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)

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/cm(3). The Nb(ss) was stable in EZ2 and EZ6 and the C14-NbCr(2) Laves phase in EZ6 and EZ8. In all four alloys, the A15-Nb(3)X (X = Al,Si,Sn) and the tetragonal and hexagonal Nb(5)Si(3) were stable. Eutectics of Nb(ss) + Nb(5)Si(3) and Nb(ss) + C14-NbCr(2) formed in the cast alloys without and with Cr addition, respectively. In all four alloys, Nb(3)Si was not formed. In the heat-treated alloys EZ5 and EZ8, A15-Nb(3)X precipitated in the Nb(5)Si(3) grains. The chemical compositions of Nb(ss) + C14-NbCr(2) eutectics and some Nb(5)Si(3) 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-Nb(3)X and Nb(ss). The hardness of the A15-Nb(3)X 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 cm(3)g(−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.