Investigation on solidification path of Ni-based single crystal superalloys with different Ru contents

Accepted Manuscript Investigation on solidification path of Ni-based single crystal superalloys with different Ru contents

Haifeng Wang, Haijun Su, Jun Zhang, Guomin, Yanbin Zhang, Quanzhao Yue, Lin Liu, Taiwen Huang, Wenchao Yang, Hengzhi Fu PII: DOI: Reference:

S1044-5803(16)31344-4 doi: 10.1016/j.matchar.2017.06.017 MTL 8718

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

19 December 2016 8 May 2017 14 June 2017

Please cite this article as: Haifeng Wang, Haijun Su, Jun Zhang, Guomin, Yanbin Zhang, Quanzhao Yue, Lin Liu, Taiwen Huang, Wenchao Yang, Hengzhi Fu , Investigation on solidification path of Ni-based single crystal superalloys with different Ru contents, Materials Characterization (2017), doi: 10.1016/j.matchar.2017.06.017

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Investigation on solidification path of Ni-based single crystal superalloys with different Ru contents

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Haifeng Wang, Haijun Su*, Jun Zhang, Guomin, Yanbin Zhang, Quanzhao Yue,

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Lin Liu, Taiwen Huang, Wenchao Yang, Hengzhi Fu State Key Laboratory of Solidification Processing, Northwestern Polytechnical

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University, Xi’an, 710072, P. R. China

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Corresponding author: [email protected] (H. Su)

ACCEPTED MANUSCRIPT Abstract The solidification path of Ni-based single crystal superalloy with different Ru contents (0-4 wt.%) was systematically investigated under planar and dendrite solid-liquid interface solidification conditions, aiming to reveal the evolution rule of solidification

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microstructure and selection mechanism of phases of the fourth generation single crystal

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superalloys. The results indicated that under plane solidification interface, the fine γ/γ´

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eutectic structure started to form after the solidification of γ phase and then further developed the coarse γ/γ´ structure. The solidification finally terminated with the

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formation of β-NiAl phase enveloped by coarse γ´ phase for Ru-free superalloy.

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However, when Ru was added in superalloy, the β-NiAl phase was firstly precipitated after the solidification of γ phase, and no fine γ/γ´ structure connecting the γ phase with

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β-NiAl phase was found. Under the dendrite solid-liquid interface condition, the

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solidification path was consistent with that in the plane one. The evolution mechanism of solidification path with different Ru contents was discussed, which was helpful to

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optimize the alloy composition design.

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Keywords: Superalloy, Solidification path, Plane solidification, Ru, β-NiAl

ACCEPTED MANUSCRIPT 1. Introduction Ni-based single crystal superalloys have been selected as high temperature application materials in aero-engines and land-based gas turbines due to their excellent creep resistance performance and microstructure stability at high temperature [1-3].

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Owing to the continually increasing temperature requirement of hot gas entering the

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turbine arrangement for the new-generation engines, a large amount of refractory

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elements, such as Re, Ru, W and Ta, have been added in new generation superalloys to promote their liquidus temperature. Particularly, the addition of Ru as a symbol element

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in the fourth- and fifth-generation Ni-based single-crystal superalloys can greatly

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improve the microstructural stability and the creep resistance [4-6]. However, some investigations indicated that the addition of Ru changed the precipitation sequence of

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the phases, which induced the reduction of the incipient melting temperature [7-9] and

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the slight increase in eutectic fraction [10]. Consequently, in light of the significance of Ru to improve the performance, it is imperative to investigate the effect of Ru on the

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composition.

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solidification path for controlling the microstructural evolution and optimizing the alloy

During the past decades, the investigations on solidification path of Ni-based single crystal superalloys have been extensively carried out. D′Souza et al. [11, 12] indicated that the microstructural characterization in the interdendritic region developed from coarse γ´ phase to a fine γ/γ´ eutectic structure with the decrease of liquid temperature. Terner et al. [13] suggested that the fine γ´ phase of interdendritic region was formed by the peritectic reaction at the primary γ dendrite surface and coarsened

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ACCEPTED MANUSCRIPT towards to the end of solidification. While, Heckl et al. [14] reported that the eutectic solidification proceeded from the fine γ/γ´ eutectic to coarse γ´ phase in the interdendritic region, and the 2-D eutectic structure depended on the cutting plane of 3-D structure. More recently, other researchers [15-17] concluded a similar sequence:

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the fine γ/γ´ eutectics were the first interdendritic constituents to solidify and then the

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coarse γ/γ´ structure appeared. However, it is known that the solidification path is

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importantly affected by the solidification conditions and alloy composition. But, the evolution mechanism of solidification path in Ni-based single crystal superalloy with

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different Ru contents under different solidification conditions is still unclear up to date.

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During directional solidification process, the solidification path plays a crucial role in determining the superalloy microstructure, phase fractions, and constituents of the

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interdendritic region. The precipitation of lowest melting point constituent with respect

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to temperature and solid fraction is associated with the incipient melting temperature during solution heat treatment. Pang et al. [17] found that the more prolonged soak time

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and solution treatment temperature were required to reduce the local compositional

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segregation and increase the local melting temperatures of these areas due to the existence of coarse γ´ phase and fine γ/γ´ eutectic. While, the solidification path can not only provide useful information for subsequently required heat treatments, but also for the solidification process itself. Liang et al. [18] found that the γ/MC eutectic reaction resulted in the formation of MC carbides under laser rapid directional solidification. When the cooling rate was low enough, γ/γ´ eutectic microstructure was formed by a peritectic transformation consuming the MC carbides. Zheng et al. [19] illustrated that

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ACCEPTED MANUSCRIPT the formation of β-NiAl phase in the as-cast microstructure at last stage of solidification gave rise to the fracture of the thin section trailing edge of blade tip. Milenkovic et al. [20] found that the change of solidification path dependent on cooling rate induced the eutectic structure transformation from γ/γ´ eutectic to γ/MC eutectic, which signally

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improved the hardness of MAR-M247 Ni-based superalloy. At present, the plane

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solidification interface technology is generally accepted to investigate the solidification

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path. Compared with the dendrite solidification, the regular distribution of solute can be obtained along the growth direction during the plane interface solidification, and the

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solidification fraction can be predicated. The evolution of microstructure is thus evident

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[21-23] and the precipitation nature of phases is also easy to identify. In the fourth- and fifth-generation single-crystal Ni-based superalloys, the addition of Ru promotes the

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creep resistance. However, the studies of its effect on solidification path are still limited.

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Therefore, a detailed knowledge of the solidification path and its associated changes with Ru content during the progression of the solidification is required.

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In order to reveal the solidification path and understand the corresponding effect

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of alloy composition and solidification conditions, this present study firstly investigated the solidification characteristics and solidification path of the fourth generation single crystal superalloy with different Ru additions by calculations based on Thermal-Calc software. The microstructural evolution with respect to different Ru additions and solidification fraction was investigated during plane interface directional solidification. Based above, the solidification path with different withdrawal rates and Ru contents was investigated under the dendritic solidification, and the evolution of microstructure was

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ACCEPTED MANUSCRIPT discussed.

2. Materials and methods The nominal chemical compositions of Ni-based single crystal superalloys investigated in this study are presented in Table 1. The TC-AB Company

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THERMO-CALC software and database TCNi7 were employed and the calculations

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were performed both in equilibrium condition and non-equilibrium solidification from

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1100 to 1700 ºC. In equilibrium calculation process, the phase equilibrium state that produced the lowest Gibbs energy was determined. Furthermore, the solidification path

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was also calculated according to Scheil model, which assumed the perfect mixing in the

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liquid phase and no solute back diffusion in the solid phase. Table 1 Chemical compositions of three investigated Ni-based single crystal superalloys

Al

Co

Cr

Ru

Mo

Re

W

Ta

Ni

1

6.1

12.1

5.0

0

1.0

5.1

5.8

8.0

Bal

2

6.0

12.0

5.0

2.0

1.0

5.2

5.9

8.1

Bal

3

6.1

12.2

5.0

4.0

1.0

5.2

5.9

8.2

Bal

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Alloy

D

(wt.%)

The directional solidification experiments were carried out in a self-developed Bridgman directional solidification furnace. The samples with 4 mm in diameter and 80 mm in length were machined from an ingot, and then the seed single crystals with its 〈001〉direction 5° misaligned to the sample axis were placed in the bottom of the samples in Al2O3 crucibles, and partially melted before the initiation of directional solidification. The samples were heated to 1500 ºC in argon atmosphere and held for 30 6

ACCEPTED MANUSCRIPT min to guarantee the uniformity of melt temperature. Two W-Re thermocouples were placed near the outside surface of the alumina crucible. One of the thermocouples was placed in the vicinity of the solid-liquid interface. The other thermocouple was placed approximately 10mm above the first thermocouples. The measured temperature gradient

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(G) at the solidification front was about 120 K/cm. The samples were directionally

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immersed into the Ga-In-Sn liquid metal bath at the withdrawal rates of 0.5, 25, 100,

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200 and 500 μm/s, respectively. In addition, the solid-liquid interfaces of 2 wt.% Ru and 4 wt.% Ru alloys at the withdrawal rate of 100 μm/s were preserved by quenching into

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the Ga-In-Sn liquid metal bath.

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The samples were cut along the transverse and longitudinal sections perpendicular to the growth direction, polished and etched with the solution of HNO3 : HF:

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glycerinum in a volume ratio of 1: 2: 3. Subsequently, the microstructures were

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observed by Leica DM-4000M Optical Microscope (OM) and ZEISS SUPERA 55 Scanning Electron Microscope (SEM). A FEI Tecnai G2 Transmission Electron

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Microscope (TEM) was utilized for phase identification. Also, the phase compositions

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were measured with a JEOL JXA-8100 electron microprobe analyzer (EPMA).

3. Results and discussion 3.1 The evolution of phase fraction of superalloys with different Ru contents Fig. 1 shows the phase fraction evolutions as a function of the heating temperature, which are calculated by thermo-calc software according to the lever rule. It can be seen that the precipitation temperatures of γ phase for three alloys are distinctly different. As the Ru content increases, the precipitation temperature of γ phase decreases from 1398

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ACCEPTED MANUSCRIPT to 1371 ºC and the solidification temperature range increases from 57 to 88 ºC. When the Ru content is up to 4 wt.%, the Gibbs free energy of B2 type β-NiAl phase (identified by TEM in experiment) is minimum as shown in the Fig. 1(d). Thus, the β-NiAl precipitates at the last stage of solidification (Fig. 1(c)). The solidification

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sequence of Ru-free and 2 wt.% Ru alloys can be described as: L → γ, but that of 4

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wt.% Ru alloy can be described as: L → γ → β-NiAl.

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Fig. 1. Equilibrium mole fraction of various phase and Gibbs free energy as a function of temperature for the Ni-based single crystal superalloys: (a) Ru-free, (b) 2 wt.%

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Ru, (c) 4 wt.% Ru, (d) Gibbs free energy of β phase in alloys . 3.2 The calculation of solidification path Fig. 2 shows the solidification process of alloys with different Ru additions predicted by the Gulliver-Scheil model. The solidification path of Ru-free alloy starts with the crystallization of γ phase. As the solidification proceeds, the γ/γ´ eutectic structure is formed when the temperature decreases from 1320 to 1285 ºC, the corresponding solidification fraction fs increases from 0.77 to 0.91. Whereas the β-NiAl 8

ACCEPTED MANUSCRIPT phase precipitates in the temperature interval of 1285 to 1275 ºC, the corresponding solidification fraction fs increases from 0.91 to 0.93. When the Ru is added into the alloy, the β-NiAl phase precipitates after the solidification of γ phase. To interpret the solidification path, the calculated liquid composition of Al, Ta and Cr elements versus

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fraction of retained liquid is plotted in Fig. 3. These elements are selected because they

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are the primary formation elements in the γ´ and β phase [7-9]. It can be seen from Fig.

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3(a) that the concentration of Al element in liquid firstly increases and then decreases as the solidification proceeds. However, the concentration of Ta and Cr elements in liquid

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generally increases. Prior to the formation of secondary phase, the Ru addition enhances

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the enrichment of Al element and but reduces the concentration of Ta and Cr elements in liquid.

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In general, when the solidification fraction is higher than 0.93, other phases also

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exist according to the calculated results, such as μ phase and σ phase, but they are rarely observed during solidification experiment of the studied alloy. Therefore, the

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solidification path of these phases has not been listed in Fig. 2.

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3.3 The evolution of solidification path of superalloys with different Ru additions under plane solidification

Fig. 4(a) shows the microstructure of Ru-free Ni-based single crystal superalloy solidified with a planar solid-liquid interface. It can be seen that there are four distinguished stages along with the growth direction: (i) γ primary phase, (ii) fine γ/γ´ eutectic separated by the protuberance of γ phase, (iii) coarse petal-like γ´ phase, and (iv) a dark contrasted phase and coarse γ´ precipitate phase. As a consequence, the

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ACCEPTED MANUSCRIPT solidification path of the Ru-free Ni-based single crystal superalloy can be described as: L → γ → γ/γ´+ γ´→ β-NiAl → γ´. This result is coincident with the result predicted in

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Fig. 2(a).

Fig. 2. Calculated solidification path with solid fraction using Gulliver-Scheil model: (a)

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Ru-free, (b) 2 wt.% Ru, (c) 4 wt.% Ru.

Fig. 3. Calculated variation of the elemental composition in wt.% for (a) Al, (b) Ta and (c) Cr elements in the liquid during solidification.

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Fig. 4. Optical micrographs of the Ru-free Ni-based single crystal superalloy directional

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solidified at the lower withdrawal rate of 0.5 µm/s. (a) solidification sequence of alloys solidified with a planar solid-liquid interface, (b), (c) and (d) are higher

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magnification micrograph of the areas marked as A, B and C, respectively, (e) the

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selected area diffraction pattern of β-NiAl phase. Generally speaking, the solidification path primarily depends on the segregation

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behaviour [24, 25]. As the solidification proceeds, the Cr, Re, W and Co elements

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segregate to the γ phase. While, the Al and Ta segregate into the liquid. Seen from Fig. 3(a) and (b), the concentration of Al and Ta elements in liquid increases before the

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appearance of eutectic for each alloy. Therefore, when the composition in liquid comes

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to the eutectic composition, the nucleation of the eutectic will occur on the surface of the growing γ phase plane, as shown in Fig. 4(b). The eutectic spreads in three dimensions, and the excess solute elements such as Cr, Re, W and Co elements rejected from the eutectic will accumulate among neighboring eutectic cells. At the front of solid-liquid interface, the concentration of Al and Ta elements decreases and the concentration of Cr, Re, W and Co elements increases with the growth of eutectic, which results in the continuous growth of the γ phase in the liquid and formation of the

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ACCEPTED MANUSCRIPT protuberance among neighboring eutectic cells, as shown in Fig. 4(a). The formation of the protuberance is schematically illustrated in Fig. 5. Moreover, it can be seen from in Fig. 3(a) that the reduction of Al accelerates with the decrease of liquid fraction from 0.23 to 0.09. It can be deduced that the eutectic morphology possibly changes. Due to

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the progressive increase of Ta as the eutectic grows and high residual lever of Al content

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(Fig. 3(a) and (b)) in liquid, the growth of the fine eutectic slows down, and γ´ phase

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can absorb more Al, Ta elements and thicken. Thus, the fine γ´ phase changes to the

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coarse petal-like γ´ phase as presented in Fig. 4(c).

Fig. 5. The schematic illustration of formation of the protuberance among neighboring

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eutectic cells: (1) the growth of only γ phase, (2) the nucleation of fine eutectic

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structure, (3) the formation of the protuberance When the solid fraction is 0.85, a dark contrasted blocky phase precipitates and is enveloped by the coarse γ´ precipitates (Fig. 4(d)). Fig. 4(e) shows a TEM image of the dark contrasted blocky phase and its corresponding selected area diffraction pattern. According to the TEM analysis, the dark contrasted blocky phase is identified as B2 type β-NiAl phase, which presents BCC structure and has been reported in Ru-containing single crystal superalloys [7-9].

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ACCEPTED MANUSCRIPT Seen from Fig. 6, when the concentration of Al element is higher than that at eutectic point, the β-NiAl phase will precipitate. However, according to the result shown in Fig 3(a), it is found that the Al content in liquid decreases with the growth of the eutectic. Thus, the formation of the β-NiAl phase is also possibly associated with other

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factors. Feng et al. [7] found that no β-NiAl phase precipitated in the baseline alloy,

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while the β-NiAl phase appeared in the alloy containing Cr element. The only

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compositional difference between the two alloys was the Cr content. This result

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suggests that Cr addition strongly affects the thermodynamics of this multi-component

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Fig. 6. The illustration of the possible solidification path in a Ni-Al binary phase diagram.

Phase

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Table 2 Compositions of the phases in Ru-free and 4 wt.% Ru superalloys (at.%).

Ni

Al

Co

Cr

Ru

Mo

Re

W

Ta

(β-NiAl)0Ru

51.9

35.96

7.36

4.25

/

0.06

0

0.11

0.33

γ´0Ru

63.86

17.68

9.88

2.96

/

0.25

0.19

0.72

4.45

(β-NiAl)4Ru

54.61

28.56

8.67

4.62

4.9

0.32

0.29

0.36

2.60

γ´4Ru

61.73

17.28

10.46

3.12

1.15

0.33

0.27

1.04

4.62

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calculated concentration of Cr element sharply increases after the formation of eutectic.

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Thereby, when the Al content reduces, it is possible that Cr will preferentially occupy

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the Al sites to balance the absence of Al in NiAl [26, 27]. Thus, the β-NiAl phase can be observed in Fig. 4(d). Subsequently, the γ´ phase is formed on the surface of NiAl phase

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by the peritectic reaction of L + β → γ´ due to the increasing Ta content.

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Fig. 7. The microstructure evolution at different solidification fractions for 2 wt.% Ru and 4 wt.% alloys with the plane solidification: (a) a schematic illustration of microstructure evolution, (b) the microstructure for 2 wt.% Ru alloy at fs = 0.82, (c) the microstructure for 4 wt.% Ru alloy at fs = 0.78. Fig. 7 shows the microstructure evolution as a function of the solidification fraction for superalloys containing 2 wt.% Ru and 4 wt.% Ru. It is clearly seen that the solidification paths are similar for the two alloys, but are distinctly different with that of 14

ACCEPTED MANUSCRIPT the Ru-free superalloy. No fine γ/γ´ structure connecting the γ phase with β-NiAl phase is found in 2 wt.% Ru and 4 wt.% Ru superalloys. After the solidification of γ dendrite, the β-NiAl phase in 4 wt.% Ru superalloy precipitates at lower solidification fraction as compared with 2 wt.% Ru superalloy (Fig. 7(a)) and is surrounded by the coarse γ´

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phase. However, the γ/γ´ eutectic structure has not been observed. Therefore, the

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solidification path of 2 wt.% Ru and 4 wt.% Ru Ni-based single crystal superalloy can

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be described as: L → γ → β-NiAl → γ´, which is in accordance with the results predicted in Fig. 2(b) and (c).

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From the calculated result shown in the Fig. 1(d), it can be seen that the addition of

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Ru reduces the Gibbs free energy of β-NiAl phase and thus promotes the stability of β-NiAl structure. Moreover, Daniel et al. [28] found that Ru dopant went to the Ni

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sublattice and a stronger Ru-Al bond forced the extra Ni atoms into the Al sublattice.

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This also illustrates that the stability of NiAl phase is improved due to the formation of Ru-Al bond. Moreover, with increasing the Ru content, the interdiffusion among atoms

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is sluggish [29], which accelerates the enrichment of Al element at the front of the

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solid-liquid interface (Fig. 3(a)). Thus, the β-NiAl phase is preferentially nucleated and further coarsened. After the formation of β-NiAl phase, the content of Al still remains at high level. The nucleation of γ´ phase will occur on the surface of β-NiAl phase, and then γ´ phase coarsens. It can be inferred from above analysis that the Al and Ru elements strongly influence the precipitation of β-NiAl phase of Ni-based single crystal superalloy. Consequently, under keeping other alloy elements content constant, the formation range

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ACCEPTED MANUSCRIPT of β-NiAl phase with respect to Al and Ru contents at the last stage of solidification is assessed by calculation, as plotted in Fig. 8. It is possibly useful in microstructure

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controlling and alloy composition optimization.

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Fig. 8. The precipitation of β-NiAl phase with respect to Al and Ru contents at the last stage of solidification.

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3.4 The effect of Ru additions on solidification path during dendritic solidification

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Fig. 9 shows the microstructures of the interdendritic regions for the Ni-based single crystal superalloy with different Ru contents. Fig. 9(a) shows the microstructure

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at high solidification fraction with the withdrawal rate of 200 μm/s. The individual γ/γ´

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eutectic islands grow together and the interfaces are clearly visible. Moreover, the coarse petal-like γ´ precipitate phase is located at the edge of the fine γ/γ´ eutectic, and the β-NiAl phase is observed in the porosity. The porosity is an excellent indication of the location of the last liquid to solidify. Therefore, it can be concluded that after the γ dendrite solidification, the solidification of the interdendritic region commences with the fine γ/γ´ morphology and is followed by a gradual transition to the formation of the coarse γ/γ´ morphology, where they impinge on adjacent γ/γ´ morphology structures

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ACCEPTED MANUSCRIPT growing from a neighboring interdendritic pool or onto the edge of porosity, and terminate with the formation of β-NiAl phase. This is coincidence with the solidification path during plane interface solidification. In Fig. 9(b) and (c), the microstructure morphologies of the interdendritic area are

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pronouncedly different from the results presented in Fig. 9(a). The β-NiAl phase is

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surrounded by the coarse γ´ precipitate phase. Between the coarse γ´ precipitate phase

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and γ dendrite, a small volume fraction of γ/γ´ eutectic is observed. In Fig. 9(d) and (e), it can be seen that the only NiAl phase precipitates in the liquid of mushy zone,

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indicating that the NiAl phase is crystallized by mean of L → NiAl, as the path 3 shown

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in Fig. 6.

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Fig. 9. The SEM images of the interdendritic regions (a-c) and the OM images of the longitudinal sections (d, e) of mushy zone for the Ni-based single crystal superalloys with different Ru contents: (a) Ru-free, (b, d) 2 wt.% Ru, (c, e) 4 wt.% Ru. Then the nucleation of the γ´ phase occurs on the surface of NiAl phase and γ´ phase coarsens. However, a fraction of eutectic forms in interdendritic region during the dendrite solidification, but is not found in plane interface solidification. It is possible

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ACCEPTED MANUSCRIPT that as the Al content decreases, the composition of residual liquid comes to the eutectic point due to the multidimensional diffusion around dendrite arms, and a small fraction of eutectic commences to form and impinges on the coarsely growing γ´ phase. 3.5 The effect of withdrawal rates on solidification path

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Fig. 10 shows that the microstructure evolution of with different withdrawal rates

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for Ru-free superalloy. At the low withdrawal rate of 25 μm/s, the fan-like γ/γ´ eutectic

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and blocky γ´ phase can be observed in Fig. 10(a). With the increasing withdrawal rate from 25 to 500 μm/s, the morphology of γ/γ´ eutectic changes from fan-like structures

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to strip-like structures, and the sizes of blocky γ´ phase distinctly reduce.

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Fig. 10. The evolution of microstructure with different withdrawal rates for Ru-free alloy: (a) 25 μm/s, (b) 100 μm/s, (c) 200 μm/s, (d) 500 μm/s

Fig. 11 shows the segregation coefficient of solute elements at different withdrawal rates. The segregation coefficient is defined as the ratio of the concentration of the elements in the dendrite core to that in the interdendritic region. It is found from Fig. 11 that the segregation of solute elements decreases with the increasing withdrawal rates from 25 to 500 μm/s. The evolution of microstructure in interdendritic regions is

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ACCEPTED MANUSCRIPT associated with the strong segregation of γ´-forming elements [30]. At the low withdrawal rate of 25 μm/s, the more γ´-forming elements segregate to liquid phase. Together with the larger primary dendrite arm spacing, the diffusion length of solute elements increases, and the γ/γ´ structure and coarse γ´ phase become coarser. With

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increasing the withdrawal rate, the segregation of solute elements decreases, the

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concentrations of Al and Ta segregated in liquid decrease. When the contents of Al and

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Ta in liquid is insufficient to continually growth of γ/γ´ structure and coarse γ´ phase, the sizes of γ/γ´ structure minishes and the morphology of γ/γ´ structure changes.

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Whereas, due to the growth of γ/γ´ structure, the contents of Al and Ta become possibly

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exhausted, the coarse γ´ phase will be not formed. Thus, as shown in Fig. 10(d), the

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coarse γ´ phase is not observed at the higher withdrawal rate of 500 μm/s.

Fig. 11. The segregation coefficient of solute elements at different withdrawal rates.

5. Conclusions The solidification path of Ni-based single crystal superalloy with different Ru contents was investigated under planer and dendrite solidification conditions. The following conclusions can be drawn:

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ACCEPTED MANUSCRIPT 1. During plane solidification of Ru-free alloy, after the solidification of γ dendrite, solidification commences with the fine γ/γ´ structure and then progressively develops into the coarse γ/γ´ structure, terminates with the formation of β-NiAl phase. 2. The Ru addition accelerates the precipitation of β-NiAl phase and the solidification

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path can be described as L → γ → β-NiAl → γ´ → γ/γ´.

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3. The solidification path during the dendrite solidification is coincident with that during

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the plane solidification. Moreover, with the increasing withdrawal rate, the segregation increases and then decreases. The morphology of γ/γ´ eutectic changes,

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and the coarse blocky γ´ phase decreases in size and even disappears.

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Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos.

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51690163, 51472200, 51331005, 51631008), The National Key Research and

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Development Program (2016YFB0701400), Aeronautical Science Foundation of China (No. 2015ZF53067), Strengthened Industrial Foundation Project of Ministry of Industry

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and Information Technology of China (TC160A310-12), Project of Shaanxi Provincial

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Youth Science and Technology Star Plan (2015KJXX-08) and Research Fund of the State Key Laboratory of Solidification Processing in NWPU (93-QZ-2014, 132-QP-2015).

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ACCEPTED MANUSCRIPT Chemistry, Microstructure, and Properties, J. Propul. Power. 22 (2006) 369. [3] R.C. Reed, Superalloys: fundaments and applications, first ed., Cambridge University Press, New York, 2006. [4] Q. Feng, T.K. Nandy, S. Tin, T.M. Pollock, Solidification of high-refractory

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ACCEPTED MANUSCRIPT Highlights

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The solidification path was investigated by plane solidification and calculation The solidification characteristic under plane solidification was revealed The β-NiAl phase was found in a Ru-free Ni-based single crystal superalloy Ru can reduces the Gibbs free energy of β-NiAl phase The solidification path with the withdrawal rates was depicted

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1. 2. 3. 4. 5.

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