Study of the effect of Ti and Ge in the microstructure of Nb–24Ti–18Si–5Ge in situ composite

The effects of Ti and Ge on the microstructure and hardness of the as cast and heat treated Nb-24Ti–18Si–5Ge (at.%) alloy (ZF3) were studied. There was macrosegregation of Si. The phases present in the as cast alloy (ZF3-AC) were the (Nb,Ti)ss, and the (Nb,Ti)3(Si,Ge), β(Nb,Ti)5(Si,Ge)3 and hexagonal (Ti,Nb)5(Si,Ge)3 silicides, with the latter forming a eutectic with the solid solution. The same phases were present after heat treatment at 1200 °C for 100 h (ZH3-HT12) but only the (Nb,Ti)ss, and the (Nb,Ti)3(Si,Ge) and (Nb,Ti)5(Si,Ge)3 silicides were present after 100 h at 1500 °C (ZF3-HT15) where TiO2 was also formed. Alloying with Ti did not stabilise the (Nb,Ti)ss + (Nb,Ti)3Si eutectic. The formation of the eutectic in ZF3-AC was strongly influenced by the partitioning behaviour of Ti in the solidifying melt that was enhanced by the presence of Ge. There were Ti rich areas in the (Nb,Ti)ss and the (Nb,Ti)3(Si,Ge) silicide only in ZF3-AC. The solubility of Ge in (Nb,Ti)3(Si,Ge) increased after heat treatment at 1500 °C. The transformation of β(Nb,Ti)5(Si,Ge)3 to α(Nb,Ti)5(Si,Ge)3 progressed from ZF3-HT12 to ZF3-HT15 but equilibrium was not reached in ZF3-HT15. The synergy of Ti with Ge resulted to a strong hardening effect and a remarkable retention of the hardness. Alloying with Ti led to a reduction of the hardness of Nb5Si3 and to an increase of the hardness of Nb3Si. The synergy of Ti with Ge resulted to a strong hardening effect for the (Nb,Ti)ss.

► Effect of Ti & Ge on macrosegregation of Si.
► Formation of (Nb,Ti)ss + (Ti,Nb)5(Si,Ge)3 eutectic.
► Hardness of alloy, Nb5Si3 and Nb3Si.