Study of the role of Fe and Sn additions in the microstructure of Nb–24Ti–18Si–5Cr silicide based alloys

The role of Fe and Sn additions in the microstructures of the as cast and heat treated alloys Nb–24Ti–18Si–5Cr–5Fe (NV3) and Nb–24Ti–18Si–5Cr–5Fe–5Sn (NV5) were studied. The microstructure of the as cast alloy NV3 consisted of the (Nb,Ti)ss, (Nb,Ti)5Si3, (Nb,Ti)3Si and metastable Nb 3-1 silicides, FeNb4Si and a Ti rich oxide. The (Nb,Ti)5Si3 exhibited severe cracking everywhere in the ingot. The microstructure of the heat treated alloy NV3 consisted of the (Nb,Ti)ss, (Nb,Ti)5Si3, FeNb4Si and metastable Nb 3-1 silicides, with the latter at the higher volume fraction. The addition of Fe in the alloy NV3 retarded the Nb3Si → Nbss + αNb5Si3 eutectoid transformation during solidification, and enhanced the sensitivity of (Nb,Ti)3Si formation on cooling rate. The phases present in the microstructure of the as cast alloy NV5 were the (Nb,Ti)ss, (Nb,Ti)3Sn, (Nb,Ti)5Si3, (Nb,Ti)3Si and metastable 3-1 silicides, FeNb4Si, Fe7Nb6 and a Ti rich oxide. The formation of the Fe7Nb6 intermetallic was sensitive to cooling rate. The microstructure of the heat treated alloy NV5 consisted of the (Nb,Ti)3Sn, metastable Nb 3-1 silicide, FeNb4Si, the Ti rich oxide and (Nb,Ti)5Si3. It is concluded that in Nb–24Ti–18Si-based alloys the addition of 5 at.% Fe enhances the stability of the (Nb,Ti)3Si and that the addition of 5 at.% Fe and 5 at.% Sn destabilises the (Nb,Ti)ss.