Study of the role of Mo and Ta additions in the microstructure of Nb-18Si-5Hf silicide based alloy

The effects of Mo and Ta on the microstructure and hardness of the as cast and heat treated Nb-18Si-5Hf-3Ta-2Mo (YG4) alloy were studied. There was macrosegregation of Mo and Ta in the as cast alloy (YG4-AC), the microstructure of which consisted of Nb-ss, Nb5Si3, Nb3Si and HfO2. In YG4-AC the volume fraction of Nb5Si3 was low, the Nb3Si was formed in its equilibrium and metastable forms, and there was a high fraction of Nb-ss + Nb3Si eutectic with Si similar to 18.1 at.%, and significant variation in the Si concentration in Nb-ss and Hf segregation in the equilibrium and metastable Nb3Si. The microstructure in the bulk of the heat treated alloy (YG4-HT) consisted of Nb-ss, Nb5Si3, Nb3Si and HfO2. Metastable Nb3Si was not present. The volume fraction of Nb5Si3 had increased owing to the transformation Nb3Si -> Nb-ss + alpha Nb5Si3 and in the lamellar microstructure the concentration of Si was similar to 24.2 at.%. There was oxygen contamination of YG4-HT, with an average concentration of oxygen in Nb-ss of 9 at.%. Only Ta and Hf exhibited solubility in Nb5Si3. Phase selection and stability in YG4 are compared with other Nb silicide alloys with/without Ti addition and alloying elements that include transition and refractory metals and Al. It is suggested that the equilibrium microstructure in YG4 is Nb-ss and alpha Nb5Si3 and that the formation of metastable Nb3Si is enhanced by the presence of Mo in the alloy. (C) 2010 Elsevier Ltd. All rights reserved.