Study of the role of Hf, Mo and W additions in the microstructure of Nb–20Si silicide based alloys

The effects of Hf, Mo and W on the microstructure and hardness of as cast and heat treated (1500 °C/100 h) Nb–20Si–5Hf–5W (YG5), Nb-20Si–5Mo–3W (YG6) and Nb–20Si–5Hf–5Mo–3W (YG8) alloys were studied. The macrosegregation of Si was strong in the as cast alloy YG6 and decreased in the order of the synergy of alloying elements as follows (Mo + W), (Hf + W) and (Hf + Mo + W). All three alloys were contaminated by oxygen. The phases present in the as cast alloys YG5, YG6 and YG8 were the Nbss, Nb5Si3 and HfO2, Nbss, Nb3Si, and Nb5Si3 and Nbss, Nb5Si3 and HfO2 respectively, with a lamellar Nbss + Nb5Si3 microstructure formed in all three alloys. In the as cast alloys YG5 and YG6 there were Nbss grains with no Si content. There was microsegregation of Hf and W in the Nbss in the as cast alloy YG8 and microsegregation of Hf in Nb5Si3 in the as cast and heat treated alloys YG5 and YG8. It is concluded that the concentrations of Si and refractory metal (RM) in the Nbss depend on the synergy of RMs with/without Hf. The Nbss with no Si and Nb5Si3 were present in all three heat treated alloys and HfO2 was formed in the heat treated alloys YG5 and YG8 via the consumption of the Hf in the Nbss. The stability of the Nbss + Nb5Si3 lamellar microstructure decreased in the order of the heat treated alloys YG5, YG8 and YG6. In the presence of Hf the transformation of βNb5Si3 to αNb5Si3 was enhanced. It is concluded that the equilibrium microstructures of the alloys consist of the Nbss with no Si and αNb5Si3 phases. Alloying with Hf caused reduction of the hardness of the alloys YG5 and YG8 after heat treatment.