Fracture and Fatigue of
Mo-Si-B
High Temperature Alloys
Developing materials for high-temperature structural applications is one of
the greatest challenges facing materials scientists today. Current nickel-based
superalloys used in turbine systems have reached the limit of working temperature
at ~1150°C. In order to continue advancement of power generation and propulsion
technology, novel high-temperature alloys must be developed.
To this end, refractory metal silicides have been suggested as potential next-generation
high-temperature materials. Unfortunately, using higher melting-point (>2000
C) materials presents many obstacles as adequate fracture, fatigue, oxidation,
and creep resistance must be maintained in inherently more brittle systems.
To meet the need multi-phase Mo-Si-B alloy systems have been proposed based
on the phases -Mo, MoSi3 and Mo5SiB2 (T2). With these alloys, oxidation resistance
is provided by a borosilicate glass scale which forms in situ on the metal surface,
while the silicide phases provide creep resistance and the -Mo phase provides
fracture resistance and ductility. However, the microstructural arrangement
of these phases is absolutely critical in determining their properties, and
a thorough understanding of how each microstructural parameter affects the material
behavior is essential to achieve a necessary balance of properties. Also, there
is a critical need to evaluate these materials, particularly with respect to
their damage-tolerant properties and fatigue behavior, at realistic temperatures,
i.e., well above 1000 C.
The goal of the current project is to determine and quantify the salient fracture
mechanisms in these alloys and the role that crack bridging, and the other toughening
mechanisms, e.g., cracking trapping and microcracking, plays. Employing our
unique capabilities to develop R-curves through in-situ measurement of crack
lengths at ultra-high temperatures, we are able to observe the fatigue and fracture
behavior of these alloys at temperatures up to 1300°C. Indeed, toughness
values in excess of 20 MPa?m have been achieved; a seven-fold increase in the
toughness over monolithic intermetallic alloys.
Figure 1. (a) highlights the ambient- and elevated-temperature
R-curve behavior of Mo-Si-B alloys, while (b) illustrates crack trapping and
bridging at the -Mo phase. The crack locally arrests at the -Mo phase, leaving
-Mo bridges in the crack wake. Uncracked a-Mo bridges left in the crack wake
account for a significant portion of the rising R-curve behavior shown in (a).
After (Metal. Mater. Trans., 36A, 2005, p. 2393)
Figure 2. Optical micrographs illustrating the crack
blunting and damage zone ahead of the crack tips after R-curve testing at 1300
°C for (a) coarse-grained (~150 m) and (b) medium-grained (~75 m) alloys.
Notice the order of magnitude difference in scale between the two photos. Furthermore,
crack bridging is seen for the medium-grained alloy in (b). Nomarski differential
image contrast was used for both images,and the nominal crack growth direction
was left to right. After (Metal. Mater. Trans., 36A, 2005, p. 2393)
Figure 3. Fatigue-crack growth rate, da/dN, data
plotted as a function of applied stress intensity range, K, for continuous
-Mo matrix Mo-Si-B alloys at room (25 °C) and elevated (1300 °C) temperatures.
The fatigue threshold clearly increases with increasing -Mo vol pct. After
(Metal. Mater. Trans., 36A, 2005, p. 2393)
Current Researchers:
J. A. Lemberg
J. H.
Schneibel (collaborator from ORNL)
R. O. Ritchie
Acknowledgments:
Work on this project conducted at Berkeley is supported by the Office of Fossil
Energy, Advanced Research Materials Program, WBS Element LBNL-2, and by the Office
of Science, Office of Basic Energy Sciences, Division of Materials Sciences and
Engineering of the U.S. Department of Energy, under contract DE-AC03-76SF0098.
Publications:
-
J. H. Schneibel, J. J. Kruzic, R. O. Ritchie, “Mo-Si-B
Alloy Development,” in Proceedings of the 17th Annual
Conference
on Fossil Energy Materials, 2003.
-
J. H. Schneibel, P. F. Tortorelli, M. J. Kramer, A. J. Thom, J. J.
Kruzic,
and R. O. Ritchie, “Optimization of Mo-Si-B Intermetallics,” in Defect
Properties and Related Phenomena in Intermetallic Alloys, E. P.
George,
M. J. Mills, H. Inui, G. Eggeler, eds., MRS Symposium Proceedings, vol.
753, Materials Research Society, Warrendale, PA, 2003. BB2.2.1-6.
-
H. Choe, D. Chen, J. H. Schneibel, R. O. Ritchie, "Fracture
and Fatigue-Crack Growth Behavior in Mo-12Si-8.5B Intermetallics at
Ambient
and Elevated Temperatures," in Fatigue and Fracture Behavior of
High Temperature Materials, P.K. Liaw, ed., TMS, Warrendale, PA,
2001.
17-24.
LBNL, MSD
* Ritchie Group * Dept.
of MSE, UC Berkeley