Alloying and Properties of C14–NbCr2 and A15–Nb3X (X = Al, Ge, Si, Sn) in Nb–Silicide-Based Alloys

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.

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 Nb ss and C14-NbCr 2 Laves phase were present at low volume fractions and were observed in the areas of dark contrast, where it was not possible to observe grain or phase boundaries. The analysis data for the Laves phase is in agreement with that from previous research [15,44]. It is likely that the analysis data of the Nb ss is not accurate owing to the size of this phase.…”

“…Alloy chemistry and processing history are important for the type, volume fraction, size, and spatial distribution of phases in the microstructure, as well as for their properties [10]. For example, the alloying of the Nb ss can improve high-temperature oxidation resistance without compromising other high-temperature mechanical properties [11][12][13], and the alloying of Nb 5 Si 3 and C14-NbCr 2 Laves phase can change the hardness and creep properties of these compounds [9,14,15].…”

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 design methodology NICE [14] utilizes relationships between the parameters δ, ∆χ, and VEC that describe the alloying behaviour of (1) Nb-silicide based alloys and (2) the phases that can be present in their microstructures [15][16][17][18][19]. NICE makes use of the relationships between the aforementioned parameters and (i) the concentrations of solutes in (a) alloy and (b) phases and (ii) the creep rate at different temperatures and stresses, (iv) the weight change in isothermal oxidation at 800 and 1200 • C, and (v) the Si macrosegregation to design alloy compositions, predict properties, and assist the selection of alloys for alloy development research.…”

“…There was macrosegregation in all three as cast alloys (Table 2). Phenomena linked with the macrosegregation of elements in Nb-silicide based alloys were discussed in [20] where the macrosegregation of Si was linked with the partitioning of other solutes between the key phases in the microstructure of Nb-silicide based alloys, namely the Nb ss , Nb 5 Si 3 , C14-NbCr 2 Laves and the Nb ss + Nb 5 Si 3 eutectic [16][17][18][19]. Tin is one of the alloying additions that have a strong effect on macrosegregation [20].…”

“…The parameters δ (related to atomic size), ∆χ (related to electronegativity), and number of valence electrons per atom filled into the valence band (VEC) describe well the alloying behaviour (a) of Nb-silicide based alloys [15] and (b) of the most important phases in their microstructures [16][17][18][19]. Also, there are relationships between the properties of these alloys (creep, weight gain in isothermal oxidation) and their phases and the alloy or phase parameters δ, ∆χ and VEC [17][18][19]. These relationships are used in the alloy design methodology NICE [14] to design new Nb-silicide based alloys.…”