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 Nbss, 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 Nbss + Nb3Si eutectic with Si ∼18.1 at.%, and significant variation in the Si concentration in Nbss and Hf segregation in the equilibrium and metastable Nb3Si. The microstructure in the bulk of the heat treated alloy (YG4-HT) consisted of Nbss, Nb5Si3, Nb3Si and HfO2. Metastable Nb3Si was not present. The volume fraction of Nb5Si3 had increased owing to the transformation Nb3Si → Nbss + αNb5Si3 and in the lamellar microstructure the concentration of Si was ∼24.2 at.%. There was oxygen contamination of YG4-HT, with an average concentration of oxygen in Nbss 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 Nbss and αNb5Si3 and that the formation of metastable Nb3Si is enhanced by the presence of Mo in the alloy.