On The Microstructures and Hardness of The Nb-24Ti-18Si-5Al-5Cr-5Ge and Nb-24Ti-18Si-5Al-5Cr-5Ge-5Hf (at.%) Silicide Based Alloys

The microstructures and hardness of the as cast and heat treated (1400 °C/100 h) alloys Nb-24Ti-18Si-5Ge-5Cr-5Al (ZF6) and Nb-24Ti-18Si-5Ge-5Cr-5Al-5Hf (ZF9) were studied. Both alloys were compared with refractory metal bcc solid solution + intermetallic High Entropy Alloys (HEAs). There was macrosegregation of Si, Ti, Cr and Al in both alloys. The roles of Ge and Hf on macrosegregation are discussed. In both alloys the primary phase was the βNb(5)Si(3). In the as cast alloy ZF6 the Nb(ss), βNb(5)Si(3) and C14-NbCr(2) Laves phase and Nb(ss) + βNb(5)Si(3) eutectic were formed. The microstructure of the as cast alloy ZF9 consisted of Nb(ss), βNb(5)Si(3), γNb(5)Si(3) and C14-NbCr(2) Laves phase. The heat-treated microstructures of the alloys ZF6 and ZF9 consisted of Nb(ss), βNb(5)Si(3) and αNb(5)Si(3) and Nb(ss), βNb(5)Si(3,) αNb(5)Si(3) and γNb(5)Si(3), respectively. The surfaces of both alloys were contaminated by oxygen where TiO(2) and HfO(2) formed respectively in the alloys ZF6 and ZF9. Alloying with Hf increased the lattice parameter of Nb(ss) and decreased the hardness of ZF9 and Nb(5)Si(3). The roles of alloying additions on the hardness of the Nb(ss) and Nb(5)Si(3) and relationships between alloy hardness and alloy parameters VEC (valence electron concentration), δ (related to atomic size) and Δχ (related to electronegativity) were discussed.