Regular eutectic and anomalous eutectic growth behavior in laser remelting of Ni-30wt%Sn alloys

Accepted Manuscript Regular eutectic and anomalous eutectic growth behavior in laser remelting of Ni30wt.%Sn alloys

Xin Lin, Yong-Qing Cao, Zhi-Tai Wang, Jun Cao, Li-Lin Wang, Wei-Dong Huang PII:

S1359-6454(16)31009-6

DOI:

10.1016/j.actamat.2016.12.061

Reference:

AM 13449

To appear in:

Acta Materialia

Received Date:

11 November 2016

Revised Date:

21 December 2016

Accepted Date:

26 December 2016

Please cite this article as: Xin Lin, Yong-Qing Cao, Zhi-Tai Wang, Jun Cao, Li-Lin Wang, Wei-Dong Huang, Regular eutectic and anomalous eutectic growth behavior in laser remelting of Ni-30wt.% Sn alloys, Acta Materialia (2016), doi: 10.1016/j.actamat.2016.12.061

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ACCEPTED MANUSCRIPT

The graphical abstract

ACCEPTED MANUSCRIPT

Regular eutectic and anomalous eutectic growth behavior in laser remelting of Ni-30wt.%Sn alloys Xin Lina,b*, Yong-Qing Caoa,b, Zhi-Tai Wanga,b, Jun Caoc, Li-Lin Wangd, Wei-Dong Huanga,b a State

Key Laboratory of Solidification Processing, Northwestern Polytechnical University, 127 Youyixilu, Xi’an, Shaanxi 710072, P. R. China; b Key Laboratory of Metal High Performance Additive Manufacturing and Innovative Design, MIIT China, Northwestern Polytechnical University, 127 Youyixilu, Xi’an, Shaanxi 710072, P. R. China; c Thermal Processing Technology Centre, Illinois Institute of Technology, 10 W 32nd St., Chicago, IL, 60616, USA; d School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, P. R. China.

Abstract Ni-30wt.%Sn alloy powder beds were remelted using a laser beam to examine regular eutectic and anomalous eutectic growth behavior. The remelted microstructure mainly consisted of primary α-Ni dendrites and refined regular lamellar eutectic in the interdendrite space. At the top of the molten pool, the competition between primary α-Ni dendrites and the regular lamellar eutectic can be explained by the maximum interface temperature criterion. Anomalous eutectic was observed at the bottom of the molten pool. The remelting of primary α-Ni dendritic arms formed in the first laser remelting scan had an important effect on the formation of the anomalous eutectic in the second laser remelting scan. This effect led to the formation of globular α-Ni particles with similar Euler's angles in the anomalous eutectic. Keywords: laser remelting; Ni-30wt.%Sn alloys; regular eutectic; anomalous eutectic

1. Introduction Eutectic solidification is an important solid-liquid phase transformation process and has been studied widely [1-4]. The performance of eutectic materials depends largely on the microstructural characteristics and orientations of their eutectic phases. Previous studies have shown that in several eutectic alloys, regular lamellar or rod eutectic transforms into anomalous eutectic via a rapid solidification process, for example, Ag-Cu [5-7], Co-Sn [8,9], Ni-Sb [10] and Ni-Sn [11-19] eutectic alloys. Nevertheless, there has been considerable controversy regarding the formation mechanism of anomalous eutectic. Kattamis and Flemings [11] first investigated the microstructure of the Ni-32wt.%Sn

*

Corresponding author. Tel: +86 29 88494510; fax: +86 29 88494001. E-mail address: [email protected] (X. Lin). 1

ACCEPTED MANUSCRIPT eutectic alloy in undercooling solidification. They found that anomalous α-Ni+Ni3Sn eutectic appeared with the increase of the melt undercooling. The solidification behavior and formation mechanism of Ni-Sn anomalous eutectic have been widely studied [12-16]. Li et al. [17,18] believed that both coupled and decoupled growth of the two phases could give rise to anomalous eutectic. Later, Yang et al. [19] verified the hypothesis of Li et al. [18] that anomalous eutectic in undercooled Ni-Sn alloys had a dual origin: it resulted from eutectic dendrites growing at low undercoolings and from single-phase α-Ni dendrites growing at high undercoolings. Recently, Wei et al. [7] determined that anomalous eutectic resulted from the remelting of the primary solid. It should be noted that most previous studies on anomalous eutectic mainly used the high undercooling solidification method. Recalescence in the undercooling melt has a great influence on the premature microstructure, which somewhat inhibits the accurate understanding of the formation mechanism of anomalous eutectic. Additionally, the solidification path of undercooled samples is poorly controlled so that measurements of the crystal growth velocity show a large scatter [20]. Laser surface remelting, as a type of rapid solidification method, has been applied in the rapid solidification research field. With this method, the recalescence of alloys can be ignored. Besides, the moving velocity of the solid/liquid interface in the molten pool can be calculated according to the shape of the molten pool [21-24]. There have been a number of studies on the formation of eutectic microstructure during laser remelting [25-28]. In general, the microstructure in the remelted region is greatly refined compared with that in the substrate material. However, there are few reports on anomalous eutectic growth during laser remelting except for our previous works [29] and [30]. Wang et al. [29] found anomalous Ni-Sn eutectic at the bottom of a sample when the Ni-33wt.%Sn alloy ingots were thoroughly remelted by a laser beam. Cao et al. [30] also found anomalous eutectic at the bottom of a specimen for a Ni-30wt.%Sn hypereutectic alloy ingot that was remelted thoroughly. Recently, Xu et al. [31] found anomalous eutectic at the top of a specimen from laser cladding of Ni-32.5wt.%Sn eutectic deposits. In particular, it was found that there were many unmelted powder particles around the anomalous eutectic. The authors concluded that the powder particles had a great influence on the formation of the anomalous eutectic. Earlier, Lin et al. [32] researched the microstructure during rapid laser-forming of Ti-Rene88DT alloy. Anomalous β(Ti)-Ti2Ni eutectic was observed in their work. In general, under the condition of laser cladding/remelting, the anomalous eutectic usually appears on the edge of the specimen. 2

ACCEPTED MANUSCRIPT Thus, it is speculated that external conditions resulting in nucleation and free growth, lead to the formation of anomalous eutectic. Up to now, there are very few studies on the formation of anomalous eutectic using the laser remelting method. It is speculated that laser remelting of powder beds and bulk ingots should lead to different epitaxial growth behavior at the bottom of the molten pool. In this paper, to further investigate the formation mechanism of the regular eutectic and anomalous eutectic in the rapid solidification, laser surface remelting was performed on Ni30wt.%Sn hypoeutectic alloy powders. The solidification condition in the molten pool was changed by varying the scanning velocity and the number of remelting laser beam runs. The evolution and competing mechanisms of the two eutectic phases were analyzed. The orientations of the eutectic phases were also characterized using electron backscatter diffraction (EBSD) analysis.

2. Experimental Procedures The Ni-30wt.%Sn alloy powders were fabricated via the plasma rotating electrode process, using Ni-30wt.%Sn hypoeutectic alloy ingots prepared from high-purity nickel (>99.99 wt.%) and tin (>99.999 wt.%). The plasma rotating electrode process was conducted in a vacuum induction furnace of intermediate frequency, with a water-cooled copper crucible. An IPGYLS-3000-CL fiber laser was used to realize selective remelting of the Ni30wt.%Sn alloy powder beds. The sizes of the Ni-30wt.%Sn alloy powders ranged from 250 μm to 710 μm, and the thickness of the powder beds was 10 mm. In the experiment, the laser beam was focused on the surface of the powder beds. The laser power was 100 W, and the scanning velocity of the laser beam was decreased from 2 mm/s to 1 mm/s. The preset Ni-Sn powder beds were remelted once or twice. Laser surface remelting was performed in an argon-shielded glove box to prevent oxidation. The remelted specimens were cut along the transverse direction, which was vertical to the scanning direction of the laser beam. After grinding and polishing, the as-remelted microstructure and its crystallographic orientations were examined using a EBSD detector installed in a TESCAN VEGAII LMH scanning electron microscope (SEM).

3. Results 3.1 As-remelted microstructure at v=2 mm/s Fig. 1 shows the SEM backscattered electron images of the typical macrostructure and microstructure of the Ni-30wt.%Sn alloys, remelted by the laser beam at a scanning velocity of 2 mm/s. Fig. 1a shows the overall appearance of the sample that has been remelted once. 3

ACCEPTED MANUSCRIPT The depth of the molten pool is approximately 2025.8 μm. As seen in Fig. 1b, the microstructure at the top of the molten pool is composed of the primary dendritic α-Ni phase and the interdendritic regular α-Ni+Ni3Sn eutectic. There is a small amount of rod eutectic surrounding the primary α-Ni dendrites. Most of the regular eutectic is lamellar. The eutectic spacing at this position is very small. The lamellar eutectic spacing at this position is 0.51± 0.04 μm, while the average rod eutectic spacing is 0.45±0.03 μm. Fig. 1c presents the microstructure in the middle of the molten pool. The microstructure is also composed of primary α-Ni dendrites and the regular eutectic, similar to the top. However, the microstructure is coarser than that at the top. The lamellar eutectic spacing is 1.16±0.12 μm, and the rod eutectic spacing is 0.79±0.04 μm. Fig. 1d shows the microstructure at the bottom of the molten pool. The morphology of the microstructure, as indicated by the black ellipse in Fig. 1d, is similar to that of the anomalous α-Ni+Ni3Sn eutectic reported in a previous undercooling solidification process [13]. Fig. 1e shows the overall appearance of a sample that has been remelted twice. It is interesting to note that there are two molten pools, although the processing parameters for these two remelting process are the same. The depth of the molten pool for the first remelting scan is approximately 1847.2 μm, while the depth of the molten pool for the second remelting scan is approximately 513.4 μm, which is much smaller than the first remelted zone. In particular, the microstructure in the molten pool of the twice-remelted sample is completely composed of regular lamellar eutectic cells, as shown in Fig. 1f. This microstructure is different from the sample remelted once. At the bottom of the molten pool of the twiceremelted sample (Fig. 1g), there are some residual coarse primary α-Ni dendrites that are formed in the first remelting scan. Fig. 1h shows the microstructure at the bottom of the specimen, which exhibits similar microstructural characteristics to those in Fig. 1d. Anomalous α-Ni+Ni3Sn eutectic also appears, as shown in the black ellipse in Fig. 1h.

3.2 As-remelted microstructure at v=1 mm/s Fig. 2 shows the backscattered electron images of the typical microstructure of Ni30wt.%Sn alloys remelted by the laser beam at a scanning velocity of 1 mm/s. It can be observed from Fig. 2a that the microstructure at the top of the molten pool after one remelting scan is composed of the regular lamellar eutectic, which is similar to the microstructure at the top of the specimen remelted twice at a scanning velocity of 2 mm/s (Fig. 1f). In the middle of the molten pool, the microstructure is composed of the primary α-Ni phase and the regular lamellar or rod α-Ni+Ni3Sn eutectic (Fig. 2b). There is also a small amount of anomalous α4

ACCEPTED MANUSCRIPT Ni+Ni3Sn eutectic microstructure at the bottom of the molten pool, as shown in the white ellipse in Fig. 2c. Fig. 2d shows the macromorphology of the Ni-30wt.%Sn remelted twice at a scanning velocity of 1 mm/s. There are two molten pools in this sample as well. The depth of the molten pool zone for the first remelting scan is approximately 2287.6 μm, while the depth of the molten pool zone for the second remelting scan is approximately 1274.6 μm. The molten pool zone for the second remelting scan is also much smaller than that for the first remelted zone, but larger than that obtained with a scanning velocity of 2 mm/s. As seen in Fig. 2e, the microstructure at the top of the molten pool of the twice-remelted sample is also mainly composed of regular lamellar eutectic cells, which is similar to the top microstructure of the molten pool of the once-remelted sample, as shown in Fig. 2a. However, there is a larger volume fraction of residual primary α-Ni dendrites, which form in the first remelting process, at the bottom of the molten pool of the twice-remelted sample (Fig. 2f). Additionally, the regular lamellar or rod eutectic in the proximity of the residual primary α-Ni dendrites is also coarser than that in the other regions. It is worth noting that the anomalous α-Ni+Ni3Sn eutectic is observed at the boundary of the twice-remelted molten pool (Fig. 2g). It is inferred that the anomalous α-Ni+Ni3Sn eutectic is related to the primary α-Ni dendrites. Coarse α-Ni dendrites are also observed, mixed with the regular rod and lamellar eutectic in the lower part of the molten pool formed in the first remelting scan (Fig. 2h). The anomalous α-Ni+Ni3Sn eutectic microstructure also exists at the bottom of the specimen (Fig. 2i). During laserremelting of the powder beds, the powders were in contact with the molten pool, which decreased the cooling rate of the molten pool due to the poor heat transfer conditions around it. In addition, the powder particles also acted as dispersed chilling points for nucleation and the free growth of grains near the boundary of the molten pool. It is inferred that homogeneous nucleation and free growth are very important for the formation of the anomalous α-Ni+Ni3Sn eutectic at the bottom of the molten pool. However, under the condition of constrained growth, the anomalous α-Ni+Ni3Sn eutectic rapidly transformed to regular α-Ni+Ni3Sn eutectic.

4. Discussion 4.1 Competition between α-Ni dendrites and regular eutectic As mentioned above, there is an obvious difference in the microstructure distribution in the molten pools between once-remelted and twice-remelted sample. Considering the competition between α-Ni dendrites and the regular lamellar/rod α-Ni+Ni3Sn eutectic, two 5

ACCEPTED MANUSCRIPT possible explanations are proposed, as follows. Generally, there are two types of eutectic instabilities: the first type consists of singlephase instabilities due to the deviation from the eutectic composition, and the second type consists of two-phase instabilities caused by other alloying elements or impurities in the alloy [33]. In the present work, the Ni-30wt.%Sn alloy is a hypoeutectic alloy. Therefore, a singlephase instability caused by the hypoeutectic composition could occur. Thus, the ratio of the imposed temperature gradient G and growth velocity v (G/v) will have a dominant effect on the instability of the growth morphology [33]. Based on the constitutional undercooling theory, if the solid/liquid interfacial energy is ignored, then the eutectic interface stability condition for the binary hypoeutectic alloy is as follows: G m  (C0  CE ) v D

(1)

where G is the temperature gradient, v is the moving velocity of the solid/liquid interface, m is the liquidus slope, C0 is the initial composition of the alloy, CE is the composition of the eutectic point and D is the diffusion coefficient of the solute in liquid. As a result, a larger G/v indicates a higher stability of the solid/liquid interface in the process of solidification. The transition from eutectic to single-phase dendritic growth will occur with a decrease of G/v. In general, G and v are related to each other through the heat flux and thermal-physical properties of the metal. In the present experiments, due to the disorder in the direction of growth of microstructure in the middle of the molten pool, v cannot be obtained accurately. Additionally, G of the molten pool varies from the bottom to the top. Therefore, the value of G/v in the middle of the molten pool cannot be obtained accurately. In addition, in most of the present specimens, the microstructure in the middle of the molten pool is composed of primary α-Ni dendrites and the interdendritic regular lamellar/rod α-Ni+Ni3Sn eutectic. Variation of the laser processing parameters leads to an obvious change of microstructure at the top of the molten pool. Therefore, only the microstructure selection mechanism of α-Ni dendrites and the regular α-Ni+Ni3Sn eutectic at the top of the molten pool is considered. At the top of the molten pool, the solidification velocity v is almost equal to the scanning velocity of the laser beam. G could not be directly measured under the present experimental conditions, and a modified Rosenthal method [34] was used to solve the temperature field in the molten pool. It was calculated that the weighted average G in the molten pool was approximately 6.09×105 K/m at a scanning velocity of 2 mm/s, and the value of G/v was 3.05×108 Ks/m2; when the scanning velocity was 1 mm/s, the weighted average G of the molten pool was approximately 5.84×105K/m, and the value of G/v was 5.84×108 Ks/m2. 6

ACCEPTED MANUSCRIPT Substituting the physical parameters for the Ni-30wt.%Sn hypoeutectic alloy [30] into Eq. (1), the critical value of G/v can be calculated as 4.95×108 Ks/m2. Therefore, when the remaining parameters are constant and only the scanning velocity decreases from 2 mm/s to 1 mm/s, the value of G/v increases and the solid/liquid interface is more stable. Therefore, the microstructure is completely lamellar eutectic. Beyond that, after one laser-remelting at a scanning velocity of 2 mm/s, the microstructure at the top of the specimen is composed of primary α-Ni and the interdendritic eutectic; after laser-remelting twice at the same scanning velocity of 2 mm/s, the microstructure in the molten pool is completely lamellar eutectic. This finding is because the twice-remelted molten pool forms in a large re-solidified solid, unlike that of remelted once forms in the powder beds. Good thermal transfer and cooling conditions in the solid lead to a second, small molten pool whose G value should be higher than that in the once-remelted molten pool in the powder beds. Thus, the increase of G/v in the molten pool leads to the disappearance of α-Ni phase dendrites and the formation of a full, fine lamellar eutectic. Competitive growth of dendrites and the eutectic microstructure can also be predicted by the maximum interface temperature criterion [32]. It is believed that the phase or pattern with a higher interface temperature dominates the kinetics and will be dominant in the final microstructure. To determine the dominant growing interface, the growth temperatures of all phases or growth patterns should be calculated. These interface temperatures, which are functions of the growth velocity and temperature gradient for a given alloy, are also called the interface response functions (IRF) [24]. It should be noted that during laser surface remelting, the temperature gradient is positive in the molten pool, which is similar to directional solidification. Therefore, the KGT model [35] describing dendrite growth and the TMK model [36] describing eutectic growth in rapid directional solidification, were used to estimate the change of the interface temperature between α-Ni dendrites and the α-Ni+Ni3Sn eutectic with growth velocity. Dendritic growth is modelled by the KGT model modified according to Burden et al. [37]. The growth temperature of dendrites Td can be written as follows: Td  Tm  mvCt* 

2 RgTm GDL  V R V0 S f V

where, mv  m0 F ( kv )

F (kv )  1  7

k0  kv 1  ln( kv k0 ) 1  k0

(2) (3) (4)

ACCEPTED MANUSCRIPT

kv 

k0  ( a0V / D ) 1  ( a0V / D )

(5)

Here, Td is the interface temperature and Tm is the melting temperature of the pure component. mv is the velocity-dependant liquidus slope and m0 is the equilibrium liquidus slope. kv is the velocity-dependant partition coefficient, in which k0 is the equilibrium partition coefficient. V is the growth velocity of dendrites, and a0 is a length scale related to the interatomic distance. D= D0exp[-Q/RgTd] is the solute diffusion across the interface, which is close to the diffusion coefficient of the solute in the liquid DL. Here, D0 is a proportionality constant and Q is the activation energy. Rg is the gas constant. ΔSf is the molar entropy of fusion. G is the temperature gradient. Γ is the Gibbs-Thomson coefficient and R is the radius of the dendrite tip, which can be obtained by the following equation for a given growth velocity, based on the marginal stability criterion: mvGCC ( Pe)  G 

  R2 *

(6)

where GC is the concentration gradient in the liquid at the dendrite tip, ξC is the stability parameter, and Pe is the solute Peclet number. These parameters have the following relationships: (1  kv )VCt* GC   DL

 c ( Pe)  1 

2k v [1  ( 2 / Pe) 2 ]1 2  1  2k v

Pe 

VR 2 DL

(7)

(8)

(9)

where Ct*  C0 / 1  (1  k v ) I V ( Pe) is the composition of the liquid at the dendrite tip. Here, IV(Pe) =Pe· exp (Pe )·E1(Pe) is the Ivantsov function and E1 is the exponential integral function. Solving equations (2) to (9), the variation of the growth temperature, Td, of α-Ni dendrites with the growth velocity V can be obtained. Eutectic growth is calculated by the TMK model [36]. It should be noted that there are two types of phase diagrams (Case 1: “cigar-shaped” phase diagram; Case 2: equal distribution coefficients which are constants for the two phases) in the TMK model. In view of the Ni-Sn phase diagram [38], the distribution coefficients of the α-Ni and Ni3Sn phases are not equal. Therefore, case 1, i.e., that of the “cigar-shaped” phase diagram in the TMK model is used below. 8

ACCEPTED MANUSCRIPT The interface temperature, Te, for eutectic growth is calculated by the following relationship： Te  TE  TE

(10)

where TE is the eutectic temperature and TE is the interface undercooling for the eutectic growth. TE can be solved by the following equations:

E2VE   L / Q L 

E TE  ma L 1  

(11)  P P   (P   ) 

(12)

where λE is the lamellar spacing and VE is the eutectic growth velocity. The definitions of the other terms are as follows:

 L   L   F (kv ) fm 1  f m      

(13)

Q0  P   P  E  DL  E 

(14)

L  2

QL 

m

Q0 

m m m  m

F (kv )

C f (1  f )

(16)

3  2  pn  1  P     sin  n f   .  n 1  n   1  1  pn2  

 P P  E   E

(15)

2

3  2     1  pn  sin n  f .             1  1  pn2   n 1  n 

(17) 2

pn 1  pn2

(18)

Here, aL and a L are capillary constants of the α phase and β phase, respectively. mα and mβ are the slopes of liquids (defined so that both are positive) corresponding to the α phase and β phase, respectively, and f is the volume fraction of the α phase in the eutectic structure. ΔC=ΔCα+ΔCβ, where ΔCα and ΔCβ are the differences in liquid and solid compositions at the α/liquid and β/liquid interfaces, respectively, and pn  2n / p and p  VE E / 2 DL , which is the solutal Peclet number for eutectic growth. Solving equations (10) to (18), we can estimate the variation of growth temperature, Te, for the lamellar α-Ni+Ni3Sn eutectic as a function of the growth velocity, VE. Fig. 3 shows the variations of interface temperatures of α-Ni dendrites and the lamellar α9

ACCEPTED MANUSCRIPT Ni+Ni3Sn eutectic as a function of the growth velocity. The physical parameters used in the calculations are obtained from references [20], [30] and [38]. It should be noted that the change of G has significant influence on the dendrite growth temperature using the KGT model. The growth temperatures of α-Ni dendrites under different G values are also calculated for varying growth velocities. As seen in Fig. 3, the intersection between the colored, dashed line (which represents for the interface temperature of α-Ni dendrites) and the solid line (which represents for the interface temperature of the lamellar α-Ni+Ni3Sn eutectic) changes with the variation of G. When G is 6.09×105 K/m, the growth velocity is ~0.17 mm/s at the intersection; when G is 1×106 K/m, the growth velocity is ~0.29 mm/s at the intersection; when G is 2×106 K/m, the growth velocity is ~0.65 mm/s at the intersection. In other words, if G is 2×106 K/m in the molten pool, then the interface temperature of the lamellar αNi+Ni3Sn eutectic is higher than that of the α-Ni dendrite when the growth velocity is less than 0.65 mm/s. Therefore, growth of the lamellar α-Ni+Ni3Sn eutectic should be dominant when the growth velocity is lower than 0.65 mm/s, while α-Ni dendritic growth should be dominant when the growth velocity is larger than 0.65 mm/s. Considering possible errors in the thermo-physical parameters, it can be observed that the theoretical prediction is in good agreement with the present experimental results obtained from laser-remelting once with different scanning velocities. Temperature gradient for the laser melting carried out on a powder bed is different from that for the block. Therefore, there is a competition between α-Ni dendrites and the lamellar eutectic in the both once- and twice- remelted molten pools at the same scanning velocity of 2 mm/s. Additionally, the interface temperature of the α-Ni dendrites changes from the bottom to the top of the molten pool with G and v change from the bottom to the top in the molten pool, and therefore the α-Ni dendrites also competes with lamellar eutectic in the same molten pool at the scanning velocity of 1 mm/s.

4.2 EBSD analysis of anomalous eutectic 4.2.1 Anomalous eutectic at the bottom of the molten pool remelted twice Fig. 4 shows the EBSD results of the anomalous eutectic zone at the bottom of the molten pool, which was remelted twice by the laser beam at a scanning velocity of 1 mm/s. As seen in Fig. 4b and 4d, the orientation of the α-Ni phase in the anomalous eutectic zone is disordered. In the EBSD results of All Euler patterns, the same color indicates that the indexed regions have the same Euler angle and the same crystal orientation. Different colors only indicate that the indexed regions have different Euler angles. However, it cannot be 10

ACCEPTED MANUSCRIPT inferred that their crystal orientations are different because of the crystal symmetry. Table 1 shows the Euler angles of the numbered α-Ni particles in Fig. 4b. The α-Ni particles numbered as 1#~6# at the bottom of the anomalous eutectic zone shown in Fig. 4b exhibit the same Euler angles, indicating that they have the same orientation. The three Euler angles of the 7# and 18# α-Ni particles are similar. The three Euler angles of the 9# α-Ni particle are close to those of 12#. The two α-Ni particles numbered as 14 # and 15# also have the same three Euler angles. A large difference is found between the Euler 1 angles of the 21# and 22# α-Ni particles, while the other two Euler angles are nearly the same. The Euler angles between the 25# and 26# α-Ni particles, as well as those between the 27# and 28# α-Ni particles follow the same trends as shown in table 1. There is a difference of approximately 14° between the Euler 3 angles of the 23# and 24# α-Ni particles, but their other two Euler angles are similar. The above results suggest that the orientation distribution of these α-Ni particles is not completely random. The α-Ni particles (No.1#~6#) at the bottom of the anomalous eutectic zone, with the same orientation, likely originate from the same primary α-Ni dendrite. Due to the cross-section effect, the arms of the α-Ni dendrites are observed as discrete particles. Globular α-Ni particles (No.20#~27#) with similar Euler angles may result from the remelting of other primary α-Ni dendrites. As seen in Fig. 4f, the blue bars represent the correlated misorientation plot, and the red bars represent the uncorrelated misorientation plot. It should be noted that the correlated misorientation plot displays the misorientation data between neighboring points in a map. On the other hand, an uncorrelated misorientation plot shows the misorientation between randomly chosen points in the data set. It is found from Fig. 4f that there is a main peak (the blue bar) between 0° and 5° in the misorientation angle distribution of α-Ni particles, which also means that the misorientation of neighboring α-Ni particles in the anomalous eutectic zone is small. In addition, there is a second peak between 55°~60° in Fig. 4f. The misorientation angle between the 4# and 9# α-Ni particles is approximately 56.7°; the misorientation angle between the 10# and 13# α-Ni particles is approximately 58.1°；the misorientation angle between the 16# and 17# α-Ni particles is approximately 54.4°. Li et al. [16] measured the orientation of the anomalous α-Ni+Ni3Sn eutectic in an undercoolingsolidified Ni-18.7at.%Sn eutectic alloy melt using the EBSD technique. In their results, approximately 50% of neighboring α-Ni particles have small misorientation angles less than 10°. Approximately 30% of neighboring α-Ni particles have misorientation angles between 50°~60°. The present work shows similar results to those of Li et al. [16]. The accurate angle between (111) and (001) in Ni is 54.7°, except for the Ni twin with a misorientation angle of 11

ACCEPTED MANUSCRIPT 60°, which may be responsible for the second important fraction with misorientation angles approximately 55°~60°, formed by rotation or displacement of the broken α-Ni particulates from local remelting. As seen in Fig. 4c and 4e, the orientation of the Ni3Sn phase in the anomalous eutectic area is consistent. The entire Ni3Sn phase in the EBSD scanning zone can be divided into green and blue regions, depending on the Euler angles. The Euler angles of the Ni3Sn phase, marked by the green region in Fig. 4c, are Euler 1=108.4°, Euler 2=149.6° and Euler 3=5.7°. The Euler angles of the Ni3Sn phase, marked by the blue region in Fig. 4c, are Euler 1=40.7°, Euler 2=36.3° and Euler 3=43.2°. The misorientation angle of these two Ni3Sn regions is approximately 60° (Indices: -7256; Offset: 5.90). It is worth noting that the misorientation angle between the 26# and 27# α-Ni particles is approximately 60°. In addition, the 25# and 26# α-Ni particles in Fig. 4b are distributed in the blue Ni3Sn phase in Fig. 4c. The 27# and 28# α-Ni particles in Fig. 4b are distributed in the green Ni3Sn phase in Fig. 4c. Thus, it is speculated that the 25# and 26# α-Ni particles come from the same α-Ni dendrite, which is surrounded by the blue Ni3Sn phase. On the other hand, the 27# and 28# α-Ni particles come from another α-Ni dendrite, which is surrounded by the green Ni3Sn phase. The misorientation angle between these two α-Ni dendrites is 60°. Therefore, the misorientation angle between the blue and green Ni3Sn phases (Fig. 4c), which could nucleate from the α-Ni dendritic arms, is also 60°. As mentioned above, the formation mechanism of the anomalous α-Ni+Ni3Sn eutectic may be as follows: the α-Ni dendrites are the primary phase at the beginning of the solidification. Then, the Ni3Sn phase nucleates and wraps around the α-Ni dendrite. Subsequently, some α-Ni dendrites are broken partially in the subsequent local remelting process. Therefore, there is small misorientation between α-Ni particles from the same α-Ni dendrite, leading to their slightly different Euler angles. In order to further clarify the growth behavior of the α-Ni and Ni3Sn phases in the anomalous eutectic, the EBSD results of the anomalous eutectic zone at the other position at the bottom of the twice-remelted molten pool are shown in Fig. 5. This position is near the coarse primary α-Ni dendrites (Fig. 5a). Although there is a random orientation distribution of some α-Ni particles far away from the coarse α-Ni dendrite, numerous globular α-Ni particles with small misorientation angles are dominant in the anomalous eutectic zone, as shown in Fig. 5f. These regularly arranged α-Ni particles near the coarse α-Ni dendritic arms in the anomalous eutectic have consistent orientations (the black ellipse in Fig. 5b), which are the same as the orientation of the coarse α-Ni dendrites. Thus, it is inferred that these globular 12

ACCEPTED MANUSCRIPT α-Ni particles in the anomalous eutectic zone could come from the remelting of these coarse α-Ni dendrites or the section effect of the α-Ni dendritic arms. At the same time, the lamellar α-Ni phase in the interdendritic α-Ni+Ni3Sn lamellar eutectic also has the same orientation as these coarse α-Ni dendrites. This finding suggests that the lamellar α-Ni phase in the interdendritic lamellar eutectic nucleates and grows from the coarse α-Ni dendrite. As seen in Fig. 5c and 5e, the orientation of the Ni3Sn phase, which is far away from the primary coarse α-Ni dendrites in the anomalous eutectic zone, is consistent. However, the orientation of the lamellar Ni3Sn phase in the interdendritic lamellar eutectic is different from the orientation of the surrounding Ni3Sn phase. 4.2.2 Anomalous eutectic at the bottom of the once-remelted molten pool Fig. 6 presents the EBSD results of the anomalous eutectic zone at the bottom of the molten pool remelted once by the laser beam at a scanning velocity of 1 mm/s. As seen in Fig. 6c and 6e, the Ni3Sn phase in the anomalous eutectic shows consistent orientation. The irregular α-Ni particles numbered as 1#~4# and the globular α-Ni particles numbered as 5#~11# have the same Euler angles (Fig. 6b). In addition, some α-Ni particles numbered as 14#~18# also have same Euler angles. Additionally, the Euler angles of the 19# α-Ni particle are exactly the same as those of the 20# α-Ni particle. From the misorientation angle distribution of the α-Ni phase in Fig. 6f, it can be observed that most of the misorientation angles among the globular α-Ni particles range between 0~15°. This result suggests that the orientation distribution of α-Ni particles is not random. This case is similar to the orientation distribution of globular α-Ni particles near the coarse α-Ni dendrite, as shown in Fig. 5b. These globular α-Ni particles, whose orientations are consistent in the anomalous eutectic, are likely the result of remelting of the same α-Ni dendrite. Fig. 7 shows the EBSD results of the anomalous eutectic zone at the other position at the bottom of the molten pool remelted once by the laser beam, at a scanning velocity of 1 mm/s. As seen in Fig. 7b and 7d, the orientation distribution of α-Ni particles in the anomalous eutectic zone seems to be random. However, the orientation of the irregular α-Ni layers, indicated by the black elliptical region, is the same as that of the globular α-Ni particles, indicated by the black triangular region in Fig. 7b. Similarly, as seen in the black elliptical area in Fig. 5b, the orientation of the α-Ni dendrites is completely consistent with the surrounding lamellar α-Ni phase. At the same time, the globular α-Ni particles (No. 5#~11#) have the same orientation as that of their surrounding lamellar/rod α-Ni phase, as shown in Fig. 6b. Additionally, the orientation of the Ni3Sn phase in the lamellar eutectic is consistent with the Ni3Sn phase in the anomalous eutectic, as shown in Fig. 7c. Thus, it is inferred that 13

ACCEPTED MANUSCRIPT the consequences of the growth of the anomalous and lamellar eutectic at this position should be as follows: at the beginning of solidification, formation of the anomalous eutectic occurs first; then, the lamellar eutectic grows from the anomalous eutectic epitaxially. As a result, the orientations of some of the α-Ni phase and the Ni3Sn phase in the lamellar eutectic are consistent with that of the anomalous eutectic. As seen in Fig. 7c and 7e, the whole Ni3Sn phase in the EBSD scanning zone can be generally divided into the dark yellow and the dark blue region, based on the Euler angles. The three Euler angles of the dark yellow region in Fig. 7c are as follows: Euler 1= 121.7°, Euler 2=99.2° and Euler 3=10.4°. The three Euler angles of the dark blue region in Fig. 7c are Euler 1=64.1°, Euler 2=104.9° and Euler 3=37.2°. The misorientation angle of the two regions is also approximately 60° (Indices: 2-31-2; Offset:1.43). These EBSD results are similar to those of the anomalous eutectic at the bottom of the molten pool remelted twice, as shown in Fig. 4c. According to the misorientation angle distribution of the α-Ni phase in Fig. 7f, there is also a main peak between 55°~60°. This result is also similar to that of Li et al. [16], indicating that the anomalous α-Ni+Ni3Sn eutectic microstructures formed in the first remelting scan and the second remelting scan are the same. Therefore, the formation mechanism of the anomalous eutectic is the same as that previously mentioned.

5. Conclusions The growth behavior of regular and anomalous eutectic in rapid solidification were investigated by laser-remelting of Ni-30wt.%Sn alloy powder beds. The growth characteristics and selection mechanisms of the primary dendritic α-Ni phase, regular eutectic, and anomalous eutectic were analyzed. The main conclusions are as follows： （1）The competition between primary α-Ni dendrites and regular lamellar α-Ni+Ni3Sn eutectic occurs at the top of the molten pool. The transition from regular lamellar α-Ni+Ni3Sn eutectic to single-phase α-Ni dendritic growth will occur with a decrease of G/v. （2）Based on the IRF, variations of the interface temperatures between α-Ni dendrites and the regular lamellar α-Ni+Ni3Sn eutectic with the growth velocity were calculated using the KGT and TMK models. There is reasonable agreement between the theoretical predictions and the present experimental results. （3）The anomalous eutectic was found at the bottom of the molten pool after one laserremelting scan of the powder beds. The distribution of the primary α-Ni phase in the onceremelted molten pool have important effects on the formation of the anomalous α-Ni+Ni3Sn eutectic at the bottom of the molten pool in the second remelting scan. Based on the EBSD results, it is speculated that α-Ni particles with similar Euler’s angles in the anomalous α14

ACCEPTED MANUSCRIPT Ni+Ni3Sn eutectic may arise from the remelting of primary α-Ni dendrites.

Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51323008, 51475380 and 51271213), the Fundamental Research Funds for the Central Universities (Grant No. 3102015BJ(II)ZS013), the Programme of Introducing Talents of Discipline to Universities, China (Grant No. 08040) and the National Key Research and Development Programme of China (Grant No. 2016YFB1100100).

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Fig. 1. SEM backscattered electron images of the typical macrostructure and microstructure of Ni-30wt.%Sn alloy remelted once and twice by the laser beam at a scanning velocity of 2 mm/s each: (a) macromorphology of specimen remelted once; (b)~(d) the typical structures, at high magnification, of the white rectangles marked as 1~3 in Fig. 1(a) respectively; (e) macromorphology of the specimen remelted twice; (f)~(h) typical structures, at high magnification, corresponding to the white rectangles marked as 1~3 in Fig. 1(e) respectively.

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Fig. 2. Backscattered electron images of the typical microstructure of Ni-30wt.%Sn alloys from one and two laser-remeltings, respectively, at a laser scanning velocity of 1 mm/s: (a)~(c) microstructure corresponding to the top, middle and bottom of the specimen remelted once; (d) macromorphology of the specimen remelted twice; (e)~(i) the typical structures, at high magnification, corresponding to the white rectangles marked as 1~5, respectively, in Fig. 2(d).

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Fig. 3. Variations of the interface temperatures of α-Ni dendrite( colorized, dashed lines, under different temperature gradient G) and the lamellar α-Ni+Ni3Sn eutectic (the solid line), as a function of the growth velocity.

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Fig. 4. EBSD results of anomalous eutectic zone at the bottom of the molten pool remelted twice by the laser beam.（a）BSE figure of anomalous eutectic structure before EBSD mapping；（b）EBSD pattern indexed to α-Ni phase；（c）EBSD pattern indexed to Ni3Sn phase；（d）{100} pole figure of α-Ni phase；（e） {0001} pole figure of Ni3Sn phase；（f）Misorientation angle distribution of α-Ni phase.（v=1mm/s）

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Fig. 5. EBSD results of anomalous eutectic zone at the other position at the bottom of the molten pool remelted twice by the laser beam.（a）BSE figure of anomalous eutectic structure before EBSD mapping；（b）EBSD pattern indexed to α-Ni phase；（c）EBSD pattern indexed to Ni3Sn phase；（d）{100} pole figure of αNi phase；（e）{0001} pole figure of Ni3Sn phase；（f）Misorientation angle distribution of α-Ni phase.（v=1mm/s）

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Fig. 6. EBSD results of anomalous eutectic zone at the bottom of the molten pool remelted once by the laser beam.（a）BSE figure of anomalous eutectic structure before EBSD mapping；（b）EBSD pattern indexed to α-Ni phase；（c）EBSD pattern indexed to Ni3Sn phase；（d）{100} pole figure of α-Ni phase；（e） {0001} pole figure of Ni3Sn phase；（f）Misorientation angle distribution of α-Ni phase.（v=1mm/s）

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Fig. 7. EBSD results of anomalous eutectic zone at the other position at the bottom of the molten pool remelted once by the laser beam.（a）BSE figure of anomalous eutectic structure before EBSD mapping；（b）EBSD pattern indexed to α-Ni phase；（c）EBSD pattern indexed to Ni3Sn phase；（d）{100} pole figure of αNi phase；（e）{0001} pole figure of Ni3Sn phase；（f）Misorientation angle distribution of α-Ni phase.（v=1mm/s）

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Table 1 Euler angles of the numbered -Ni particles (7#~28#) in Fig. 7 (b). Number Euler1 （°） 7# 265.2

Euler2 （°） 32.8

Euler3 （°） 61.0

Number Euler1 （°） 8# 171.8

Euler2 （°） 20.1

Euler3 （°） 59.6

9#

291.8

27.8

30.9

10#

292.7

16.8

11.6

11#

285.6

37.3

31.5

12#

297.9

25.3

24.3

13#

231.5

45.8

19.8

14#

170.1

41.8

0.5

15#

170.4

41.4

0.6

16#

163.6

37.7

26.2

17#

218.5

41.8

25.4

18#

267.3

37.5

58.4

19#

230.5

36.2

45.3

20#

103.9

42.8

37.6

21#

321.9

31.1

87.9

22#

254.2

30.9

88.9

23#

200.9

49.6

57.4

24#

203.1

44.3

43.8

25#

166.5

45.1

62.4

26#

71.2

45.6

69.7

27#

342.1

16.0

24.6

28#

199.2

16.6

24.1

Note: The numbers or Euler angles with the same colours have relationships as mentioned above.