation of high-temperature structural materials for
more efficient 21st century civil transport systems.
The two major disadvantages that have historically prevented the
application of NiAl as a high-temperature structural material
are its poor creep resistance and low room-temperature ductility.
Alloying strategies similar to those used for nickel-base superalloys
are being used to improve the high-temperature strength via solid-solution
and precipitate-hardening effects. This study highlights the potent
role of Ti and Hf as potential solid-solution strengtheners in
NiAl and also the added effect of second-phase particles when
Ti and Hf are both used.
The high-temperature deformation behavior of <001> NiAl-3Ti,
NiAl-0.2Hf, and NiAl-3Ti-0.5Hf (at.%) single crystals was studied
at the NASA Lewis Research Center in the temperature range ~800
to 1150 °C. The bar graph shows steady-state creep strengths
of these alloys at 1027 °C (1880 °F) at a strain rate
of ~10-6 sec-1. Comparison of these alloys
with unalloyed NiAl and a first-generation single-crystal superalloy
shows that the strengths of the NiAl alloys are higher than those
of binary NiAl and that they approach that of the superalloy.
This increase in strength (to that of the superalloy) was accomplished
with total alloying additions of less than four atomic percent.
Furthermore, because the NiAl-3Ti-0.5Hf alloy is much lighter
than the superalloy, it would exhibit about a 30-percent greater
density-compensated creep strength in comparison to the superalloy
under the test conditions described in the bar graph.
Creep strength comparison of binary NiAl, alloyed NiAl
single crystals, and a first-generation single-crystal nickel-base
superalloy made at 1880 °F and a strain rate of 1x10-6
sec-1.
Transmission electron microscopy observations showed essentially
no precipitation in the NiAl-3Ti and NiAl-0.2Hf alloys. Additional
analysis revealed that strengthening in these alloys is due solely
to solid-solution effects through a significant increase in the
drag force on dislocations. When both Hf and Ti are added to the
alloy, the individual solubility for each element decreases, resulting
in significant precipitation. Consequently, the NiAl-3Ti-0.5Hf
alloy contained a high density of very fine Heusler (Ni2AlTi)
precipitates and a lower density of heterogeneously nucleated
and somewhat coarser Heusler (Ni2Al(Hf,Ti)) precipitates within
the NiAl matrix, as shown in the photomicrograph. A higher level
of strength was achieved in this precipitate-containing alloy
because of the combined strengthening effects associated with
solute drag on the dislocations plus impedance due to the presence
of precipitate particles. The strength level of this alloy approaches
that of the superalloy single crystal, whereas even higher levels
of these alloying additions would increase the amount of precipitate
phase and the strength of the NiAl even further.
Microstructure of a creep-resistant NiAl-3Ti-0.5Hf
single-crystal alloy.
Consequently, through combined solid-solution and precipitation-strengthening
mechanisms, NiAl alloys can be developed with creep strengths
equivalent or superior to conventional nickel-base superalloys,
while at the same time providing a 33-percent weight savings and
higher temperature capability.
Lewis contacts: Dr. Ronald D. Noebe, (216) 433-2093, Ronald.D.Noebe@grc.nasa.gov;
Dr. Anita Garg, (216) 433-8908, Anita.Garg@grc.nasa.gov;
and Dr. J. Daniel Whittenberger, (216) 433-3196, jdwhittenberger@grc.nasa.gov
Author: Dr. Ronald D. Noebe
Headquarters Program Office: OA
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Last updated April 30, 1997
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