Study of the role of Al, Cr and Ti additions in the microstructure of Nb–18Si–5Hf base alloys

The effects of Cr, Al and Ti on the microstructure and hardness of as cast and heat treated Nb–18Si–5Hf–5Cr (YG1), Nb–18Si–5Hf–5Al (YG2) and Nb–18Si–5Hf–24Ti (YG3) alloys were studied. The phases present in the as cast alloys were the Nbss, Nb3Si, Nb5Si3 and HfO2 in YG1-AC, the Nbss, Nb5Si3 and HfO2 in YG2-AC and the Nbss, Nb3Si and Nb5Si3 in YG3-AC. In the YG1 the formation of Nb3Si was sensitive to cooling rate, in the YG3 the volume fraction of Nb5Si3 was low and in the YG3 and YG1 Ti and Hf rich areas were formed respectively in the Nbss and Nb5Si3. The phases present in the heat treated alloys were the Nbss, Nb5Si3 and HfO2 in YG1-HT and YG2-HT and the Nbss, Nb5Si3 (including the hexagonal γNb5Si3), Nb3Si and HfO2 in YG3-HT. Nb2N and TiN formed near the surfaces of YG1-HT and YG3-HT. Compared to YG1, in the microstructure of YG2 (i) there was a significantly higher volume fraction of a thermally stable lamellar Nbss and Nb5Si3 microstructure and (ii) a lower volume fraction of Nbss. In YG1 the eutectoid transformation of Nb3Si was enhanced and the solubility of Si in Nbss was reduced significantly. In the Nbss a reduction in the Hf solubility led respectively to increase and decrease in the Cr and Al solubility in YG1 and YG2. In YG3 the synergy of Hf and Ti stabilised the 5–3 silicide and the transformation Nb3Si → Nbss + αNb5Si3 (tI32) was replaced by an alternate eutectoid transformation in which the 5–3 silicide had the hexagonal (hP16) structure. In YG3-HT the consumption of Ti to ≤12 at% to form TiN near the surface led to the formation of a heavily contaminated fine lamellar structure via the eutectoid decomposition (Nb,Ti)3Si → (Nb,Ti)ss + (Nb,Ti)5Si3. The hardness of Nb5Si3 was reduced as the solubilities of Hf and Al in the silicide increased.