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Effect of Solidification Rate on Grain Structure Evolution During Directional Solidification of a Ni-based Superalloy
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J. Mater. Sci. Technol., 2013, 29(9), 879e883
Effect of Solidification Rate on Grain Structure Evolution During Directional Solidification of a Ni-based Superalloy Xiaoli Zhang1), Yizhou Zhou1)*, Tao Jin1), Xiaofeng Sun1), Lin Liu2) 1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2) State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China [Manuscript received July 2, 2012, in revised form September 3, 2012, Available online 18 April 2013]
The effect of solidification rate on grain structure evolution during directional solidification (DS) of a Ni-based superalloy was explored. It was found that a high solidification rate led to sharper <001> texture and smaller grain size in the DS samples. One of the most important findings in this work was that such result was not in accordance with the general concept, and the sharper <001> texture was accompanied by the larger grain size. To explain the contradiction, the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples. KEY WORDS: Solidification rate; Grain orientation; Grain size; Competitive grain growth
1. Introduction Since it has been found that the creep properties are improved markedly when the perpendicular grain boundaries are eliminated, directional solidification (DS) has been widely used to produce the columnar grain structure in as-cast turbine blades of Ni-base superalloys[1]. According to the generally accepted model which has been used to explain the development of columnar grains, columnar grains are produced by the competitive grain growth during the DS. The competitive grain growth is dominated by the dendrite tip undercooling, a function of the dendrite solidification rate. Thus, the dendrite tip undercooling increases with increasing solidification rate. It follows that the misaligned dendrites keeping up with the perfectly aligned ones must grow at a greater undercooling and hence at the rear of the growth front. This creates the condition for the secondary and tertiary arms of the well-aligned dendrites to suppress the primary trunks of the misaligned dendrites. The net result is a reduction of grain numbers and the establishment of solidification texture[2]. Schematic diagram of this mechanism is shown in literature[2e6]. In cubic alloys, the grains with <001> parallel to the thermal gradient have the fastest solidification rate, thus the columnar grains have the characteristic of <001> texture[7e10]. Thermal gradient and withdrawal speed are the key factors dominating the grain structure evolution. The withdrawal speed * Corresponding author. Prof., Ph.D.; Tel.: þ86 24 23971767; Fax: þ86 24 23971758; E-mail address: [email protected] (Y. Zhou). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.04.016
is the speed of ceramic mold moving downward in DS furnace. The solidification rate is the rate of liquid alloy solidifying to solid. As the withdrawal speed increases, the solidification rate increases simultaneously. The effect of solidification rate on <001> texture development in Ni-base superalloys has been studied[11e13]. It was reported that a higher solidification rate led to a sharp <001> texture. The phenomenon was explained in literature[11] based on the above idea of undercooling. At higher solidification rate, the better aligned grain was more advanced than the misaligned grain and the branching effect was thereby greater. Consequently, the better aligned grain overgrew the misaligned grain more quickly and led to sharp <001> texture. It is clear from this explanation and the generally accepted model that sharp <001> texture must be corresponding to less number of columnar grains. However, a recent study found that higher solidification rate led to sharp <001> texture but could not result in less number of columnar grains, i.e. more columnar grains remained at higher solidification rate[14]. To explore the contradiction between the generally accepted model and the experimental results in literature[14], different withdrawal speeds were used to produce DS samples and the modeling five-crystal (FC) samples in the present work. Since the orientations in FC samples were well controlled, structure evolution at different solidification rates could be shown clearly. By comparing the structure evolutions in DS and FC samples, a better insight into grain structure evolution in DS process can be obtained. 2. Experimental The compositions of the superalloy SRR99 used in the present work are listed in Table 1. To produce the modeling FC samples,
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X. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(9), 879e883
Table 1 Nominal composition of the superalloy SRR99 (wt%) Cr 8.4
Co 5.0
Mo e
W 9.5
Al 5.5
Ti 2.1
Ta 2.9
Ni Bal.
five single crystal (SC) seeds A, B, C, D and E were cut from the same SRR99 SC plate to control the grain numbers and orientations of the casting samples and the five seeds were placed from the left to the right in turn. To simplify analysis of microstructure, the [001] directions of these seeds were kept on the same plane and their [100] or [010] directions were nearly parallel to each other. The [001] directions of seeds A, C, and E were well aligned to the withdrawal direction, while those of seeds B and D were misaligned to the thermal gradient by an angle of 14 1 . Casting experiments were performed in a Bridgman high rate solidification (HRS) furnace with a twin-zone resistance heater. The temperatures of the upper and lower zones were 1500 and 1550 C, respectively. The alloys were superheated to 1550 C and kept at the temperature for 5 min to homogenize the melt. The melt was then poured into the preheated mold. Following 10 min holding time to ensure the system to attain a thermal equilibrium, the mold was withdrawn at a constant withdrawal speed of 1 or 6 mm/min. The details of casting experiments are listed in Table 2. The microstructure of DS samples was studied by means of optical microscopy and electron back scattered diffraction (EBSD) analysis. Microstructure development close to the chill end was examined on longitudinal section. The solidification texture evolution, the columnar grain size (d) and the primary dendrite arm spacing (l1) were analyzed on horizontal sections. d was determined by counting the number (NG) of grains within a specified area (A) and calculating the average area (a) of one grain: a ¼ A/ NG. Assuming that the grains are circular, the average grain size (d) was obtained according to: a ¼ p(d/2)2. In the process of EBSD analysis, the sample surfaces were scanned by using step sizes of 5 and 50 mm on cross sections close to and farther from the chill, respectively. Both step sizes were much less than l1. Microstructure evolution of the FC samples was studied by means of optical microscopy on longitudinal and cross sections, respectively. The grain boundary (GB) misorientation from the withdrawal axis (qGB) was determined by, tanqGB ¼ R/L, where R is the traveling distance of the GB from the initial position (determined on the cross section) and L is the corresponding distance from the melteback interface to the determined cross section. To reduce the experimental error, L was as long as possible. 3. Results The structural transition of a DS casting sample at the initial stage of the withdrawal is shown in Fig. 1. The fine and Table 2 Structural characteristics and casting conditions of the samples Exp.
Nature of sample
qA, qC and qE ( )
qB and qD ( )
V (mm/min)
1 (a) 1 (b) 2 (a) 2 (b)
DS cylinder DS cylinder FC plate FC plate
e e 01 01
e e 14 1 14 1
1 6 1 6
Notes: Vdwithdrawal speed; qdmisorientation (axial orientation) of single crystal.
Fig. 1 Optical micrograph showing the longitudinal microstructure development before and after directional solidification.
indistinct grains at the bottom end are formed by quenching before the initiation of continuous withdrawal. The DS structure then develops from the quenched solid. Grain coarsening as a result of grain overgrowth can be seen clearly in the solid formed during withdrawal. The start of continuous withdrawal can be distinguished by the different grain morphologies and the approximate position was marked by a straight line on the figure. The height of the quenched solid was about 300 mm. Since the quenched solid was formed at the same solidification conditions, the microstructures of the quenched solids at different solidification rates were the same. This was confirmed by horizontal sections at a height of 0.1 mm from the chill end (below the start of DS) in Exp. 1 (a) and (b) and the results of EBSD analysis are shown in Fig. 2(a) and (c). The values of the average grain size (d) and <001> deviation of both samples are summarized in Table 3 and are nearly the same. Furthermore, the orientations in both cases are randomly distributed. However, when the columnar grain structure is well developed, the structure characteristics are different at different solidification rates as shown in Fig. 2(b) and (d). It is clear that the high solidification rate leads to smaller grain size and sharper <001> texture. The values of d and <001> deviation are shown in Table 3. Fig. 3 shows the microstructure evolution initiated by five SC seeds at different withdrawal speeds. At 1 mm/min withdrawal speed, the grain evolution can be observed clearly on the longitudinal section as shown in Fig. 3(a). Grain A gradually overgrows grain B from the left and grain E gradually overgrows grain D from the right simultaneously. Although grain C is well aligned to the thermal gradient, it is overgrown by grains B and D from the left and right, respectively. The GB misorientations AB DE BC CD are jqGB jzjqGB jz6 and jqGB jzjqGB jz5 (superscripts A, B, C, D and E refer to different grains). In the process of competitive grain growth, grain C disappears firstly, grains B and D disappear secondly, and grains A and E survive at the top of the sample. Consequently, five grains A, B, C, D and E are observed on the cross section at the bottom of sample (Fig. 3(c)) and two grains A and E are observed on the cross section at the top of sample (Fig. 3(e)).
X. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(9), 879e883
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Fig. 2 Transverse grain maps and corresponding inverse pole figures at different distances from the chill end: (a) and (c) 0.1 mm; (b) and (d) 50 mm.
Although the corresponding seeds have the same size (Fig. 3(c) and (d)), the grain overgrowth process is different when the withdrawal speed is varied. At 6 mm/min withdrawal speed (Fig. 3(b)), the overgrowth process of grains A and E is identical with that at 1 mm/min withdrawal speed. However, the overgrowth process of the converging grains B and C or C and D at 6 mm/min withdrawal speed is different from that at 1 mm/ min withdrawal speed. The grains B and D are not able to overgrow the grain C at 6 mm/min withdrawal speed. The GB AB DE BC CD misorientations are jqGB jzjqGB jz6 and jqGB jzjqGB jz1 . Consequently, the grain C does not disappear in the process of competitive grain growth, which is different from that the grain C disappearing at 1 mm/min withdrawal speed. In short, for the five grains at the initial stage of the DS (Fig. 3(d)), the grains B and D disappear over some distance and three grains A, C and E survived finally at the top of sample (Fig. 3(f)). 4. Discussion In the progress of DS (under conditions of reasonable thermal gradient and withdrawal speed), crystal nucleation only occurs at the stage of melt pouring before the initiation of continuous withdrawal and there is no further crystal nucleation after the withdrawal. In the FC samples, we did not find additional grains formed in the DS process. This indicates our experiments were carried out under the conditions without crystal nucleation after withdrawal. As shown above, although the <001> orientations in the DS samples was distributed within smaller range, more columnar grains were obtained at higher withdrawal speed. Consequently, the columnar grain size was reduced as the withdrawal speed was increased. This result is attributed to the overgrowth process of the converging grains as shown by FC samples. Compared with the converging grains, the overgrowth of diverging grains has much less influence. The grain structure Table 3 Average grain size and <001> deviation in DS samples V (mm/min)
1 6
0.1 mm from the chill end
50 mm from the chill end
d (mm)
<001> deviation ( )
d (mm)
<001> deviation ( )
59 58
Random Random
1336 1070
0e20 0e13
evolution on the longitudinal sections in the DS process at low and high solidification rate is illustrated schematically in Fig. 4, respectively. In the case of low solidification rate (Fig. 4(a)), since grains B and D are able to expand to grain C, grains A and E have to take longer distance to overgrow them. In the case of high solidification rate (Fig. 4(b)), since grains B and D are blocked by grain C, they are overgrown by grains A and E at an earlier stage. This is why high solidification rate accelerates the formation of <001> texture and at the same time leads to smaller columnar grain size in DS process. Although branching effect was used to explain the effect of solidification rate on grain structure evolution as mentioned in the introduction, a reasonable illustration how solidification rate influences grain structure evolution cannot be found. Furthermore, nobody used the generally accepted model to explain this issue in detail. The generally accepted model suggests that the GB of converging grains lies parallel to the dendrite trunks of better aligned grain and it is independent on the solidification rate. Following this idea, it can be concluded that high solidification rate must accelerate the overgrowth rate of the diverging grains to obtain a sharp <001> texture and it is impossible that high solidification rate results in more columnar grains in the DS process. Obviously, the present results are not in accordance with the predictions of the generally accepted model completely. The problem of the generally accepted model comes from the issue about which GB of converging grains lies parallel to the dendrite trunks of the better aligned grain. This issue has been challenged by number of experiments in the recent studies. In previous work[15e17], grain structure evolution of bi-crystal samples during DS processing was explored in an attempt to explain the mechanism of competitive grain growth. In the case of converging grains, the abnormal blocking behavior that the unfavorably oriented dendrites were able to block the favorably oriented dendrites was usually observed, which resulted in the overgrown of the favorably oriented grain by unfavorably oriented grain. Namely, the favorably oriented grain enabled to be eliminated and the unfavorably oriented grain survived during the DS. In previous work[18,19], bi-crystals samples were produced to study the effect of solidification rate on competitive grain growth. For converging grains, the abnormal blocking behavior was also present and the abnormal behavior became unobvious as withdrawal speed increased. Our experimental results are in well agreement with above views. Interaction between the solute fields of neighboring dendrite tips is the most probable factor for the abnormal
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Fig. 3 Optical micrographs showing the microstructure evolution of FC samples: (a) and (b) longitudinal micrographs; (c) and (d) transverse micrographs of 7 mm from the melteback interface of seeds; (e) and (f) transverse micrographs of 90 mm from the melteback interface of seeds.
Fig. 4 Schematic illustration of the grain structure evolution of FC samples: (a) low solidification rate; (b) high solidification rate.
blocking behavior[20]. Since the solute field range d of a dendrite tip reduces with increasing withdrawal speed ðdfV 1 Þ, the solute field overlap of neighboring dendrite tips is smaller at the higher withdrawal speed. Thereby the influence of the solute field on the converging GB dendrites is weaker at the higher withdrawal speed. The weaker the influence of the solute field, the less frequent the abnormal blocking behavior. Therefore, the grain boundary misorientation of converging grains is smaller at higher withBC CD drawal speed, which is shown by jqGB jzjqGB jz5 and BC CD jqGB jzjqGB jz1 at 1 and 6 mm/min withdrawal speeds, respectively. However, at diverging grains, the overgrowth rate was invariant with changing of the solidification rate. Owing to the presence of the gap between neighboring GB dendrites, the solute fields of the neighboring GB dendrites hardly overlap. The solute field cannot affect the branching behavior at the diverging GB. The grain boundary misorientation is thus constant at different withdrawal speeds.
X. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(9), 879e883
5. Conclusion The effect of solidification rate on grain structure evolution during directional solidification is the same in the DS samples and modeling samples with five grains. A high solidification rate produces sharper <001> texture and smaller grain size, which differs from the general concept. The contradiction derives from the competitive grain growth of converging grains rather than diverging grains. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. U1037601 and 50931004); the National Basic Research Program of China (Grant No. 2010CB631206); the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201113) and the Program of “One Hundred Talented People” of the Chinese Academy of Sciences. REFERENCES [1] [2] [3] [4]
F.L. Versnyder, M.E. Shank, Mater. Sci. Eng. 6 (1970) 213e247. D. Walton, B. Chalmers, Trans. Met. Soc. AIME 215 (1959) 447e456. Ch.A. Gandin, M. Rappaza, Acta Metall. Mater. 42 (1994) 2233e2246. M. Rappaz, Ch.A. Gandin, J.L. Desbiolles, P. Thevoz, Metall. Mater. Trans. A 27 (1996) 695e705. [5] H. Esaka, Ecole Polytechnique Federale de Lausanne, Ph.D. Thesis, Switzerland, 1986, pp. 101e116.
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[6] A.G. Borisov, J. Cryst. Growth 156 (1995) 296e302. [7] P.N. Quested, M. Mclean, Mater. Sci. Eng. 65 (1984) 171e180. [8] P. Carter, D.C. Cox, C.A. Gandin, R.C. Reed, Mater. Sci. Eng. A 280 (2000) 233e246. [9] N. D’Souza, M.G. Ardakani, M. McLean, B.A. Shollock, Metall. Mater. Trans. A 31 (2000) 2877e2886. [10] Ch.A. Gandin, M. Rappaza, D. West, B.L. Adams, Metall. Mater. Trans. A 26 (1995) 1543e1551. [11] M.G. Ardakani, N. D’Souza, A. Wagner, B.A. Shollock, M. McLean, Superalloys, TMS, 2000, pp. 219e228. [12] M. Mclean, Directionally Solidified Materials for High Temperature Service, Metals Society, London, 1983, p. 161. [13] M. McLean, P.D. Lee, B.A. Shollock, Advance Materials and Processes for Gas Turbines, TMS, Warrendale, PA, 2003, pp. 83e90. [14] Y.Z. Zhou, A. Volek, R.F. Singer, Metall. Mater. Trans. A 36 (2005) 651e656. [15] Y.Z. Zhou, A. Volek, N.R. Green, Acta Mater. 56 (2008) 2631e 2637. [16] Y.Z. Zhou, N.R. Green, Superalloys, in: R.C. Reed, K.A. Green, P. Caron, T.P. Gabb, M.G. Fahrmann, E.S. Huron, S.A. Woodard (Eds.), TMS, Warrendale, PA, 2008, pp. 317e323. [17] Y.Z. Zhou, T. Jin, X.F. Sun, Acta Metall. Sin. 46 (2010) 1327e 1334 (in Chinese). [18] Y.Z. Zhou, X.F. Sun, Sci. China Technol. Sci. 55 (2012) 1327e 1334. [19] J.J. Li, Z.J. Wang, Y.Q. Wang, J.C. Wang, Acta Mater. 60 (2012) 1478e1493. [20] X.B. Meng, Q. Lu, X.L. Zhang, J.G. Li, Z.Q. Chen, Y.H. Wang, Y. Z. Zhou, T. Jin, X.F. Sun, Z.Q. Hu, Acta Mater. 60 (2012) 3965e 3975.
J. Mater. Sci. Technol., 2013, 29(9), 879e883
Effect of Solidification Rate on Grain Structure Evolution During Directional Solidification of a Ni-based Superalloy Xiaoli Zhang1), Yizhou Zhou1)*, Tao Jin1), Xiaofeng Sun1), Lin Liu2) 1) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2) State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China [Manuscript received July 2, 2012, in revised form September 3, 2012, Available online 18 April 2013]
The effect of solidification rate on grain structure evolution during directional solidification (DS) of a Ni-based superalloy was explored. It was found that a high solidification rate led to sharper <001> texture and smaller grain size in the DS samples. One of the most important findings in this work was that such result was not in accordance with the general concept, and the sharper <001> texture was accompanied by the larger grain size. To explain the contradiction, the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples. KEY WORDS: Solidification rate; Grain orientation; Grain size; Competitive grain growth
1. Introduction Since it has been found that the creep properties are improved markedly when the perpendicular grain boundaries are eliminated, directional solidification (DS) has been widely used to produce the columnar grain structure in as-cast turbine blades of Ni-base superalloys[1]. According to the generally accepted model which has been used to explain the development of columnar grains, columnar grains are produced by the competitive grain growth during the DS. The competitive grain growth is dominated by the dendrite tip undercooling, a function of the dendrite solidification rate. Thus, the dendrite tip undercooling increases with increasing solidification rate. It follows that the misaligned dendrites keeping up with the perfectly aligned ones must grow at a greater undercooling and hence at the rear of the growth front. This creates the condition for the secondary and tertiary arms of the well-aligned dendrites to suppress the primary trunks of the misaligned dendrites. The net result is a reduction of grain numbers and the establishment of solidification texture[2]. Schematic diagram of this mechanism is shown in literature[2e6]. In cubic alloys, the grains with <001> parallel to the thermal gradient have the fastest solidification rate, thus the columnar grains have the characteristic of <001> texture[7e10]. Thermal gradient and withdrawal speed are the key factors dominating the grain structure evolution. The withdrawal speed * Corresponding author. Prof., Ph.D.; Tel.: þ86 24 23971767; Fax: þ86 24 23971758; E-mail address: [email protected] (Y. Zhou). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.04.016
is the speed of ceramic mold moving downward in DS furnace. The solidification rate is the rate of liquid alloy solidifying to solid. As the withdrawal speed increases, the solidification rate increases simultaneously. The effect of solidification rate on <001> texture development in Ni-base superalloys has been studied[11e13]. It was reported that a higher solidification rate led to a sharp <001> texture. The phenomenon was explained in literature[11] based on the above idea of undercooling. At higher solidification rate, the better aligned grain was more advanced than the misaligned grain and the branching effect was thereby greater. Consequently, the better aligned grain overgrew the misaligned grain more quickly and led to sharp <001> texture. It is clear from this explanation and the generally accepted model that sharp <001> texture must be corresponding to less number of columnar grains. However, a recent study found that higher solidification rate led to sharp <001> texture but could not result in less number of columnar grains, i.e. more columnar grains remained at higher solidification rate[14]. To explore the contradiction between the generally accepted model and the experimental results in literature[14], different withdrawal speeds were used to produce DS samples and the modeling five-crystal (FC) samples in the present work. Since the orientations in FC samples were well controlled, structure evolution at different solidification rates could be shown clearly. By comparing the structure evolutions in DS and FC samples, a better insight into grain structure evolution in DS process can be obtained. 2. Experimental The compositions of the superalloy SRR99 used in the present work are listed in Table 1. To produce the modeling FC samples,
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X. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(9), 879e883
Table 1 Nominal composition of the superalloy SRR99 (wt%) Cr 8.4
Co 5.0
Mo e
W 9.5
Al 5.5
Ti 2.1
Ta 2.9
Ni Bal.
five single crystal (SC) seeds A, B, C, D and E were cut from the same SRR99 SC plate to control the grain numbers and orientations of the casting samples and the five seeds were placed from the left to the right in turn. To simplify analysis of microstructure, the [001] directions of these seeds were kept on the same plane and their [100] or [010] directions were nearly parallel to each other. The [001] directions of seeds A, C, and E were well aligned to the withdrawal direction, while those of seeds B and D were misaligned to the thermal gradient by an angle of 14 1 . Casting experiments were performed in a Bridgman high rate solidification (HRS) furnace with a twin-zone resistance heater. The temperatures of the upper and lower zones were 1500 and 1550 C, respectively. The alloys were superheated to 1550 C and kept at the temperature for 5 min to homogenize the melt. The melt was then poured into the preheated mold. Following 10 min holding time to ensure the system to attain a thermal equilibrium, the mold was withdrawn at a constant withdrawal speed of 1 or 6 mm/min. The details of casting experiments are listed in Table 2. The microstructure of DS samples was studied by means of optical microscopy and electron back scattered diffraction (EBSD) analysis. Microstructure development close to the chill end was examined on longitudinal section. The solidification texture evolution, the columnar grain size (d) and the primary dendrite arm spacing (l1) were analyzed on horizontal sections. d was determined by counting the number (NG) of grains within a specified area (A) and calculating the average area (a) of one grain: a ¼ A/ NG. Assuming that the grains are circular, the average grain size (d) was obtained according to: a ¼ p(d/2)2. In the process of EBSD analysis, the sample surfaces were scanned by using step sizes of 5 and 50 mm on cross sections close to and farther from the chill, respectively. Both step sizes were much less than l1. Microstructure evolution of the FC samples was studied by means of optical microscopy on longitudinal and cross sections, respectively. The grain boundary (GB) misorientation from the withdrawal axis (qGB) was determined by, tanqGB ¼ R/L, where R is the traveling distance of the GB from the initial position (determined on the cross section) and L is the corresponding distance from the melteback interface to the determined cross section. To reduce the experimental error, L was as long as possible. 3. Results The structural transition of a DS casting sample at the initial stage of the withdrawal is shown in Fig. 1. The fine and Table 2 Structural characteristics and casting conditions of the samples Exp.
Nature of sample
qA, qC and qE ( )
qB and qD ( )
V (mm/min)
1 (a) 1 (b) 2 (a) 2 (b)
DS cylinder DS cylinder FC plate FC plate
e e 01 01
e e 14 1 14 1
1 6 1 6
Notes: Vdwithdrawal speed; qdmisorientation (axial orientation) of single crystal.
Fig. 1 Optical micrograph showing the longitudinal microstructure development before and after directional solidification.
indistinct grains at the bottom end are formed by quenching before the initiation of continuous withdrawal. The DS structure then develops from the quenched solid. Grain coarsening as a result of grain overgrowth can be seen clearly in the solid formed during withdrawal. The start of continuous withdrawal can be distinguished by the different grain morphologies and the approximate position was marked by a straight line on the figure. The height of the quenched solid was about 300 mm. Since the quenched solid was formed at the same solidification conditions, the microstructures of the quenched solids at different solidification rates were the same. This was confirmed by horizontal sections at a height of 0.1 mm from the chill end (below the start of DS) in Exp. 1 (a) and (b) and the results of EBSD analysis are shown in Fig. 2(a) and (c). The values of the average grain size (d) and <001> deviation of both samples are summarized in Table 3 and are nearly the same. Furthermore, the orientations in both cases are randomly distributed. However, when the columnar grain structure is well developed, the structure characteristics are different at different solidification rates as shown in Fig. 2(b) and (d). It is clear that the high solidification rate leads to smaller grain size and sharper <001> texture. The values of d and <001> deviation are shown in Table 3. Fig. 3 shows the microstructure evolution initiated by five SC seeds at different withdrawal speeds. At 1 mm/min withdrawal speed, the grain evolution can be observed clearly on the longitudinal section as shown in Fig. 3(a). Grain A gradually overgrows grain B from the left and grain E gradually overgrows grain D from the right simultaneously. Although grain C is well aligned to the thermal gradient, it is overgrown by grains B and D from the left and right, respectively. The GB misorientations AB DE BC CD are jqGB jzjqGB jz6 and jqGB jzjqGB jz5 (superscripts A, B, C, D and E refer to different grains). In the process of competitive grain growth, grain C disappears firstly, grains B and D disappear secondly, and grains A and E survive at the top of the sample. Consequently, five grains A, B, C, D and E are observed on the cross section at the bottom of sample (Fig. 3(c)) and two grains A and E are observed on the cross section at the top of sample (Fig. 3(e)).
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Fig. 2 Transverse grain maps and corresponding inverse pole figures at different distances from the chill end: (a) and (c) 0.1 mm; (b) and (d) 50 mm.
Although the corresponding seeds have the same size (Fig. 3(c) and (d)), the grain overgrowth process is different when the withdrawal speed is varied. At 6 mm/min withdrawal speed (Fig. 3(b)), the overgrowth process of grains A and E is identical with that at 1 mm/min withdrawal speed. However, the overgrowth process of the converging grains B and C or C and D at 6 mm/min withdrawal speed is different from that at 1 mm/ min withdrawal speed. The grains B and D are not able to overgrow the grain C at 6 mm/min withdrawal speed. The GB AB DE BC CD misorientations are jqGB jzjqGB jz6 and jqGB jzjqGB jz1 . Consequently, the grain C does not disappear in the process of competitive grain growth, which is different from that the grain C disappearing at 1 mm/min withdrawal speed. In short, for the five grains at the initial stage of the DS (Fig. 3(d)), the grains B and D disappear over some distance and three grains A, C and E survived finally at the top of sample (Fig. 3(f)). 4. Discussion In the progress of DS (under conditions of reasonable thermal gradient and withdrawal speed), crystal nucleation only occurs at the stage of melt pouring before the initiation of continuous withdrawal and there is no further crystal nucleation after the withdrawal. In the FC samples, we did not find additional grains formed in the DS process. This indicates our experiments were carried out under the conditions without crystal nucleation after withdrawal. As shown above, although the <001> orientations in the DS samples was distributed within smaller range, more columnar grains were obtained at higher withdrawal speed. Consequently, the columnar grain size was reduced as the withdrawal speed was increased. This result is attributed to the overgrowth process of the converging grains as shown by FC samples. Compared with the converging grains, the overgrowth of diverging grains has much less influence. The grain structure Table 3 Average grain size and <001> deviation in DS samples V (mm/min)
1 6
0.1 mm from the chill end
50 mm from the chill end
d (mm)
<001> deviation ( )
d (mm)
<001> deviation ( )
59 58
Random Random
1336 1070
0e20 0e13
evolution on the longitudinal sections in the DS process at low and high solidification rate is illustrated schematically in Fig. 4, respectively. In the case of low solidification rate (Fig. 4(a)), since grains B and D are able to expand to grain C, grains A and E have to take longer distance to overgrow them. In the case of high solidification rate (Fig. 4(b)), since grains B and D are blocked by grain C, they are overgrown by grains A and E at an earlier stage. This is why high solidification rate accelerates the formation of <001> texture and at the same time leads to smaller columnar grain size in DS process. Although branching effect was used to explain the effect of solidification rate on grain structure evolution as mentioned in the introduction, a reasonable illustration how solidification rate influences grain structure evolution cannot be found. Furthermore, nobody used the generally accepted model to explain this issue in detail. The generally accepted model suggests that the GB of converging grains lies parallel to the dendrite trunks of better aligned grain and it is independent on the solidification rate. Following this idea, it can be concluded that high solidification rate must accelerate the overgrowth rate of the diverging grains to obtain a sharp <001> texture and it is impossible that high solidification rate results in more columnar grains in the DS process. Obviously, the present results are not in accordance with the predictions of the generally accepted model completely. The problem of the generally accepted model comes from the issue about which GB of converging grains lies parallel to the dendrite trunks of the better aligned grain. This issue has been challenged by number of experiments in the recent studies. In previous work[15e17], grain structure evolution of bi-crystal samples during DS processing was explored in an attempt to explain the mechanism of competitive grain growth. In the case of converging grains, the abnormal blocking behavior that the unfavorably oriented dendrites were able to block the favorably oriented dendrites was usually observed, which resulted in the overgrown of the favorably oriented grain by unfavorably oriented grain. Namely, the favorably oriented grain enabled to be eliminated and the unfavorably oriented grain survived during the DS. In previous work[18,19], bi-crystals samples were produced to study the effect of solidification rate on competitive grain growth. For converging grains, the abnormal blocking behavior was also present and the abnormal behavior became unobvious as withdrawal speed increased. Our experimental results are in well agreement with above views. Interaction between the solute fields of neighboring dendrite tips is the most probable factor for the abnormal
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Fig. 3 Optical micrographs showing the microstructure evolution of FC samples: (a) and (b) longitudinal micrographs; (c) and (d) transverse micrographs of 7 mm from the melteback interface of seeds; (e) and (f) transverse micrographs of 90 mm from the melteback interface of seeds.
Fig. 4 Schematic illustration of the grain structure evolution of FC samples: (a) low solidification rate; (b) high solidification rate.
blocking behavior[20]. Since the solute field range d of a dendrite tip reduces with increasing withdrawal speed ðdfV 1 Þ, the solute field overlap of neighboring dendrite tips is smaller at the higher withdrawal speed. Thereby the influence of the solute field on the converging GB dendrites is weaker at the higher withdrawal speed. The weaker the influence of the solute field, the less frequent the abnormal blocking behavior. Therefore, the grain boundary misorientation of converging grains is smaller at higher withBC CD drawal speed, which is shown by jqGB jzjqGB jz5 and BC CD jqGB jzjqGB jz1 at 1 and 6 mm/min withdrawal speeds, respectively. However, at diverging grains, the overgrowth rate was invariant with changing of the solidification rate. Owing to the presence of the gap between neighboring GB dendrites, the solute fields of the neighboring GB dendrites hardly overlap. The solute field cannot affect the branching behavior at the diverging GB. The grain boundary misorientation is thus constant at different withdrawal speeds.
X. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(9), 879e883
5. Conclusion The effect of solidification rate on grain structure evolution during directional solidification is the same in the DS samples and modeling samples with five grains. A high solidification rate produces sharper <001> texture and smaller grain size, which differs from the general concept. The contradiction derives from the competitive grain growth of converging grains rather than diverging grains. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. U1037601 and 50931004); the National Basic Research Program of China (Grant No. 2010CB631206); the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201113) and the Program of “One Hundred Talented People” of the Chinese Academy of Sciences. REFERENCES [1] [2] [3] [4]
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