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Microsegregation and Rayleigh number variation during the solidification of superalloy Inconel 718
Journal of University of Science and Technology Beijing Volume 15, Number 5, October 2008, Page 594
Materials
Microsegregation and Rayleigh number variation during the solidification of superalloy Inconel 718 Ling Wang1, 2), Jianxin Dong1), Yuliang Tian1), and Lei Zhang2) 1) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2) College of Mathematics and Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China (Received 2007-10-19)
Abstract: The microstructure and composition of the residual liquid at different temperatures were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDX) associated with the Thermo-calc software calculation of the equilibrium phase diagrams of Inconel 718 and segregated liquid. The liquid density difference and Rayleigh number variation during solidification were estimated as well. It is found that the heavy segregation of Nb in liquid prompts the precipitation of G and Laves phase directly from liquid and the resultant quenched liquid microstructure consists of pro-eutectic J+eutectic, or complete eutectic according to the content of Nb from low to high. The liquid density increases with decreasing temperature during the solidification of Inconel 718 and the liquid density difference is positive. The largest relative Rayleigh number occurs at 1320qC when the liquid fraction is about 40vol%. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: superalloy Inconel 718; microsegregation; liquid density difference; Rayleigh number
1. Introduction Inconel 718 is a Ni-Cr-Fe based superalloy whose main alloying elements are Nb, Mo, Ti, and Al. The alloy is strengthened by precipitates of Jc with the face centered A3B type structure and body centered tetragonal Jcc. Because of its excellent properties at medium temperature, Inconel 718 has been found extensive applications in aerospace industry, nuclear power plants, and petrochemical engineering as well. However, Inconel 718 has high contents of Nb and Mo (5wt% Nb and 3wt% Mo) and is susceptible to heavy segregation and even macrodefects, such as freckles [-9]. Freckle is a kind of macrosegregation with a diameter of more than 1 mm and it is difficult to eliminate by heat treatment and preprocessing. So the ingots with freckles are scrapped, which limits the scale-up of the size and development of superalloy castings [10-16]. According to the traditional freckling mechanism, freckles form by thermosolutal convection driven by the density difference between the lighter liquid at the dendrite bottom and the heavier liquid at the dendrite tips. Based on this theory, sevCorresponding author: Ling Wang, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
eral versions of freckle criteria were proposed. The criterion given by Flemings and co-workers was found to predict the freckle formation satisfactorily according to Ref. [9]. In this article, the liquid density and Rayleigh number were calculated by the method provided by Refs. [17-18] and the freckling tendency in Inconel 718 was analyzed.
2. Experimental The chemical composition (wt%) of Inconel 718 is C 0.021, Nb 5.36, Ti 0.97, Al 0.56, Mo 2.98; Fe 19.93, Cr 17.72, and Ni balanced. The raw experimental materials were vacuum induction melted and vacuum arc remelted beforehand. The remelting process in the experiment was designed according to the melting temperature range of Inconel 718 which was 1260-1336qC. All the samples are columns of I25 mmu35 mm. They were heated to 1400qC in alumina crucibles, held for 20 min for soaking and then cooled in the furnace to a temperature in freezing range; held for 20 min followed by quenching in water. The temperatures at which the samples were quenched were designed as 1360, 1340, Also available online at www.sciencedirect.com
L. Wang et al., Microsegregation and Rayleigh number variation during…
1320, 1300, 1280, 1260, 1240, 1200, 1180, and 1140 qC, respectively. Oxidization scale on the remelt-treated samples was removed by the grinder. Then these samples were ground to 1000-grit followed by being mechanically polished and then electro etched in a solution of CrO3, H2SO4, and H3PO4 at a voltage of 10 V for 10 s. The microstructure of these etched samples was investigated by an S250 scanning electron microscope (SEM). The composition of liquid, dendrites, and some phases in the microstructure at different temperatures was analyzed by EDX.
3. Results and discussion 3.1. Solidification microstructure and segregation The SEM micrographs of some remelted samples are shown in Fig. 1. The alloy liquidus of Inconel 718 is below 1340qC and it is complete liquid in the microstructure of Fig. 1(a). Below 1340qC, the liquid transforms into solid quickly and it can be seen from Fig. 1(b) that there is less than 50vol% residual liquid in the microstructure at 1320qC. When the tempera-
Fig. 1.
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ture decreases to 1280qC, the interdendritic liquid network becomes discontinuous (Fig. 1(c)). The residual liquid at 1180qC distributes as strips and short bars in the microstructure as shown in Fig. 1(d). There is about 5vol% liquid that remains in the microstructure quenched at 1180qC. The composition variation in the interdendritic liquid was investigated. At least five times of measurements of the liquid composition at each temperature were made by EDX. The segregation of alloying elements during solidification is shown in Fig. 2(a). The liquid fraction is shown by the dash dotted line. It can be seen that the content of Nb increases quickly and greatly as solidification. There is about 16wt% Nb in the final liquid although there is only 5wt% Nb in the nominal composition. The increasing Nb content concentrates at a temperature above 1300qC when the liquid fraction decreases from 100vol% to less than 20vol%. The content of Mo and Ti in the liquid has a light increase during solidification. But the content of Fe and Cr significantly decreases in the residual liquid with decreasing temperature.
Solidification microstructures at (a) 1340qC; (b) 1320qC; (c) 1280qC; (d) 1180qC.
To accurately and completely investigate the segregation behavior during the solidification of Inconel 718, the segregation ratios (SR) at different temperatures were calculated and the result is shown in Fig. 3(b). For the alloying elements of positive segregation, the higher the content of Nb in the interdendritic liquid, the higher the SR and the heavier the segregation. But for the alloying elements of negative segregation, the lower the SR, the less the depletion of the elements in the solid. Fig. 3(b) shows that the SR of Nb
increases greatly and quickly as the temperature changes from 1340 to 1300qC but the increase becomes slowly under 1300qC and the SR at 1180qC has a slight increase. The SR of other positive segregation alloying elements, such as Mo and Ti, has a similar change tendency during solidification but their value is much lower than that of Nb. The maximum SR of Nb reaches 5.5. The SR curves of negative segregation alloying elements, Cr and Fe, are very similar to each other and their value is lower than 1. The SR of Ni is
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about 1 and its curve is a straight line. During the solidification of Inconel 718, the alloying elements will be redistributed between the solid and the liquid and the elements of positive segregation, such as Nb, Mo, and Ti are rejected into liquid from solid. So the content of them in liquid will increase with the liquid fraction decreasing. On the other hand, the negative segregated elements, such as Fe and Cr,
Fig. 2.
Profiles of liquid composition (a) and segregation ratio during solidification (b).
3.2. Microstructure of the liquid quenched at different temperatures The variation in the content of Nb in the liquid changes the equilibrium phase transformation which results in different microstructures of the quenched liquid. The equilibrium phase diagrams of Inconel 718 and the segregated liquid quenched at 1280qC were calculated and their partial enlarged diagrams are shown in Fig. 3. As shown in Fig. 3(a), the main phase transformation in the freezing range is the liquid
Fig. 3.
congregate in solid. Because of the large atom size and low diffusion coefficient, Nb segregation is the most serious, the content of Nb exceeds 15wt% in the final solidified liquid and the maximum SR reaches 5.5 in this experiment. Mo segregation is not very serious and the Mo content in the final solidified liquid does not reach 10wt% because the content of Mo in the alloy is not very high.
transforming into J phase and very little MC. The G and Laves phase form at the temperature below solidus. But phase transformation becomes very different in the segregated liquid. According to Fig. 3(b), both the liquidus and solidus decrease to lower temperatures and at the same time, the G and Laves phase precipitate directly from liquid other than solid. The higher the Nb content, the lower the solidus and the more the quantity of high-Nb phases, such as G and Laves, precipitating directly from liquid.
Partial enlarged equilibrium diagrams of Inconel 718 (a) and the segregated liquid quenched at 1280qC (b).
During solidification, the phase transformation and the resultant microstructure of the segregated liquid are very different from those of the liquid with the original composition of Inconel 718 as the above calculated results indicated. The liquid microstructures quenched at different temperatures were observed and are shown in Fig. 4. Due to not a very high content of Nb, the residual liquid at 1320qC transforms into
pro-eutectic phase of J plus eutectic during quenching in water as given in Fig. 4(a). According to the calculated results, the eutectic is mainly (J+G/Laves) and pseudo-eutectic is not excluded because of the fast cooling. As the temperature decreases below 1300qC and the residual liquid becomes less than 20vol%, the Nb content increases close to eutectic concentration. Such liquid microstructure completely consists of
L. Wang et al., Microsegregation and Rayleigh number variation during…
eutectic or pseudo-eutectic after quenching (Fig. 4(b)). Then solidification becomes slow and the quantity of eutectic transformed from the residual liquid gradually becomes less as the quenching temperature decreases. Fig. 4(c) shows the microstructure quenched at 1200qC. There is a small block of NbC in the microstructure and a Gҏ-depleted area surrounding the carbide. Laves eutectic forms during late solidification as shown in Fig. 4(d).
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content of Nb in it. At the beginning of solidification, the content of Nb in the liquid is not too high to form complete eutectic but the higher the content of Nb, the more the eutectic microstructure. As the content of Nb increases to or is close to the eutectic content, the residual liquid forms complete eutectic or pseudo-eutectic. During late solidification, there are MC and Laves in the liquid and the content of Nb and Mo decreases slightly.
The liquid microstructure is closely related to the
Fig. 4.
Residual liquid microstructures quenched at different temperatures: (a) 1320qC˗(b) 1280qC; (c) 1200qC; (d) 1140qC.
3.3. Calculation of liquid density difference and Rayleigh number Inconel 718 is a kind of superalloy prone to freckling. According to the most recognized theory, freckles form by thermo-solutal convection driven by the density difference between the lighter liquid at the dendrite bottom and the heavier liquid at the dendrite tips. Based on this, Rayleigh number (Ra), derived by Flemings and co-workers, as given in Eq. (1), was found to predict the freckle formation satisfactorily [2]. Ra
'U g 3 1 Q fL R
(1)
where 'U is the liquid density difference, g the acceleration of gravity, 3 the permeability, Q the liquid viscosity, fL the liquid fraction, and R the crystal growth speed. In this article, only the compositional effect on freckle formation is discussed and the viscosity changed during solidification is not considered, because the viscosity does not vary significantly in the mushy zone temperature range for superalloys [1-2, 10-13]. Therefore, a simplified equation was used to
calculate the relative Rayleigh number Rar: Ra r
'U 3 fL
(2)
The liquid density calculation was according to the method provided by Refs. [17-18] and 'U was calculated by the following equation: 'U
U7 U 0 U0
(3)
where U 7 is the liquid density at temperature T and U 0 is that at liquidus. The variation in 'U during solidification is shown in Fig. 5. Inconel 718 has an obvious positive density difference and the values of 'U increases as the temperature decreasing. Obviously, the liquid density difference is closely related to the initial chemical composition and liquid composition variation during solidification. Inconel 718 has a high level of Nb and Mo and a low level of Ti and Al elements. As Fig. 2(a) shows, the content of Nb in liquid increases dramatically as the temperature decreases. Therefore, the density of Nb-enriched liquid becomes heavier and heavier with decreasing
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temperature. As a result, the liquid density difference generates great variation during solidification.
Fig. 5. Variation in liquid density difference during solidification.
The method used to calculate the permeability was initially proposed by Poirier and modified by Yang et al. [2]. According to Refs. [1-2], the liquid flow will be affected seriously by gravity if the liquid density is positive, such as Inconel 718, and the flow direction will be perpendicular to the primary dendrite. Generally, the permeability can be calculated by the following equation if the liquid flow is perpendicular to the primary dendrite arm:
3
'T 'T
³0
f L3.34dT
(4)
Fig. 6 gives the calculated Rar during the solidification of Inconel 718. The calculated result shows that Inconel 718 alloy has the largest Rar at 1320qC when the liquid volume fraction is about 40%. This is in agreement with that the freckling liquid fraction range is 40vol%-60vol% reported in Ref. [11]. As the liquid fraction decreases, the liquid density increases, but the permeability decreases. As a result, the maximum Rar appears at the liquid fraction of about 40vol% to 60vol%.
Fig. 6.
Relative Rayleigh number (Rar) of Inconel 718.
The tendency to form freckles in an alloy is related to the maximum Ra through solidification. The bigger the absolute value of Ra, the higher the freckle formation tendency. During slow cooling, heavy Nb segregation will be resulted and large liquid density difference generates. Meanwhile, the liquid fraction has not decreased to a very low value because the solidification is slowed down due to the high content of low-melting point elements in the liquid. Thus the resultant Ra is bigger than that under fast cooling and the system is more unstable. This is consistent with that freckles are more prone to formation in large ingots under a slower cooling. The maximum value of Rar here is not the critical Rar for freckle formation but it shows when and where the freckling tendency is the largest in the mushy zone.
4. Conclusions (1) During the solidification of Inconel 718, the most serious segregation alloying element is Nb. The maximum content of Nb in liquid reaches 16wt% and the maximum SR is 5.5. (2) Segregation changes the phase transformation in liquid. The constituent of the liquid structure quenched in water varies from pro-eutectic J+eutectic to complete eutectic as the Nb content in liquid increases; there are G, Laves and MC forming during late solidification. (3) The liquid density difference of Inconel 718 alloy is positive and the largest relative Rayleigh number occurs at 1320qC when the liquid fraction is about 40vol%.
References [1] Z.D. Long, X.B. Liu, and W.H. Yang, Thermodynamic assessment of liquid composition change during solidification and its effect on freckle formation in superalloys, Mater. Sci. Eng. A, 386A(2004), p.254. [2] W.H. Yang, J. deBarbadillo, and K. Morita, Freckle criterion for the solidification of superalloys with a tilted solidification front, JOM, 9(2004), p.56. [3] P. Aubertin, T. Wang, and S.L. Cockcroft, Freckle formation and freckle criterion in superalloy castings, Metall. Trans. B, 31B(2001), p.801. [4] J.X. Dong, M.C. Zhang, and Y.P. Zeng, Microstructure behavior and freckle characteristics for GH706 superalloy, Rare Met. Mater. Eng. (in Chinese), 35(2006), No.2, p.176. [5] L. Wang, J.X. Dong, and C.Q. Li, Effect of cooling rate on the segregation and Rayleigh number of IN718 during solidification, J. Univ. Sci. Technol. Beijing (in Chinese), 29(2007), No.12, p.1222. [6] J.X. Dong, M.C. Zhang, and X.S. Xie, Interfacial segrega-
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tion and cosegregation behavior in a nickel-base alloy 718, Mater. Sci. Eng. A, 328A(2002), p.8. B. Bottger, U. Grafe, D. Ma, et al., Prediction and measurement of microsegregation and microstructural evolution in directionally solidified superalloys, [in] R.D. Kissinge ed. Superalloys 2000, Warrendale, 2000, p.313. L. Wang, J.X. Dong, and Y.L. Tian, Segregation and liquid density variation during solidification of Waspaloy alloy, Rare Met. Mater. Eng. (in Chinese), 35(2006), No.12, p.1408. X.F. Sun, F.S. Yin, and J.G. Li, Microsegregation behavior of a cast nickel-base single crystal superalloy, Acta Metall. Sin. (in Chinese), 39(2003), No.1, p.27. W.H. Yang, W. Chen, and K.-M. Chang, Freckles in remelted niobium containing superalloys, [in] E.A. Loria ed. Superalloys 718, 625, 706 and Various Derivatives, Warrendale, 2001, p.113. W.H. Yang, W. Chen, and K.M. Chang, Freckle criteria for the upward directional solidification of alloys, Mater., Metall. Trans. A, 31A(2001), p.397. W.H. Yang, W. Chen, K.M. Chang, et al., Segregation and solid evolution during the solidification of niobium-containing superalloys, [in] T.M. Pollock, R.D.
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Kissinger, R.R. Browan, et al. eds. Superalloys 2000, Warrendale, 2000, p.75. Z.D. Long, W.H. Yang, and K.M. Chang, Freckle formation and thermodynamic assessment for Nb-bearing superalloys, [in] Proceedings of the International Symposium on Superalloys and Various Derivatives, Warrendale, 2001, p.745. L.C. Li, Microstructural Development and Segregation Effects in Directionally Solidified Ni-Based Superalloy PWA1484 [Dissertation], Auburn University, 2002. C. Frueh, D.R. Poirier, and R.G. Erdmann, Simulation of directional solidification with steep thermal gradients, Mater. Sci. Eng. A, 328A(2003), p.245. C. Frueh, D.R. Poirier, and R.G. Erdmann, On the length-scale and location of channel nucleation in directional solidification, Mater. Sci. Eng. A, 345A(2003), p.72. P. Auburtin and A. Mithchell, Liquid density inversion during the solidification of superalloys and their relationship to freckle formation in castings, [in] E.A. Loria ed. Superalloys 1996, Warrendale, 1996, p.443. P.K. Sung, D.R. Poirier, and E. Mcbride, Estimating densities of liquid transition-metals and Ni-base superalloys, Mater. Sci. Eng. A., 231A(1997), p.189.
Materials
Microsegregation and Rayleigh number variation during the solidification of superalloy Inconel 718 Ling Wang1, 2), Jianxin Dong1), Yuliang Tian1), and Lei Zhang2) 1) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2) College of Mathematics and Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China (Received 2007-10-19)
Abstract: The microstructure and composition of the residual liquid at different temperatures were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDX) associated with the Thermo-calc software calculation of the equilibrium phase diagrams of Inconel 718 and segregated liquid. The liquid density difference and Rayleigh number variation during solidification were estimated as well. It is found that the heavy segregation of Nb in liquid prompts the precipitation of G and Laves phase directly from liquid and the resultant quenched liquid microstructure consists of pro-eutectic J+eutectic, or complete eutectic according to the content of Nb from low to high. The liquid density increases with decreasing temperature during the solidification of Inconel 718 and the liquid density difference is positive. The largest relative Rayleigh number occurs at 1320qC when the liquid fraction is about 40vol%. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: superalloy Inconel 718; microsegregation; liquid density difference; Rayleigh number
1. Introduction Inconel 718 is a Ni-Cr-Fe based superalloy whose main alloying elements are Nb, Mo, Ti, and Al. The alloy is strengthened by precipitates of Jc with the face centered A3B type structure and body centered tetragonal Jcc. Because of its excellent properties at medium temperature, Inconel 718 has been found extensive applications in aerospace industry, nuclear power plants, and petrochemical engineering as well. However, Inconel 718 has high contents of Nb and Mo (5wt% Nb and 3wt% Mo) and is susceptible to heavy segregation and even macrodefects, such as freckles [-9]. Freckle is a kind of macrosegregation with a diameter of more than 1 mm and it is difficult to eliminate by heat treatment and preprocessing. So the ingots with freckles are scrapped, which limits the scale-up of the size and development of superalloy castings [10-16]. According to the traditional freckling mechanism, freckles form by thermosolutal convection driven by the density difference between the lighter liquid at the dendrite bottom and the heavier liquid at the dendrite tips. Based on this theory, sevCorresponding author: Ling Wang, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
eral versions of freckle criteria were proposed. The criterion given by Flemings and co-workers was found to predict the freckle formation satisfactorily according to Ref. [9]. In this article, the liquid density and Rayleigh number were calculated by the method provided by Refs. [17-18] and the freckling tendency in Inconel 718 was analyzed.
2. Experimental The chemical composition (wt%) of Inconel 718 is C 0.021, Nb 5.36, Ti 0.97, Al 0.56, Mo 2.98; Fe 19.93, Cr 17.72, and Ni balanced. The raw experimental materials were vacuum induction melted and vacuum arc remelted beforehand. The remelting process in the experiment was designed according to the melting temperature range of Inconel 718 which was 1260-1336qC. All the samples are columns of I25 mmu35 mm. They were heated to 1400qC in alumina crucibles, held for 20 min for soaking and then cooled in the furnace to a temperature in freezing range; held for 20 min followed by quenching in water. The temperatures at which the samples were quenched were designed as 1360, 1340, Also available online at www.sciencedirect.com
L. Wang et al., Microsegregation and Rayleigh number variation during…
1320, 1300, 1280, 1260, 1240, 1200, 1180, and 1140 qC, respectively. Oxidization scale on the remelt-treated samples was removed by the grinder. Then these samples were ground to 1000-grit followed by being mechanically polished and then electro etched in a solution of CrO3, H2SO4, and H3PO4 at a voltage of 10 V for 10 s. The microstructure of these etched samples was investigated by an S250 scanning electron microscope (SEM). The composition of liquid, dendrites, and some phases in the microstructure at different temperatures was analyzed by EDX.
3. Results and discussion 3.1. Solidification microstructure and segregation The SEM micrographs of some remelted samples are shown in Fig. 1. The alloy liquidus of Inconel 718 is below 1340qC and it is complete liquid in the microstructure of Fig. 1(a). Below 1340qC, the liquid transforms into solid quickly and it can be seen from Fig. 1(b) that there is less than 50vol% residual liquid in the microstructure at 1320qC. When the tempera-
Fig. 1.
595
ture decreases to 1280qC, the interdendritic liquid network becomes discontinuous (Fig. 1(c)). The residual liquid at 1180qC distributes as strips and short bars in the microstructure as shown in Fig. 1(d). There is about 5vol% liquid that remains in the microstructure quenched at 1180qC. The composition variation in the interdendritic liquid was investigated. At least five times of measurements of the liquid composition at each temperature were made by EDX. The segregation of alloying elements during solidification is shown in Fig. 2(a). The liquid fraction is shown by the dash dotted line. It can be seen that the content of Nb increases quickly and greatly as solidification. There is about 16wt% Nb in the final liquid although there is only 5wt% Nb in the nominal composition. The increasing Nb content concentrates at a temperature above 1300qC when the liquid fraction decreases from 100vol% to less than 20vol%. The content of Mo and Ti in the liquid has a light increase during solidification. But the content of Fe and Cr significantly decreases in the residual liquid with decreasing temperature.
Solidification microstructures at (a) 1340qC; (b) 1320qC; (c) 1280qC; (d) 1180qC.
To accurately and completely investigate the segregation behavior during the solidification of Inconel 718, the segregation ratios (SR) at different temperatures were calculated and the result is shown in Fig. 3(b). For the alloying elements of positive segregation, the higher the content of Nb in the interdendritic liquid, the higher the SR and the heavier the segregation. But for the alloying elements of negative segregation, the lower the SR, the less the depletion of the elements in the solid. Fig. 3(b) shows that the SR of Nb
increases greatly and quickly as the temperature changes from 1340 to 1300qC but the increase becomes slowly under 1300qC and the SR at 1180qC has a slight increase. The SR of other positive segregation alloying elements, such as Mo and Ti, has a similar change tendency during solidification but their value is much lower than that of Nb. The maximum SR of Nb reaches 5.5. The SR curves of negative segregation alloying elements, Cr and Fe, are very similar to each other and their value is lower than 1. The SR of Ni is
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about 1 and its curve is a straight line. During the solidification of Inconel 718, the alloying elements will be redistributed between the solid and the liquid and the elements of positive segregation, such as Nb, Mo, and Ti are rejected into liquid from solid. So the content of them in liquid will increase with the liquid fraction decreasing. On the other hand, the negative segregated elements, such as Fe and Cr,
Fig. 2.
Profiles of liquid composition (a) and segregation ratio during solidification (b).
3.2. Microstructure of the liquid quenched at different temperatures The variation in the content of Nb in the liquid changes the equilibrium phase transformation which results in different microstructures of the quenched liquid. The equilibrium phase diagrams of Inconel 718 and the segregated liquid quenched at 1280qC were calculated and their partial enlarged diagrams are shown in Fig. 3. As shown in Fig. 3(a), the main phase transformation in the freezing range is the liquid
Fig. 3.
congregate in solid. Because of the large atom size and low diffusion coefficient, Nb segregation is the most serious, the content of Nb exceeds 15wt% in the final solidified liquid and the maximum SR reaches 5.5 in this experiment. Mo segregation is not very serious and the Mo content in the final solidified liquid does not reach 10wt% because the content of Mo in the alloy is not very high.
transforming into J phase and very little MC. The G and Laves phase form at the temperature below solidus. But phase transformation becomes very different in the segregated liquid. According to Fig. 3(b), both the liquidus and solidus decrease to lower temperatures and at the same time, the G and Laves phase precipitate directly from liquid other than solid. The higher the Nb content, the lower the solidus and the more the quantity of high-Nb phases, such as G and Laves, precipitating directly from liquid.
Partial enlarged equilibrium diagrams of Inconel 718 (a) and the segregated liquid quenched at 1280qC (b).
During solidification, the phase transformation and the resultant microstructure of the segregated liquid are very different from those of the liquid with the original composition of Inconel 718 as the above calculated results indicated. The liquid microstructures quenched at different temperatures were observed and are shown in Fig. 4. Due to not a very high content of Nb, the residual liquid at 1320qC transforms into
pro-eutectic phase of J plus eutectic during quenching in water as given in Fig. 4(a). According to the calculated results, the eutectic is mainly (J+G/Laves) and pseudo-eutectic is not excluded because of the fast cooling. As the temperature decreases below 1300qC and the residual liquid becomes less than 20vol%, the Nb content increases close to eutectic concentration. Such liquid microstructure completely consists of
L. Wang et al., Microsegregation and Rayleigh number variation during…
eutectic or pseudo-eutectic after quenching (Fig. 4(b)). Then solidification becomes slow and the quantity of eutectic transformed from the residual liquid gradually becomes less as the quenching temperature decreases. Fig. 4(c) shows the microstructure quenched at 1200qC. There is a small block of NbC in the microstructure and a Gҏ-depleted area surrounding the carbide. Laves eutectic forms during late solidification as shown in Fig. 4(d).
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content of Nb in it. At the beginning of solidification, the content of Nb in the liquid is not too high to form complete eutectic but the higher the content of Nb, the more the eutectic microstructure. As the content of Nb increases to or is close to the eutectic content, the residual liquid forms complete eutectic or pseudo-eutectic. During late solidification, there are MC and Laves in the liquid and the content of Nb and Mo decreases slightly.
The liquid microstructure is closely related to the
Fig. 4.
Residual liquid microstructures quenched at different temperatures: (a) 1320qC˗(b) 1280qC; (c) 1200qC; (d) 1140qC.
3.3. Calculation of liquid density difference and Rayleigh number Inconel 718 is a kind of superalloy prone to freckling. According to the most recognized theory, freckles form by thermo-solutal convection driven by the density difference between the lighter liquid at the dendrite bottom and the heavier liquid at the dendrite tips. Based on this, Rayleigh number (Ra), derived by Flemings and co-workers, as given in Eq. (1), was found to predict the freckle formation satisfactorily [2]. Ra
'U g 3 1 Q fL R
(1)
where 'U is the liquid density difference, g the acceleration of gravity, 3 the permeability, Q the liquid viscosity, fL the liquid fraction, and R the crystal growth speed. In this article, only the compositional effect on freckle formation is discussed and the viscosity changed during solidification is not considered, because the viscosity does not vary significantly in the mushy zone temperature range for superalloys [1-2, 10-13]. Therefore, a simplified equation was used to
calculate the relative Rayleigh number Rar: Ra r
'U 3 fL
(2)
The liquid density calculation was according to the method provided by Refs. [17-18] and 'U was calculated by the following equation: 'U
U7 U 0 U0
(3)
where U 7 is the liquid density at temperature T and U 0 is that at liquidus. The variation in 'U during solidification is shown in Fig. 5. Inconel 718 has an obvious positive density difference and the values of 'U increases as the temperature decreasing. Obviously, the liquid density difference is closely related to the initial chemical composition and liquid composition variation during solidification. Inconel 718 has a high level of Nb and Mo and a low level of Ti and Al elements. As Fig. 2(a) shows, the content of Nb in liquid increases dramatically as the temperature decreases. Therefore, the density of Nb-enriched liquid becomes heavier and heavier with decreasing
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temperature. As a result, the liquid density difference generates great variation during solidification.
Fig. 5. Variation in liquid density difference during solidification.
The method used to calculate the permeability was initially proposed by Poirier and modified by Yang et al. [2]. According to Refs. [1-2], the liquid flow will be affected seriously by gravity if the liquid density is positive, such as Inconel 718, and the flow direction will be perpendicular to the primary dendrite. Generally, the permeability can be calculated by the following equation if the liquid flow is perpendicular to the primary dendrite arm:
3
'T 'T
³0
f L3.34dT
(4)
Fig. 6 gives the calculated Rar during the solidification of Inconel 718. The calculated result shows that Inconel 718 alloy has the largest Rar at 1320qC when the liquid volume fraction is about 40%. This is in agreement with that the freckling liquid fraction range is 40vol%-60vol% reported in Ref. [11]. As the liquid fraction decreases, the liquid density increases, but the permeability decreases. As a result, the maximum Rar appears at the liquid fraction of about 40vol% to 60vol%.
Fig. 6.
Relative Rayleigh number (Rar) of Inconel 718.
The tendency to form freckles in an alloy is related to the maximum Ra through solidification. The bigger the absolute value of Ra, the higher the freckle formation tendency. During slow cooling, heavy Nb segregation will be resulted and large liquid density difference generates. Meanwhile, the liquid fraction has not decreased to a very low value because the solidification is slowed down due to the high content of low-melting point elements in the liquid. Thus the resultant Ra is bigger than that under fast cooling and the system is more unstable. This is consistent with that freckles are more prone to formation in large ingots under a slower cooling. The maximum value of Rar here is not the critical Rar for freckle formation but it shows when and where the freckling tendency is the largest in the mushy zone.
4. Conclusions (1) During the solidification of Inconel 718, the most serious segregation alloying element is Nb. The maximum content of Nb in liquid reaches 16wt% and the maximum SR is 5.5. (2) Segregation changes the phase transformation in liquid. The constituent of the liquid structure quenched in water varies from pro-eutectic J+eutectic to complete eutectic as the Nb content in liquid increases; there are G, Laves and MC forming during late solidification. (3) The liquid density difference of Inconel 718 alloy is positive and the largest relative Rayleigh number occurs at 1320qC when the liquid fraction is about 40vol%.
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