The role of Sn and Ti additions in the microstructure of Nb-18Si base alloys

The effects of Sn and Ti on the microstructure and hardness of the as cast and heat treated Nb-18Si-5Sn (NV9) and Nb-24Ti-18Si-5Sn (NV6) alloys were studied. In both alloys the phases present in the as cast and heat treated microstructures were Nbss, Nb3Sn and Nb5Si3. In NV9, Sn suppressed the formation of Nb3Si, partitioned in Nbss stronger than in Nb5Si3 and did not affect significantly the solubility of Si in the Nbss. In NV6, the solubility of Ti in (Nb,Ti)ss increased in the presence of Sn, the concentration of Ti in Nb5Si3 was sensitive to cooling rate and the solubility of Sn in Nb5Si3 decreased as the concentration of Ti increased. The Ti controlled the partitioning of Si between (Nb,Ti)ss and Nb3Sn and was considered responsible for the macrosegregation of Si in the as cast ingot. The transformation of β to α Nb5Si3 was enhanced by the synergy of Sn and Ti. The addition of Ti did not destabilise the Nb3Sn. Silicon increased the hardness of Nb3Sn significantly, Sn did not affect the hardness of Nb5Si3 and Ti reduced the hardness of Nb3Sn and Nb5Si3 significantly. The hardness of NV9 and NV6 decreased and increased, respectively, by heat treatment. The reduction of the hardness of NV6-AC compared to NV9-AC is attributed to the strong effect of Ti on the hardness of Nb3Sn and Nb5Si3.