A study of the effects of Hf and Sn additions on the microstructure of Nb ss/Nb 5Si 3 based in situ composites

Abstract

The phase stability and microstructures of Nb ss/Nb 5Si 3 based in situ composites alloyed with Hf and Sn have been investigated. The Nb 5Si 3 silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb 3Si phase was suppressed by the addition of Hf. The structure of the Nb 5Si 3 phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb 5Si 3. This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb 5Si 3 was higher than that in the Nb 5Si 3. The Si concentration in the Nb 5Si 3 phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb 5Si 3 and Nb ss, respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb ss, the Sn addition promoted the formation of Si-rich C14 Cr 2Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO 2 were formed below the surfaces of the heat treated alloys.