The impact of Ti and temperature on the stability of Nb(5)Si(3) phases: a first-principles study

Nb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb(5)Si(3) polymorphs are known, namely αNb(5)Si(3) (tI32 Cr(5)B(3)-type, D8(l)), βNb(5)Si(3) (tI32 W(5)Si(3)-type, D8(m)) and γNb(5)Si(3) (hP16 Mn(5)Si(3)-type, D8(8)). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb(5)Si(3) depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb(5)Si(3) silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb(5)Si(3) was found to decrease significantly with increasing Ti content. The γNb(5)Si(3) was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb(5)Si(3) and γNb(5)Si(3) and decreased for βNb(5)Si(3). With the addition of Ti the αNb(5)Si(3) and γNb(5)Si(3) became less ductile, whereas the βNb(5)Si(3) became more ductile. When Ti was added in the αNb(5)Si(3) and βNb(5)Si(3) the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly.