Utilization of VN particles for grain refinement and mechanical properties of AZ31 magnesium alloy

Utilization of VN particles for grain refinement and mechanical properties of AZ31 magnesium alloy

Journal of Alloys and Compounds 781 (2019) 1150e1158 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: htt...

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Journal of Alloys and Compounds 781 (2019) 1150e1158

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Utilization of VN particles for grain refinement and mechanical properties of AZ31 magnesium alloy Wei Qiu*, Zhiqiang Liu, Rongzong Yu, Jian Chen, Yanjie Ren, Jianjun He, Wei Li, Cong Li School of Energy and Power Engineering, Changsha University of Science and Technology, Changsha, Hunan 410114, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 October 2018 Received in revised form 7 December 2018 Accepted 8 December 2018 Available online 10 December 2018

The influence of VN particles on the grain refinement and mechanical properties of AZ31 magnesium alloy was investigated and the refining and reinforcing mechanisms were also discussed. These particles present remarkable grain refining efficiency on AZ31 alloy. Addition of 0.5 wt% VN into AZ31 alloy led to decreasing the average grain size from 115.7 mm to 62.4 mm. X-Ray Diffraction and energy dispersive spectroscopy showed the existence of AlN, which was in situ produced through the reaction of VN and Al. Based on the phase identification and disregistry calculation, it was confirmed that the refinement of AZ31 alloy was mainly ascribed to duplex phase composed of AlN and VN acting as nucleating substrates of a-Mg during solidification. Improved mechanical properties were achieved after addition of VN. Yield strength (YS) increased from 39.5 MPa to 50.3 MPa with addition of 1 wt% VN. 0.5 wt% VN into AZ31 alloy exhibited the best mechanical properties with a YS of 47.1 MPa, an ultimate tensile strength (UTS) of 197.4 MPa and an elongation (EI) of 17.8%, enhancing the AZ31 alloy by about 19.2%, 40.3% and 67.9%, respectively. © 2018 Elsevier B.V. All rights reserved.

Keywords: AZ31 alloy VN AlN Microstructure Grain refinement Mechanical properties

1. Introduction As the lightest structural metal, Mg alloys, especially Mg-Al based alloys (AZ31, AZ61 and AZ91), are widely used in fields of automotive, aerospace and telecommunication industries [1e3]. However, polycrystalline material needs to activate five independent slip systems in order to deform without cracks at room temperature, but Mg only possesses three slip systems from the basal plane due to the hexagonal closed packed (HCP) crystalline structure, which resulted that magnesium alloys usually exhibited a poor ductility and formability at ambient temperature, hindering their practical application as structure components [4,5]. Recent studies have focused on how to improve its strength and ductility, and it is well documented that the strength of magnesium alloys are directly related to grain size [6]. A reduction in grain size can enhance the ductility of magnesium alloys without sacrificing their strength [7,8], so various potential grain refiners were employed on casting Mg-Al based alloys such as carbon inoculation [9e12], adding alloying elements [13e17] and adding particles etc. The most commonly accepted theory for carbon inoculation on the

* Corresponding author. E-mail address: [email protected] (W. Qiu). https://doi.org/10.1016/j.jallcom.2018.12.124 0925-8388/© 2018 Elsevier B.V. All rights reserved.

grain refinement is that Al4C3 act as heterogeneous nuclei for the aMg phase during solidification [11]. For example, Han et al. [10] reported that grain size of AZ31 alloys with ball milled Al-C deceased from 850 mm to 180 mm. Liu et al. [12] added a new Mg50%Al4C3 master alloy into an AZ91 alloy and achieved remarkable grain refinement. Alloying elements like Sr, Ca, Sn and Sm with a high growth restriction factor Q can also refine the grains by providing rapid development of constitutional supercooling, which restricts the growth of existing grains allowing additional nucleation events to occur [13]. For example, Sr addition can improve the GRF (grain growth restriction factor) values of Al and Zn to the AZ31 magnesium alloys and hence improve the grain refinement efficiency by Al and Zn [16]. The mechanical improvements caused by adding alloying elements can be ascribed to the refined grains and the intermetallic phases formed in the alloys. Particle reinforcements such as the employment of AlN [18e23], SiC [24e26] and TiB2 [27e29] are found to be simpler and more effective regarding the grain refining. Compared with SiC and TiB2, AlN as a novel ceramic material has a simple HCP structure with lattice parameters of a ¼ 0.3113 nm and c ¼ 0.4981 nm, which are

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very close to the lattice parameters of the Mg matrix with HCP structure of a ¼ 0.3209 nm and c ¼ 0.5211 nm [18]. The disregistry between Mg and AlN is approximately 3.1% between the (1010) planes and 2.7% along the (0001) planes, suggesting that growth of Mg on an AlN particle is relatively easy and does not require accommodation of large strain, and the differences in lattice parameter can be overcome locally in the first layers of Mg that grow on the AlN particles [23]. Besides, AlN is very stable in molten magnesium alloys, which means that it can provide a clear interfacial bond between matrix and reinforcement and act as potential heterogeneous nucleation agents of a-Mg during solidification. Moreover, it is worth noting that AlN powder is available commercially at low cost and process of preparing the master alloy is simple [19]. H.M. Fu et al. [19] reported that an addition of 0.5 wt % AlN reduced the grain size of Mg-3wt.% Al alloy from 450 mm to 120 mm at melted temperature of 765  C, but the mechanism of grain refinement was not explained in detail. To improve the wear resistance of magnesium matrix and AlN particles and make it homogeneously distributed in the matrix, in situ synthesis process is considered an attractive alternative to fabricate Mg-Al alloy reinforced by AlN [18,20,22]. C.L. Yang et al. [20] fabricated in-situ AlN/AZ91 composites prepared by liquid nitriding method, where AlN particles were produced in the alloy melt by N2 þ 3Mg/Mg3 N2 and Mg3 N2 þ 2Al ¼ 2AlN þ 3Mg, and reported that the grain sizes of a-Mg were significantly refined and the ultimate tensile strength of experimentally prepared alloys increased from 143 MPa to 240 MPa. However, the bubbling nitrogen gas method involved complex equipment and procedures, thus imposing a relatively high cost. H.L. Zhao et al. [22] reported an experimental study on the in situ formation of hard AlN particles in AZ31 alloy by externally adding Mg3N2 powders, and results showed the grains were refined and the ultimate tensile strength of AZ31 Mg alloy increased up to 198.7 MPa. However, the elongation just slightly increased, which is 11.8%. Therefore, it is required that other compounds be utilized for in situ producing AlN, and VN could be one of the promising agent. In this work, VN particles for grain refinement and mechanical enhancement of AZ31 alloy were investigated. The reaction of in situ producing AlN in AZ31 alloy containing VN was discussed. The refining and enhancing mechanisms were also discussed.

2. Experimental AZ31 alloy with the addition of 0, 0.5, 1 and 2 wt% VN particles were respectively fabricated by melting commercial Mg (>99.9%), Al (>99.9%), Zn (>99.9%), Mn (>99.9%) and addition of VN powder (1 mm in diameter and wrapped with aluminum foil) together in the electromagnetic induction furnace. The melt was manual stirred at 750  C for 15 min and held for 10 min, then poured into graphite mould under a CO2 þ SF6 atmosphere. To facilitate the microstructure observation, samples cut at the location of 10 mm from the bottom of the as-cast alloys were subjected to solution treatment at 400  C for 24 h followed by cold water quenching. Samples for optical microscope (OM) observation were initially ground using different grades of polishing paper, and subsequently polished with 0.5 mm diamond paste and then etched in a solution of 1 ml nitric acid, 1 ml acetic acid, 1 ml oxalic acid and 97 ml distilled water. Microstructure analysis was characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The phases in the experimentally prepared alloys and VN particles were identified by X-ray diffraction (XRD) using monochromatic CuKa radiation. The tensile properties of the as-cast samples prepared as per ASTM E8 specifications from different castings were evaluated using a universal

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testing machine at a tensile speed of 1 mm/min at room temperature and ten samples were tested for each alloy. 3. Results and discussion 3.1. Microstructure and phase analysis Fig. 1 presents the XRD pattern of VN particles. Fig. 2 presents XRD patterns of as-cast AZ31 alloy with and without VN particle additions. The AZ31 alloy was mainly composed of a-Mg. Second phase Mg17Al12 could not be detected owning to the very low content. Extra peaks were identified to be AlN with VN into AZ31 alloy. In addition, both AlN and VN were identified In AZ31 alloy containing 2 wt% VN, as shown in Fig. 2. It is worth noting that mainly AlN rather than VN was identified in experimentally prepared alloys. Fig. 3 shows the as-cast (a-d) and solution treated (e-h) optical microstructure of AZ31 alloys with different contents of VN. It should be emphasized that the grain boundary became more distinct after solution treated, providing a clear vision on the grain size. Fig. 4 illustrates the variation of the average grain size measured using the linear intercept method. As seen in Fig. 3(a,e), the grains of AZ31 alloy without VN are coarse and their average grain size is about 115.7 mm. However, it decreased dramatically to 62.4 mm, 67.5 mm and 69.8 mm with 0.5, 1 and 2 wt% VN into AZ31 alloy, respectively. The finest grains appeared in AZ31 alloy containing 0.5 wt% VN. Nevertheless, it indicates that no further refinement in the grains would be generated with increasing the VN concentration above 1 wt%. In addition, black spots observed by OM segregated along the grain boundary with addition lever of 1 wt% and 2 wt%, which are indicated with the white circle in Fig. 3(c-d,g-h). To further investigate the second phases especially the segregated spots appeared in Fig. 3(c and d), microstructure of AZ31 alloy containing VN particle additions were characterized by SEM with EDS. Fig. 5(a) shows the SEM image of as-cast AZ31 alloy, it can be seen that the white coarse phase were distributed along the boundaries. Some of them were point-analyzed by EDS and the results are illustrated in Table 1. It can be seen that spot 1 and spot 2 are mainly rich in Al. Ref. [1] shows the similar microstructure of AZ31 alloy, the second phase was identified to be Mg17Al12. Fig. 5(b) shows the SEM image of as-cast AZ31 alloy containing 0.5 wt% VN. It can be seen that the white coarse phases along the grain boundary were replaced by small rod like precipitates (marked as 3,

Fig. 1. XRD pattern of VN particles.

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Fig. 2. XRD patterns of as-cast alloys. (aed) AZ31 þ xwt.%VN (x¼0, 0.5, 1, 2).

4 and 5) homogeneously dispersed within grains. Table 1 shows the elements composition of spots 3, 4 and 5. It is found that spot 3 is rich in Al and Mn, and spot 4 is rich in Al and N, and spot 5 is rich in Al, V and N. The results above indicate that the introduced VN particles led to the refined second phases and the increasing Al-Mn and Al-N and Al-V-N phases. Liu et al. [12] reported that Al-Mn phases in AZ31 alloys with carbon additions were believed to be Al0.89Mn0.11, the atomic ratio of which is close to EDS result in Table 1. According to Fig. 1, it is conformed that the Al-N phase is AlN. Fig. 5(c and d) shows the SEM image of as-cast AZ31 alloy containing 1 wt% and 2 wt% VN, where some segregated irregular phases were found among the grains. Fig. 6(a) shows the high magnification SEM image of as-cast AZ31 alloy containing 1 wt% VN, where some small spots (marked as 6 and 7) were point-analyzed by EDS and the results are illustrated in Table 1 and the elementary composition is similar to that of spots 3 and 4, respectively. Fig. 6(b) shows the magnified coarse irregular phase observed in Fig. 6(a). It actually consists of some large stone-shaped particles (marked as 9) surrounded by clusters (marked as 8). According to Table 1, spot 9 is rich in V and spot 8 is rich in Al and N. It indicates that the stone-shaped particle might be elementary V and the clusters might be Al-N compound. Fig. 6(c) shows the surface scanning elements distribution in Fig. 6(a), which further confirmed the elementary composition of the precipitates and the large irregular phases. Based on Figs. 2, 5, and 6 and Table 1, it is presumed that VN particles react with Al producing AlN in the melts according to the following reaction:

VN þ Al ¼ AlN þ V

(1)

The varieties of Gibbs free energy (DG) and reaction formation enthalpy (DH) based on Reaction (1) were theoretically calculated according to the thermodynamic data [30], results of which are shown in Fig. 7 as a function of temperature. In the temperature range of 700  C ~ 850  C, varieties of Gibbs free energy (DG) and

reaction formation enthalpy (DH) are in the range of 82.30 kJ/ mol ~ 78.21 kJ/mol and 116.77 kJ/mol ~ -115.78 kJ/mol, respectively, indicating that the reaction might take place spontaneously to generate AlN at alloy melted temperature. In other words, AlN is more stable than VN in Al-V-N system. According to Fig. 2, the majority of VN particles were consumed by Al in the alloy containing 0.5 wt% and 1 wt% VN, thus VN could not be identified. However, when more VN particles were added to AZ31 alloy, it was identified. Lee et al. [31] reported Al content is very important to refine Mg-Al alloys, and increasing the Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5%wt Al. The consumption of Al to form AlN would deteriorate the grain refining effect of VN particles addition. So the grain size reached a relatively constant with increasing the content of VN particles.

3.2. Grain refining mechanism It is well known that the grain refinement of polycrystalline materials is mainly determined by enhancing the nucleation rate in the melt and/or reducing the growth of grains [32]. Therefore, the main approaches of grain refinement are adding potent nucleating agent, increasing undercooling and holding back grain boundary sliding. In this work, three kinds of compound are likely to be potent nucleating substrates for primary a-Mg, which are Al0.89Mn0.11, AlN and VN. As demonstrated in this section, Al0.89Mn0.11, which has great thermal stability and is homogeneously distributed within the grains in the alloy containing VN, seems to be an effective nucleating substrate. However, Liu et al. [12] proved that it had low nucleating efficiency in AZ31 alloy. Just like Al0.89Mn0.11, AlN is also homogeneously distributed within the grains, indicating that AlN might serve as an effective nucleation site for the primary a-Mg, thus leading to the refinement in AZ31 alloy containing VN.

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Fig. 3. Optical microstructure of experimentally prepared alloys. (aed) As-cast AZ31 þ xwt.%VN (x¼0, 0.5, 1, 2) alloys; (eeh) Solution treated AZ31 þ xwt.%VN (x¼0, 0.5, 1, 2) alloys.

VN has a FCC structure with lattice parameters of a ¼ 0.4137 nm [33]. According to the disregistry model of two-dimensional lattices proposed by Bramiftt [34], the formula is as following:

s dðhklÞ ðhklÞn

( ) 3 . X ¼ jd½uvwi cos q  d½uvwi j d½uvwi =3  100% i¼1

s

n

(2)

n

where ðhklÞs and ðhklÞn are the low index planes of the matrix and nucleus, respectively, ½uvws and ½uvwn are the low index orientations in ðhklÞs and ðhklÞn , respectively, d½uvws and d½uvwn are atomic spacing distances along ½uvw, and q is the angle between ½uvws and ½uvwn . Fig. 8 shows examples of three crystallographic relationships for a-Mg and VN used for planar disregistry calculations,

results of which are illustrated in Table 2. It can be seen that the smallest disregistry between a-Mg and VN is 6.85% while their crystallographic orientation relationship is (0001)a-Mg//(111)VN, which is less than 10%. Fig. 9 shows Surface scanning elements distribution of VN particle in AZ31 alloy containing 0.5 wt% VN. Discussion above means that the unreacted VN particles can also act as nucleation of a-Mg during solidification. Based on the phase identification and disregistry calculating results, it is confirmed that duplex phases composed of VN and AlN act as nucleation substrates during solidification of a-Mg. Liu et al. [24] also concluded that duplex phases Al4C3 and SiC acted as nucleation of a-Mg. It should be noted that AlN and VN have a melting point of 2200  C and 2320  C [30], respectively, which are much higher than

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W. Qiu et al. / Journal of Alloys and Compounds 781 (2019) 1150e1158 Table 1 EDS analysis results of spots in Figs. 5 and 6 (at%). Fig. 5(a)

Mg Al Zn Mn V N

Fig. 4. Effect of VN addition rate on average grain size of AZ31 alloy.

experimental temperature. Hence it is believed that the refiners are in the solid state before casting or at least solidify prior to a-Mg during solidification process. On the one hand, as discussed above, the small homogeneously distributed refiners are in the solid state, providing a solid-liquid interface for a-Mg in the liquid state to solidify. The efficiency of solidification is related to the nucleation undercooling and particle size, on which more work is still needed to do to understand it deeply. On the other hand, the observed Al-VN compounds with slightly larger size have great thermal stability, restraining the growth of grains during solidification. As seen in Fig. 3(c-d,g-h), it is obvious that areas containing these particles have finer grains. 3.3. Mechanical properties Fig. 10 shows the engineering stress-engineering strain curve and corresponding mechanical properties of as-cast AZ31 þ xwt.%

Fig. 5(b)

Fig. 6(a)

Fig. 6(b)

1

2

3

4

5

6

7

8

9

74.64 19.62 5.32 0.42 e e

66.65 27.87 5.42 0.06 e e

70.26 26.03 0.42 3.18 0.09 0.12

40.13 23.38 1.06 1.09 0.04 34.30

57.49 17.97 0.52 0.37 10.18 13.47

62.86 31.22 0.41 4.26 0.13 1.12

68.57 16.97 0.99 0.38 0.11 12.98

53.89 18.88 0.28 0.21 4.31 22.43

24.44 3.83 0.81 0.25 68.43 2.24

VN (x¼0, 0.5, 1, 2) alloys at room temperature. As seen in Fig. 10, the tensile properties of AZ31 alloy are poor and its YS, UTS and elongation are 39.5 MPa, 140.7 MPa and 10.6%, respectively. It can be found that the addition of VN particles effectively improved the tensile properties. AZ31 containing 0.5 wt% VN exhibited the best comprehensive properties with a YS of 47.1 MPa, an UTS of 197.4 MPa and an elongation of 17.8%, enhancing the AZ31 alloy by about 19.2%, 40.3% and 67.9%, respectively. When addition of VN is 1 wt%, YS reached their peak value 50.3 MPa, enhancing the AZ31 alloy by about 27.3%, but the elongation decreased dramatically compared with AZ31 containing 0.5 wt% VN. However, after 2 wt% VN particles were added into AZ31 alloy, YS still keep a high value while both UST and elongation decreased, which is even worse than unrefined AZ31 alloy. Important factors such as the grain size [6,35] can strongly affect on the change of mechanical properties. A reduction in grain size serves to enhance yield strength as formulated by well-known Hall-Petch relationship [6]:

sy ¼ s0 þ kd1=2

(3)

where sy is the yield stress, s0 is the friction stress when dislocations glide on the slip plane, d is the average grain size, and k is the stress concentration factor. According to Equation (3), the smaller of the average grain size, the better of the yield strength. A HallPetch diagram is shown in Fig. 11, which presents the relationship

Fig. 5. SEM image of as-cast alloys. (aed) AZ31 þ xwt.%VN (x¼0, 0.5, 1, 2) alloys.

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Fig. 6. SEM image of (a) as-cast AZ31þ1 wt%VN; (b) Magnified image of the white rectangle area in image (a); (c) Surface scanning elements distribution in image (a).

Fig. 7. The varieties of Gibbs free energy (DG) and reaction formation enthalpy (DH) as a function of temperature for the reaction (1).

between the yield strength and the reciprocal of the square root of grain size (d) for as-cast AZ31 and AZ31 containing VN particles. It is found from the Hall-Petch fitted curve that s0 is 15.5 MPa and k is 257.3 MPa mm1/2. Accordingly, the strengthening contribution from grain size reduction can be calculated by equation blow [36]:



DsGR ¼ k d1 1=2  d0 1=2



(4)

where d0 and d1 are the grain size in the reinforced and unreinforced alloy, respectively. Taking the grain size from Fig. 4 and k to be 257.3 MPa mm1/2, the strengthening contribution from grain size according to Equation (4) is 8.63 MPa, 7.36 MPa and 6.88 MPa for the as-cast AZ31 alloys containing 0.5, 1 and 2 wt% VN, respectively, which is very close to the measured yield strength increase.

On the other hand, secondary phase strengthening can also explain the improvement on mechanical properties [37]. As shown in Fig. 5(a), the coarse second phases Mg17Al12 distribute along the boundaries, which is prone to generate stress concentration and crack, thus leading to the poor mechanical properties. However, after VN particles were added into AZ31 alloy, both quantity and size of Mg17Al12 decreased, reducing the stress concentration. Moreover, some of the refined rod-like Mg-Al second phase and increased Al-Mn, Al-N and Al-V-N compounds found both inside and on the boundaries could impede dislocation movement during deformation, giving the as-cast AZ31 alloy considerable properties due to the precipitation strengthening. As the hexagonal metals, it is well known that Mg alloy only possesses three slip system from the basal (0002) and due to the large difference in critical resolved shear stress (CRSS) between the basal and non basal slip system, cracking tend to occur before the non-basal slip system being activated, leading to a low deformability [4,38]. The grain refinement is the most effective method to improve the ductility due to more crystal orientation distribution after grain refining [18,39]. Table 3 illustrated the XRD peakintensity ratio of as-cast AZ31 containing 0, 0.5, 1 and 2 wt% VN, where it is found that the peak-intensity ratio of basal plane (0002) decreased while pyramidal plane (10e11) increased, suggesting that there are more grains whose basal plane inclined to loading direction after addition of VN (the surface of XRD specimen is vertical to loading direction). There is a certain angle between basal plane of most grains and loading direction, which indicates most grains are soft grains or they are not hard grains. It has been reported that the Al-Sr particle, W-phase and CNTs could lead to a weakened texture in Mg alloys [40e42], indicating that the increasing intensity ratio of plane (10e11) caused by adding VN particles may result in weakening the basal texture and lead to high Schmid factor and high resolved shear stress in the basal plane, hence basal plane slip systems will activated easily to give better

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Fig. 8. Examples of three crystallographic relationships for a-Mg and VN at different matching interface for planar disregistry calculations. (a) ð0001ÞaMg ==ð100ÞVN ; (b) ð0001ÞaMg ==ð110ÞVN ; (c) ð0001ÞaMg ==ð111ÞVN .

Table 2 Calculated values of planar disregistry between a-Mg and VN phase. Matching interface

ð0001ÞaMg ==ð100ÞVN

½uvwaMg

½1210 ½010

½1010 ½001

½2110 ½011

0 20.8%

0

15

½uvwVN

q=ð Þ ½hkl

d½hklaMg

ð0001ÞaMg ==ð110ÞVN ½1210 ½110 0 31.2%

½1010 ½001 0

ð0001ÞaMg ==ð111ÞVN ½2110

½1210

½1120

½2110

½112 6.15

½011 0 6.85%

½110 0

½101 0

VN

Fig. 9. Surface scanning elements distribution of VN particle in AZ31 alloy containing 0.5 wt% VN.

ductility to the alloys with VN addition than as-cast AZ31 alloy, on which further work like EBSD is underway. In addition, as is well known, the precipitates in alloys play an important role in mechanical properties. The Mg17Al12 detrimentally influences the ductility of the Mg-Al alloy because Mg17Al12 is a very hard and brittle phases [43]. The shape, size and distribution of Mg17Al12 will change, normally improve mechanical properties of Mg alloys after heat treatment or alloying [15,17]. In the present work, size of the Mg17Al12 phase decreased and shape changed after VN was added into AZ31 alloy, hence decreasing the stress concentration at the crack front which initiates near Mg17Al12 phase, and resulting increase of ductility. As mentioned above, the grain refining of Mg alloys resulted from the interaction of the AlN, VN particles and the Al content. The alloys present similar grain size and yield strength after addition of VN. However, the ultimate strength and elongation increased first then decreased with increasing the content of VN particles from 0.5 wt% to 2 wt%. The large segregated Al-V-N compound generate after smaller one clustering together as shown in Fig. 10(d), which

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Fig. 10. Engineering stress-engineering strain curve (a) and corresponding mechanical properties (bed) of as-cast AZ31 þ xwt.%VN (x¼0, 0.5, 1, 2) alloys at room temperature; (b) Yield strength; (c) Ultimate tensile strength; (d) Elongation.

ultimate strength and the ductility. 4. Conclusions Remarkable refinement was achieved by adding VN particles into AZ31 alloy. The finest grain appears in AZ31 alloy containing 0.5 wt% VN particles. AlN were in situ produced through the reaction of VN and Al and homogeneously distributed within grains. Duplex phases composed of AlN and VN act as nucleation substrates during solidification of a-Mg. Improved mechanical properties were also achieved. Yield strength (YS) increased from 39.5 MPa to 50.3 MPa with addition of 1 wt% VN. 0.5 wt% VN into AZ31 alloy exhibited the best mechanical properties with a YS of 47.1 MPa, an UTS of 197.4 MPa and an elongation of 17.8%. Acknowledgements Fig. 11. Hall-Petch plot of yield strength over the reciprocal of the square root of grain size d following Equation (3).

Table 3 Comparison of XRD peak-intensity ratio of AZ31 containing VN. Crystal plane

References

Peak-intensity ratio AZ31

AZ31 þ 0.5 wt%VN

AZ31þ1 wt%VN

AZ31þ2 wt%VN

13.6

2.4

23.4

17.1

ð1011Þ

100 45.8

13.8 100

11.1 100

52.2 100

ð1012Þ

5.4

9.1

3.3

14.3

ð1120Þ

11.3

4.7

2.7

7.9

ð1013Þ

16.8

54.8

10.6

13.7

ð1010Þ ð0002Þ

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 51301025 and 51141001).

promoted the dislocations pile-up and stress concentration. Initiation and propagation of cracks led to the adverse effect on the

[1] S.C. Wang, C.L. Gan, X.H. Li, K.H. Zheng, W.J. Qi, Microstructure of Al-4.99Zr1.1B master alloy and its grain refinement effect on AZ31 magnesium alloy, Rare Metal Mater. Eng. 43 (2014) 2567e2571. [2] L.Q. Wu, R.Z. Wu, L.G. Hou, J.H. Zhang, M.L. Zhang, Microstructure, mechanical properties and wear performance of AZ31 matrix composites reinforced by graphene nanoplatelets (GNPs), J. Alloys Compd. 750 (2018) 530e536. [3] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, et al., What is going on in magnesium alloys? Mater. Sci. Technol. 34 (2018) 245e247. [4] U.M. Chaudry, T.H. Kim, S.D. Park, Y.S. Kim, K. Hamad, J.G. Kim, Effects of calcium on the activity of slip systems in AZ31 magnesium alloy, Mater. Sci. Eng. A 739 (2019) 289e294. [5] T.Z. Han, G.S. Huang, Q.Y. Deng, G.G. Wang, B. Jiang, et al., Grain refining and mechanical properties of AZ31 alloy processed by accumulated extrusion bonding, J. Alloys Compd. 745 (2018) 599e608. [6] H.H. Yu, Y.C. Xin, M.Y. Wang, Q. Liu, Hall-Petch relationship in Mg alloys: a

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review, Mater. Sci. Technol. 34 (2018) 248e256. [7] R. Zheng, T. Bhattacharjee, A. Shibata, T. Sasaki, K. Hono, M. Joshi, N. Tsuji, Simultaneously enhanced strength and ductility of Mg-Zn-Zr-Ca alloy with fully recrystallized ultrafine grained structures, Scr. Mater. 131 (2017) 1e5. [8] S.M. Fatemi, A. Zarei-Hanzaki, Microband/twin recrystallization during back extrusion of AZ31 magnesium, Mater. Sci. Eng. A 708 (2017) 230e236. [9] G. Han, X.F. Liu, Duplex nucleation in Mg-Al-Zn-Mn alloys with carbon inoculation, J. Alloys Compd. 487 (2009) 194e197. [10] H.M. Ding, X.F. Liu, The grain refinement efficiency of Ni-C on Mg-Al alloys, Mater. Lett. 63 (2009) 635e637. [11] M. Suresh, A. Srinivasan, U.T.S. Pillai, B.C. Pai, The effect of charcoal addition on the grain refinement and ageing response of magnesium alloy AZ91, Mater. Sci. Eng. A 528 (2011) 8573e8578. [12] S. Liu, Y. Chen, H. Han, Grain refinement of AZ91D magnesium alloy by a new Mg-50%Al4C3 master alloy, J. Alloys Compd. 624 (2015) 266e269. [13] X.Y. Hu, P.H. Fu, D. StJohn, L.M. Peng, M. Sun, M.X. Zhang, On grain coarsening and refining of the Mg-3Al alloy by Sm, J. Alloys Compd. 663 (2016) 387e394. [14] A.M. ZHANG, H. HAO, X.T. LIU, X.G. ZHANG, Effects of precipitates on grain size and mechanical properties of AZ31-x%Nd magnesium alloy, J. Rare Earths 32 (2014) 451e457. [15] Y.A. Chen, L. JIN, D. Fang, Y. Song, R.Y. Ye, Effects of calcium, samarium addition on microstructure and mechanical properties of AZ61 magnesium alloy, J. Rare Earths 33 (2015) 86e92. [16] R.J. Cheng, F.S. Pan, S. Jiang, C. Li, B. Jiang, X.G. Jiang, Effect of Sr addition on the grain refinement of AZ31 magnesium alloys, PNS:MI 23 (2013) 7e12. [17] H. Lin, M.B. Yang, H. Tang, F.S. Pan, Effect of minor Sc on the microstructure and mechanical properties of AZ91 magnesium alloy, PNS:MI 28 (2018) 66e73. [18] C.L. Yang, H.B. Lü, G.Y. Chen, F. Liu, In situ synthesis and formation mechanism of AlN in Mg-Al alloys, Rare Metal Mater. Eng. 45 (2016) 18e22. [19] H.M. Fu, M.X. Zhang, D. Qiu, P.M. Kelly, J.A. Taylor, Grain refinement by AlN particles in Mg-Al based alloys, J. Alloys Compd. 478 (2009) 809e812. [20] C.L. Yang, B. Zhang, D.C. Zhao, H.B. Lü, T.G. Zhai, F. Liu, Microstructure and mechanical properties of AlN particles in situ reinforced Mg matrix composites, Mater. Sci. Eng. A 674 (2016) 158e163. [21] J. Chen, C.G. Bao, W.H. Chen, L. Zhang, J.L. Liu, Mechanical properties and fracture behavior of Mg-Al/AlN composites with different particle contents, Mater. Sci. Technol. 33 (2017) 668e674. [22] H.L. Zhao, Z.X. Zhou, X.D. Liu, S.K. Guan, Influence of Mg3N2 powder on microstructures and mechanical properties of AZ31 Mg alloy, J. Cent. S. Univ. Technol. 04 (2008) 459e462. cs, M. Horstmann, M. Wolff, et al., [23] H. Dieringa, L. Katsarou, R. Buzolin, G. Szaka Ultrasound assisted casting of an AM60 based metal matrix nanocomposite, its properties, and recyclability, Metals 7 (2017) 388. [24] Y.H. Liu, X.F. Liu, X.F. Bian, Grain refinement of Mg-Al alloys with Al4C3-SiC/Al master alloy, Mater. Lett. 58 (2004) 1282e1287. [25] K.K. Deng, K. Wu, Y.W. Wu, K.B. Nie, M.Y. Zheng, Effect of submicron size SiC particulates on microstructure and mechanical properties of AZ91 magnesium matrix composites, J. Alloys Compd. 504 (2010) 542e547.

[26] X.J. Wang, W.Q. Liu, X.S. Hu, K. Wu, Microstructural modification and strength enhancement by SiC nanoparticles in AZ31 magnesium alloy during hot rolling, Mater. Sci. Eng. A 715 (2018) 49e61. [27] H.Y. Wang, Q.C. Jiang, Y. Wang, B.X. Ma, F. Zhao, Fabrication of TiB2 particulate reinforced magnesium matrix composites by powder metallurgy, Mater. Lett. 58 (2004) 3509e3513. [28] P. Xiao, Y.M. Gao, X.R. Yang, F.X. Xu, C.C. Yang, B. Li, Y.F. Li, Z.W. Liu, Q.L. Zheng, Processing, microstructure and ageing behavior of in-situ submicron TiB2 particles reinforced AZ91 Mg matrix composites, J. Alloys Compd. 764 (2018) 96e106. [29] S.F. Liu, Y. Zhang, H. Han, B. Li, Effect of Mg-TiB2 master alloy on the grain refinement of AZ91D magnesium alloy, J. Alloys Compd. 487 (2009) 202e205. [30] D.L. Ye, Thermodynamics Date Handbook of Practical Inorganic Matter, Metallurgical Industry Press Beijing, 1981. [31] Y.C. Lee, A.K. Dahle, D.H. StJohn, The role of solute in grain refinement of magnesium, Metall. Mater. Trans. A 31 (2000) 2895e2906. [32] Y. Ali, D. Qiu, B. Jiang, F.S. Pan, M.X. Zhang, Current research progress in grain refinement of cast magnesium alloys: a review article, J. Alloys Compd. 619 (2015) 639e651.  ski, A. Urbanowicz, W. Gulbin  ski, Preferentially oriented [33] T. Suszko, W. Gulbin vanadium nitride films deposited by magnetron sputtering, Mater. Lett. 65 (2011) 2146e2148. [34] B.L. Bramfft, The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron, Metall. Trans. 6 (1971) 1258. [35] Y.A. Chen, Y. Wang, J.J. Gao, Microstructure and mechanical properties of ascast Mg-Sn-Zn-Y alloys, J. Alloys Compd. 740 (2018) 727e734. [36] C.S. Kim, I. Sohn, M. Nezafati, J.B. Ferguson, B.F. Schultz, Z. Bajestani-Gohari, et al., Prediction models for the yield strength of particle-reinforced unimodal pure magnesium (Mg) metal matrix nanocomposites (MMNCs), J. Mater. Sci. 48 (2013) 4191e4204. [37] D. Liu, J.F. Song, B. Jiang, Y. Zeng, Q.H. Wang, Z.T. Jiang, B. Liu, G.S. Huang, F.S. Pan, Effect of Al content on microstructure and mechanical properties of as-cast Mg-5Nd alloys, J. Alloys Compd. 737 (2018) 263e270. € Duygulu, Plastic anisotropy and the role of non-basal slip in [38] S.R. Agnew, O. magnesium alloy AZ31B, Int. J. Plast. 21 (2005) 1161e1193. [39] W.D. Callister, D.G. Rethwisch, Materials Science and Engineering: an Introduction, ninth ed., John Wiley & Sons, Inc., New York, 2007. [40] A. Sadeghi, M. Hoseini, M. Pekguleryuz, Effect of Sr addition on texture evolution of Mg-3Al-1Zn (AZ31) alloy during extrusion, Mater. Sci. Eng. A 528 (2011) 3096e3104. [41] Q.F. Wang, K. Liu, Z.H. Wang, S.B. Li, W.B. Du, Microstructure, texture and mechanical properties of as-extruded Mg-Zn-Er alloys containing W-phase, J. Alloys Compd. 602 (2014) 32e39. [42] G.Q. Han, J.H. Shen, X.X. Ye, B. Chen, H. Imai, K. Kondoh, W.B. Du, The influence of CNTs on the microstructure and ductility of CNT/Mg composites, Mater. Lett. 181 (2016) 300e304. [43] B. Çiçek, H. Ahlatç, Y. Sun, Wear behaviours of Pb added Mg-Al-Si composites reinforced with in situ Mg2Si particles, Mater. Des. 50 (2013) 929e935.