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A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y 2 0 1 5
2 4
Transverse properties of ODS al-
loys. In addition to lower ductility, men-
tioned previously, the creep strength of
ODS alloys is noticeably weak in trans-
verse directions. It is not clear how se-
rious this issue is, as many successful
ground engine tests of ODS alloys[30]
and directionally solidified eutectics
(also with large creep anisotropy) were
accomplished in the past. If transverse
properties are a serious issue, one route
for mitigation could be improved un-
derstanding of recrystallization meth-
ods. The ability to control recrystalli-
zation to produce single crystals[31] in
wafers, for example, would eliminate
the transverse-property debit. The en-
tire topic of recrystallization would be
an excellent focus for future integrated
computational materials engineering
(ICME) study. Models to predict recrys-
tallization response as a function of
alloy composition, oxide dispersion pa-
rameters, and thermomechanical pro-
cessing history would accelerate ODS
alloy development.
Inspection limits. An unpublished
reason for the lack of success for
MA6000 as a turbine blade material is
its sensitivity to small unrecrystallized
regions that could serve as sites for pre-
mature failure. These regions could not
be reliably detected by nondestructive
inspection methods. Future work must
revisit this issue and quantify its ef-
fect. Adopting a wafer-blade processing
scheme should enable easier inspec-
tion of wafers prior to bonding.
SUMMARY
The case for reemphasizing ODS
alloys to improve turbine blade tem-
perature capability beyond that of
superalloy single crystals appears to
be quite strong. Experimental alloys
already exist that exceed the best sin-
gle crystals, and further improvements
are possible through updated alloy de-
sign. Exceptional creep properties were
achieved without relying on expensive,
strategic elements such as rhenium.
Compared with ceramics, interme-
tallics, and cobalt alloys, the risks are
modest and the payoffs significant. In
contrasttothesealternatives,technology
solutions either already exist or have
been identified to achieve the bal-
ance of properties required for a high-
pressure turbine blade including cost,
strength, toughness, impact resistance,
oxidation and corrosion resistance, TBC
compatibility, and manufacturability.
A program directed at develop-
ing ODS alloys for use in turbine blade
applications is recommended. In ad-
dition to alloy development, concerns
that must be addressed in future de-
velopment efforts include ductility,
thermomechanical fatigue, and pro-
cessability. Use of modern modeling
and design tools could accelerate mat-
uration of this promising technology.
~AM&P
For more information: Mike Nathal
(retired) was chief of the Advanced Me-
tallics Branch, Structures and Materials
Division, NASA Glenn Research Center,
,
Acknowledgment
The author thanks Tom Glasgow
and Ray Benn for helpful discussions.
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