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 degrees 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 beta Nb5Si3. In the as cast alloy ZF6 the Nb-ss, beta Nb5Si3 and C14-NbCr2 Laves phase and Nb-ss + beta Nb5Si3 eutectic were formed. The microstructure of the as cast alloy ZF9 consisted of Nb-ss, beta Nb5Si3, gamma Nb5Si3 and C14-NbCr2 Laves phase. The heat-treated microstructures of the alloys ZF6 and ZF9 consisted of Nb-ss, beta Nb5Si3 and alpha Nb5Si3 and Nb-ss, beta Nb5Si3, alpha Nb5Si3 and gamma Nb5Si3, respectively. The surfaces of both alloys were contaminated by oxygen where TiO2 and HfO2 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 Nb5Si3. The roles of alloying additions on the hardness of the Nb-ss and Nb5Si3 and relationships between alloy hardness and alloy parameters VEC (valence electron concentration), delta (related to atomic size) and Delta chi (related to electronegativity) were discussed.