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

The effects of Hf, Mo and W on the microstructure and hardness of as cast and heat treated (1500 degrees 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 Nb-ss, Nb5Si3 and HfO2, Nb-ss, Nb3Si, and Nb5Si3 and Nb-ss, Nb5Si3 and HfO2 respectively, with a lamellar Nb-ss + Nb5Si3 microstructure formed in all three alloys. In the as cast alloys YG5 and YG6 there were Nb-ss grains with no Si content. There was microsegregation of Hf and W in the Nb-ss 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 Nb-ss depend on the synergy of RMs with/without Hf. The Nb-ss 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 Nb-ss. The stability of the Nb-ss + Nb5Si3 lamellar microstructure decreased in the order of the heat treated alloys YG5, YG8 and YG6. In the presence of Hf the transformation of beta Nb5Si3 to alpha Nb5Si3 was enhanced. It is concluded that the equilibrium microstructures of the alloys consist of the Nb-ss with no Si and alpha Nb5Si3 phases. Alloying with Hf caused reduction of the hardness of the alloys YG5 and YG8 after heat treatment. (C) 2011 Elsevier Ltd. All rights reserved.