The impact of Ti and temperature on the stability of Nb₅Si₃ phases: a first-principles study

Abstract

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 Nb5Si3 polymorphs are known, namely αNb5Si3 (tI32 Cr5B3-type, D8l), βNb5Si3 (tI32 W5Si3-type, D8m) and γNb5Si3 (hP16 Mn5Si3-type, D88). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb5Si3 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 Nb5Si3 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 Nb5Si3 was found to decrease significantly with increasing Ti content. The γNb5Si3 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 αNb5Si3 and γNb5Si3 and decreased for βNb5Si3. With the addition of Ti the αNb5Si3 and γNb5Si3 became less ductile, whereas the βNb5Si3 became more ductile. When Ti was added in the αNb5Si3 and βNb5Si3 the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly.

A Study of the Effects of Al, Cr, Hf, and Ti Additions on the Microstructure and Oxidation of Nb-24Ti-18Si Silicide Based Alloys

“…The Young’s moduli of Nb, αNb 5 Si 3 , and βNb 5 Si 3 , respectively are 101.9, 291, and 269 GPa [ 40 ]. The data in Table 3 shows that the synergy of Al, Cr, and Ti had a stronger negative effect compared with the synergies between Al and Ti and between Al, Cr, Hf, and Ti.…”

“…Furthermore, the data for the Nb 5 Si 3 in Table 3 is in agreement with the calculations that showed higher modulus for binary (unalloyed) αNb 5 Si 3 than βNb 5 Si 3 . Calculations in [ 40 ] for Nb 5 Si 3 alloyed with Ti showed that when 12.5 at.% Ti substitutes Nb in the Nb 5 Si 3 , the moduli of α(Nb 50 Ti 12.5 )Si 37.5 and β(Nb 50 Ti 12.5 )Si 37.5 were 313.8 and 238.5 GPa, respectively. The data in Table 3 for the alloy KZ7 would suggest that the substitution of Nb by Ti, and Si by Al, reduced the modulus of αNb 5 Si 3 .…”

On the Microstructure and Isothermal Oxidation at 800 and 1200 °C of the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn (at.%) Silicide-Based Alloy

“…The same is the case when Ge is in synergy with Al, Cr, and Hf without transition and refractory metals [34], but not the case without Hf and other transition and refractory metals [34]. The type of Nb 5 Si 3 in the microstructure is important owing to the different properties and CTE of the binary and alloyed βNb 5 Si 3 and αNb 5 Si 3 silicides [14,39].…”

“…The latter was not observed in ZF6. The Nb 5 Ge 3 has lower shear (G) and Young's (E) moduli and higher Poisson's ratio (ν) than βNb 5 Si 3 [54,59], as well as G, E, and ν values closer to the values of Ti-alloyed βNb 5 Si 3 , particularly as the Ti concentration in the silicide increases [39]. Furthermore, in the IOZ of OHS1, the concentrations of Cr and Ti were reduced significantly in the areas that were enriched in Ge and Sn below the said interface.…”

A Study of the Effect of 5 at.% Sn on the Micro-Structure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si Based Alloys with Al and/or Cr Additions

“…The CTE (coefficient of thermal expansion) of Nb is 7.6 × 10 −6 K −1 and of Nb 3 Sn is 10.3 × 10 −6 K −1 [43]. The βNb 5 Si 3 and αNb 5 Si 3 exhibit anisotropy in CTE and the ratio of CTE values along the a and c axes of their lattices (i.e., the CTE anisotropy ratio) is different for each phase [44]. All the aforementioned values are for the unalloyed phases and are expected to change as they become alloyed [44].…”