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ScienceDirect www.materialstoday.com/proceedings Materials Today: Proceedings 1 (2014) 3 – 10

The 1st International Joint Mini-Symposium on Advanced Coatings between Indiana UniversityPurdue University Indianapolis and Changwon National University

Directional Solidification Microstructure Control in CM247LC Superalloy J.S. Lee, J.H. Gu, H.M. Jung, E.H. Kim, Y.G. Jung, and J.H. Lee* Department of Materials Science and Engineering, Changwon National University, 20 Changwondaehakro, Uichang-gu, Changwon, Gyeongnam 641-773, Korea

Abstract Directional solidification of the superalloy CM247LC has been carried out to investigate the microstructural evolution under various solidification conditions. It can be clearly seen that the solid/liquid interface morphology depends primarily on the growth rate and temperature gradient. The planar interface was stable at low velocities and the increased velocity encouraged the development of dendritic interface. As a result, the conditions for the planar to cellular transition and the cellular to dendritic transition were obtained and therefore the solidification microstructure map was constructed based on these results. The experimental results were in good agreement with theoretical predictions for this alloy. However, the condition for the dendritic to equiaxed transition was discussed in the light of the heat balance at the solid/liquid interface. © TheAuthors. Authors.Published Published Elsevier © 2014 2014 The byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the Chairs of The 1st International Joint Mini-Symposium on Advanced Coatings between (http://creativecommons.org/licenses/by-nc-nd/3.0/). Indiana University Indianapolis and Changwon National University, Indianapolis. SelectionUniversity-Purdue and Peer-review under responsibility of the Chairs of The 1st International Joint Mini-Symposium onThis is an open access under the CCbetween BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/). article Advanced Coatings Indianalicense University-Purdue University Indianapolis and Changwon National University, Indianapolis. Keywords: Directional solidification ; CM247LC ; Microstructure selection ; Growth rate : Temperature gradient

* Corresponding author. Tel.: +82-55-213-3695 ; fax: +82-55-261-7017. E-mail address: [email protected]

2214-7853 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and Peer-review under responsibility of the Chairs of The 1st International Joint Mini-Symposium on Advanced Coatings between Indiana University-Purdue University Indianapolis and Changwon National University, Indianapolis. doi:10.1016/j.matpr.2014.09.002

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J.S. Lee et al. / Materials Today: Proceedings 1 (2014) 3 – 10

1. Introduction Ni-based superalloys have been extensively used for turbine blades and other engine parts due to their good creep strength and oxidation resistance at elevated temperatures [1,2]. Polycrystalline superalloys show brittle fracture along the grain boundaries because the stress concentration results in grain boundary sliding and the resultant strength decreases with an increase of the grain size. Directional solidification techniques have been successfully applied to blades in gas turbine engines because the columnar grain structures can be aligned parallel to the major stress axes [3,4]. During directional solidification, the solid/liquid interface can adopt planar or cellular or dendritic shape through control of solidification variables such as the solidification rate (V) and the temperature gradient (G) for a given alloy composition (C0). The solidification rate controls the evolution of the interface morphology which determines the final microstructures, whereas the temperature gradient determines the direction of heat flow which affects the grain size and the grain orientation. The interface morphology is known to be far more sensitive to the solidification rate, even though the temperature gradient is also involved. Thus, G and V govern the interface morphology for a given alloy. A ratio of G/V value represents the planar, cellular and dendritic microstructures, i.e. the interface is changed from planar to dendritic with decreasing G/V value. The low G/V value facilitates the constitutional undercooling [5,6] which leads to the interface instabilities. Meanwhile, the product of G and V represents the cooling rate which determines the scale of microstructures. As a higher ratio of GV is imposed, the interface favors a finer microstructure without changing the morphology. Unlike the directional solidification, during ingot casting, the temperature gradient continues to decrease with time and approaches zero, so that the exquiaxed grains can form in the central portion of the ingot. However, the dendritic (or columnar) to equiaxed transition will be considered in terms of heat balance here because the temperature gradient keeps constant during directional solidification as mentioned above. In the present study, the directional solidification of the CM247LC superalloy was carried out over a wide range of solidification rate and temperature gradient conditions in order to investigate the dependence of solidification variables on the microstructure selection. By altering the two main variables, the microstructure selection map was established for planar to cellular and cellular to dendritic transitions during solidification. 2. Experimental The chemical composition of the CM247LC superalloy used in this study is listed in Table. 1. Directional solidification experiments were carried out in the modified Bridgman type furnace, consisting of the super Kanthal heating element as heating source and the copper cold finger cooled by water at the bottom of the furnace, as shown in Fig.1. An alloy rod with an inner diameter of 4.7 mm was placed into an alumina tube with an inner diameter of 5 mm and a length of 700 mm. The alumina tube containing the alloy rod was fixed at the top of the furnace set and directly mounted into the cold finger. The alumina tube was first evacuated and then was back filled with highpurity argon gas to a pressure of 10 psi. This process was repeatedly conducted more than four times. Directional solidification experiments were carried out by translating the furnace upwards at various solidification rates between 1 and 300 µm/s and temperature gradients between 3.0 and 17.1 K/mm. Right after an upward growth of 40 mm in a given solidification condition, the alumina tube was dropped into a water bath to preserve the growing solid/liquid interface. The longitudinal and transverse sections of directionally solidified specimens were polished and etched with Kalling's reagent (5 g CuCl2 + 100 ml HCl + 100 ml C2H5OH). Microstructural observations were made by using an optical microscope and a scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). The differential scanning calorimetry (DSC) measurements under controlled heating and cooling rates of 10 K/minute were performed in NETZSCH model DSC 404C Pegasus. Table 1. Nominal composition of the superalloy CM247LC used in this study. Elements

Cr

Al

Ti

C

Ta

W

Mo

B

Zr

Hf

Co

Ni

wt.%

8.0

5.54

0.72

0.07

3.3

9.4

0.48

0.016

0.02

1.4

9.5

Bal.


G

Fig. 1. Schematic drawing of the modified Bridgman type directional solidification setup.

3. Results and Discussion 3.1. Solid/liquid interface morphology Fig. 2 shows the typical microstructures near the solid/liquid interface with different solidification rates under a given temperature gradient, G=10.5 K/mm. The planar interface was stable at a solidification rate of 1 µm/s and transformed into the dendritic form with well-developed side branches when it grew at rates higher than 5 µm/s. As seen in Fig. 2, finer dendritic microstructures can be obtained by increasing the solidification rate at a given temperature gradient. Table 2 summarizes all data sets obtained from the directional solidification experiments of the CM247LC superalloy. The solidification front was planar at the G/V values higher than 17.1u103 Ks/mm2. The dendritic interface was observed when the G/V values is fewer than 5.25u103 Ks/mm2. It is clearly shown that the specific values of G/V determine the interface microstructure such as plane front, cells and dendrites, as indicated by their similar proportions in the G/V. 3.2. DSC analysis Fig. 3 represents the result of DSC measurements during heating and cooling of the as-received CM247LC sample. In this figure, TJ, TMC and TJ/Jc are the temperature where the J phase, the MC type carbide and the J/Jc phases begin to form, respectively, and TL denotes the liquidus temperature. A downward peak required for melting was drawn during heating, whereas an upward peak came from the latent heat released during solidification. Both curves show the three main reactions related to the phase transformations of J, MC type carbide and J/Jc. The J/Jc reaction in the Ni-based superalloys is known as the eutectic reaction. Each reaction temperature was 1380.6 °C for TL, corresponding to the melting temperature of J phase, 1355.1 °C for TMC, 1269.1 °C for TJ/Jc, respectively and determined from the heating curve due to the absence of undercooling. From the DSC cooling curve in Fig. 3, the solidification sequence of the present superalloy can be described as the development of J dendrites o J dendrites + the formation of MC type carbides between J dendrites o J dendrites + MC type carbides + the growth of J/Jc phases in residual interdendritic liquid.

5

6


(b)

(a)

(c)

(d)

Fig. 2. Optical micrographs showing the longitudinal sections with solidification rate: (a) 1, (b) 5, (c) 25 and (d) 100 µm/s. Table 2. Experimental data obtained from the directional solidification of the superalloy CM247LC. G (K/mm) 3.0

5.7

10.5

V (µm/s)

G/V (Ks/mm2)

Interface morphology

100

0.03u103

Dendritic

300

3

Dendritic

200

0.029u10

3

Dendritic

300

0.019u103

Dendritic

350

3

Dendritic

0.016u10 3

0.5

21u10

1

10.5u103

Cellular

2

5.25u103

Dendritic

5

2.1u103

Dendritic

3

Dendritic

15

17.1

0.01u10

Planar

0.7u10

3

Dendritic

50

0.21u10

1

17.1u103

2

8.55u10

3

Cellular

3

Dendritic

Planar

5

3.42u10

25

0.684u103

Dendritic

50

0.342u10

3

Dendritic

100

0.171u103

Dendritic

200

3

Dendritic

0.086u10

Fig. 3. DSC curves for the superalloy CM247LC.

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Fig. 4. SEM micrographs showing the interdendritic carbides and J/Jc phases observed on the transverse sections of the samples solidified at the rates of (a) 5 µm/s and (b) 50 µm/s, respectively.

Fig. 4 shows some examples of the interdendritic microstructures solidified at different rates under a constant temperature gradient, G=17.1 K/mm. MC type carbides and J/Jc eutectic phases are found to form among the J dendrites. The interdendritic MC type carbides mentioned above were identified from EDS analysis and they were rich in Ta, Hf, Ti and W whose concentration was constantly detected in all solidified samples irrespective of the solidification rate. An increase in solidification rate led to a change in carbide morphology from a coarse blocky type to a fine script one and a decrease in the size of the J/Jc eutectic pool. It is suggested that the carbide morphology and the eutectic pool size depend on the G/V value. 3.3. Microstructure selection map During the directional solidification of alloys, the interface shape depends strongly on the solidification rate and the temperature gradient. A solidification rate versus temperature gradient plot for the superalloy CM247LC is drawn in Fig. 5. The experimental results as a function of various solidification variables and the theoretical calculations for the planar to cellular transition and the cellular to dendritic transition were compared in the figure. 3.3. 1. Planar to cellular transition For a given alloy, the G/V value controls the interface morphology. In the present study, the planar interface was stable at the high value of G/V where the value is more than 17.1u103 Ks/mm2 and it became unstable with decreasing G/V value. Thus, the critical growth condition, G/V, determines the planar interface to remain stable. The plane front destabilizes above the critical velocity for respective gradients and eventually it changes into cells, growing at low constitutional undercooling. According to the constitutional undercooling criterion [5], the critical velocity, Vc, for the planar interface instability is expressed as:

Vc

GD 'T0

(1)

where D is the solute diffusion coefficient in liquid, D=3.6u10-9 m2/s and 'T0 is the temperature difference between liquidus and solidus, 'T0=52.4 K, obtained on the DSC heating curve in Fig. 3. The critical velocity was calculated to be 0.20 µm/s for G=3.0 K/mm, 0.39 µm/s for G=5.7 K/mm, 0.72 µm/s for G=10.5 K/mm and 1.17 µm/s for G=17.1 K/mm, respectively. These four points are connected with a dotted line on the graph of Fig. 5. The planar interface was experimentally kept stable when it grew at 1 µm/s under G=17.1 K/mm and at 0.5 µm/s under G=10.5 K/mm, respectively, showing good agreement with the calculated results. Therefore, the above calculation gives the condition that the planar interface is expected to break down when the G/V values are at below 14.94u103 Ks/mm2.

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Fig. 5. Microstructure selection map for the superalloy CM247LC directionally solidified at various solidification rates and temperature gradients.

3.3.2 Cellular to dendritic transition The cellular interface was observed in a very narrow range of solidification rates and temperature gradients and it transformed into dendritic interface in the case that the G/V value is less than 5.25u103 Ks/mm2. At solidification rates faster than the critical velocity, cells undergo the noticeable change in shape, e. g. the side-branch formation involved with constitutional undercooling in the transverse direction due to the difference in the liquid concentration gradient at intercells. It is generally known that the cellular to dendritic transition is found in an array where its local spacing is larger than the maximum cell spacing and smaller than the minimum dendrite spacing in the cell-dendrite coexistence region. Kurz and Fisher [7] proposed that the transition from cell to dendrite occurs at low velocities where the solute diffusion length, D/V, is less than the thermal length, 'T0/G. The transition velocity, VCDT, was calculated by the following equation.

VCDT

GD k'T0

(2)

where k is the dimensionless partition coefficient, k=0.49, obtained from the phase diagrams [8,9]. The transition velocity was calculated to be 0.41, 0.77, 1.43, 2.33 for the applied temperature gradient, 3.0, 5.7, 10.5, 17.1 K/mm, respectively, as depicted by a dashed line in Fig. 5. In the present results, the cell-dendrite transition can be anticipated to occur at rates between 1 and 2 µm/s under G=10.5 K/mm and at rates between 2 and 5 µm/s under G= 17.1 K/mm. These experimental results are consistent well with the calculated results. The calculation by equation (2) suggests that when the G/V values are less than 7.32u103 Ks/mm2, the cellular array changes into the dendritic one. 3.3.3 Dendritic (columnar) to equiaxed transition During solidification, the critical parameters in determining the interface morphology include the contribution of temperature gradient. A positive gradient in the liquid ahead of the solidifying front dominates the extraction of the latent heat antiparallel to the growth direction. However, it is interesting to note that the introduction of a negative temperature gradient gives rise to the formation of equiaxed grains or freckles because the crystals grow in the same

9


direction of heat extraction [4]. Considering the heat balance at the growing solid/liquid interface [10], one can obtain the condition for the dendritic to equiaxed transition.

K S GS K L G L

U S HV

(3)

where K is the thermal conductivity, U the density, H the heat of fusion, and subscript S and L refer to solid and liquid, respectively. In the above equation, the temperature gradients can be controlled independently of the solidification rate. If the gradients continue to reduce with time, the contribution of the terms on the left-hand side in equation (3) may become negative. Under these circumstances, the dendrite tips no longer advance into an undercooled liquid and therefore the equiaxed growth can occur ahead of the dendrite tips, preventing the growth of the dendrites. The dendrites grow at a tip undercooling which depends on solidification rate. The dendritic growth at GL=0 exhibits a maximum growth rate, Vmax, and the equation (3) can be rewritten as [10]:

Vmax

K S GS U S H

(4)

When the liquid temperature gradient goes to negative values, the dendrite tip undercooling is greater than the undercooling for equiaxed growth and thereby it is possible to account for the nucleation and growth of equiaxed dendrites ahead of the growing columnar dendrites [11-15]. Accordingly, the directional growth would be attained at velocities below Vmax. On the other hand, the temperature gradient in the present furnace assembly keeps constant during the solidification process, as shown in Fig. 1. Hence, it is unlikely that the equiaxed crystals nucleate ahead of the advancing dendrites. The equiaxed grains are generally believed to deteriorate the mechanical properties of as-cast products due to the grain-boundary effects and the loss of benefits coming from columnar grains elongated in the main stress and they should be avoided in the directionally solidified superalloys. 4. Conclusion Directional solidification experiments of the Ni-based superalloy CM247LC have been accomplished on the microstructural evolution under various solidification conditions. For a given alloy, the solid/liquid interface morphology depended on the solidification rate and the temperature gradient, more precisely the ratio G/V. The solidification microstructure map as functions of G and V was established for the planar to cellular and the cellular to dendritic transitions. The planar interface was stable at high G/V values more than 14.94u103 Ks/mm2 and the cellular interface was transformed sharply into the dendritic structure when the G/V values are fewer than 7.32u103 Ks/mm2. Comparison made showed good agreement between the experimental results and the theoretical calculations. However, the dendritic to equiaxed transition is not observed in the present investigation. It appears to be responsible for the constant temperature gradient imposed along the growth axis during solidification. The negative temperature gradients can encourage the competition between the nucleation and growth of equiaxed crystals ahead of the dendrite tips. Therefore, the nucleation undercooling of equiaxed grains should be taken into account for the quantitative analysis of the dendritic-equiaxed transition.

Acknowledgements This research was financially supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. 2011-0030058) and by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grants funded by the Korea Ministry of Knowledge Economy (2011T100200224).

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