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.
Highlights
The development of high-temperature engineering alloys that can operate at temperatures above those of the latest generations of Ni-based superalloys is a priority in current metallurgical research to enable future gas turbine technologies to meet environmental and performance targets [1]
The Nb-silicide based alloys have higher melting temperatures, lower densities and better creep properties and are stable at higher temperatures than the Ni-based superalloys. These new alloys are known as Nb in situ composites, and their microstructures consist of Nb solid solution that provides toughness and intermetallics that give low- and high-temperature strength and creep resistance [2]
Higher Ti concentrations of 37.5 at. % and 50 at. % were considered in order to study the effect of the Ti concentration on the stability of the hexagonal silicide, and provide an estimation of the critical Ti concentration to form γNb5Si3
Summary
The development of high-temperature engineering alloys that can operate at temperatures above those of the latest generations of Ni-based superalloys is a priority in current metallurgical research to enable future gas turbine technologies to meet environmental and performance targets [1]. Nb-silicide based alloys at temperatures below 1500 °C [7,8] The latter is undesirable because the hexagonal 5–3 silicide is reported to have inferior creep properties than the αNb5Si3 and βNb5Si3 [1,2]. Nb5Si3 solidification area without specifying whether the primary silicide was the βNb5Si3 or the αNb5Si3 or the hexagonal Ti5Si3 based 5–3 silicide [10]. Li et al [12] revised the Nb-Ti–Si liquidus projection based on a study of ternary alloys in the Nb5Si3-Ti5Si3 region. %, the liquidus projection has a very narrow primary αNb5Si3 solidification area and indicates that at higher concentrations primary hexagonal Ti5Si3 will form during solidification. In this work first-principles calculations are used to study the stability and physical properties of the three polymorphs, αNb5Si3, βNb5Si3 and γNb5Si3 alloyed with Ti The paper provides new data that advance current understanding of the stability of complex Nb-silicide based alloys and the design and development of new alloys
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