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),beta(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 degrees 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 degrees C (ZF3-HT-15) where TiO(2) was also formed. Alloying with Ti did not stabilise the (Nb,Ti)(ss) + (Nb,Ti)(3)Si 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 G e. 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 degrees C. The transformation of beta(Nb,Ti)(5)(Si,Ge)(3) to a(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 Nb(5)Si(3) and to an increase of the hardness of Nb(3)Si. The synergy of Ti with Ge resulted to a strong hardening effect for the (Nb,Ti)ss. (C) 2011 Elsevier Ltd. All rights reserved.