Study of the role of Al and Cr additions in the microstructure of Nb–Ti–Si in situ composites

Additions of Cr and Al in Nb–24Ti–18Si have been studied either alone or together at Cr/Al≥1. It is shown that the βNb5Si3 is the primary phase in the Nb–24Ti–18Si–5Al, Nb–24Ti–18Si–5Cr–5Al and Nb–24Ti–18Si–8Cr–4Al alloys, which are classified as hypereutectic and that the niobium solid solution, (Nb,Ti)ss, is the primary phase in the Nb–24Ti–18Si and Nb–24Ti–18Si–5Cr alloys that are classified as hypoeutectic. In the presence of Ti the solubility of Cr and Al in the (Nb,Ti)ss increased. Titanium stabilized the (Nb,Ti)3Si phase which was destabilized by Cr and Al with the latter being more potent. The Cr additions in Nb–24Ti–18Si–5Al with Cr/Al≥1 caused the formation of the C14 Laves phase without destabilizing the βNb5Si3 phase. The C14 Laves phase with Si>2.5at.% is the ternary ρ phase proposed by Goldschmidt and Brant and its formation in the as cast alloys was associated with the partitioning of Ti and Cr during the solidification of (Nb,Ti)ss dendrites. In the heat treated hypereutectic alloys, the βNb5Si3 phase transformed to αNb5Si3 according to the precipitation reaction βNb5Si3→αNb5Si3+(Nb,Ti)ss. In the presence of Cr the diffusion of Al from the 5-3 silicide was retarded and the transformation of βNb5Si3 to αNb5Si3 and (Nb,Ti)ss was slowed down. Titanium nitride particles were formed near the edges of the heat treated specimens of all the alloys.