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

Abstract

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 Nb ss and βNb 5Si 3. The latter silicide was the primary phase and the two phases formed high volume fractions of Nb ss + βNb 5Si 3 eutectic. The Ge addition destabilized the Nb 3Si and the Nb ss + Nb 3Si eutectic. After heat treatment at 1200 °C for 100 h, the βNb 5Si 3 was partially transformed to the αNb 5Si 3 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 Nb ss and the Nb 5Ge 3 based 5-3 silicide (isomorphous with αNb 5Si 3). The Ge partitioned to the Nb 5Si 3 rather than the Nb ss, and substituted for Si atoms in the lattice of the Nb 5Si 3. The addition of Ge increased the microhardness of the Nb 5Si 3; the two phase lamellar mixture played a dominant role in determining the macrohardness of the as cast and heat treated alloys.