Effects of substitution of Al for Si on the lattice variations and thermal expansion of Mo(Si,Al) 2 Tatsuo Tabaru a, *, Kazuhisa Shobu a , Michiru Sakamoto a , Shuji Hanada b a Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST), 807-1, Shuku, Tosu, Saga 841-0052, Japan b Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan Received 29 May 2003; accepted 30 July 2003 Abstract The lattice variations and thermal expansion of Mo(Si 1x ,Al x ) 2 with x=0 to 0.6 were investigated, and the results were discussed in terms of the substitution ratio of Al–Si. The crystal structure of Mo(Si,Al) 2 alters from C11 b to C40, and further to orthorhombic C54 with the increasing Al substitution. The lattice of each structure generally expands with the increasing substitution ratio due to the larger atomic radius of Al compared to Si. To clarify the lattice expansion in a wide composition range including C11 b , C40 and C54, new parameters corresponding to the nearest neighbor interatomic distance were considered, and a relation between the parameters and the substitution ratio was indicated. The mean values of the coefficients of thermal expansion (CTE) in the tem- perature range from 298 to 1723 K vary from 9.210 6 K 1 for MoSi 2 to 10.410 6 K 1 for Mo(Si 0.4 ,Al 0.6 ) 2 . However, the CTE variation is mostly attributed to the phase constitutions, and not to the Al substitution ratio. There is only a slight change in the C40 single-phase region. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Molybdenum silicides; C. Powder metallurgy; Coatings 1. Introduction Molybdenum aluminosilicide Mo(Si,Al) 2 with hex- agonal C40-type structure is a promising candidate for oxidation-resistance coating materials. The material exhibits a good oxidation resistance at high tempera- tures due to the sluggish alumina (Al 2 O 3 ) scale growth, and no pest disintegration at intermediate temperatures around 780 K [1–3] in contrast to MoSi 2 . Although MoSi 2 forms a silica (SiO 2 ) scale with substantially slower growth rates compared to Al 2 O 3 , the problems of destructive pest oxidation and scale deterioration in a water–vapor containing atmosphere are of critical con- cern [4,5]. Therefore, Mo(Si,Al) 2 is considered to have definite advantages over MoSi 2 for oxidation-resistance coatings at service temperatures up to about 1673 K. One of the possible applications of Mo(Si,Al) 2 would be as a coating on Nb-base structural materials, which have been extensively studied due to their excellent mechanical properties [6–8]. However, severe oxidation of the Nb-base alloys at elevated temperatures over 800 K restricts their practical use only in an inert atmos- phere, unless an oxidation-resistance coating is employed [9]. When oxidation-resistance coatings are developed with adequate durability and reliability, the Nb-base structural materials will be further used in various fields. We believe Mo(Si,Al) 2 will be the prime candidate for this purpose. The coefficient of thermal expansion (CTE) of an Al containing Mo(Si,Al) 2 has never been reported, while those of Nb, Al 2 O 3 and MoSi 2 have been reported to be close to each other, i.e., (7–10)10 6 K 1 [10]. The CTE of Mo(Si,Al) 2 is expected to be close to that of MoSi 2 , and therefore to Nb, so that the thermal stress due to the CTE mismatch would be minimal in the system. However, a detailed study is apparently needed, espe- cially for the Al substitution dependence of the CTE of Mo(Si,Al) 2 . In addition to the hexagonal C40 structure, tetragonal C11 b represented by MoSi 2 and orthorhombic C54 0966-9795/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2003.07.002 Intermetallics 12 (2004) 33–41 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).