Ambient- to elevated-temperature fracture and fatigue properties of Mo-Si-B alloys: Role of microstructure
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. INTRODUCTION
A major limitation in the technological advancement of aerospace engines and power generation is the lack of highertemperature structural materials to replace conventional nickel-base superalloys. Indeed, single-crystal nickel-base superalloys have essentially reached their temperature limit and are unsuitable for structural use above 1100 °C.[1] Unfortunately, using higher melting-point (2000 °C) materials presents several obstacles as adequate fracture, fatigue, oxidation, and creep resistance must be maintained. For example, materials based on refractory metals such as molybdenum typically suffer from poor oxidation and creep resistance. While the formation of molybdenum silicides and borosilicides significantly improves these properties,[2,3,4] such intermetallic compounds are invariably brittle and provide little resistance to fracture. To meet the need of developing useful structural molybdenum-based alloys, two distinct multiphase Mo-Si-B alloy systems have been proposed, based on the phases: (1) Mo3Si, Mo5SiB2 (T2), and Mo5Si3 (T1), by Akinc and co-workers;[2–5] and (2) -Mo, Mo3Si, and Mo5SiB2 (T2), by Berczik.[6,7] With regard to fracture resistance, the latter alloys hold more promise since they contain the relatively ductile -Mo phase, with Si and B in solid solution, in addition to the brittle intermetallic phases Mo3Si and Mo5SiB2. The microstructural arrangement of these phases is critical in determining the properties. Indeed, while alloys containing molybdenum particles surrounded by the hard but brittle intermetallic phases display definitive improvements in toughness compared to monolithic silicides,[8,9] larger J.J. KRUZIC, Assistant Professor, is with the Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331. Contact e-mail: [email protected] J.H. SCHNEIBEL, Senior Research Staff Member, is with Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831. R.O. RITCHIE, Professor, is with the Materials Sciences Division, Lawrence Berkeley National Laboratory, and the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Manuscript submitted January 3, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
improvements in fracture and fatigue resistance have been achieved using a continuous -Mo matrix.[10] Although these -Mo matrix alloys resemble nickel-base (/) superalloys in their similarly high volume fraction of intermetallic phase, an additional difficulty for Mo-Si-B materials is that, unlike the Ni3Al phase, Mo3Si and Mo5SiB2 exhibit essentially no ductility at ambient temperatures. These factors suggest that making the most effective use of the relatively ductile molybdenum phase is important for achieving high fracture and fatigue resistance. However, since the development of serviceable Mo-Si-B alloys will depend on compromises between several material properties,[11] there is a need to understand more specifically how different microstructural parameters affect the
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