Fatigue and Fracture of Titanium Aluminides Program
Program Highlights
The efficiency of high-performance gas turbines is limited by the temperature
capabilities of materials used for engine components. These applications
demand materials with higher specific stiffness, strength and damage tolerance
at higher operating temperatures. In this regard, gamma-TiAl based intermetallic
alloys show particular promise due primarily to their lower density compared
to the Ti- and Ni-base alloys currently used in engines. In addition, due
to their adequate creep resistance and improved strength retention at elevated
temperatures, gamma-TiAl alloys have become attractive candidates to replace
conventional titanium alloys in the compressor sections of engines. Although
various microstructures have been developed, optimal mechanical properties
have been obtained to date by modifying the composition and thermomechanical
processing to achieve a two phase, (TiAl + Ti3Al), structure. The two phases
can be distributed as fine or coarse lamellar microstructures, with ~100-2000
micron-sized colonies of alternating TiAl and Ti3Al layers, or as duplex
microstructures with fine equiaxed (~10-40 micron-sized) TiAl and Ti3Al
grains. Accordingly, in the present program of study, ambient and high
temperature fatigue-crack growth behavior in titanium aluminide alloys
is studied with consideration of the role of environment and microstructure.
Specifically, microstructural effects are examined in terms of the role
of extrinsic toughening mechanisms on crack-growth resistance, principally
uncracked ligament bridging and roughness-induced crack closure. Additionally,
the role of crack size is examined by comparing the growth-rate behavior
of "small" (~70-500 micron) surface cracks and "long" (> 5 mm) through-thickness
cracks.
As gamma-TiAl alloys are being developed for applications requiring
temperatures up to 800 C, it is important to evaluate the fatigue properties
at these high temperatures. This graph shows the fatigue-crack growth rate
vs. the applied stress intensity range for Ti-47Al-2Nb-0.8Mn + 1 vol. %
TiB2 particles at three temperatures: 25, 600, and 800 C.
Researchers
Josh Campbell
Jay Kruzic
Aindrea McKelvey
K.T. Rao
Prof.
Robert Ritchie
Publications
-
A. L. McKelvey, K. T. Venkateswara Rao, R. O. Ritchie, "High temperature
fracture and fatigue-crack growth behavior of an XD gamma based titanium
aluminide intermetallic alloy"; submitted to Metallurgical and Materials
Transactions, 1997.
-
J.P. Campbell, K.T. Venkateswara Rao, R.O. Ritchie, "The Effect of Microstructure
on Fracture Toughness and Fatigue-Crack Growth Behavior in g-based
Titanium Aluminide Intermetallics", submitted to Metall. Trans. A,
1997.
-
J.P. Campbell, K.T. Venkateswara Rao, R.O. Ritchie, "On the Role of Microstructure
in Fatigue-Crack Growth of g-based Titanium
Aluminides", Mater. Sci. Eng. A, in press, 1997.
-
A. L. McKelvey, K. T. Venkateswara Rao, R. O. Ritchie, "On the anomalous
temperature dependence of fatigue-crack growth in g-based
titanium aluminides"; Scripta Metallurgica et Materialia, vol. 37:11,
p.1797, 1997.
-
J.P. Campbell, J.J. Kruzic, S. Lillibridge, K.T. Venkateswara Rao, R.O.
Ritchie, "On the Growth of Small Fatigue Cracks in g-based
Titanium Aluminides", Scripta Mat., vol. 37, p. 707, 1997.
-
J.P. Campbell, A.L. McKelvey, S. Lillibridge, K.T. Venkateswara Rao, R.O.
Ritchie, "Fracture and Fatigue in g-based TiAl
Alloys with Fine Lamellar Microstructures: Long and Small Crack Behavior",
in Deformation and Fracture of Ordered Intermetallic Materials III,
eds. W.O. Soboyejo, T.S. Srivatsan, and H.L. Fraser, TMS, Warrendale PA,
p. 141, 1996.
-
J.P. Campbell, K.T. Venkateswara Rao, R.O. Ritchie, "Fatigue-Crack Growth
and Fracture Behavior in a XDTM g-TiAl
Alloy with a Fine Lamellar Microstructure", in Fatigue '96, Proceedings
of the Sixth International Fatigue Congress, eds. G. Lütjering
and H. Nowack, Pergamon, Oxford, p. 1779, 1996.
-
A.L. McKelvey, J.P. Campbell, K.T. Venkateswara Rao, R.O. Ritchie, "High
Temperature Fatigue Crack Growth Behavior in an XDTM
g-TiAl Intermetallic Alloy" in Fatigue '96,
Proceedings of the Sixth International Fatigue Congress, eds. G. Lütjering
and H. Nowack, Pergamon, Oxford, p. 1743, 1996.