The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)ss, (Nb,Ti)3Sn, (Nb,Ti)5Si3, (Nb,Ti)3Si, FeNb4Si, and Fe2Nb3 and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)3Si, (Nb,Ti)3Sn and the Ti rich oxide. In NV8 the formation of Nb5Si3 was destabilised, the stability of Nb3Si was enhanced and the eutectic between Nb5Si3 and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb4Si and Fe2Nb3 phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)3Sn, β(Ti,Nb)ss, (Ti,Nb)3Si and (Ti,Nb)5Si3 phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)ss + (Ti,Nb)5Si3 → (Ti,Nb)3Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb3Sn, Ti3Sn, and Nb3Si, Ti3Si, and Ti and Nb base 5-3 silicides are discussed.