Introduction

One of the most difficult problems in materials engineering today is the development of higher temperature structural materials for use in applications such as gas-turbine engines.  The current material of choice, single-crystal nickel-based superalloys, has reached its technological limit; indeed, as these alloys melt at temperatures between 1200°-1400°C, they are unsuitable for structural use above ~1100°C (Fig. 1).   High melting-point (>2000°C) materials, based on refractory metals such as molybdenum, represent a higher-temperature alternative but have been plagued by oxidation and brittleness problems.   Additions of silicon and boron to molybdenum to form silicides and borosilicides have shown promise in improving the oxidation resistance; however, the silicide compounds are quite brittle and will provide little fracture resistance for most structural applications without significant active toughening mechanisms.  While several alloys have been produced containing the more ductile α-molybdenum phase in addition the hard but brittle intermetallic phases Mo3Si and Mo5SiB2 (the T2 phase), many of these show only marginal improvements in toughness relative to the monolithic intermetallic phases which have fracture toughnesses of ~ 3 - 4 MPa√

m (e.g., Metal. Mater. Trans., 34A, 2003, p. 225).  This suggests that the key to achieving high fracture resistance in these materials may lie in making more effective use of the "ductile" α-Mo phase, in a manner not unlike the way that nickel-based (γ-γ') superalloys obtain high fracture toughness with a similarly high fraction of intermetallic (γ') precipitates.

Figure 1 Plot showing the improvement since 1940 in the temperature capacity of metallic alloys, specifically nickel-based superalloys, for gas-turbine engine applications and demonstrating the need for new materials, such as molybdenum based superalloys, in order to achieve further technological gains.

 

Previous Research

Accordingly, to achieve improved fracture resistance, the approach currently being undertaken is to develop alloys where the intermetallic phases are completely surrounded by a continuous "ductile" α-Mo phase.  Molybdenum-based alloys have been processed with Mo3Si and T2 particles in a continuous α-Mo matrix using a novel powder processing route.  Specifically, to obtain the continuous α-Mo phase, ground powders of Mo3Si and T2 phase (composition Mo-20Si-10B at%) were vacuum annealed to remove silicon from the surface and leave a α-Mo coating on each particle.  These surface-modified powders were then hot isostatically pressed to achieve alloys with a continuous α-Mo matrix, reinforced by the intermetallic phases, Mo3Si and T2 (Fig. 2a).  Full processing details may be found in (Scripta Mater., 46, 2002, p. 217).  Preliminary results have indicated fracture toughness values in excess of 20 MPa√m may be achieved, while future work will focus on understanding the specific relations between the microstructure and the fracture and fatigue behavior of these alloys.

 

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Figure 2 (a) Microstructure of a Mo-Si-B alloy with a continuous α-Mo matrix (~ 46% vol.) produced by the surface modified powder metallurgy method. (b) "Quasi-continuous duplex" microstructure with 50% vol α-Mo phase produced using the ULTMAT process.

Current Work

The focus of this work is to make a significant advance in the development of Mo-Si-B alloys, specifically by tailoring the composition, morphology and volume fraction of the major phases of these alloys (α-Mo, MoSi3 and Mo5SiB2 (T2)) to achieve an optimum balance of low- and high-temperature damage-tolerance with creep and oxidation resistance.  Unlike Mo-Si-B materials based entirely on intermetallic compounds, these alloys contain the metallic α-Mo phase which provides some degree of fracture resistance and ductility. Furthermore, the silicide and borosilicide phases provide creep and oxidation resistance, the latter of which is the result of a borosilicate glass scale which forms in situ on the metal surface.

 

Newer processing routes, developed as part of ONERA's ULTMAT program, produce alloys with compositions near those of the work above, namely Mo-3wt%Si-1wt%B.  The resulting alloys consist of roughly 50vol% α-Mo and 50vol% intermetallic phases in a "quasi-continuous duplex" microstructure.  See Figure 2b.  Grains in this material are much smaller (15-20 mm) than previous materials.  Nanoscale yttria particles have been dispersed in the grain boundaries to limit grain growth and impede creep.  See Jehanno et. al., Materials Science and Engineering A 463 (2007) pgs. 216-223 for full processing details.

 

A third processing route, described by Middlemas and Cochran, JOM July (2008) pg 19-24, will also be studied.  The microstructure of these Mo-3wt%Si-1wt%B alloys is nearly identical to that of the ULTMAT materials.

 

Preliminary results show initiation toughnesses on the order of 8.5 MPa√m for the ULTMAT material and 7.2 MPa√m for the Middlemas & Cochran (GT material), with no stable crack growth apparent for either material.  Compare these results with the results of Kruzic, et. al., Metall Mater Trans A 36A (2005), where similar volume fractions of α-Mo produced initiation toughnesses up to 12 MPa√m and significant rising R-curve behavior.  The difference in behavior becomes more pronounced at higher temperatures.  Consult Figure 3 below.

 

 Figure 3: Initiation toughness for ULTMAT (blue) GT (red) and Kruzic, et. al. materials.  The ULTMAT and GT materials show no stable crack growth, while Kruzic et. al. were able to grow cracks stably for several millimeters, even at room temperature.  They achieved room temperature toughnesses above 20 MPa√m.

Fractography of the fracture surfaces and crack path-microstructure studies will be performed to better understand the origin of these disparate crack growth behaviors.

Current Researchers:

                            J. A. Lemberg
                            J. H. Schneibel (collaborator from ORNL)
                            M. R. Middlemas (collaborator from Georgia Tech)

                            J. K. Cochran (collaborator from Georgia Tech)

    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: