Study of the effect of Cr and Ti additions in the microstructure of Nb–18Si–5Ge based in-situ composites

The microstructure and hardness of as cast and heat treated (100 h/1400 °C) alloys Nb–18Si–5Cr–5Ge (ZF7) and Nb–24Ti–18Si–5Cr–5Ge (ZF4) were studied. Titanium in synergy with Cr had a very strong effect on the macrosegregation of Si that increased significantly. The microstructures of the as cast alloys ZF7 and ZF4 respectively consisted of Nbss and βNb5Si3 and Nbss, βNb5Si3, and C14-Cr2Nb Laves phase. In both alloys the βNb5Si3 was the primary phase and the Nbss and βNb5Si3 formed a eutectic. The latter and the Laves phase were not formed in the bottom of the ingots (highest cooling rate) of the alloys ZF7 and ZF4, respectively. After heat treatment at 1400 °C for 100 h, the βNb5Si3 was partially transformed to the αNb5Si3 in both alloys, the two-phase Nbss + βNb5Si3 (prior eutectic) microstructure was present only in ZF7 and the Laves phase was absent in ZF4. The Ge, Cr and Ti additions caused contraction of the lattice of the Nbss. The Ge and Cr increased the hardness of the Nbss and Nb5Si3 but the Ti caused a decrease in hardness. The hardness of both alloys increased after the heat treatment.

► Effect of Ti, Cr & Ge on macrosegregation of Si.
► Formation and stability of NbCr2 Laves phase.
► Formation of Nbss + Nb5Si3 eutectic.
► Lattice parameters of Nbss.
► Hardness of alloy, Nb5Si3 and Nbss.