LatticeparametersandthermalexpansionoftheT 2 -phaseofthe Nb–Si–B system investigated by high-temperature X-ray diffraction Geovani Rodrigues*,CarlosAngeloNunes,PauloAtsushiSuzuki, Gilberto Carvalho Coelho Faculdade de Engenharia Quı´mica de Lorena (FAENQUIL), Departamento de Engenharia de Materiais (DEMAR), Polo Urbo-Industrial, Gleba AI-6 s/n o , Caixa Postal 116, 12600-970, Lorena (SP), Brazil Received 1 March 2003; accepted 17 September 2003 Abstract Thelatticeparametersandthermalexpansioncoefficients( a and  c )oftheT 2 -phaseoftheNb–Si–Bsystemwithdifferentboron contents were determined from high temperature X-ray diffraction data (298–1473 K). Alloys with nominal compositions Nb 62.5 Si 37.5 ,Nb 64 Si 30 B 6 and Nb 64 Si 14.4 B 21.6 (at.%) were prepared from high-purity materials through arc melting and heat-treat- mentundervacuum.Both a and c latticeparametersdecreasewithincreasingboroncontent.Asignificantdecreasein  c isobserved with increasing boron content while  a remains practically unchanged, diminishing the anisotropy ratio  c / a from approximately 1.2atthebinarycompositiontoapproximately0.7atNb 64 Si 14.4 B 21.6 . # 2004ElsevierLtd.Allrightsreserved. Keywords: A.Silicidesvarious;A.Ternaryalloysystems;B.Anisotropy;B.Thermalproperties;F.Diffraction 1. Introduction Alloys of the RM–Si–B (RM—Refractory Metal) systems,especiallyMo–Si–B,havebeenevaluatedunder several aspects aiming at their use in high temperature structural parts [1–7]. In the Mo–Si–B system, one of the motivations for these studies is the existence of a high melting point ternary phase (T 2 -phase with approximate Mo 5 SiB 2 stoichiometry) [8,9] presenting good creep and oxidation resistance [10–12]. This T 2 - phase has a Cr 5 B 3 -type tetragonal cell, with 32 atoms per unit cell (space group I4/mcm, Wyckoff positions: Si1=4a,Mo1=4c,B1=8h,Mo2=16l) [13] andshowsa narrow homogeneity range. In noncubic crystal struc- turesthecoefficientsofthermalexpansion(CTE)canbe highly anisotropic resulting either in high degree of strain or microcracking. A recent study [14] shows that the thermal expansion anisotropy ratio ( c / a ) of this T 2 -phase is significantly smaller than that of Mo 5 Si 3 (T 1 )phase(0.8–1.1,increasingwithtemperature,forT 2 , versus 2.2 for Mo 5 Si 3 ). The knowledge of these quan- tities constitutes an important parameter for the devel- opment of high-temperature structural materials. The T 2 -phase is already stable in the Nb–Si system (aNb 5 Si 3 -phase) [15] in which metal atoms occupy the RM1=4c andRM2=16l andSiatomstheSi1=4a and Si2=8h Wyckoff positions [13]. The isothermal section in Fig. 1 [16] indicates that this phase shows significant boron solubility in the Nb–Si–B system (up to 23 at.% at 1600  C). Based on the currently accepted Nb–Si phase diagram shown in Fig. 2 [15], the T 2 -phase homogeneityregionshownin Fig.1 shouldextendupto aNb 5 Si 3 in the Nb–Si binary. This large homogeneity range may be explained by boron substitution for sili- conintheSi2positions.Similarlytoitsthermalstability in the Mo–Si–B system, this phase presents a high melting point (above 2150  C) in the Nb–Si–B system [17]. In the present investigation we have evaluated the effect of boron substitution for silicon on the lattice parameters and thermal expansion behavior of the T 2 - phase of the Nb–Si–B system by means of High Tem- perature X-ray Diffraction (HTXRD) measurements. 0966-9795/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2003.09.015 Intermetallics 12 (2004) 181–188 www.elsevier.com/locate/intermet * Corresponding author. Tel.: +55-12-3159-9912; fax: +55-12- 3153-3006. E-mail address: geovani@ppgem.faenquil.br (G. Rodrigues).