Effect of Nb Content on Solidification Characteristics and Microsegregation in Cast Ti-48Al-xNb Alloys

doi:10.1007/s40195-016-0435-9

Abstract

Abstract:

Microstructural evolution in nonequilibrium solidification of Ti-48Al-xNb alloys with Nb contents ranging from 2 to 8 at% has been studied by containerless electromagnetic levitation. Levitated drops of controlled undercooling were quenched onto chill copper substrates and subjected to phase and microstructure analysis. With increasing Nb content, the solidification path changes gradually from hyperperitectic solidification to hypoperitectic solidification and both solidification segregation (S-segregation) and β-solidification gradually increase. A transition from typical hypoperitectic solidification to a sole solidification of the β phase beyond a critical undercooling is revealed for the Ti-48Al-8Nb hypoperitectic alloy. For the Ti-48Al-2Nb alloy, the morphologies of the primary β dendrites are not observed. With increasing undercooling, the coarsening of the lamellar colonies occurs, which can be attributed to the transition of the primary β dendritic morphology. Furthermore, the solute concentration profiles for the final solidification microstructure are obtained to examine the segregation behaviors of alloying elements. With increasing Nb content, the undercooling eliminating S-segregation gradually increases.

Key words: TiAl alloy, Undercooling, Microsegregation, Microstructure

Tan He,Rui Hu,Tie-Bang Zhang,Jin-Shan Li. Effect of Nb Content on Solidification Characteristics and Microsegregation in Cast Ti-48Al-xNb Alloys[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(8): 714-721.

Figures/Tables 9

Fig. 1 Schematic diagram of electromagnetic levitation facility

Fig. 2 Section of the phase diagram of TiAl-8Nb [12]

Fig. 3 SEM micrographs taken in BSE mode of the cast microstructures: a Ti-48Al-2Nb alloy, b Ti-48Al-4Nb alloy, c Ti-48Al-6Nb alloy, d Ti-48Al-8Nb alloy

Fig. 4 XRD patterns of the Ti-48Al-8Nb alloy at different undercoolings

Fig. 5 BSE images of the Ti-48Al-8Nb alloy at different undercoolings: a ΔT = 10 K, b ΔT = 59 K, c ΔT = 84 K, d ΔT = 193 K, e ΔT = 211 K, f ΔT = 300 K

Fig. 6 XRD patterns of the Ti-48Al-2Nb alloy at different undercoolings

Fig. 7 BSE images of the Ti-48Al-2Nb alloy at different undercoolings: a ΔT = 10 K, b ΔT = 75 K, c ΔT = 93 K, d ΔT = 184 K

Fig. 8 Calculated dendrite tip radius of β dendrite versus the undercooling for Ti-48Al-2Nb alloy

Fig. 9 Partition coefficients measured on the sample quenched at different undercoolings

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