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 ^oC/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.