Effect of Mo and Si on morphology and volume fraction of eutectic in Nb–Si–Mo alloys

The eutectic morphology of ternary Nb–Si–Mo alloys with hypereutectic (Nb–19Si–5Mo, and Nb–18Si–26Mo) and hypoeutectic (Nb–13Si–4Mo, and Nb–12Si–15Mo) compositions has been examined. The eutectic in these materials has been found to consist of a mixture of Nb solid solution and Nb5Si3 with lamellar or non-lamellar morphology. For similar concentration of Si, the volume fraction of non-lamellar eutectic in the hypoeutectic alloys increases with Mo content. However, the concentration of Mo has little effect on the non-lamellar eutectic content of hypereutectic alloys. Hypereutectic compositions, having higher concentration of Si, show a much larger volume fraction of non-lamellar eutectic compared to that found in the hypoeutectic compositions. The lamellar or non-lamellar character of the eutectic is attributed to the composition-dependent under-cooling behaviour of the ternary Nb–Si–Mo alloys during solidification and their volume fractions in these alloys assist to explain some of the structure–property relations.