Ultrahigh strength Mg-Al-Ca-Mn extrusion alloys with various aluminum contents

Accepted Manuscript Ultrahigh strength Mg-Al-Ca-Mn extrusion alloys with various aluminum contents Z.T. Li, X.G. Qiao, C. Xu, S. Kamado, M.Y. Zheng, A.A. Luo PII:

S0925-8388(19)31157-0

DOI:

https://doi.org/10.1016/j.jallcom.2019.03.319

Reference:

JALCOM 50083

To appear in:

Journal of Alloys and Compounds

Received Date: 27 September 2018 Revised Date:

15 February 2019

Accepted Date: 22 March 2019

Please cite this article as: Z.T. Li, X.G. Qiao, C. Xu, S. Kamado, M.Y. Zheng, A.A. Luo, Ultrahigh strength Mg-Al-Ca-Mn extrusion alloys with various aluminum contents, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.03.319. 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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Graphical abstract

ACCEPTED MANUSCRIPT Ultrahigh strength Mg-Al-Ca-Mn extrusion alloys with various aluminum contents Z.T. Li1, 2, X.G. Qiao1, C. Xu1, S. Kamado3, M.Y. Zheng1*, A.A. Luo2, 4 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

2

Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210,

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1

USA

Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188,

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3

4

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Japan

Department of Integrated Systems Engineering, The Ohio State University, Columbus, OH 43210,

USA

Abstract:

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* [email protected], Tel: 86-451-86402291, Fax: 86-451-86413922

In this work, Mg-(5, 6, 7) Al-3Ca-0.3Mn (wt.%) alloys were fabricated by permanent mould

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direct chill casting followed by indirect extrusion, to achieve ultrahigh strength. The microstructure and

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mechanical properties of these alloys were systematically investigated. While C36-(Mg, Al)2Ca was identified to be the major eutectic phase in the as-cast alloys, and a bimodal microstructure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed grains was observed in the as-extruded alloys. The as-extruded Mg-5Al-3Ca-0.3Mn (wt.%) alloy exhibits a yield strength of 420 MPa and ultimate tensile strength of 451 MPa with an elongation to failure of 4.1%. The ultrahigh strength is mainly due to dense homogeneously dispersed Mg2Ca nano-precipitates, grain refinement and strong basal texture. Further Al addition substantially decreases the number density of Mg2Ca

ACCEPTED MANUSCRIPT precipitates, causing a decrease in strength. Fracture observations indicate that the low ductility of the alloys is likely caused by non-uniform distribution of the fragmented eutectic particles.

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Keywords: Ultrahigh strength magnesium alloys; Extrusion; Nano-precipitates; Mechanical properties; fracture behaviors. 1. Introduction.

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The development of high strength wrought magnesium alloys has drawn great attentions due to

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the increased interest in using lightweight materials as automotive and aerospace applications [1, 2]. Extraordinarily high yield strength (YS) has been obtained in Mg-Gd-Y-Zn [3-9] and Mg-Y-Zn [10] alloys. However, large amounts of rare earth element addition results in high costs, which hinders their widespread industrial applications.

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Researches on the develop of RE-free high strength wrought Mg alloys indicate that Mg-Zn-Ca [11-13], Mg-Sn [14-16] Mg-Al-Ca [17-19] alloys are promising candidates for the next generation high-strength low-cost wrought alloys. Among them, as-extruded Mg-Al-Ca alloys exhibit ultra-high

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strength. A yield strength of 410 MPa and ultimate tensile strength (UTS) of 420 MPa have been

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achieved in an as-extruded Mg-3.5Al-3.3Ca-0.3Mn (all compositions are expressed in weight % in this paper unless otherwise specified) alloy [17]. There are two types of Al-Ca eutectic Laves phases formed during the solidification process in as-cast Mg-Al-Ca alloys, i.e. C36-(Mg, Al)2Ca and C14-Mg2Ca [20]. The type of the eutectic phases in as-cast Mg-Al-Ca-Mn alloys is dependent on the Ca/Al mass ratio [18, 20]. C36 is formed in the Mg-Al-Ca alloys with Ca/Al mass ratio lower than 0.5, while C14 is formed in the alloys with Ca/Al mass ratio higher than 1.3. If Ca/Al mass ratio is between 0.5 and 1.3, both C14 and C36 are formed. Our previous work [18] indicates that the type and

ACCEPTED MANUSCRIPT morphology of the primary phases significantly affect the mechanical properties of as-cast and as-extruded Mg-Al-Ca-Mn alloys. As-extruded Mg-2.7Al-3.5Ca-0.4Mn alloy containing fine lamellar C14 phase exhibits an ultrahigh yield strength of 438 MPa and an elongation to failure of 2.5 %. While

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as-extruded Mg-4.4Al-1.1Ca-0.4Mn alloy containing coarse divorced eutectic C36 primary phase has a lower yield strength of 190 MPa and a very high elongation to failure of 30 %, which may be due to the large inter-particle distance of the broken second phase after extrusion [18]. Therefore, in order to

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improve the strength and ductility of Mg-Al-Ca-Mn wrought alloys, it is important to modify the type

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and amount of eutectic phases by control of alloy composition and processing.

In addition to primary phases, multiple nano-scale precipitates, such as C15-Al2Ca, G.P. zones and Al-Mn particles have been observed in deformed [17-19], aged [21, 22] and crept [23, 24] Mg-Al-Ca-Mn alloys. G.P. zones enriched with Al and Ca elements lying on the basal planes of α-Mg

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are observed in T6 treated Mg-0.3Al-0.5Ca alloy. The G.P. zones transform into equilibrium Al2Ca phase after over-aging treatment [21]. Plate Al2Ca phase is distributed along the basal planes in the unrecrystallized regions of as-extruded Mg-Al-Ca-Mn alloys [17-19]. Nano-scale Al-Mn spherical

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particles are formed in as-extruded [17, 19] and as-crept Mg-Al-Ca-Mn alloys [23, 24]. These

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nano-scale precipitates have a significant strengthening effect on Mg-Al-Ca-Mn alloys. An increment of 100 MPa in yield strength is achieved by aging treatment of dilute Mg-1.3Al-0.3Ca-0.4Mn alloy due to the formation of uniformly dispersed G.P. zones with a very high number density [25]. While the ductility of as-aged Mg-1.3Al-0.3Ca-0.4Mn alloy is not obviously decreased since the G.P. zones can facilitate cross slip and do not act as crack initiators or propagation sites [25]. In this research, in order to further optimize the microstructure and improve the mechanical properties of the Mg-Al-Ca-Mn wrought alloys, the effect of Al content on the microstructure and

ACCEPTED MANUSCRIPT mechanical properties of as-cast and as-extruded Mg-(5~7)Al-3Ca-0.3Mn alloys is investigated. Ca content is kept at 3 wt. % to obtain a high volume fraction of Al-Ca eutectic second phase according to our previous work [18]. The Ca/Al mass ratio of the three alloys is selected to be 0.4~ 0.7 in order to

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make C36 phase as the main primary second phase in the as-cast alloys, since previous research suggests that a higher ductility can be expected in Mg-Al-Ca-Mn alloys with C36 as main eutectic

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phase [18].

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2. Experimental procedures

Three Mg-(5~7)Al-3Ca-0.3Mn alloys with different Al content were prepared by permanent mold direct-chill casting [10] from pure Mg, pure Al, Mg-20wt.% Ca and Mg-2wt.% Mn master alloys. The alloy melt was held at 720 ℃ for 20 min in a cylinder steel crucible, followed by immerging the

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crucible into cooling water. Ingots with a diameter of 60 mm and a height of 140 mm were produced. The chemical composition of ingots was analyzed by X-ray fluorescence (XRF) spectroscopy and the results are listed in Table 1 (the alloys are denoted as AXM5303, AXM6303 and AXM7303,

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respectively.). Rod billets with a diameter of 42 mm and a height of 35 mm were machined from the

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ingots and then subjected to indirect-extrusion at 300℃ with an extrusion ratio of 12 and a ram speed of 0.1mm/s without homogenization. Prior to extrusion, the billets were preheated at the extrusion temperature for 10 min. The extrusion rods with a diameter of 12 mm and length of 230 mm were produced. As-extruded AXM5303 alloy was subjected to annealing at 350oC for 1 h. The microstructure of the alloys was observed by ZEISS Supra 55 SAPPHIRE scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscope (EDX) operated at 20 kV and FEI-TECHNAI G2 F30 transmission electron microscope (TEM) with energy dispersive spectrometer

ACCEPTED MANUSCRIPT (EDS) operated at 200 kV. Samples for OM and SEM observation were grounded using emery papers up to #4000 and then etched in a solution of acetic picral (10ml water + 3 g picric acid + 5 ml acetic acid + 100 ml ethanol). Thin foils for TEM observation were mechanically polished to ~50 µm,

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punched into discs of 3 mm in diameter and then milled using Gatan plasma ion polisher. The texture was examined by an EDAX-TSL electron backscattered diffraction (EBSD) system, and the data were analyzed with OIM software. Tensile specimens with a gauge length of 15 mm and a cross-sectional

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area of 6 mm × 2 mm were machined from the as-extruded rods. The tensile tests were performed at

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room temperature using an Instron 5569 universal test machine at a crosshead speed of 1mm/min (corresponding to the initial strain rate 1.67 × 10-3 s-1). The tensile direction was parallel to extrusion direction (ED).

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Table 1. Nomenclatures and chemical compositions of alloys used in this work (in wt.%). Al

Ca

Mn

Mg

AXM5303

5.1

3.1

0.3

Bal.

AXM6303

5.9

3.1

0.3

Bal.

AXM7303

6.9

3.3

0.3

Bal.

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Alloy

3. Results and discussion

3.1 Microstructure of as-cast Mg-Al-Ca-Mn alloys Fig. 1 shows a SEM image of as-cast Mg-Al-Ca-Mn alloys with different Al contents. As shown in Fig. 1a-c, the microstructure of all three alloys consist of dendritic α-Mg grains and lamellar intermetallic compounds in the interdendritic areas, forming a continuous network. Fig. 1 d-f are the

ACCEPTED MANUSCRIPT enlarged images of the interdendritic areas. The total area fractions of second phases in as-cast AXM5303, AXM6307 and AXM7303 alloys are 19%, 22% and 26%, respectively, indicating that the amount of the second phases increases with increasing Al content. In AXM5303 alloy (Fig. 1a), there

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are two types of lamellar eutectic compounds in the Mg matrix: a low volume fraction of coarse lamellar compound formed in the triple junction areas (marked as 2 in Fig.1a) and a high volume fraction of fine lamellar compound (marked as 1 in Fig. 1a). There is only fine lamellar structure in the

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interdendritic areas of AXM6303 and AXM7303 alloys (Fig. 1b, c, e and f). Additionally, a blocky

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second phase with bright contrast is observed in all the three alloys (indicated by 5 in Fig. 1c). Table 2 summarizes the chemical compositions of the phases marked in Fig. 1 by EDX analysis. Both coarse and fine lamellar phases contain Al and Ca, while the blocky phase contains Al and Mn. The fine lamellar phases in all the three alloys have similar Ca/Al atomic ratios of ~ 0.50, while the coarse

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lamellar phase has a much higher Ca/Al atomic ratio. According to Rzychoń’s research on as-cast Mg-5Al-3Ca-0.2Mn-0.7Sr alloy [26] and our previous study on as-cast Mg-3.0Al-2.7Ca-0.4Mn alloy [18], the fine and coarse lamellar phases are determined to be (Mg, Al)2Ca and Mg2Ca, respectively.

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The bright blocky phase enriched with Al and Mn is Al8Mn5 phase with a rhombohedral structure,

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which is usually observed in Mg-Al-Mn alloys [17, 23, 27].

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Fig. 1. SEM images of as-cast Mg-Al-Ca-Mn alloys: (a, d) AXM5303; (b, e) AXM6303; (c, f)

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AXM7303.

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Table 2. EDS analysis of the second phases formed in as-cast Mg-Al-Ca-Mn alloys (at. %)

Position

Elements

Morphology

Ca/Al ratio Al

Ca

Mn

Mg

Phase

1

Fine lamellar

19.8±1.1

9.8±0.7

< 0.1

Bal.

0.50

(Mg, Al)2Ca

2

Coarse lamellar

13.6±0.4

14.7±0.7

< 0.1

Bal.

1.1

Mg2Ca

3

Fine lamellar

21.8±0.6

11.6±0.8

< 0.1

Bal.

0.53

(Mg, Al)2Ca

4

Fine lamellar

15.0±0.5

7.7±0.2

< 0.1

Bal.

0.51

(Mg, Al)2Ca

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Blocky

40.9±2.1

0.6±0.1

24.2±0.8

Bal.

-

Al8Mn5

Fig. 2 shows TEM images of coarse (Fig. 2a) and fine (Fig. 2b) lamellar phases in as-cast

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AXM5303 alloy. According to the selected area diffraction patterns (SADPs), the coarse and fine lamellar phases are identified to be Mg2Ca with C14 (hexagonal) structure and (Mg, Al)2Ca with C36

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(dihexagonal) structure [20, 28], respectively.

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Al)2Ca

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Fig. 2. TEM images of the eutectic phases in as-cast AXM5303 alloy: (a) C14-Mg2Ca; (b) C36-(Mg,

Fig. 3 shows the liquidus projection of Mg-Al-Ca system based on Suzuki and Cao’s research on

phase equilibria of Mg-Al-Ca alloys [20, 29]. The solidification paths of the three Mg-Al-Ca-Mn alloys in the present work are indicated in Fig. 3.

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Fig. 3. Liquidus projection in Mg-rich corner of Mg-Al-Ca system [20, 29] and solidification paths of three Mg-Al-Ca-Mn alloys in this study.

As shown in Fig. 3, the Mg-rich corner is surrounded by three eutectic transformations: L →

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α-Mg + C14-Mg2Ca, L → α-Mg + C36-(Mg, Al)2Ca and L → α-Mg + A12-Al17Mg12, in an order of increasing Al content. For L → α-Mg + C36-(Mg, Al)2Ca eutectic transformation, a saddle point is

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located at the composition of Mg-14.5Al-7.5Ca with a Ca/Al mass ratio of 0.52 and a maximum temperature of 534oC (Point S1 in Fig. 3). For AXM5303 alloy with a Ca/Al ratio greater than 0.52, the

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solidification begins with the nucleation of primary magnesium (α-Mg). When the melt temperature reaches the binary eutectic temperature, C36 is formed due to L → α-Mg + C36 eutectic transformation. The solidification ends with the ternary eutectic reaction L → α-Mg + C36 + C14 (point E in Fig. 3), during which a small amount of C14 is formed [30]. For AXM6303 and AXM7303 alloys with Ca/Al ratios lower than 0.52, the solidification paths are different. After primary α-Mg and C36 are formed, the solidification ends with the peritectic reaction L + C36 → α + A12 (Point U in Fig. 3) at around 452℃.

ACCEPTED MANUSCRIPT For all the three alloys in this study, C36-(Mg, Al)2Ca with a fine lamellar structure is the main second phase (Fig. 1). In AXM5303 alloy, a small amount of C14-Mg2Ca phase with a coarse lamellar structure was observed (Fig. 1a and d). C14 is formed in the last stage of solidification, which is

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consistent with the solidification path shown in Fig. 3. As for AXM6303 and AXM7303 alloys with Ca/Al ratios lower than 0.52, C36 phase is the only observed phase except Al8Mn5 (Fig. 1b, c, d and f). A12-Mg17Al12 is not observed by SEM, which is not in agreement with the solidification paths in Fig. 3.

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This may be due to the fact that high cooling rate during permanent mould direct-chill casting leads to

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a very short reaction period, which restricts the formation of A12. Our result is consistent with Suzuki’s research [20] that only C36 eutectic phase was observed in as-cast Mg-8Al-3Ca alloy, and A12-Al8Mn5 phase was formed when Al content was increased to 16 wt.%. In our previous work [18], Mg17Al12 phase was not observed in Mg-4.4Al-1.1Ca-0.4Mn and Mg-4.0Al-2.0Ca-0.4Mn alloys with lower

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Ca/Al mass ratios.

3.2 Microstructure of as-extruded Mg-Al-Ca-Mn alloys.

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Fig. 4 shows SEM microstructure of as-extruded Mg-Al-Ca-Mn alloys. After extrusion, the

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lamellar eutectic phases are fragmented into fine particles, forming bands along the extrusion direction (ED). According to The isolated coarse particles with brighter contrast are Al8Mn5 phase. The volume fraction of second phases increases with increasing Al content. Since the major second phase is fine lamellar C36 phase in all the Mg-Al-Ca-Mn alloys, the fragmented C36 particles show similar size and distribution after extrusion. As shown in Fig. 4d, e and f, the average size of the broken second phase particles in the three as-extruded Mg-Al-Ca-Mn alloy is only about 0.9 µm. Also, these broken fine particles distribute compactly, with average inter-particle distance less than 0.5 µm. Cracks along the

ACCEPTED MANUSCRIPT transverse direction can be seen on some of the fragmented second phase particles (marked by circles in Fig 4. d, e and f). Fig. 5 shows TEM microstructure of as-extruded AXM5303 alloy. Typical bimodal structure

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consisting of unDRXed region and DRXed region was observed. In DRXed regions, fragmented eutectic particles with a dark contrast and an irregular shape (marked by red arrows) can be found. DRXed grains have a moderate contrast and a polygon shape (marked by orange arrows), their size is

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only several hundred nanometers. While the unDRXed regions are composed of elongated large grains

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with a high dislocation density. Compared with DRXed regions, the unDRXed regions are free of compound particles, suggesting that dynamic recrystallization might be facilitated by the second phase particles via particle stimulated nucleation (PSN) [31, 32]. During extrusion, misorientation is accumulated in the matrix surrounding the hard fragmented eutectic phase particles, forming sub-grain

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boundaries in these regions. The migration of sub-grain boundaries promotes the nucleation of DRXed grains. Thus, the DRXed grains are formed in the vicinity of fragmented eutectic phase particles, while the regions far away from the particle bands remain unDRXed due to the lack of misorientation

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gradient. As a result, bimodal structure is formed during hot extrusion. According to the

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composition analysis by EDS, the small fragmented second phase particles are mainly C36 phase with a similar composition in table 2. The Al concentration in α-Mg of all the three alloys is 0.8-1.1wt.%, the Ca and Mn concentrations are below 0.3 wt.% and 0.1 wt.%, respectively.

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ED Fig. 4. SEM images of as-extruded Mg-Al-Ca-Mn alloys: (a), (d) AXM5303; (b), (e) AXM6303; (c), (f)

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AXM7303.

Fig. 5. TEM image of as-extruded AXM5303 alloy showing a bimodal grain structure with DRXed

ACCEPTED MANUSCRIPT regions and unDRXed regions. 3.3. Precipitation in as-extruded Mg-Al-Ca-Mn alloys. Fig. 6 shows TEM microstructure of as-extruded AXM5303 and AXM7303 alloys. As shown in

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Fig. 6a and c, dense fine spherical precipitates are observed in both DRXed and unDRXed areas in as-extruded AXM5303 alloy, and the number density of the spherical particles is higher in the unDRXed area. In both DRXed and unDRXed regions, the spherical precipitates are dispersed

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homogenously. As seen from Fig. 6b, spherical precipitates are also formed in DRXed regions of

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as-extruded AXM7303 alloy, but the number density is much lower compared with those in as-extruded AXM5303 alloy. Fig. 6d shows that planar phases are precipitated in unDRXed region of as-extruded AXM7303 alloy. These planar phase particles are parallel to each other. The selected area electron diffraction (SAED) patterns taken with an incident beam along [1120] direction of Mg matrix

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inserted in Fig. 6d shows faint streaks along [0001] direction, indicating that these planar precipitates are formed along basal planes. Some of the planar precipitates are connected with each other, forming “dashed lines” on basal planes. This indicates that the formation of the planar precipitates may be

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facilitated by dislocations on basal planes. These planar precipitates along basal planes are previously

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reported in as-extruded Mg-3.5Al-3.3Ca-0.4Mn [17] and over-aged Mg-0.3Al-0.5Ca alloys [21]. Since the microstructure and distribution are similar to those in Refs. [17, 21], the planar phase in unDRXed regions of as-extruded AXM7303 alloy is considered to be Al2Ca phase with a C15 structure. Homogenously distributed spherical nano-precipitates are also observed in unDRXed grains, but their number density is much lower than those in as-extruded AXM5303 alloys.

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Fig. 6. TEM images of DRXed and unDRXed regions of as-extruded Mg-Al-Ca-Mn alloys: (a, c) AXM5303; (b, d) AXM7303. SAED patterns inserted in both (c) and (d) were taken with an incident

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electron beam along the [1120] zone axis of the Mg matrix.

To identify the spherical nano-precipitates in as-extruded AXM5303 and AXM7303 alloys,

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high-angle annular dark field (HAADF) imaging and high-resolution transmission electron microscopy

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(HRTEM) techniques were used. Fig. 7a shows a HAADF image of DRXed region in as-extruded AXM5303 alloy. It can be seen that most of the nano-scale precipitates in the unDRXed regions are spherical phase with a diameter smaller than 25 nm (marked by red arrows). In addition, a small amount of precipitates are rod-like and have a length of ~30 nm (indicated by yellow arrows). The EDS elemental mappings shown in Fig. 7b-e indicate that both spherical and rod-like precipitates are enriched with Al and Ca, which may be Mg-Al-Ca compounds. Similar nano-spherical and rod-like precipitates were observed in as-crept Mg-5Sn-3Ca wt.% alloy and identified to be Mg2Ca

ACCEPTED MANUSCRIPT according to SADP [33]. A large number of spherical nano-particles were also observed in DRXed regions of as-extruded Mg-1Ca alloy, and they were confirmed to be Mg2Ca by EDS and Fast Fourier Transformation (FFT) patterns [34]. Therefore, the spherical precipitates in AXM5303 and AXM7303

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alloys might be Mg2Ca phase with Al dissolved in it.

Fig. 7. Nano precipitates in DRXed regions of as-extruded AXM5303 alloy, (a) HAADF image; (b-e)

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elemental mapping of Mg, Al, Ca and Mn of this area.

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Fig. 8 gives HRTEM images of nano-scale precipitates in DRXed regions of as-extruded AXM5303 alloy with the incident electron beam along the [1010] zone axis of Mg matrix. The HRTEM images of the spherical and short rod-like precipitates and their corresponding FFT spectra are inserted in Fig. 8a and b, respectively. From Fig. 8a, the spherical precipitate particle is coherent with α-Mg matrix. Based on the FFT spectra, this spherical precipitate is identified to be C14-Mg2Ca with Al dissolved in it, with an orientation relationship of [1210]Mg2Ca // [0110]Mg, (0001)Mg2Ca // (0001)Mg. Spherical nano Mg2Ca precipitates were also reported by Pan et al. in an as-extruded Mg-1Ca

ACCEPTED MANUSCRIPT alloy, with an orientation relationship of [1120]Mg2Ca // [1210]Mg , (0001) Mg2Ca // (1011) Mg [34]. The orientation relationship between Mg2Ca precipitates and α-Mg in our work is consistent with Langelier [35] and Oh’s [36] reports on aged Mg-Zn-Ca alloys.

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From Fig. 8b, a short rod-like precipitate is formed along the basal plane. It is also coherent with the α-Mg matrix. The FFT spectra show extra reflections forming streaks along the [0001] direction. The FFT spectra in Fig. 9b are consistent with the SADP of rod-like Mg2Ca precipitates [35, 36]. Thus

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the rod-like precipitate is also identified as Mg2Ca phase. Mg2Ca precipitates with both spherical and

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rod-like morphologies have been reported previously [33, 34, 36]. Since the morphology and distribution of the spherical nano-precipitates in both the DRXed and unDRXed regions of as-extruded

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AXM7303 are similar with those in as-extruded AXM5303, they are also identified to be Mg2Ca phase.

Fig. 8. HRTEM images along with the FFT spectra of nano-precipitates in DRXed regions of as-extruded AXM5303 alloys: (a) spherical precipitate; (b) rod-like precipitate.

As shown in Fig. 6, Al content has a significant influence on the precipitate phases in as-extruded Mg-Al-Ca-Mn alloy. With Al content increasing from 5 to 7 wt.%, the number density of Mg2Ca phase

ACCEPTED MANUSCRIPT decreases significantly, and Al2Ca is formed, which is probably due to different concentration of Al in α-Mg. Fig. 9 shows the variation of average concentration of Al, Ca and Mn elements in α-Mg with temperature during solidification of AMX5303 and AXM7303 alloys. The calculations were performed

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using Scheil model in Pandat 2017 software package with PanMg 2017 database. It can be seen that the solidification range is 609oC to 515oC for the AXM5303 and 601oC to 436oC for AXM7303. The concentration of Ca and Mn is limited in α-Mg due to their low solubility. At the end of the

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solidification, the average Al concentration in α-Mg of AXM5303 and AXM7303 is 1.8 % and 2.8 %,

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respectively. When the alloys were extruded at 300oC, the solubility of all the alloying elements in α-Mg is decreased, resulting in precipitation of second phases. Low Al concentration in the α-Mg of AXM5303 leads to the precipitation of dense nano-scale Mg2Ca particles other than Al2Ca. While plate

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Al2Ca phase are formed in as-extruded AXM7303 due to the higher Al concentration in the matrix.

Fig. 9. Variation of Al, Ca and Mn solute concentration in α-Mg of (a) Mg-5Al-3Ca-0.3Mn and (b) Mg-7Al-3Ca-0.3Mn alloy with temperature during solidification.

According to previous researches on as-extruded Mg-Al-Ca-Mn alloys, plate-like Al2Ca precipitates or G.P. zones are observed in the unDRXed regions [17-19], while spherical Al-Mn

ACCEPTED MANUSCRIPT nano-precipitates are observed in the DRXed regions [17, 18, 25]. However, the present research indicates that the spherical and rod-like precipitates in as-extruded AXM5303 are Mg2Ca dissolved with Al, no Al-Mn nano-precipitate is observed. To clarify the precipitation condition of Al-Mn nano

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precipitate, as-extruded AXM5303 was subjected to annealing at 350oC for 1h, with TEM images of the precipitates shown in Fig. 10. High density of spherical nano-precipitates enriched with Al and Mn are observed, indicating the precipitation of nano Al-Mn precipitates during annealing at 350oC.

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The formation of precipitate requires long-range diffusion of solute atoms in the matrix [37]. In the

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present work, the extrusion temperature is 300oC, which is lower than those applied in other reports [17, 18, 25]. The diffusion coefficient of Mn in Mg matrix at 300 oC is approximately 5×10-20 m2/s, which is 4 and 3 orders of magnitude lower than that of Ca (approx. 6×10-16 m2/s) and Al (approx. 6×10-17 m2/s), respectively [38]. The low extrusion temperature and very short preheating time (10 min) would

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suppress the diffusion of Mn. As a result, Mg2Ca is preferentially precipitated rather than Al-Mn phase during extrusion. On the other hand, the high diffusion coefficient of Mn in Mg matrix (approx. 1×10-18m2/s) at 350oC [38] and sufficient diffusion time promotes the diffusion of Mn in the matrix,

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leading to the precipitation of Al-Mn nano-phase during annealing at 350oC. Mg2Ca precipitates were

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not detected in the annealed samples, which indicates Mg2Ca is unstable at 350

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Fig. 10. Nano precipitates in the annealed AXM5303 alloys: (a) TEM bright field image; (b) HAADF image (c-f) Elemental mapping of Mg, Al, Ca and Mn.

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3.4. Texture of as-extruded Mg-Al-Ca-Mn alloys.

The inverse pole figure (IPF) maps of as-extruded Mg-Al-Ca-Mn alloys are shown in Fig. 11. All as-extruded alloys exhibit a bimodal structure consisting of fine equiaxed dynamic recrystallized

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(DRXed) grains and coarse elongated unDRXed grains. The area fractions of DRXed regions in

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AXM5303, AXM6303 and AXM7303 alloys are 50%, 46% and 50%, respectively. The DRXed grains show a relatively random texture, while the unDRXed grains show a strong basal texture. For all three alloys, the DRXed regions are distributed near the strips of fragmented second phase particles. As shown in Fig. 5 a, b and c, the DRXed grain size in the Mg-Al-Ca-Mn alloys increases with increasing Al content. The average DRXed grain size is 0.75 µm in AXM5303, which is significantly smaller than that in AXM6303 or AXM7303 alloys. As discussed in Section 3.2, the nucleation of DRXed grains in AXM5303 alloy is facilitated by the

ACCEPTED MANUSCRIPT fragmented eutectic phase particles via PSN mechanism. The nucleation sites are located in vicinity of the particles. From Fig. 4d, e and f, the size and interspacing of the fragmented particles of all three alloys are similar. Therefore, the influence of second phase particles on the DRXed grain size can be

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excluded. From Fig.6a and b, spherical nano-scale Mg2Ca precipitates are formed in the DRXed regions of as-extruded Mg-Al-Ca-Mn alloys. These precipitates can impede the motion of dislocations and sub-grain boundaries, and the transition from sub-grains into the DRXed grains are retarded [39].

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Consequently, DRXed grains with very small grain size are formed in the as-extruded Mg-Al-Ca-Mn

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alloys. The number density Mg2Ca precipitates in the DRXed regions of as-extruded AXM5303 alloy is higher than that in as-extruded AXM7303 alloys. As a result, the grain size in as-extruded AXM5303 is

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smaller than that in as-extruded AXM7303 alloys.

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Fig. 11. IPF maps of as-extruded (a) AXM5303, (b) AXM6303 and (c) AXM7303 alloys. Grain size

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distribution of the DRXed areas in (d) AXM5303, (e) AXM6303 and (f) AXM7303 alloys.

Fig. 12 shows inverse pole figures along the extrusion direction of as-extruded Mg-Al-Ca-Mn

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alloys. Both unDRXed and DRXed regions show basal fiber texture with the <10-10> direction of Mg

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matrix parallel to the extrusion direction, which is typical texture in extruded magnesium alloys [13, 17, 25, 40, 41]. The intensity of <10-10> pole is much lower in DRXed regions than that in unDRXed regions. For DRXed regions, the texture component of AXM5303 is concentrated on <10-10>, the intensity of <10-10> pole is the highest among all three extruded alloys. While the texture component of DRXed regions of AXM6303 and AXM7303 exhibits a dispersed distribution. This suggests that the basal planes of DRXed grains in AXM5303 are more strongly aligned parallel to the extrusion direction than the other two alloys. The research of Agnew et al. on recrystallization of rolled AZ31B alloy

ACCEPTED MANUSCRIPT indicates that texture component is dependent on the recrystallized grain size, larger recrystallized grains possess more <11-20> texture component [39]. The larger DRXed grain size of AXM6303 and AXM7303 than that of AXM5303 contributes to the weak <10-10> texture component and a dispersed

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texture component distribution from <10-10> to <11-20> pole. The average Schmid factors for {0002}<1120> basal slip along extrusion direction of AXM5303, AXM6303 and AXM7303 alloys are 0.107, 0.121 and 0.129, respectively. All three as-extruded alloys

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have very low average Schmid factors due to their high volume fractions of unDRXed regions which

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have very strong basal texture. As-extruded AXM5303 alloy has the lowest average Schmid factor because of its strongest basal texture intensity in DRXed regions. The low average Schmid factors for the basal slip in the as-extruded alloys indicates that the basal slip is suppressed during tensile test

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along the ED, which leads to high yield strength.

Fig. 12. Inverse pore figures of unDRXed and DRXed regions of as-extruded Mg-Al-Ca-Mn alloys: (a, d) AXM5330; (b, e) AXM6303; (c, f) AXM7303.

3.5. Mechanical properties of Mg-Al-Ca-Mn alloys.

ACCEPTED MANUSCRIPT Nominal tensile strain-stress curves of as-cast and the as-extruded Mg-Al-Ca-Mn alloys are presented in Fig. 13. Corresponding tensile properties are listed in Table 3. The as-cast alloys exhibit both low strength and ductility. Both yield strength (YS) and ultimate tensile strength (UTS) of the

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as-cast alloys increase with increasing Al content. This may be due to the increasing amount of eutectic (Mg, Al)2Ca phase with increasing Al content. As-cast AXM7303 has a yield strength of 112 MPa, ultimate tensile strength of 128 MPa, and elongation to failure of 0.7 %. After extrusion, both strength

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and elongation of Mg-Al-Ca-Mn alloys are greatly increased. As-extruded AXM5303 alloy exhibits a

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YS of 420 MPa, UTS of 451 MPa and elongation to failure of 4.1%. Table 3 also shows the tensile properties of other experimental high strength wrought Mg-Al-Ca and Mg-RE alloys reported in literatures [17, 24, 25, 42, 43, 44, 45]. Among all the developed Mg-Al-Ca-Mn alloys with an elongation to failure higher than 4%, AXM5303 has the highest yield strength and ultimate tensile

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strength. Furthermore, the yield strength of AXM5303 is even higher than some ultrahigh strength Mg-RE wrought alloys containing high concentration of Gd and Y. This indicates that Mg-Al-Ca-Mn alloys have great potential as low cost ultra-high strength wrought RE-free Mg alloys. The as-annealed

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AXM5303 alloy exhibits a YS, UTS and an elongation of 315 MPa, 335 MPa and 5.4%, respectively.

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The as-extruded AXM5303 alloy shows a large decrease in strength but only 1.3% percent increase in elongation after annealing.

It is interesting to note that both YS and UTS of the as-extruded Mg-Al-Ca-Mn alloys decrease with increasing Al content, which is contrary to the as-cast Mg-Al-Ca-Mn alloys. Among all three as-extruded Mg-Al-Ca-Mn alloys, AXM5303 exhibits the smallest average DRXed grain size of 0.76 µm, which contributes to higher yield strength according to Hall-Petch equation. Moreover, all three alloys have a high area fraction (about 50 %) of unDRXed regions which have a very strong basal

ACCEPTED MANUSCRIPT texture. Basal dislocations are greatly suppressed in these regions during tensile tests, which contributes to the high YS. As for the DRXed regions, all three as-extruded alloys exhibit much weaker texture, and the AXM5303 alloy exhibits a higher texture intensity (Fig. 12) than the other two alloys,

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which may also result in a higher strength. More importantly, the profuse homogeneously distributed spherical Mg2Ca nano-precipitates are formed in both the DRXed and the unDRXed regions in the as-extruded AXM5303 alloy. These nano-precipitates can effectively resist dislocation motion during

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tensile test, resulting in significant improvement on yield strength. In the as-extruded AXM5303 alloy,

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Mg2Ca precipitates have a much higher number density and a smaller interspacing than the spherical Mg2Ca and planar Al2Ca precipitates in AXM7303, which contributes to higher YS for AXM5303. However, the solid solution strengthening effect would not be significant in this discussion, due to the low Al, Ca and Mn concentration in the Mg matrix of all the as-extruded alloys. Therefore, the

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ultrahigh strength of the as-extruded AXM5303 alloy is primarily attributed to dense homogeneously

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dispersed Mg2Ca nano-precipitates, grain refinement and strong basal texture.

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500

as-extruded AXM5303 as-extruded AXM6303 as-extruded AXM7303

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300

200

as-annealed AXM5303 as-cast AXM5303 as-cast AXM6303 as-cast AXM7303

100

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Stress (MPa)

400

0 1

2

3

4

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0

5

6

Strain (%)

Fig. 13. Nominal tensile strain-stress curves of the as-cast and as-extruded Mg-Al-Ca-Mn alloys.

alloys. Alloy

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Table 3. Comparison of tensile properties of Mg-Al-Ca-Mn alloys and RE-containing magnesium

YS

UTS

Elongation

Condition

EP

(wt.%)

Ref. (MPa)

(MPa)

(%)

As-cast

88±4

121±4

0.9±0.2

AXM6303

As-cast

92±6

127±5

1.0±0.1

AXM7303

As-cast

112±4

128±8

0.7±0.2

This

AXM5303

As-extruded

420±8

451±5

4.1±0.5

work

AXM6303

As-extruded

389±6

423±3

3.60±0.3

AXM7303

As-extruded

360±8

414±2

4.4±0.4

AXM5303

As-annealed

315±4

335±3

5.4±0.3

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AXM5303

ACCEPTED MANUSCRIPT As-extruded

438

457

2.5

[24]

Mg-3.5Al-3.3Ca-0.4Mn

As-extruded

410

420

5.1

[17]

Mg-1.3Al-0.3Ca-0.5Mn

Extrusion+T5

287

306

20

[25]

Mg-2.3Al-1.7Ca

As-extruded

275

324

10.2

[42]

Mg-10.1Gd-5.7Y-1.6Zn-0.

As-extruded

419

461

Extrusion+T5

350

440

Forging+T5

391

448

[43]

4

[44]

3.9

[45]

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Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr

3.6

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Mg–12.1Y-7.0Zn-0.4Zr

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Mg-2.7Al-3.5Ca-0.4Mn

Although all three as-extruded Mg-Al-Ca-Mn alloys exhibit high strength, it is worth noting that the elongation to failure of the three alloys is less than 5 % even after extrusion. It is important to investigate the mechanism for the poor ductility of the present Mg-Al-Ca-Mn alloys. Fig. 14 shows

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SEM microstructure of longitudinal section near tensile fracture surface of the as-cast and the as-extruded AXM5303 alloy. Fig. 14a shows that cracks are formed in the lamellar phase network of as-cast AXM5303 alloy and propagated along the network. While in as-extruded AXM5303 alloy,

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cracks initiate at the strips composed of the broken second phase particles, then propagate across the

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second phase particle strips, as shown in Fig. 14b. In both as-cast and as-extruded alloys, brittle eutectic second phases (Mg2Ca and (Mg, Al)2Ca) can cause loss of tensile ductility by providing microcrack initiation sites, accelerating the flow localization process in the matrix, and creating an easy path for crack propagation. During tensile test, the stress is concentrated around these second phases. The microcracks initiate and coalesce, then propagate along the second phase networks or pass through the particle strips, causing failure of the tensile samples.

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ACCEPTED MANUSCRIPT

Fig. 14. SEM microstructure of longitudinal section near tensile fracture surface of the (a) as-cast and (b) as-extruded AXM5303 alloys.

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Brittle eutectic phases act as microcrack initiation sites and cause early fracture. Thus these phases can be considered as crack forming phases (CFPs) [46]. In as-extruded Mg-Al-Ca-Mn alloys, microcracks along the transverse direction are formed on some of the fragmented eutectic phase

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particles (shown in Fig. 4d, e, f). Assuming that microcracks arrange in a cubic array, macroscopic fracture strain of alloy ε , can be obtained as follows [47, 48]:  

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ε =







  .









 1

  



[1]

where λ and ! are the interspacing and radius of the microcacks, respectively (as shown in Fig.

15); "̃$ θ is the effective value for the proportionality constant "̃&' θ , which is a dimensionless function of the angular variable θ in the polar coordinates, and is a constant when for θ = 0; I and h are both the functions of the strain hardening exponent n. "̃ is the critical local strain in the middle of the ligament connecting the two most neighboring microcracks. [49, 50].

ACCEPTED MANUSCRIPT Assuming that the second phase particles are also in cubic arrangement, the volume fraction of particles φ) can be obtained as [48]: 

φ) = 2+, - . / .

0

[2]

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where !) and λ) interspacing and radius of the particles, , is the aspect ratio of the particles, and equals 1 for spherical particles.

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Since not all the particles will crack during deformation, cracking fraction of the particles is

assumed to be F. As shown in Fig.15, the microcrack is formed across the particle, the radius of the

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crack ! equals to the radius of the particle !) , thus the volume of crack forming particles φ should be: 

0

φ = 2+, 1  2 = 3φ) 

[3]









  .



or

  .

45 9

6

78.

 



;

<

9

. √7

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ε =



 

 1:

 

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ε =



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By combining Eq. [1], [2] and [3], the macroscopic fracture strain of alloy ε can be expressed as:

 1>





  



[4]

[5]

From Eq. [4] and [5], amount, size, distribution, morphology and fracture fraction of second phase

particles have influence on the ductility for alloys containing brittle phase which can initiate microcracks. Increasing the volume fraction of second phase φ) is detrimental to the ductility. Jiang et al.’s [42] research on the as-extruded Mg-Al-Ca alloys containing 1, 2, 4 and 6 wt.% of Al2Ca phase indicates that the elongation to failure of the Mg-Al-Ca alloys decrease with increasing Al2Ca fraction.

ACCEPTED MANUSCRIPT From Eq. [5], the ratio of the interspacing to the radius of the second phase particles λ) /!) is positively correlated with the ductility. In the present work, the eutectic phases in the as-cast alloys show a continuous distribution, thus the λ) /!) value is considered to be 2. In this case, the

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microcracks can propagate easily along the brittle second phase network. After extrusion, the eutectic phases are fragmented into fine particles with a discontinuous distribution. As a result, the propagation of microcrack is suppressed. According to Fig. 4, !) is about 0.45 µm and λ) is about 1.4 µm for the

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from 2 to 3.1, which is consistent with Eq. 5.

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extruded alloys. After extrusion, the elongation is increased from ~1% to ~4% as λ) /!) increases

After extrusion, fragmented second phase particles are clustered in strips and have a very small interspacing of less than 0.5 µm. During tensile test, microcacks are formed in the strips, propagate and connect easily. Thus larger cracks are formed across the whole strip (as shown in Fig.14b) and finally

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cause fracture of the sample. This indicates that the brittleness of the as-extruded Mg-Al-Ca-Mn alloys is mainly induced by the inhomogeneously compact distribution of the fragmented brittle primary phases. Annealing usually decreases the intensity of basal texture and the density of dislocations in the

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matrix, which can significantly increase the ductility of the wrought Mg alloys [51, 52]. However,

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annealing at 350oC for 1h has little effect on improving the ductility of the as-extruded AXM5303 alloy. This is probably due to the high thermal stability of the eutectic C36 and C14 phases [53, 54], the annealing time is not sufficient to change the large fraction or the compact distribution of the brittle fragmented eutectic particles. To improve the ductility of the wrought Mg-Al-Ca-Mn alloys, decreasing the Al-Ca content to reduce the fraction of eutectic phases and optimizing the plastic deformation processing parameters to obtain a uniform distribution of the fine fragmented second phases particles with a larger interspacing are both potential ways.

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ACCEPTED MANUSCRIPT

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Fig. 15. Schematic illustration of the local fracture behavior in Mg-Al-Ca-Mn alloys. 4. Conclusions.

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In this study, wrought Mg-(5, 6, 7)Al-3Ca-0.3Mn wt.% alloys processed by conventional casting and extrusion were developed. Conclusions are drawn as follows: 1.

Fine lamellar C36-(Mg, Al)2Ca eutectic phase is the major second phase for all three as-cast alloys. A small amount of C14-Mg2Ca eutectic phase is observed in AXM5303. After extrusion, these

2.

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phases are fragmented into fine particles which can facilitate recrystallization. Dense Mg2Ca nano-precipitates are formed homogeneously in as-extruded AXM5303 alloy. While

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both Mg2Ca and Al2Ca are precipitated in as-extruded AXM7303 alloy. The number density of the precipitates in as-extruded AXM7303 alloy is much lower than that in as-extruded AXM5303

3.

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alloy.

As-extruded AXM5303 alloy exhibits a yield strength of 420 MPa, ultimate tensile strength of 451 MPa and elongation to failure of 4.1%. The strength of as-extruded Mg-(5-7)Al-3Ca-0.3Mn alloys decreased with increasing Al content due to larger DRXed grain size and lower number density of precipitates in the alloys with higher concentration of Al.

4.

The brittle eutectic phases act as crack initiation and propagation sites for both as-cast and as-extruded Mg-Al-Ca-Mn alloys during tensile loading, which causes premature failure. The

ACCEPTED MANUSCRIPT limited ductility for as-extruded alloys is caused by the inhomogeneously compact distribution of the broken eutectic particles.

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Acknowledgements This research was supported by National Natural Science Foundation of China (51771062 and

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his stay at The Ohio State University as a visiting scholar.

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51571068). Z. Li would also like to express his gratitude to China Scholarship Council for supporting

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Highlights： ： Mg-5Al-3Ca-0.3Mn alloy with a YS of 420 MPa is developed by hot extrusion.

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Ultrafine DRXed and coarse unDRXed grains are formed in the as-extruded alloy.

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Dense Mg2Ca nano-precipitates are homogeneously dispersed in the grains.

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The mechanism of premature fracture in the as-extruded alloy is discussed.

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