Study of the effects of Ge addition on the microstructure of Nb–18Si in situ composites

The effects of Ge addition on the microstructure and hardness of the as cast and heat treated (1200 °C/100 h and 1500 °C/100 h) Nb–18Si–5Ge (ZF1) and Nb–18Si–10Ge (ZF2) (at%) alloys were studied. The as cast microstructures of both alloys consisted of Nbss and βNb5Si3. The latter silicide was the primary phase and the two phases formed high volume fractions of Nbss + βNb5Si3 eutectic. The Ge addition destabilized the Nb3Si and the Nbss + Nb3Si eutectic. After heat treatment at 1200 °C for 100 h, the βNb5Si3 was partially transformed to the αNb5Si3 in both alloys, and the two phase lamellar mixture was stable. After heat treatment at 1500 °C for 100 h, the microstructure of both alloys had become coarser and consisted of the Nbss and the Nb5Ge3 based 5-3 silicide (isomorphous with αNb5Si3). The Ge partitioned to the Nb5Si3 rather than the Nbss, and substituted for Si atoms in the lattice of the Nb5Si3. The addition of Ge increased the microhardness of the Nb5Si3; the two phase lamellar mixture played a dominant role in determining the macrohardness of the as cast and heat treated alloys.