Optimization of Mo-Si-B Intermetallics
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Optimization of Mo-Si-B Intermetallics Joachim H. Schneibel and Peter F. Tortorelli Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A Matthew J. Kramer and Andrew J. Thom Iowa State University, Ames Laboratory, Ames, Iowa 50011-3020, U.S.A. Jamie J. Kruzic and Robert O. Ritchie Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, U.S.A. ABSTRACT Mo-Si-B intermetallics consisting of the phases Mo3Si, Mo5SiB2, and α-Mo (Mo solid solution) can be designed to exhibit some degree of oxidation resistance, fracture toughness, and creep strength, but not necessarily all of these at the same time. For example, microstructures that enhance the oxidation resistance are typically associated with low fracture toughness. Examples will be given illustrating the oxidation resistance, fracture toughness, and creep strength of Mo-Si-B intermetallics as a function of their phase volume fractions as well as the topology and length scale of their microstructures. Microstructures containing either individual α-Mo particles or a continuous α-Mo matrix will be described. The examples provide possible ways to control the composition and microstructure of Mo-Si-B alloys such as to optimize the desired balance of properties. INTRODUCTION The Mo-Si-B ternary phase diagram exhibits two regions that have been of interest for several years. One region, which was the subject of pioneering research by Akinc and collaborators [1], consists of Mo5Si3, the T2 phase Mo5SiB2, and the A15 phase Mo3Si, as shown in the light-gray triangle in Fig. 1. Alloys in this region of the phase diagram exhibit excellent oxidation resistance at elevated temperatures, e.g., 1300°C. Without the boron additions, much higher silicon concentrations, such as in MoSi2, would be required [2,3]. The other alloying region, which is the subject of this paper, follows from the work of Berczik et al. [4,5]. It consists of the phases α-Mo, Mo3Si, and T2 (see dark gray triangle in Fig. 1). While alloys containing a Mo solid solution phase are not as oxidation resistant as Mo5Si3-T2-Mo3Si alloys, they exhibit a higher fracture toughness. The fracture toughness tends to increase with an increasing volume fraction of α-Mo and, for a given fraction, will be higher if the α-Mo forms as a continuous matrix instead of individual particles [6]. Since the creep strength of α-Mo is lower than that of Mo3Si and T2, the creep strength of these alloys will depend on their α-Mo volume fraction. It should also depend on the topology of the microstructure. For example, if the α-Mo is distributed as a continuous matrix or “binder” phase instead of isolated particles, the creep strength should be relatively low. EXPERIMENTAL DETAILS There are several options for processing Mo-Si-B alloys. The first approach involves arcmelting elemental materials in argon followed by drop casting into a water-cooled copper crucible [7]. If the α-Mo volume fra
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