A Study of the Effects of Hf and Sn on the Microstructure, Hardness and Oxidation of Nb-18Si Silicide-Based Alloys-RM(Nb)ICs with Ti Addition and Comparison with Refractory Complex Concentrated Alloys (RCCAs)

Abstract

In this paper, we present a systematic study of the as-cast and heat-treated microstructures of three refractory metal intermetallic composites based on Nb (i.e., RM(Nb)ICs), namely the alloys EZ2, EZ5, and EZ6, and one RM(Nb)IC/RCCA (refractory complex concentrated alloy), namely the alloy EZ8. We also examine the hardness and phases of these alloys. The nominal compositions (at.%) of the alloys were Nb-24Ti-18Si-5Hf-5Sn (EZ2), Nb-24Ti-18Si-5Al-5Hf-5Sn (EZ5), Nb-24Ti-18Si-5Cr-5Hf-5Sn (EZ6), and Nb-24Ti-18Si-5Al-5Cr-5Hf-5Sn (EZ8). All four alloys had density less than 7.3 g/cm³. The Nb_ss was stable in EZ2 and EZ6 and the C14-NbCr₂ Laves phase in EZ6 and EZ8. In all four alloys, the A15-Nb₃X (X = Al,Si,Sn) and the tetragonal and hexagonal Nb₅Si₃ were stable. Eutectics of Nb_ss + Nb₅Si₃ and Nb_ss + C14-NbCr₂ formed in the cast alloys without and with Cr addition, respectively. In all four alloys, Nb₃Si was not formed. In the heat-treated alloys EZ5 and EZ8, A15-Nb₃X precipitated in the Nb₅Si₃ grains. The chemical compositions of Nb_ss + C14-NbCr₂ eutectics and some Nb₅Si₃ silicides and lamellar microstructures corresponded to high-entropy or complex concentrated phases (compositionally complex phases). Microstructures and properties were considered from the perspective of the alloy design methodology NICE. The vol.% Nb_ss increased with increasing Δχ_Nbss. The hardness of the alloys respectively increased and decreased with increasing vol.% of A15-Nb₃X and Nb_ss. The hardness of the A15-Nb₃X increased with its parameter Δχ, and the hardness of the Nb_ss increased with its parameters δ and Δχ. The room-temperature-specific strength of the alloys was in the range 271.7 to 416.5 MPa cm³g^-1. The effect of the synergy of Hf and Sn, or Hf and B, or Hf and Ge on the macrosegregation of solutes, microstructures, and properties of RM(Nb)ICs/RCCAs from this study and others is compared. Phase transformations involving compositionally complex phases are discussed.

Keywords: Nb silicide-based alloys; alloy design; complex concentrated alloys; complex concentrated phases; compositionally complex phases; high-entropy phases; refractory metal intermetallic composites.