Crack Growth Behavior in a Two-Phase Mo-Si-B Alloy
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0980-II06-01
Crack Growth Behavior in a Two-Phase Mo-Si-B Alloy Sharvan Kumar1 and Amruthavalli Pallavi Alur2 1 Division of Engineering, Brown University, 182 Hope Street, Providence, RI, 02912 2 Intel Corporation, 5000 W. Chandler Blvd, CH5-159 (M/S), Chandler, AZ, 85226 ABSTRACT Mo-rich Mo-Si-B multiphase alloys are currently being explored for their potential as hightemperature structural materials for components in hot sections in aircraft engines. In this paper, we present crack growth behavior in one such two-phase alloy consisting of a Mo solid solution matrix in which is dispersed approximately 40 volume percent of the Mo5SiB2 (T2) phase. Crack growth under monotonic and cyclic loading is considered over a temperature range spanning 20°C to 1400°C. The effects of loading rate (in monotonic loading) and dwell times at maximum stress (in cyclic loading) at high temperatures on crack growth were examined to understand the contribution from creep. Results confirm a gradual increase in fracture toughness upto 1000°C, beyond which the increase is more substantial with temperature; fatigue susceptibility was also observed in excess of 900°C and crack-tip-stresses-driven microstructural instability is evident at 1400°C. At this temperature, slow loading rates or dwell times at maximum stress lead to crack-tip recrystallization and creep cavitation that together degrade the material’s properties. INTRODUCTION Multiphase Nb-based alloys and Mo-based alloys containing a matrix metallic phase with the potential to provide damage tolerance, and a significant volume fraction of an intermetallic second phase(s) that is projected to enhance the creep resistance of the alloy, are being considered promising alternatives to monolithic intermetallics and ceramics for ultra-high temperature applications. An overview on each of these alloy systems was published in 2003 [1,2] and more recently, a chapter on the structure and properties of Mo-Si-B alloys captures the current status [3]. Within the subject of Mo-based alloys, there have been several recent studies on the microstructure and properties of single- and multiphase alloys in the Mo-Si-B system [4-20]. These have included i) establishing ternary phase equilibria [4-7] and understanding how they change with quaternary alloying [4,8,9], ii) detailed characterization of the resulting microstructures [8,9], iii) understanding and developing strategies for improving hightemperature oxidation resistance [11-15], iv) characterizing the compression and tension response as a function of temperature and strain rate of several ternary and quaternary alloy compositions, some in single crystal forms but most as polycrystals [16-29], and vi) in one instance, characterizing the cyclic crack growth response of a ternary Mo-Si-B alloy at ambient temperature, 800°C, 1200°C and 1300°C [30-32]. At elevated temperatures, crack growth behavior in most metallic materials will be influenced by creep and/or by environment. These effects have been examined in specialty steels and in Nibase superal
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