Abstract
The oxidation of Nb–silicide-based alloys is improved with Al, Cr, Ge or Sn addition(s). Depending on addition(s) and its(their) concentration(s), alloyed C14-AB2 Laves and A15-A3X phases can be stable in the microstructures of the alloys. In both phases, A is the transition metal(s), and B and X respectively can be Cr, Al, Ge, Si or Sn, and Al, Ge, Si or Sn. The alloying, creep and hardness of these phases were studied using the composition weighted differences in electronegativity (∆χ), average valence electron concentrations (VEC) and atomic sizes. For the Laves phase (i) the VEC and ∆χ were in the ranges 4.976 < VEC < 5.358 and −0.503 < ∆χ < −0.107; (ii) the concentration of B (=Al + Cr + Ge + Si + Sn) varied from 50.9 to 64.5 at %; and (iii) the Cr concentration was in the range of 35.8 < Cr < 51.6 at %. Maps of ∆χ versus Cr, ∆χ versus VEC, and VEC versus atomic size separated the alloying behaviours of the elements. Compared with unalloyed NbCr2, the VEC decreased and ∆χ increased in Nb(Cr,Si)2, and the changes in both parameters increased when Nb was substituted by Ti, and Cr by Si and Al, or Si and Ge, or Si and Sn. For the A15 phase (i) the VEC and ∆χ were in the ranges 4.38 < VEC < 4.89 and 0.857 < ∆χ < 1.04, with no VEC values between 4.63 and 4.72 and (ii) the concentration of X (=Al + Ge + Si + Sn) varied from 16.3 to 22.7 at %. The VEC versus ∆χ map separated the alloying behaviours of elements. The hardness of A15-Nb3X was correlated with the parameters ∆χ and VEC. The hardness increased with increases in ∆χ and VEC. Compared with Nb3Sn, the ∆χ and hardness of Nb3(Si,Sn) increased. The substitution of Nb by Cr had the same effect on ∆χ and hardness as Hf or Ti. The ∆χ and hardness increased with Ti concentration. The addition of Al in Nb3(Si,Sn,Al) decreased the ∆χ and increased the hardness. When Ti and Hf, or Ti, Hf and Cr, were simultaneously present with Al, the ∆χ was decreased and the hardness was unchanged. The better creep of Nb(Cr,Si)2 compared with the unalloyed Laves phase was related to the decrease in the VEC and ∆χ parameters.
Highlights
The inherent temperature capability of Ni-based superalloys is limited by the melting point of Ni.Currently, new high temperature alloys are developed based on refractory metals
The C14–NbCr2 Laves and A15–Nb3 X intermetallic phases studied in this paper were in cast (AC) and/or heat treated (HT) microstructures of Nb–silicide-based alloys that were prepared in earlier research, meaning no new experimental data were created during the course of this study
Laves phases where Cr is substituted by Fe in Nb–silicide-based alloys); (b) that in the alloyed NbCr2, the Cr concentration was in the range 35.8 < Cr < 51.6 at % and increased after heat treatment, from 35.8 < Cr < 49.2 at % to 44.7 < Cr < 51.6 at % and (c) that the solubilities of the elements that substituted Cr in the Laves phase were Al ≤ 11 at %, Ge ≤ 3.1 at %, 5 < Si < 12.6 at % and Sn ≤ 2.8 at %
Summary
The inherent temperature capability of Ni-based superalloys is limited by the melting point of Ni. Alloying additions that substitute Nb strengthen the Nb3 Al at high temperatures in the sequence W, Mo, Ta, Ti, and the stronger effect of the refractory metals has been attributed to their low diffusivities in it [27]. The refractory metals, Mo, Ta and W, provide solid solution strengthening and improve the high temperature strength and creep of Nb–silicide-based alloys. Phase equilibria data for Nb–Si based systems where C14 NbCr2 Laves and A15-Nb3 X phases can form is insufficient. What can we learn about the alloying behaviours of the C14–NbCr2 Laves and A15–Nb3 X phases in Nb–silicide-based alloys from data concerning the chemical compositions of these phases in developmental alloys?. The structure of the paper is as follows: First, the alloying of the C14–NbCr2 Laves phase is discussed using maps that are based on electronegativity, valence electron concentration and atomic size. The creep of C14–NbCr2 Laves and A15 Nb3 Al phases is compared
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