Microstructure Development in High-Temperature Mo-Si-B Alloys
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MICROSTRUCTURE DEVELOPMENT IN HIGH-TEMPERATURE Mo-Si-B ALLOYS R. Sakidja and J. H. Perepezko Dept. of Materials Science and Engineering, University of Wisconsin-Madison 1509 University Ave., Madison WI 53706, USA
Abstract Mo-Si-B alloys are considered as potential high temperature structural materials due to their high melting points (above 2000oC) and excellent oxidation resistance attributed to their self-healing characteristics over an extended temperature range. In the current study, the effect of alloying additions to achieve lower weight density and microstructure stability has been examined. The critical factor to the alloying additions appears to be the stability of the high melting ternary-based T2 borosilicide phase. Introduction The challenges of a high temperature environment (T>1400°C) impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In this respect, the multiphase microstructures developed from the Mo-Si-B system offer an attractive option [1, 2]. Two phase alloys based upon the coexistence of the high melting (>2100°C) and creep resistant ternary intermetallic Mo5SiB2 or the T2 phase with relatively ductile Mo solid solution (BCC phase) allow for in-situ toughening and a further possibility for strengthening through a precipitation of Mo within the T2 phase[3]. Three phase alloys comprised of Mo, T2 and Mo3Si offer a promising balance of oxidation resistance and mechanical properties [4, 5]. The excellent oxidation stems from the formation of borosilica at low temperature and almost pure silica at high temperature (due to the volatility of B2O3 above 1000oC) on the surface during oxidation in air. Furthermore, there is large alloying substitution of molybdenum with other transition metals which potentially allow for significant improvement in the materials properties. One critical aspect of interest has been to lower the weight density of the overall Mo-Si-B alloys. In this respect, substitution of molybdenum with elements such as titanium is an attractive option the Mo-Si-B alloys. As shown in Figure 1, for a Mo-20Si-
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10 9
Ni
8
g/cm 3
7 6 5 4 3 2 Mo-20Si10B
Mo-5Ti20Si-10B
Mo-20Ti20Si-10B
Mo-50Ti20Si-10B
Figure 1 The effect of Ti substitution for Mo on the weight density. 10B (at. %) alloys which are comprised of a two-phase Mo3Si + T2, substitution of Mo with Ti will enables the weight density to drop from slightly above 9 g/cm3 to close to 6 g/cm3 (after about 70 % substitution). Nb or Zr substitution can also achieve a similar type of weight density reduction although to lesser extent. The important issue in this regard is how the alloying substitution would impact the critical properties such as the melting point and the oxidation resistance. The melting point and high-temperature properties in Mo-Si-B alloys are very much tied to the stability of the ternary-based intermetallic Mo5SiB2 (T2 phase) and the phase equilibrium that maintains the two-phase field of the ductile BCC phase with the
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