Effect of Al content on microstructure and mechanical properties of as-cast Mg-5Nd alloys

Accepted Manuscript Effect of Al content on microstructure and mechanical properties of as-cast Mg-5Nd alloys Dan Liu, Jiangfeng Song, Bin Jiang, Ying Zeng, Qinghang Wang, Zhongtao Jiang, Bo Liu, Guangsheng Huang, Fusheng Pan PII:

S0925-8388(17)33900-2

DOI:

10.1016/j.jallcom.2017.11.143

Reference:

JALCOM 43836

To appear in:

Journal of Alloys and Compounds

Received Date: 27 July 2017 Revised Date:

9 November 2017

Accepted Date: 10 November 2017

Please cite this article as: D. Liu, J. Song, B. Jiang, Y. Zeng, Q. Wang, Z. Jiang, B. Liu, G. Huang, F. Pan, Effect of Al content on microstructure and mechanical properties of as-cast Mg-5Nd alloys, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.143. 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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Effect of Al content on microstructure and mechanical properties of as-cast Mg-5Nd alloys

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Dan Liu a, Jiangfeng Song a,*, Bin Jiang a,b,**, Ying Zeng a, Qinghang Wang a, Zhongtao Jiang a, Bo Liu c, Guangsheng Huang a,b, Fusheng Pan a,b

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a State Key Laboratory of Mechanical Transmissions, College of Materials Science

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and Engineering, Chongqing University, Chongqing 400044, China

b Chongqing Academy of Science and Technology, Chongqing 401123, China c Chongqing Chang-an Automobile Co., Ltd, Chongqing 400023, China

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* Corresponding author. Tel.: +86 18223616225

E-mail address: [email protected] (J. Song) ** Corresponding author. Tel.: +86 13594190166; fax: +86 65111140

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E-mail address: [email protected] (B. jiang)

Abstract

The microstructures and mechanical properties of as-cast Mg-5Nd-xAl (x = 0,

0.8, 1.6, 2.4 and 3.0 wt.%) were investigated by the optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and uniaxial tension test. The addition of 3.0 wt.% Al led to the most significant grain refinement of Mg-5Nd alloy with the reduction of average grain size from 448 µm to 68 µm. The grain refinement

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ACCEPTED MANUSCRIPT was attributed to the formation of Al2Nd particles as the Al2Nd particles acted as effective grain refiners for the Mg matrix confirmed by XRD analysis, SEM observation, and crystallography calculation. The mechanical properties of as-cast

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Mg-5Nd alloy increased obviously after the addition of Al. The yield strength, ultimate tensile strength and elongation of the as-cast Mg-5Nd-3.0Al alloy were enhanced by about 45.3%, 72.4% and 264.0%, respectively, compared to as-cast

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refined grains and secondary phase strengthening.

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Mg-5Nd alloy. The improvement of mechanical properties was mainly ascribed to the

Keywords

Mg-Nd alloys; Microstructure; Al2Nd particles; Grain refinement; Mechanical

1. Introduction

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properties

Magnesium (Mg) alloys, as the lightest metal structural material, have excellent

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properties, such as low density, high specific strength, superior damping capacity,

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good electromagnetic shielding characteristics and excellent machinability [1-4]. Thus, Mg alloys exhibit great potential to be widely employed in various fields including automotive, electronics and aerospace industries. However, the large scale industrial applications of Mg alloys are limited by their undesirable strength and ductility [5]. Mg-RE (rare earth, RE) alloys have high mechanical properties and thus have achieved considerable interest in recent years [6]. Among various Mg-RE alloys, Mg-Nd

based

alloys

have

attracted

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more

and

more

attention,

e.g.

ACCEPTED MANUSCRIPT Mg-2.7Nd-0.6Zn-0.5Zr (wt.%) alloy [7] and Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr (wt.%) alloy [8] have been developed and they exhibit good mechanical properties. Hence, Mg-Nd based alloys appear to be promising for industrial applications.

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Additionally, grain refinement has been proven to be an effective approach to improve the castability, ductility and strength of alloys [9]. For as-cast Mg-RE alloys without Al, it has been realized that grain refinement can be greatly achieved through

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Zr addition. However, this approach still exposes many issues in industrial

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applications, like low utilization rate of Zr element, high price [10].

Therefore, numerous research works have been performed to develop low cost, new and effective grain refiners for Mg alloys. Recently, some studies revealed that the substantial grain refinement of Mg-RE alloys, such as Mg-Y [11], Mg-Gd [12],

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Mg-Sm [13], Mg-Ce [14], can be obtained by the addition of Al. This is attributed to the in-situ formation of Al2RE (Al2Y, Al2Gd, Al2Sm, or Al2Ce) particles which can act as nucleation sites for Mg matrix. Moreover, according to edge-to-edge matching

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(E2EM) model, Qiu et al. [15] predicted that some other Al2RE particles referring to

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Al2La, Al2Sc, Al2Dy and Al2Nd can also act as heterogeneous nucleation sites for Mg matrix. Although Al2Nd particle was generally observed in Mg-Al-Nd alloys [16-18], there was no systematic study about its grain refinement effect on Mg alloys. Therefore, Mg-5Nd-xAl alloys were designed in this study to study the effect of Al addition on grain refinement and mechanical properties of as-cast Mg-5Nd alloys in detail. In addition, it is expected that the Al refined Mg-5Nd alloys will exhibit better mechanical properties and have great potential for automotive and aerospace

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ACCEPTED MANUSCRIPT applications. The current work is conducted to clarify whether Al2Nd particles can act as heterogeneous nucleation sites for Mg matrix and whether the as-cast Mg-5Nd alloys

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added with Al element show the obvious grain refinement and excellent mechanical properties. The effect of Al contents on grain size of as-cast Mg-5Nd alloys was investigated. Crystallographic matching between Al2Nd and the α-Mg matrix was

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calculated. The mechanical properties of the Al refined Mg-5Nd alloy were also

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studied. The results of this study will provide a reference to understand the effect of different Al contents on Mg-5Nd alloy and develop a new grain refiner.

2. Experimental procedures

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Commercially pure Mg (99.99 wt.%), pure Al (99.99 wt.%) and Mg-30 wt.% Nd master alloys were used to prepare the Mg-5Nd-xAl (x = 0, 0.8, 1.6, 2.4 and 3.0 wt.%) alloys. All prepared materials were melted in an electric-resistant furnace under the

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protection of an anti-oxidizing flux mixed gas of CO2 and SF6. Pure Al was added into

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the Mg-5Nd melt at 730 °C, according to the stoichiometric ratio of the designed alloys. The melt was held at 730 °C for 30 min after the alloying elements were completely dissolved, and then the melt was poured into the cylindrical steel mold (φ 20×90 mm) preheated to 300 °C. After cooled to room temperature, the as-cast Mg-5Nd-xAl (x = 0, 0.8, 1.6, 2.4 and 3.0 wt.%) alloy bars were obtained. The actual chemical compositions were determined by an X-ray fluorescence spectrometer (XRF, 800CCDE), and the results were listed in Table 1.

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ACCEPTED MANUSCRIPT Table 1 The actual compositions of experimental alloys (wt.%). Nd

Al

Mg

Mg-5Nd Mg-5Nd-0.8Al Mg-5Nd-1.6Al Mg-5Nd-2.4Al Mg-5Nd-3.0Al

4.9730 4.8526 4.7711 5.0742 4.9884

0 0.7814 1.6186 2.3839 2.9757

Bal. Bal. Bal. Bal. Bal.

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Alloy

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All as-cast samples for optical microscopy (OM) observation were cut at the location of 10 mm from the bottom of the as-cast Mg-5Nd-xAl (x = 0, 0.8, 1.6, 2.4

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and 3.0 wt.%) alloy bars, mechanically polished, followed by etching in a picric ethanol solution (30 ml ethanol, 5 ml acetic and 2 g picric acid). The grain sizes were examined by an optical microscope (OM) under polarized light and were measured by a linear interception method. The microstructures of all samples were further

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examined by scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) detector. The phase identification was performed by an

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X-ray diffraction (XRD). Tensile test was conducted by a universal testing machine (NEW SANSI CMT-5105) at a tensile speed of 1 mm/min at room temperature. The

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gage dimensions were φ5×25 mm for tensile specimen. Four pieces of tensile specimen were tested for each alloy and the values were averaged in order to assure the accurate tensile properties. The statistical result of size distribution of Al2Nd particles at grain centers was measured by image analysis software of SmileView.

3. Results and discussion 3.1 Grain refinement of Mg-5Nd alloys

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dendrites with plenty of intermetallic compounds distributed at interdendrictic regions. It is shown in Fig. 1(a) that the grains of the as-cast Mg-5Nd alloy are coarse and their average grain size is about 488 µm. After the addition of 0.8 wt.% Al, as seen in Fig.

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1(b), the grain size of the as-cast Mg-5Nd alloy increases significantly from 448 µm

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to 879 µm, and the microstructure still consists of coarse dendrites and network intermetallic compounds. As the addition of Al into Mg-5Nd alloy increases to 1.6 wt.%, 2.4 wt.% and 3.0 wt.%, as shown in Fig. 1(c-e), the grains are refined gradually and the average grain size of the as-cast Mg-5Nd alloy decreases to 278 µm, 84 µm

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and 68 µm, respectively. The finest grains appear at the 3.0 wt.% Al addition into Mg-5Nd alloy. The variation in average grain size as a function of Al content in Mg-5Nd alloy (Fig. 1(f)) is quite similar to that of Mg-6Sm [13] and Mg-6Ce [14]

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binary alloys with Al additions.

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Fig.1. Optical micrographs of as-cast Mg-5Nd alloys with different Al contents: (a) 0 wt.%, (b) 0.8 wt.%, (c) 1.6 wt.%, (d) 2.4 wt.%, (e) 3.0 wt.% and (f) variation in average grain size as a function of Al content.

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3.2 Phase identification and microstructure analysis

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Fig. 2 shows that the XRD patterns of the as-cast Mg-5Nd alloys with different Al contents. From the diffraction pattern of the as-cast Mg-5Nd alloy (Fig. 2(a)), it can be observed that the phases are comprised of α-Mg and Mg12Nd. The Mg12Nd phase

is

also

observed

in

Mg-4.2Y-2.5Nd-1Gd-0.6Zr

alloy

[19]

and

Mg-2.0Nd-0.3Zn-1.0Zr alloy [20]. When Al is added to Mg-5Nd alloy, Al-Nd intermetallic compounds is more likely to form than Mg-Nd and Mg-Al compounds, because standard molar enthalpy of formation of Al-Nd compounds is much lower

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When Al content is above 1.6 wt.%, Al2Nd compound appears and its amount increases gradually, while the content of Mg12Nd compound decreases gradually and Mg12Nd is hardly observed in Mg-5Nd-3.0Al alloy. Therefore, according to the XRD

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analysis, the evolution of intermetallic compounds in Mg-5Nd alloy with Al addition

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is as follows: Mg12Nd (Mg-5Nd alloy) → Mg12Nd + Al11Nd3 (Mg-5Nd-0.8Al alloy) → Mg12Nd + Al11Nd3 + Al2Nd (Mg-5Nd-(1.6-2.4)Al alloy) → Al11Nd3 + Al2Nd

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(Mg-5Nd-3.0Al alloy).

Fig.2. XRD patterns of as-cast Mg-5Nd alloys with different Al contents: (a) 0 wt.%, (b) 0.8 wt.%, (c) 1.6 wt.%, (d) 2.4 wt.% and (e) 3.0 wt.%.

Fig. 3 shows the back-scattered electron (BSE) images of Mg-5Nd alloys with

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α-Mg + Mg12Nd. As seen in Fig. 3(a), the network eutectic Mg12Nd phase distributes along the interdendritic boundaries. Therefore, the microstructure of as-cast Mg-5Nd alloy consists of α-Mg primary phase and α-Mg+Mg12Nd eutectic.

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When 0.8 wt.% Al is added, the lamellar Al11Nd3 phase appears at interdendritic

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regions and partly replaces the eutectic Mg12Nd phase. As the amount of Al reaches to 1.6 wt.%, it can be seen that some polygonal Al2Nd particles occur inside the Mg matrix and the morphology of some Al11Nd3 phase transforms into acicular structure, as shown in Fig. 3(c). When Al content is between 2.4 wt.% and 3.0 wt.%, much more

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polygonal Al2Nd particles with much bigger size range can be observed, compared with the as-cast Mg-5Nd-1.6Al alloy. Meanwhile, the morphology of Al11Nd3 phase

gradually.

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fully transforms into acicular structure and the acicular Al11Nd3 phase widens

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With the addition of Al ranging from 1.6 wt.% to 3.0 wt.%, the location of polygonal Al2Nd phase and acicular Al11Nd3 phase is associated with phase formation reaction during solidification. According to the existed Al-Mg-Nd ternary phase diagram [23, 24] and the report of Zhang et al. [25], it is considered that the Al2Nd phase might be directly formed from the liquid phase prior to the formation of α-Mg phase. It is most likely that the peritectic reaction of L + Al2Nd → α-Mg occurs as the temperature reduces. Such similar phase formation reaction has also been found in

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ACCEPTED MANUSCRIPT Mg-Al-Y [26] and Mg-Al-Sm [27] alloy systems. Therefore, the nucleation of α-Mg is likely to conduct on the surface of Al2Nd [15, 28, 29]. In non-equilibrium cooling situations in this experiment, some Al2Nd particles still remain after solidification is

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finished and these Al2Nd particles are surrounded by α-Mg phase. Additionally, with further reducing the temperature, according to the Al-Mg-Nd ternary phase diagram [23, 24], the Al11Nd3 phase may form through eutectic reaction and results in obvious

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eutectic feature distributed along interdendritic boundaries. Therefore, the actual

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solidification process and the succession of the phase reactions of Mg-5Nd-3.0Al alloy during solidification can be assumed as L → L + Al2Nd → L + α-Mg → (α-Mg + Al11Nd3) + α-Mg, although more deep work is still needed to do. Fig. 4(a) shows a magnification image of typical microstructure of

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Mg-5Nd-3.0Al alloy, and some polygonal and acicular particles are observed in the microstructure. The EDS results (Fig. 4(b-c)) indicate that the atomic ratio of Al and Nd in the polygonal particle (marked as A in Fig. 4(a)) and the acicular particle

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(marked as B in Fig. 4(a)) is 2.05 and 3.55, close to the stoichiometric ratio of 2:1 and

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11:3, respectively. Combining with the results of XRD analysis (Fig. 2(b-e)), the polygonal and acicular particles are identified as Al2Nd and Al11Nd3 phase, respectively. These phases have been found in existed Al-Mg-Nd ternary phase diagram [23, 24] and Mg-Al-Nd alloys [30, 31].

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Fig.3. Back scattered electron (BSE) SEM images of as-cast Mg-5Nd alloys with different Al contents: (a) 0 wt.%, (b) 0.8 wt.%, (c) 1.6 wt.%, (d) 2.4 wt.% and (e) 3.0 wt.%.

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Fig.4. (a) High magnification BSE-SEM image of typical microstructure of Mg-5Nd-3.0Al alloy, (b) EDS spectrum of the polygonal particle(marked as A in (a)) and (c) EDS spectrum of the acicular particle (marked as B in (a)).

3.3 Grain refinement mechanism

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Crystallographic matching between the particles and the metal matrix is often

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used to identify whether the particles can act as effective nucleation sites, consequently resulting in the grain refinement [32]. The edge-to-edge matching (E2EM) model [33] was widely considered as a simple and effective approach to predict the orientation relationships (ORs) between nucleant particle and the matrix, which is consistent with experimental results [34-38]. Hence, this method is used to interpret the grain refinement mechanism in the present work. It is observed that some Al2Nd particles appear in the grain center, as shown in Fig. 3(c-e). It can be concluded

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ACCEPTED MANUSCRIPT that Al2Nd particles play a major role in grain refinement of Mg matrix. In order to identify that the Al2Nd particle can serve as nucleation site for α-Mg, it is necessary to verify whether Al2Nd particle has the good crystallographic matching with the Mg

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matrix. The E2EM model indicates that there should be at least a pair of close-packed atomic rows called matching rows, along which the interatomic misfit (fr) is less than

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10% between the particle and the matrix. Moreover, there should be one more pair of

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close-packed planes containing the close-packed atomic rows whose interplanar spacing mismatch (fd) is less than 6%, which are called matching planes. According to the crystallography database and X-ray powder diffraction, it is found that Al2Nd is face-centered cubic structure, its three close-packed planes are (111), (220), (311)

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planes, and those of Mg are (10-10), (0002), (10-11). Based on the calculation method [33], only a pair of the matching plane meets the E2EM requirement and only one interatomic misfit (fr) value of the matching planes is less than 10%, as shown in

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Table 2 and Table 3. The matching plane of Al2Nd and Mg matrix is {311}Al2Nd

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│{10-11}α-Mg , which has fd =1.6%. Possible matching direction with less than 10% interactomic misfit (fr) value between the plane of {311}Al2Nd and {10-11}α-Mg only includes the direction pair of <1-2-1> Al2Nd│<-2113>α-Mg, which has fr =7.8%. By combining the matching row pair with the plane pair containing the matching row, the orientation relationship (OR) is predicted as follows: [1-2-1]Al2Nd

[-2113] α-Mg (311) Al2Nd

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(10-11) α-Mg

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1.6

{10-11}α-Mg

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{311}Al2Nd

Mismatch (%)

Table 3 The interatomic spacing misfit (fr) (%) along the possible matching directions between the plane of {311}Al2Nd and {10-11}α-Mg.

Misfit (%)

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Direction pairs

7.8

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<1-2-1> Al2Nd <-2113>α-Mg

According to the E2EM model, the matching direction pair is parallel to each other but the matching plane pair will rotate about the direction. Hence, the roughly

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predicted OR can be further refined using the ∆g parallelism criterion [39, 40] to specify the rotation of the matching plane pair about the associated matching row.

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After being refined, the predicted OR between Al2Nd and Mg is obtained as follows: [1-2-1]Al2Nd

[-2113] α-Mg (311) Al2Nd 15.39º away from (10-11) α-Mg

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Fig. 5 shows the simulated spot diffraction patterns of Al2Nd and Mg matrix

along the [1-2-1]Al2Nd

[-2113] α-Mg zone axis with a set of parallel ∆gs. The calculated

result implies that the Al2Nd can act as effective heterogeneous nucleation site for Mg matrix.

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Fig.5. Simulated spot diffraction patterns of Al2Nd and Mg matrix along [1-2-1]Al2Nd

[-2113] α-Mg

zone axis, indicating a set of parallel ∆gs. Dashed line shows the habit plane.

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Generally, the final grain size of polycrystalline materials is mainly determined

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by both the potency of nucleant particles and the segregating power of the solute atoms (defined as the growth restriction factor, Q) [41, 42]. The larger Q value indicates the more powerful growth restriction will be and the higher possibility to form smaller grains. Hence, the effective approaches for grain refinement are composed of increasing potent nucleation sites and adding solute elements with higher Q value [41]. Adding 0.8 wt.% Al into Mg-5Nd alloy leads to grain coarsen, which is also

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ACCEPTED MANUSCRIPT observed in Mg-6Sm [13] and Mg-6Ce [14] alloys when small Al content is added. The possible reasons are as follows: (i) when the 0.8 wt.% Al is added into Mg-5Nd alloy, Al will react with Nd to form the Al11Nd3 compound, confirmed by the research

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of Zou et al. [43] about Mg-5Zn-2Al-xNd alloy. Additionally, Al11Nd3 compound has a melt point of about 1235 °C and exists stably in the alloy melt temperature of 730 °C [17, 44]. Therefore, the addition of Al into Mg-5Nd alloy will consume some

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of Nd content in Mg-5Nd alloy due to the formation of stable Al11Nd3 compound. Consequently, the concentration of Nd in the Mg-5Nd alloy melt will be reduced at

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some extent. According to the formula [41, 42] Q = ∑ mi coi (ki − 1) , where mi is the i

slope of the liquidus line on the binary phase diagram, ki is the solute distribution coefficient, and coi is the solute concentration of element i, the reduction of Nd

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concentration in Mg-5Nd alloy will decrease the Q value; (ii) the formed Al11Nd3 compound cannot act as heterogeneous nucleation site for α-Mg due to the large crystallographic mismatch between Al11Nd3 compound and α-Mg matrix [45], which

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is consistent with the E2EM calculation, although it is stable compound in the

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Mg-5Nd-0.8Al alloy melt. Consequently, with a minor addition of Al, Al11Nd3 compound with no heterogeneous nucleation potential will form and it consumes the Nd content which will decrease the Q value of Mg-5Nd alloy, and hence results in coarsened grains.

The remarkable grain refinement is achieved at 2.4~3.0 wt.% Al addition into Mg-5Nd alloy which are associated with the formation of Al2Nd particles. In addition, besides the lowest crystallographic mismatch between nucleant particles and the metal

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Fig.6. Size distribution of the active Al2Nd particles in Mg-5Nd-xAl (x = 0, 1.6, 2.4 and 3.0 wt.%) alloys.

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matrix, the size of active nucleant particles also plays a critical role in grain refinement. The size distributions of the active Al2Nd particles at the grain centers in

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Mg-5Nd-xAl (x = 1.6, 2.4 and 3.0 wt.%) alloys are shown in Fig. 6. It can be seen in Mg-5Nd-1.6Al alloy that about 72% of the active Al2Nd particles have the size of 2~4

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µm. On the contrary, in Mg-5Nd-2.4Al and Mg-5Nd-3.0Al alloys, over 75% of the active Al2Nd particles have the size range of 4~7 µm. Therefore, with the increase of Al addition, the larger active Al2Nd nucleant particles form. The similar results were also observed in the Mg-Y-Al [15, 46], Al-Zr [47], Mg-Sm-Al [13] and Mg-Ce-Al [14] alloys. Greer et al. [48, 49] elucidated that when nucleant particles are effective for the melt alloys, the critical supercooling for free growth to occur is inversely proportional to the nucleant particle size. This means that increasing the nucleant 17

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gradually on the smaller particles due to the increase of undercooling in the nucleation free zone caused by latent heat release and/or due to the constitutional supercooling (CS) produced by the segregation of the solute atoms. Therefore, the formation of a

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large amount of larger nucleant particles can promote the reduction of grain size.

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Besides, the grain refinement effect is also affected by the number density of active nucleant particles. The number density of possible active Al2Nd particles in the Mg-5Nd-xAl (x = 1.6, 2.4 and 3.0 wt.%) gradually increase. Therefore, significant grain refinement achieved in the Mg-5Nd-3.0Al alloy results from large amount of

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high potency Al2Nd particles promoting heterogeneous nucleation. 3.4 Mechanical properties

Fig. 7 shows the room temperature tensile properties of as-cast Mg-5Nd-xAl (x =

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0, 1.6, 2.4 and 3.0 wt.%) alloys by the true stress-strain curves marked with yield

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strength (YS), ultimate tensile strength (UTS) and elongation. For the Mg-5Nd alloy, the mechanical properties were poor and the YS, UTS and elongation were 64 MPa, 123 MPa and 2.5%, respectively. It is clear that the YS, UTS and elongation of Mg-5Nd alloys gradually increase with increasing the Al content from 1.6 wt.% to 3.0 wt.%. When the Al content increases to 3.0 wt.%, the Mg-5Nd-3.0Al alloy exhibits the best comprehensive mechanical properties with a UTS of 212 MPa, a YS of 93 MPa, and an elongation of 9.1%. The YS, UTS and elongation of Mg-5Nd-3.0Al alloy

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Mg-5Nd alloy.

Fig.7. The true stress-strain curve of as-cast Mg-5Nd-xAl (x = 0, 1.6, 2.4 and 3.0 wt.%) alloys.

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Usually, the change in mechanical property is closely related to the microstructure evolutions of alloys. In Mg-5Nd alloy, the continuous network Mg12Nd

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phase distributed along the interdendritic boundaries is prone to generate stress concentration and crack during the deformation process, leading to the poor

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mechanical property. Moreover, based on the well-known Hall-Petch relationship [50], the coarse grain size (about 488 µm) also weakens the mechanical properties. However, the Mg-5Nd alloys containing Al exhibit the higher strength and elongation, which is ascribed to three factors as follows: 1) grain boundary strengthening, 2) solid solution strengthening and 3) secondary phase strengthening. With the addition of 1.6 wt.% Al, the grain size of the alloy becomes smaller than that of as-cast Mg-5Nd alloy and the acicular Al11Nd3 phase as the main Al-Nd compounds can pin dislocation 19

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formation of Al11Nd3 and Al2Nd intermetallic compounds reduces the solubility of Nd in Mg alloy [45, 52], resulting in weak solid solution strengthening. It appears that the combination of increase in grain boundary strengthening and the Al2Nd and Al11Nd3

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intermetallic strengthening is higher than the loss of the solid solution strengthening.

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When the Al content is 2.4 wt.%, especially 3.0 wt.%, large amount of Al2Nd particles were formed, causing the extensive refinement of grain size from 448 µm to 68 µm. In addition, the morphology of acicular Al11Nd3 phase (Fig. 4) gradually involved into the reinforced-fiber with a diameter of 200-400 nm and a length to diameter ratio of

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over 50 widely observed in the nano-fiber strengthened composite materials [53, 54]. Such reinforcement can further enhance the strength and ductility. Therefore, the increase of the mechanical properties is mostly attributed to the grain refinement and

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the formation of the secondary phases like Al2Nd and Al11Nd3.

4. Conclusions

1. The grain size of Mg-5Nd alloy was remarkably refined by additions of 2.4~3.0 wt.% Al. The grain refinement was attributed to the heterogeneous nucleation role of Al2Nd particles for Mg-5Nd alloy, confirmed by the E2EM crystallography calculation. 2. The highest number density of active Al2Nd particles with a size of 4~7 µm could

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attributed to the combination effect of grain boundary strengthening and secondary phase strengthening.

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Acknowledgments

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The authors are grateful for the financial supports from the National Key Research and Development Program of China [2016YFB0301104]; the National Natural Science Foundation of China [51531002, 51474043]; and Chongqing Science and Technology Commission [CSTC2014jcyjjq0041, cstc2015zdcy-ztzx50003]; and

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Education Commission of Chongqing Municipality [KJZH14101]; and the

References:

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Fundamental Research Funds the Central Universities [106112016CDJZR138801].

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• The grains of Mg-5Nd alloy were refined by addition of Al. • The mechanical properties of Al refined Mg-5Nd alloys were improved.

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• Crystallographic matching between Al2Nd and α-Mg was calculated.

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