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Study on segregation behavior of alloying elements in titanium alloys during solidification Kawakami, Akira

Abstract

A fundamental study was conducted on segregation behavior of alloying elements in titanium alloys to clarify the formation mechanism of "beta-flecks", melt-related defects enriched in beta stabilizing elements, which can cause a decrease in mechanical performance. Commercial titanium alloys, which are prone to the beta-fleck formation, such as 10-2-3 (Ti-10%V-2%Fe-3%Al), Ti-17(Ti-5%Al-2%Sn-2%Zr-4%Mo-4%Cr) and 6242 (Ti-6%Al-2%Sn-4%Zr-2%Mo) were used. Solidification parameters, such as dendrite arm spacing, distribution coefficients and densities of solid/liquid phase during solidification, were investigated in these alloys. Electron Probe Micro Analysis (EPMA) revealed that periodicity in distribution profiles of alloying elements corresponded to the primary or secondary dendrite arm spacing both in laboratory melted 10-2-3 ingots and in production 10-2-3 and Ti-17 ingots. This result indicates that EPMA is an effective method to clarify the dimensions of dendrite structures in titanium alloys(no good etching technique has been demonstrated that resolves the original dendritic structure). Distribution coefficients of alloying elements in 10-2-3, Ti-17 and 6242 were experimentally obtained using a zone-melting furnace. Distribution coefficients for iron in 10-2-3, zirconium and molybdenum in 6242 were deviated from the equilibrium distribution coefficients calculated from the binary phase diagrams. The fraction solidified (f[sub s]) at the initiation of beta-flecks was estimated to be 0.9 in 10-2-3 and Ti-17 using the Scheil equation, in which experimentally obtained distribution coefficients were used. The density of liquid and solid metal at around the melting point was estimated with the calculation software "METALS" and it was clarified that solid metal is heavier than liquid enriched with iron in 10-2-3 and that enriched with chromium in Ti-17. The Rayleigh number was calculated to exceed 1 when the periodicity of chromium observed in a Ti-17 production ingot was assumed to be primary dendrite arm spacing. This fact suggests that the density-driven downward flow of liquid metal can occur. This in turn could cause channels perpendicular to the solidification direction and lead to the formation of beta-flecks, and supports the proposed mechanism. However, there are still some questions about the mechanism, such as the possibility of fluid flow at the final stage of solidification and the validity of considering the periodicity as primary dendrite arm spacing.

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