The Effect of Ge and Ti Additions on the Microstructures and Properties of Nb-18Si Based Alloys

Abstract

The effects of Ge and Ti additions on the microstructure, hardness and oxidation behaviour of the alloys Nb-18Si-5Ge (ZF1) and Nb-24Ti-18Si-5Ge (ZF3) were studied. The as cast microstructure of the alloy ZF1 consisted of Nb ss (cI2), and Nb 5 Si 3 (tI32) with the latter being the primary phase and the two phases forming high volume fractions of Nb ss + Nb 5 Si 3 eutectic. The Ge addition stabilised the Nb 5 Si 3 (tI32), and destabilised the Nb 3 Si (tP32) and the Nb ss + Nb 3 Si eutectic. After heat treatment at 1200 °C for 100 h the Nb 5 Si 3 (tI32) was partially transformed to the Nb 5 Si 3 (tI32), and equilibrium was reached after heat treatment at 1500 °C for 100 h. The phases present in the as cast alloy ZF3 were the Nb ss (cI2), and the Nb 3 Si (tP32), Nb 5 Si 3 (tI32) and Ti 5 Si 3 (hP16) 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 but only the Nb ss , and the Nb 3 Si and Nb 5 Si 3 silicides were present after 100 h at 1500 °C where TiO 2 was also formed. The Ge addition increased the microhardness of the Nb 5 Si 3 . The synergy of Ti with Ge resulted in a strong hardening effect and a remarkable retention of the hardness of the alloy ZF3. The additions of Ge and Ti to the Nb-18Si alloy improved the oxidation resistance at 800 °C, but pest oxidation behaviour was not eliminated.