The effect of Gd element and solution treatment on the microstructure of AZ31 magnesium alloy and its kinetic model

Accepted Manuscript The effect of Gd element and solution treatment on the microstructure of AZ31 magnesium alloy and its kinetic model Jin-ling Zhang, Yang-li Liu, Jun Liu, Yan-chong Yu, She-bin Wang PII:

S0925-8388(15)31924-1

DOI:

10.1016/j.jallcom.2015.12.133

Reference:

JALCOM 36222

To appear in:

Journal of Alloys and Compounds

Received Date: 24 November 2015 Revised Date:

16 December 2015

Accepted Date: 17 December 2015

Please cite this article as: J.-l. Zhang, Y.-l. Liu, J. Liu, Y.-c. Yu, S.-b. Wang, The effect of Gd element and solution treatment on the microstructure of AZ31 magnesium alloy and its kinetic model, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.133. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effect of Gd element and solution treatment on the microstructure of AZ31 magnesium alloy and its kinetic model

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, Yang-li LIU , Jun LIU , Yan-chongYU , She-bin WANG

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Jin-ling ZHANG

College of Material Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China 2

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan 030024, PR China

Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, PR China

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Corresponding author. E-mail: [email protected] (Jin-ling ZHANG). Tel. /fax : +86 351 6010311.

Abstract

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The influence of Gd element and solution treatment on the microstructure of AZ31 alloy was investigated in this paper. The microstructure and phase composition were characterized by OM, SEM, EDS and XRD. The results indicate that Mg17Al12 eutectic phase, scattered along the grain boundary, is suppressed effectively with Gd addition and the acicular Al2Gd super lattice with lamellar distribution is formed. Furthermore, the grain size of AZ31 alloy is also refined with Gd addition. In the subsequent process of solution, the migration and growth of Al2Gd phase conforms to Ostwald Ripening Theory. The morphology of Al2Gd phase becomes granular and scatterd in matrix uniformly after solution for 16 hours at 813K.

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Key Words: solution treatment; microstructure; Ostwald Ripening Theory; kinetic model 1. Introduction

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Magnesium alloy has occupied the focus in automotive and aerospace industries due to low density and high specific intensity, especially in the situation that resources and energies are reducing very quickly[1-3]. AZ31 magnesium alloy, the representative of wrought magnesium alloys, is always given close attention by researchers. However, it is a key problem to impede engineering application of AZ31 magnesium alloy for poor absolute intensity[4]. At present, most researchers take advantage of alloying and heat treatment to improve the property of AZ31 magnesium alloy[5-7]. The structure of rare earth (RE) elements is the same as magnesium, namely hcp structure, at room temperature. Furthermore, the Mg-RE metastable phase is a superlattice like the type of D019 structure[8-11] and the RE elements can improve the mechanical properties of magnesium alloys obviously[12,13]. The relationship betweeen metastable phase and Mg matrix is coherency/half coherency[14]. Ming 1

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2. Experimental Procedure

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Sun[15] et al. found that adding Sm to AZ31 alloy will improve the mechanical properties due to yielding better thermal stable Al-RE phases and inhibiting the β-Mg17Al12 phrase. Qiu[16,17] et al. also found Al2Y intermetallic refines matrix very obviously with good thermodynamic stability. Li[18]et al. found the grains of Mg-6Al-0.6Zn alloy are refined intensely for the formation of Al2Sm particles. Peng Qiuming[19]et al. found that Mg3Gd is so stable a intermetallic compound in Mg-Gd binary alloy that will not disappear during annealing at 823K. Xudong Wang[20]et al. reported that the tension property of Mg-2Al-1Zn alloy at room and high temperature would be improved for (Mg, Al)3Gd and Al2Gd phases were produced by adding 1wt.%-4wt.% Gd element. However, the influence of heat treatment on magnesium alloy and kinetic model of Al2Gd phase growth are scarcely studied. This paper aims at discussing the formation of Al2Gd phase in Mg-3Al-1Zn-5Gd alloy (denoted as AZG315 alloy), the kinetic process of Al2Gd phase during solution treatment at 813K.

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The ingot-1 of about 0.4kg AZ31 alloy was melt from raw materials of pure Mg(>99.9%), pure Al(>99.9%), pure Zn(>99.9%) and pure Mn(>99.9%) in an electric furnace under the protection of Ar gas. At 1013K, the melt was manually stirred for 5-10min by a stainless steel paddle and then held for 15min to refine. When temperature decreased to 953K, the melt was poured into a permanent metallic mold preheated to 473K after refinement and the ingot-1 was produced. Similarly, the ingot-2 of AZG315 alloy was obtained in the same way but at 813K the Mg-30Gd master alloy was added to the melt and initiated refinement until the temperature regained 813K. The chemical composition of ingot-1 and ingot-2 was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) listed in Table 1. As cast samples for optical microscopy(OM) and SEM observations were cut by an electric-spark wire-cutting machine. The samples of Ingot-2 were carried out solution treatment at 813K for 1h, 3h, 6h, 9h, 12h, 16h, 20h, 24h and 28h, respectively. The samples were polished and then etched with an E1(8g picric acid, 10ml water, 5ml acetic acid and 100ml ethanol). The microstructure of as cast and as solutionized samples was determined by a Zessi OM and a FEI Quanta 250 field emission scanning electron microscope (SEM with EDS attached). And the component of phases was determined by TD-3500 XRD. 3. Results and discussion 3.1 The microstructures of as cast alloys The typical SEM morphology and EDS results of various phases in as-cast AZ31 alloy and AZG315 alloy (noted as C-AZ31 alloy and C-AZG315 alloy, respectively) are shown in Fig.1 respectively. Fig. 2 shows XRD patterns of the C-AZ31 alloy and C-AZG315 alloy. It can be seen that C-AZ31 alloy contains α-Mg matrix and β-Mg17Al12 phase. And C-AZG315 alloy contains α-Mg matrix and β-Al2Gd phase. 2

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The optical microstructures of C-AZ31 alloy and C-AZG315 alloy are shown in Fig.3. Combining with Fig.3a, it shows that the granular β-Mg17Al12 phase distributes in α-Mg matrix while the short-rod Mg17Al12 phase scatters along the grain boundary discontinuously in AZ31 alloy. But, the phase composition will change tremendously by adding 5wt.% Gd into AZ31 alloy. There are also two phases in C-AZG315 alloy. One is α-Mg solid solution being gray in OM; the other is acicular black Al2Gd phase (seen in Fig.3b). But the Mg17Al12 phase does not be found in C-AZG315 alloy, which is in accord with the results of W.P.Li[21]et al. That is why Mg17Al12 phase are restrained or eliminated in C-AZG315 alloy. The reason is that the electronegativity difference between Gd and Al is stronger than that between Gd and Mg (shown in Table 2). As a result, it is prior to interact between Gd and Al which will restrain or eliminate the generation of Mg17Al12 eutectic phase.

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3.2 Grain size According to Fig.3, the grain size of C-AZ31 alloy and C-AZG315 alloy was measured by Nano Measure software. The results show that the average grain size of C-AZ31 and C-AZG315 alloy is about 206.5µm and 152.6µm respectively. The conclusion can be attained that the average gain size of AZ31 alloy will be refined by 26% via adding 5wt.% Gd to matrix. The reason is that acicular Al2Gd phase is stable with high melting point that can pin the growth of α-Mg matrix effectively [22-24]. When α-Mg matrix grows with dendrite method, Gd atoms and Al atoms will be concentrated at the interface between solid and liquid because the solid solubility of Gd and Al element in Mg crystal decreases rapidly with the temperature reducing. The atoms combination for Gd and Al to Al2Gd particles will be resulted. Since the temperature of melt is quite lower than the melting point of Al2Gd phase which results in a natural super cooling for Al2Gd phase, the atoms of Al and Gd around Al2Gd particle will diffuse, adhere and make the Al2Gd particle grow to melt so rapidly that Al2Gd phase can anchor the α-Mg matrix. Because the atoms of Al and Gd diffuse directionally, the concentration of Al and Gd atoms nearby Al2Gd phase is less than that far from Al2Gd phase. So the acicular Al2Gd phase shows discontinuous distribution at intergranular and every needle is perpendicular to the interface between solid and liquid growing to melt (as shown in Fig. 4b arrow). In addition, it is advantageous to make nucleation for α-Mg matrix due to Gd atoms in the solid solution increasing the super-cooling degree of matrix [25]. Consequently, the grain size is refined. 3.3 The morphology evolution of Al2Gd phase during solution treatment Fig.4 shows the SEM images of as cast and as solutionized AZG315 alloy. It can be seen that the change of α-Mg solid solution mainly contains two stages during solution treatment. A large number of the second phase precipitated in the process of solution for 1 hour. With the extension of solution time, the second phase in grain immediately 3

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3.4 The precipitation and migration of Al2Gd phase

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began to dissolve. The second phase in grain completed to dissolve into matrix basically and a little of short-rod phase was left which is located at special orientation after solution for 9 hours. The change of intergranular Al2Gd phase lies in the morphology. During solution treatment, the morphology of the intergranular Al2Gd phase is needle, rod-like, short-bar and granule in turn. And the interspace among Al2Gd phase will be larger and larger with the extension of solution time.

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It can be seen that a large number of the second intracrystalline phase precipitates at the grey area that shows by OM (seen in Fig.3b) after solution for 1 hour at 813K and the Fig.4b is obvious. The reason is that a lot of Gd, Al atoms solutionize into Mg matrix in the process of refinement (the maximum solid solubility of Al atom and Gd atom in Mg matrix is 11.5wt% (710K), 23.71wt% (821K), respectively. While only a little of Gd atom collides with Al atom into Al2Gd superlattice phase. When the melt was poured into cast mould, a lot of heat would be released for the heat capacity of Mg matix is high(6.156×104kJ/mol·K). And the solid solution of Al and Gd in Mg matrix decreases rapidly with the temperature going down. So the supersaturated α-Mg solid solution would not precipitate the second phase but be in the growth of dendrite solidification at the short time when the temperature decreases quickly. Consequently, the supersaturated solid solution is gray in OM. And some dot small black particles can be seen in the gray dendrites namely as Al2Gd phase that is the isolated white dots in SEM (as shown in Fig.4a box). Exsolution reaction occurs in supersaturated α-Mg solid solution which is stimulated by high temperature during solution treatment at 813K. Thus much granular Al2Gd precipitates in grain. The EDS results and XRD pattern is shown in Fig.5 and Fig.6 respectively. Fig.7 is a magnified SEM image of AZG315 alloy solution for 6 hours and 12 hours. The morphology of Al2Gd phase starts to transform from rod-like to short bar after solution for 6 hours. The arrow here displays the dissolving and breaking process of Al2Gd phase in Fig.7a. Al2Gd phase transforms from short bar to granule in Fig.7b. It can be seen every short bar is twisted during solution treatment. And it is easy to break at node for serious stress concentration. The change for average thickness (AT) and average interspaces (AS) of intergranular Al2Gd phase with solution time is shown in Fig. 9. The smooth curve is fitting curve. The equation of fitting curve is: x x y = K1 exp(− ) + K 2 exp(− ) (1) a b where K1, K2, a and b are constants as -0.72, -1.02, 0.81, 3.52 respectively. It shows that the AT and AS for as cast AZG315 alloy is 0.2µm, 0.5µm respectively and the maximum AT and AS for as solutionized is 0.6µm, 3.0µm. The thickness ratio is 3 and the interface ratio is 6 for two states. That is the results of acicular Al2Gd phase migrating and thickening by 3D twist rather than simple combination, as shown in Fig. 8b. 4

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3.5 The model of diffusion growth and Ostwald Ripening Theory

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Fig. 10 is a model diagram of diffusion growth. The black block stands for Al2Gd phase and the red points are for Al atoms or Gd atoms. Firstly some Al atoms will dissolve into Mg matrix and diffuse towards nearby Al2Gd phase in the process of solution treatment. In the same way, Gd atoms lossing Al atoms will also dissolve into Mg matrix then diffuse towards nearby Al2Gd. As a result, the nearby Al2Gd will thicken. The diffusion of Gd atoms is assumed in the form of plane source. The restrictive factor in diffusion process is the diffusion rate of Gd atoms, so the diffusion distance of Gd atoms in α-Mg solid solution is calculated by the below formula[26]

L = Dv ⋅ t

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Where, L is planar diffusion distance of Gd atoms in α-Mg solution, (nm); Dv is diffusion coefficient of Gd atom,(nm2/s); t stands for diffusion time, (s). The AT and AS of Al2Gd phase from cast, solution for one hour and maximum are listed in Table 3. Needle-like of Al2Gd phase becomes rod-like in diffusion thickening way during solution for one hour, because the AS of solutionized Al2Gd phase (AS(s)) is 4 times as the cast according to Table 3. AS s (3) L= 2 t=3.6×103 s Dv =105.06 nm2/s, Dv is the diffusion coefficient of Gd atom in α-Mg matrix at 813K. Similarly, when the thickness of Al2Gd phase is maximum, the result (Dv =105.06 nm2/s) is substituted in Eq. (2), while the diffusion time is obtained as 5.56 hours which is equal to the result (6 hours) from the fitting function. Namely, the thickness of Al2Gd phase keeps the same after solution for 6 hours. Hence, the change of thickness and interspaces of Al2Gd phase is in accordance with “diffusion and growth” model in solution process at 813K. The model is actually Ostwald Ripening Theory in magnesium alloy. When the α-Mg solid solution grows in dendrite way, the Al2Gd phase is formed like the pearlite in steel. Since the size of Al2Gd phase is not uniform, the solution concentrations near little size Al2Gd phase is higher than that near big size Al2Gd phase. Under the driving force from concentration difference, the Al atoms and Gd atoms near litter size Al2Gd phase will diffuse to the big size Al2Gd directionally during solution treatment. As a consequence, the litter size Al2Gd will become less and disappear but the big size Al2Gd will grow up. As it is well known, the Gibbs energy of ball structure material is least and the granular second phase is more stable. So the Al2Gd phase thickens at first stage and the thickness keeps the same after solution for 6 hours. Subsequently its morphology tends to be particulate from short-bar (as the Fig.4g shown).

4. Conclusion 5

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It is effective to inhibit the formation of Mg17Al12 eutectic phase along boundary by adding 5wt.% Gd to AZ31 alloy. The Gd atom and Al atom dissolve into the melt during refinement. Al2Gd superlattice is formed by Gd atoms and Al atoms concentrate at the interface between solid and liquid when the matrix grows with dendrite method. And acicular Al2Gd phase distributes discontinuously due to the directional diffusion of Gd atoms and Al atoms. The diffusion rate of Gd atom is 105.06 nm2/s at 813K. The acicular Al2Gd phase will diffuse and thicken in solution process then the thickness will remain the same after solution for 6 hours. The morphology evolution of Al2Gd phase is coincident with the Ostwald Ripening Theory during solution treatment 813K.

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Acknowledgements

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This research was supported by the National Natural Science Foundation of China (No. 51371122), Shanxi Province Science Foundation (No.2015011068) and The Open Project of State Key Laboratory of New Technology in Iron and Steel Metallurgy (No.KF13-06).

References

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[1] Q.Wei, K.K.Deng, J.Y.Shi, W.Liang. Mater. Sci. Eng. A 609(2014) 1-6 [2] Mordike BL, Ebert T. Mater. Sci. Eng. 302 (2001) 37-45 [3] I. J. Polmear. Mater. Sci. and Tech. 10(1994)1-16 [4] Knezevic M, Levinson A, Harris R. Acta Mater. 58(2010) 6230-42 [5] Deng K.K, Li J.C, Fan J.F, Wang X.J, Xu B.S. Acta Metall. Sin. 27(2014) 885-893 [6] Li P J ,Tang B, Kandal ova E G . Mater. Lett. 59(2005) 671-679 [7] Zeng X.Q, Wu Y.Y, Peng L.M, Ding W.J. Acta Metall. Sin. 46(2010) 1041-1046 [8] Zhang J.L, Liu Y.L, Wang S.B. J. Alloys Compd. 629 (2015) 290–296. [9] X.B.Liu, R.S.Chen, E.H.Han. J. Alloys Compd. 465(2008): 232-238 [10] R.Mahmudi, F.Kabirian, Z.Nematollahi. Mater. Des. 32(2011)2583-2589 [11] Wu G.H, Zhang Y, Liu W.C, Ding W.J. J. Mag. Alloys. 1(2013) 39-46 [12] M.Paramsothy, J.Chan, R.Kwok, M.Gupta. J. Alloys Compd. 509(2011)7572-7578 [13] J.W.Qiao, Y.Zhang, P.K.Liaw, B.S.Xu. 100(2012) 121902 [14] Wang S.B, Qi X.Y, Zhang J.L, Xu B.S. Acta Metall. Sin. 47(2011):743-750 [15] Sun M. Hu X.Y, Peng L.M, Peng Y.H. Mater. Sci. Eng. A 620(2015) 89-96 [16] Qiu D, Zhang M.X. J. Alloys Compd. 586(2014) 39-44 [17] Qiu D, Zhang M.X, J. A. Taylor, P. M. Kelly. Acta Mater. 57(2009)3052-3059 [18] K.J.Li, Q.A.Li, X.T.Jing, J.Chen, X.Y.Zhang,Q.Zhang. Scr. Mater. 60(2009) 1101–1104. [19] Peng Q.M, Ma N, Li H. J. Rare Earths. 30(2012):1064-1068. [20] Wang X.D, Du W.B, Liu K, Li S.B. J. Alloys Compd. 522(2012)78-84 [21] Li,W.P. Zhou,H. Li,Z.F. J. Alloys Compd. 475(2009)227-232. [22] J. Hadorn, K. Hantzsche, S. Yi, J. Bohlen, D. Letzig, J. A. Wollmershauser, S.R. Agnew. Metall. Trans. A . 4(2012)1347-1362 [23] Nicole Stanford, Dale Atwell, Matthew R. Barnett. Acta Mater. 58 (2010) 6773–6783 [24] N. Stanford, G. Sha, J.H. Xia, S.P. Ringer, M.R. Barnett. Scripta Mater 65(2001) 919-921 6

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[25] Wang S.B, Zhang J.L, Qi X.Y, Xu B.S. J. Mater. Eng. S1(2009)303-307 [26] Liu Liufa, Ding Hanlin, Ding Wenjiang. Rare Metal Mater Eng. 38(2009)104-109

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Table 1. The chemical composition of ingots

[wt.%]

Gd

Al

Zn

Mn

Fe

Cu

Si

Mg

Ingot-1

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3.05

0.92

0.28

0.031

0.002

0.027

bal

Ingot-2

4.83

3.17

0.73

0.311

0.046

0.007

0.035

bal

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Element

Table 2. The electronegativity among Mg, Al and Gd Electronegativity

Electronegativity difference with Mg

Electronegativity difference with Al

Mg Al Gd

1.31 1.61 1.20

-------0.30 0.11

0.30 -------0.41

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Table 3. The average thickness and interspace [nm]

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AT AS

cast 190 353

solutionized 353 1230

max 640 2900

P1 at.% 39.6 19.2 11.5 2.7 27.0

element Mg K Al K Gd K Zn K O K

P2 at.% 58.8 9.9 5.9 1.6 23.8

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P1 at.% 54.1 24.5 21.4

element Mg K Al K Zn K

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P2 at.% 46.7 28.4 24.9

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Fig.1. The SEM morphologies and the EDS results for relevant points of C-AZ31 alloy and C-AZG315 alloy a: C-AZ31 alloy; b: C-AZG315 alloy




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2θ (degree)

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α-Mg Al2Gd

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Mg Al 17 12

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Intensity (A.U.)

3000

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α-Mg











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2θ ( degree)

Fig.2. The XRD patterns of C-AZ31 alloy and C-AZG315 alloy.

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Fig.3 The optical microstructures. a: C-AZ31 alloy; b: C-AZG315 alloy.

Fig.4. The SEM images of as cast and as solutionized AZG315 alloy a: as cast; b: solution for 1h; c: solution for 3h; d: solution for 6h; e: solution for 9h; f: solution for 12h; g: solution for 16h; h: solution for 20h; i: solution for 24h; j: solution for 28h.

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4

2000

α-Mg Al2Gd







P2 at.% 96.7 1.5 0.9 0.9

P3 at.% 96.3 1.6 1.2 0.9

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element Mg K Al K Gd K Zn K

P1 at.% 16.8 50.7 30.1 2.4

Intensity (A.U.)

1500












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0.6 2.5

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0.3

Interspace (µm)

Thickness (µm)

80

Fig.6. The XRD for solution for 1h

Fig.7. The amplifications of SEM images for as solutionized AZG315 alloy: a: solution for 6h; b: solution for 12h.

1.0

Phase thickness Phase interspace 0.2

0.5 3

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2θ (degree)

Fig.5. The amplification SEM image of solution for 1h

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Solution Time (h)

Fig.8. The thickness and interspace of intergranular Al2Gd phase change with solution time.

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Fig.9. The model diagram of diffusion growth

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Highlights:


The effect of Gd and solution treatments on the microstructure evolution of AZ31 was investigated. Analyzing the acicular Al2Gd phase formation mechanism.




Calculating the diffusion rate of Gd atom.




Established the kinetic model of microstructure evolution in the process of heat

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treatment.

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