On the Stability of Complex Concentrated (CC)/High Entropy (HE) Solid Solutions and the Contamination with Oxygen of Solid Solutions in Refractory Metal Intermetallic Composites (RM(Nb)ICs) and Refractory Complex Concentrated Alloys (RCCAs)

In as-cast (AC) or heat-treated (HT) metallic ultra-high temperature materials often “conventional” and complex-concentrated (CC) or high-entropy (HE) solid solutions (sss) are observed. Refractory metal containing bcc sss also are contaminated with oxygen. This paper studied the stability of CC/HE Nb(ss) and the contamination with oxygen of Nb(ss) in RM(INb)ICs, RM(Nb)ICs/RCCAs and RM(Nb)ICs/RHEAs. “Conventional” and CC/HE Nb(ss) were compared. “Conventional” Nb(ss) can be Ti-rich only in AC alloys. Ti-rich Nb(ss) is not observed in HT alloys. In B containing alloys the Ti-rich Nb(ss) is usually CC/HE. The CC/HE Nb(ss) is stable in HT alloys with simultaneous addition of Mo, W with Hf, Ge+Sn. The implications for alloy design of correlations between the parameter δ of “conventional” and CC/HE Nb(ss) with the B or the Ge+Sn concentration in the Nb(ss) and of relationships of other solutes with the B or Ge+Sn content are discussed. The CC/HE Nb(ss) has low Δχ, VEC and Ω and high ΔS(mix), |ΔH(mix)| and δ parameters, and is formed in alloys that have high entropy of mixing. These parameters are compared with those of single-phase bcc ss HEAs and differences in ΔH(mix), δ, Δχ and Ω, and similarities in ΔS(mix) and VEC are discussed. Relationships between the parameters of alloy and “conventional” Nb(ss) also apply for CC/HE Nb(ss). The parameters δ(ss) and Ω(ss), and VEC(ss) and VEC(alloy) can differentiate between types of alloying additions and their concentrations and are key regarding the formation or not of CC/HE Nb(ss). After isothermal oxidation at a pest temperature (800 (o)C/100 h) the contaminated with oxygen Nb(ss) in the diffusion zone is CC/HE Nb(ss), whereas the Nb(ss) in the bulk can be “conventional” Nb(ss) or CC/HE Nb(ss). The parameters of “uncontaminated” and contaminated with oxygen sss are linked with linear relationships. There are correlations between the oxygen concentration in contaminated sss in the diffusion zone and the bulk of alloys with the parameters Δχ(Nbss), δ(Nbss) and VEC(Nbss), the values of which increase with increasing oxygen concentration in the ss. The effects of contamination with oxygen of the near surface areas of a HT RM(Nb)IC with Al, Cr, Hf, Si, Sn, Ti and V additions and a high vol.% Nb(ss) on the hardness and Young’s modulus of the Nb(ss), and contributions to the hardness of the Nb(ss) in B free or B containing alloys are discussed. The hardness and Young’s modulus of the bcc ss increased linearly with its oxygen concentration and the change in hardness and Young’s modulus due to contamination increased linearly with [O](2/3).