Abstract
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 βNb 5Si 3 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 βNb 5Si 3 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 βNb 5Si 3 phase transformed to αNb 5Si 3 according to the precipitation reaction βNb 5Si 3→αNb 5Si 3+(Nb,Ti) ss. In the presence of Cr the diffusion of Al from the 5-3 silicide was retarded and the transformation of βNb 5Si 3 to αNb 5Si 3 and (Nb,Ti) ss was slowed down. Titanium nitride particles were formed near the edges of the heat treated specimens of all the alloys.
Published Version
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