Influences of Al content and secondary phase of Mo 5 (Si,Al) 3 on the oxidation resistance of Al-rich Mo(Si,Al) 2 -base composites Tatsuo Tabaru a, *, Kazuhisa Shobu, Hisatoshi Hirai, Shuji Hanada b a Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, 807-1 Shuku, Tosu, Saga 841-0052, Japan b Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai, Miyagi 980-8577, Japan Received 12 February 2003; accepted 17 April 2003 Abstract The microstructures and oxidation behavior of Mo(Si,Al) 2 -base composites were investigated. A high Al content up to 38 mol% and the effect of the secondary Mo 5 (Si,Al) 3 phase were specifically examined. Microstructure observations indicated that the Al solubility limit at 1680 K extends to 43 and 10 mol% for Mo(Si,Al) 2 and Mo 5 (Si,Al) 3 , respectively. The composites containing about 10 vol.% Mo 5 (Si,Al) 3 exhibits excellent oxidation resistance in the temperature range between 780 and 1780 K, so that the Mo 5 (Si,Al) 3 up to about 10 vol.% is acceptable and has no detrimental influence on the oxidation resistance. At temperatures from 1480 to 1780 K, a continuous alumina layer develops on the Mo 5 (Si,Al) 3 as well as on the Mo(Si,Al) 2 . The alumina growth rate is nondistinctive in the Al range investigated, and it is controlled by inward oxygen diffusion through the alumina at 1580 K and higher. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Molybdenum silicides; B. Oxidation; C. Coating 1. Introduction Niobium (Nb)-base composites have been extensively studied for structural applications at elevated tempera- tures over the service range of Ni-base superalloys. The mechanical properties such as high temperature strength, creep properties and room temperature frac- ture toughness have been significantly improved in this decade [1–10]. However, catastrophic oxidation above 700 K remains a serious problem despite of some inten- sive attempts [11–13]. Oxidation resistance coatings are indispensable, and only when a suitable coating, including materials and coating design, is developed, will the Nb-base materials be put in practical use. The authors believe that Mo(Si,Al) 2 is one of the key materials to provide sufficient oxidation resistance to Nb-base materials due to its excellent oxidation resis- tance: i.e., a protective alumina scale is formed by high temperatureoxidationabove1300K,andanamorphous Mo–Si–Al–O forms at low temperatures around 770 K which prevents pest disintegration in contrast to MoSi 2 . To ensure successful and prolonged oxidation resis- tance, a higher Al content is favored. There have already been several studies on the oxida- tion behavior of Mo(Si,Al) 2 . Mitra and Rao has inves- tigated the oxidation behavior of Al alloyed MoSi 2 with a tetragonal C11 b structure [14], Ramberg and Worrell have reported the long-term oxidation kinetics of Mo(Al,Si) 2 with Al contents of 0.7 and 6.6 mol% [15]. As for Mo(Si,Al) 2 with higher Al contents, where the crystal structure changes to the hexagonal C40 type, Stergiou et al. and Yanagihara et al. have explored the behavior in the region with the Al content varying up to 30 mol% [16,17]. According to the studies on the phase relationship around the C40 type Mo(Si,Al) 2 [18], the Al content of the Mo(Si,Al) 2 extends to about 37 mol% at 1670 K, whereas that of the Mo 5 (Si,Al) 3 extends to about 8 mol%. The Mo(Si,Al) 2 has a narrow Mo composition range, and equilibrates with Mo 5 (Si,Al) 3 and the liqui- dus phase at the Mo-rich and-poor side, respectively. Hence, from a practical point of view, Mo(Si,Al) 2 -base 0966-9795/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0966-9795(03)00072-4 Intermetallics 11 (2003) 721–733 www.elsevier.com/locate/intermet * Corresponding author. Tel.: +81-942-81-3679; fax: +81-942-81- 3698. E-mail address: t-tabaru@aist.go.jp (T. Tabaru).