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

Abstract

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, Nb 5Si 3, Nb 3Si and HfO 2. In YG4-AC the volume fraction of Nb 5Si 3 was low, the Nb 3Si was formed in its equilibrium and metastable forms, and there was a high fraction of Nb ss + Nb 3Si eutectic with Si ∼18.1 at.%, and significant variation in the Si concentration in Nb ss and Hf segregation in the equilibrium and metastable Nb 3Si. The microstructure in the bulk of the heat treated alloy (YG4-HT) consisted of Nb ss, Nb 5Si 3, Nb 3Si and HfO 2. Metastable Nb 3Si was not present. The volume fraction of Nb 5Si 3 had increased owing to the transformation Nb 3Si → Nb ss + αNb 5Si 3 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 Nb ss of ∼9 at.%. Only Ta and Hf exhibited solubility in Nb 5Si 3. 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 αNb 5Si 3 and that the formation of metastable Nb 3Si is enhanced by the presence of Mo in the alloy.