Role of Microstructure in Promoting Fracture and Fatigue Resistance in Mo-Si-B Alloys
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Role of Microstructure in Promoting Fracture and Fatigue Resistance in Mo-Si-B Alloys J. J. Kruzic* Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331, U.S.A. J. H. Schneibel Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831, U.S.A. R. O. Ritchie Department of Materials Science and Engineering, University of California, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. ABSTRACT An investigation of how microstructural features affect the fracture and fatigue properties of a promising class of high temperature Mo-Si-B based alloys is presented. Fracture toughness and fatigue-crack growth properties are measured at 25º and 1300ºC for five Mo-Mo3Si-Mo5SiB2 containing alloys produced by powder metallurgy with α-Mo matrices. Results are compared with previous studies on intermetallic-matrix microstructures in alloys with similar compositions. It is found that increasing the α-Mo phase volume fraction (17 – 49%) or ductility (by increasing the temperature) benefits the fracture resistance; in addition, α-Mo matrix materials show significant improvements over intermetallic-matrix alloys. Fatigue thresholds were also increased with increasing α-Mo phase content, until a transition to more ductile fatigue behavior occurred with large amounts of α-Mo phase (49%) and ductility (i.e., at 1300°C). The beneficial role of such microstructural variables are attributed to the promotion of the observed toughening mechanisms of crack trapping and bridging by the relatively ductile α-Mo phase. INTRODUCTION Intermetallic based Mo-Si-B alloys have been targeted for high temperature turbine engine applications as potential replacements for nickel based superalloys. Two specific Mo-Si-B alloy systems developed by Akinc et al. [1-4] and Berczik [5,6] have received recent attention. While the former is composed entirely of intermetallic compounds, the latter utilizes the relatively ductile α-Mo phase to impart some ductility and fracture resistance to a three phase microstructure also containing Mo3Si and Mo5SiB2 (T2). For any of these alloys to be successful, adequate resistance to oxidation, creep, fracture, and fatigue must be achieved; however, it is recognized that microstructural features which promote improvements in one property are often detrimental to others [7,8]. For example, while a continuous α-Mo matrix with high volume fraction may be beneficial to the fracture and fatigue behavior [9], this tends to compromise the oxidation and creep resistance [7,8,10-12]. Accordingly, a thorough understanding of how microstructure affects each property is needed so that appropriate tradeoffs can be made in the optimization of these alloys. Consequently, this present paper seeks to characterize the specific mechanistic role of microstructure in determining the fracture and fatigue resistance of alloys based on the α-Mo, Mo3Si, and T2 phases, with the objective of
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