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 degrees C/100 h and 1500 degrees 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 Nb-ss and beta Nb5Si3. The latter silicide was the primary phase and the two phases formed high volume fractions of Nb-ss + beta Nb5Si3 eutectic. The Ge addition destabilized the Nb3Si and the Nb-ss + Nb3Si eutectic. After heat treatment at 1200 degrees C for 100 h, the beta Nb5Si3 was partially transformed to the alpha Nb5Si3 in both alloys, and the two phase lamellar mixture was stable. After heat treatment at 1500 degrees C for 100 h, the microstructure of both alloys had become coarser and consisted of the Nb-ss and the Nb5Ge3 based 5-3 silicide (isomorphous with alpha Nb5Si3). The Ge partitioned to the Nb5Si3 rather than the Nb-ss, 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. (C) 2010 Elsevier Ltd. All rights reserved.