Effect of Mg–10Sr master alloy on grain refinement of AZ31 magnesium alloy

Effect of Mg–10Sr master alloy on grain refinement of AZ31 magnesium alloy

Materials Science and Engineering A 491 (2008) 440–445 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepag...

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Materials Science and Engineering A 491 (2008) 440–445

Contents lists available at ScienceDirect

Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea

Effect of Mg–10Sr master alloy on grain refinement of AZ31 magnesium alloy Mingbo Yang a,b,∗ , Fusheng Pan b , Renju Cheng b , Aitao Tang b a b

Materials Science & Engineering College, Chongqing Institute of Technology, Chongqing 400050, China Materials Science & Engineering College, Chongqing University, Chongqing 400030, China

a r t i c l e

i n f o

Article history: Received 19 June 2007 Received in revised form 13 February 2008 Accepted 13 February 2008 Keywords: Mg–10Sr master alloy AZ31 magnesium alloy Grain refinement Mg17 Sr2 phase

a b s t r a c t The effect of Mg–10Sr master alloy on the grain refinement of AZ31 magnesium alloy is investigated. The results indicate that the various Mg–10Sr master alloys (original, rolled, aged and remelted) can effectively refine the grains of AZ31 alloy. Furthermore, for a given 60-min melt holding time, the average grain size of AZ31 alloy treated with the various Mg–10Sr master alloys gradually decreases with the Sr amount increasing from 0.01 to 0.1%. In addition, for a given Mg–10Sr master alloy and 0.1% Sr, the average grain size of the AZ31 alloy treated for 20-min melt holding time is larger than that of the AZ31 alloy treated for 40, 60 or 80 min, and the effect of melt holding time on the average grain size becomes unobvious after 20 min. The grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys may be explained by the GRF (growth restriction factor) mechanism. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In the last decade, the Mg–Al-based magnesium alloys have been widely applied in industrial production, but their mechanical properties and performance still do not meet the needs of some important parts in vehicles and other applications. Therefore, many ways are being investigated in the world in order to further improve the mechanical properties and performance of Mg–Al-based magnesium alloys [1–4]. It is known that the grain refinement is an important method of elevating the properties and improving the formability for magnesium alloys. Recent investigations [5–7] indicated that the Sr additions which have been widely used in industrial practice especially for the modification of Al–Si alloys, are an effective refiner for Mg–Al-based alloys. For example, Srinivasan et al. [8] and Nam et al. [9] reported that adding an Al–10Sr master alloy to a Si-containing AZ91 alloy appeared to refine the microstructure by promoting a smaller grain size, and similar results in AM50 and AZ91 alloys were obtained by Zhao et al. [10] and Hirai et al. [11]. In addition, adding pure Sr or Al–10Sr alloy to AZ31 alloy could effectively reduce the grain size of the alloy was reported by Zeng et al. [12] and Cheng et al. [13]. It is well known that adding pure Sr to magnesium alloys would produce serious burning loss thus the amount of Sr is difficult to control. Therefore, the Sr-containing master alloys are thought to be an ideal Sr addition for the grain refinement

∗ Corresponding author at: Materials Science & Engineering College, Chongqing Institute of Technology, Chongqing 400050, China. Tel.: +86 23 68667455; fax: +86 23 68667714. E-mail address: [email protected] (M. Yang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.02.017

of magnesium alloys. But up to now, only the effects of Al–Sr master alloys on the grain refinement of magnesium alloys are reported. The investigation about the effects of other Sr-containing master alloys such as Mg–Sr master alloys on the grain refinement of magnesium alloys is very scarce in the literatures. In recent studies, we [14] preliminarily found that the Mg–10Sr master alloy produced by a conventional casting method can effectively refine the grains of AZ31 magnesium alloy. In spite of the above studies, the investigation about the effects of Mg–10Sr master alloys on the grain refinement of magnesium alloys is not very systematical because one problem whether the rolled, heat-treated and remelted Mg–10Sr master alloys are also an effective refiner for magnesium alloys still remains. Due to the above-mentioned reasons, the present work first investigates the effects of aging, rolling and remelting treatments on the microstructure of Mg–10Sr master alloy produced by a conventional casting method. Then the effects of these Mg–10Sr master alloys on the grain refinement of AZ31 magnesium alloy are analyzed. 2. Experimental The original Mg–10Sr master alloy was prepared by a conventional casting method. The pure Mg was first melted in a crucible resistance furnace and protected by a flux addition. When the melt temperature reached approximately 993 K, the pure Sr was rapidly added to the melt. After holding at 1023 K for 60 min, the melts were poured into a permanent mould with a cavity of size ˚20 mm × 90 mm. Furthermore, the sample of original Mg–10Sr master alloy was aged, rolled and remelted, respectively. The aging treatment was carried out at 623 K for 24 h and followed by water quenching. The rolling was carried out at 673 K for a total reduc-

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Table 1 Chemical compositions of the Mg–10Sr master alloy (wt%) Types of master alloys

Sr

Fe

Si

Mg

Original Mg–10Sr alloy Remelted Mg–10Sr alloy

10.1 9.92

0.17 0.32

0.15 0.21

Balance Balance

tion level of 20%. The rolling procedure included two steps. First, a 10% reduction level was achieved by rolling at 673 K. Then, the sample was again heated to 673 K and rolled in order to achieve the remanent reduction level of 10%. The remelting was carried out in an electrical resistance furnace by using a graphite crucible. After the original Mg–10Sr master alloy was melted and held at 1013 K for 60 min, the melts were poured into a water-cooling permanent mould with a cavity of size 50 cm × 0.5 cm × 60 cm. Obviously, the solidification rate of remelted Mg–10Sr master alloy is rapider than that of the original Mg–10Sr master alloy. Table 1 lists the chemical compositions of original and remelted Mg–10Sr master alloys. The original, aged, rolled and remelted Mg–10Sr master alloys were used to refine AZ31 alloy, respectively. The refining treatment was carried out in an electrical resistance furnace by using a graphite crucible and protected by a flux addition. When the melt temperature of AZ31 alloy reached approximately 1013 K, the melts were treated with 0.01, 0.05 and 0.1 wt% Sr using the various Mg–10Sr master alloys, respectively. After holding at 1013 K for 20, 40, 60 and 80 min, respectively, the melts were poured into a permanent mould with a cavity of size ˚20 mm × 90 mm. Furthermore, the samples of AZ31 alloy treated with the various Mg–10Sr master alloys were subjected to an annealing treatment (688 K/12 h, water cooled) in order to clearly reveal the grain boundaries. Table 2 lists the chemical compositions of AZ31 alloy treated with the various Mg–10Sr master alloys. The samples of Mg–10Sr master alloy and AZ31 alloy were mechanically polished and etched with an 8% nitric acid distilled water solution, and then examined by an Olympus optical microscope and JOEL/JSM-6460LV type scanning electron microscope (SEM) equipped with Oxford energy dispersive spectroscope (EDS) with an operating voltage of 20 kV. The grain size was measured by the standard linear intercept method using an Olympus stereomicroscope. The phases in the Mg–10Sr master alloy and AZ31 alloy were also analyzed by D/Max-1200X type X-ray diffraction (XRD) operated at 40 kV and 30 mA. 3. Results and discussion 3.1. Microstructures of various Mg–10Sr master alloys Fig. 1 shows the XRD results of original and remelted Mg–10Sr master alloys. It is found from Fig. 1 that the two master alloys are composed of ␣-Mg and Mg17 Sr2 phases. Since in the present investigations the aging and rolling treatments usually do not cause the formation of any new phases, it is believed that the original, aged, rolled and remelted Mg–10Sr master alloys are composed of ␣-Mg and Mg17 Sr2 phases. Fig. 2 shows the SEM images of vari-

Fig. 1. XRD results of the various Mg–10Sr master alloys: (a) original and (b) remelted.

ous Mg–10Sr master alloys. It is found from Fig. 2a and d that the Mg17 Sr2 phases in the original and remelted Mg–10Sr master alloys mainly exhibit fine particles, and the size of Mg17 Sr2 phases in the two master alloys is close (≈1.4 ␮m in diameter). Furthermore, it is observed from Fig. 2b and c that the Mg17 Sr2 phases in the rolled and aged Mg–10Sr master alloys exhibit relatively coarse particles, and the size of Mg17 Sr2 phases in the two master alloys is also close (≈3.1 ␮m in diameter). 3.2. Microstructural refinement of AZ31 alloys treated with various Mg–10Sr master alloys Fig. 3 shows the as-cast microstructures of AZ31 alloy without Sr modification. Fig. 4 shows the as-cast microstructures of AZ31 alloy treated with the various Mg–10Sr master alloys for 0.1% Sr and 60-min melt holding time. It is observed from Figs. 3 and 4 that the primary ␣-Mg phases in these AZ31alloys mainly display a dendrite configuration. However, it is found from Table 3 that adding the various Mg–10Sr master alloys to AZ31 alloy can obviously reduce the secondary dendrite arm spacings (DAS) of primary ␣-Mg phases in the alloy. Fig. 5 shows the effects of Sr amount and melt holding time on the average grain size of solutionized AZ31 alloy treated with the various Mg–10Sr master alloys. It is found from Fig. 5 that adding the various Mg–10Sr master alloys to AZ31 alloy can effectively reduce the grain size of the alloy. Fig. 6 presents an overview of solution-

Table 2 Chemical compositions of the as-cast AZ31 alloy treated with the various Mg–10Sr master alloys Nominal alloys

AZ31 AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr

Melt holding time (min)

60 60 60 60 60

Types of Mg–10Sr master alloys

Untreatment Original Rolled Aged Remelted

Elements (wt%) Al

Zn

Mn

Sr

Mg

2.852 2.876 2.875 2.884 2.865

0.763 0.748 0.761 0.758 0.755

0.171 0.162 0.165 0.173 0.164

– 0.086 0.087 0.087 0.086

Balance Balance Balance Balance Balance

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Fig. 2. SEM images of the various Mg–10Sr master alloys: (a) original, (b) rolled, (c) aged, and (d) remelted.

amount increasing from 0.01 to 0.1%. In addition, it is also observed from Fig. 5b that for a given Mg–10Sr master alloy and Sr amount of 0.1%, the average grain size of AZ31 alloy treated for 20-min melt holding time is larger than that of the AZ31 alloy treated for 40, 60 or 80 min, and the effect of melt holding time on the average grain size of AZ31 alloy becomes unobvious after 20 min. 3.3. Discussion

Fig. 3. As-cast microstructure of the AZ31 alloy without Sr modification.

ized AZ31 alloy treated with the various Mg–10Sr master alloys for 0.1% Sr and 60-min melt holding time, corresponding to the observations shown in Fig. 5. Based the above results, it is inferred that the original, aged, rolled and remelted Mg–10Sr master alloys are an effective refiner for AZ31 alloy. Furthermore, it is found from Fig. 5a that for a given 60-min melt holding time, the average grain size of AZ31 alloy treated with the various Mg–10Sr master alloys gradually decreases with the Sr

As listed in Table 1, the Fe element is detected in the original and remelted Mg–10Sr master alloys. The previous investigations [15] indicated that the Fe addition is an effective grain refiner for Mg–Albased alloys. Therefore, one problem arises whether the grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys is related to the Fe element. In order to solve the problem, the microstructure and micro-elemental analysis of precipitates in the as-cast AZ31 alloys treated with the various Mg–10Sr master alloys were examined (Fig. 7 and Table 4). Combining Tables 2 and 4, it is believed that in the present investigations the effect of Fe may be neglected due to the following reason: the Fe element is not detected in the AZ31 alloys treated with the various Mg–10Sr master alloys, which might be related to the manganese in the experimental alloys since the Fe can be removed by the precipitation and setting of intermetallic particles containing iron and manganese during the melting. Therefore, the grain refinement of

Table 3 DAS of the as-cast AZ31 alloy treated with the various Mg–10Sr master alloys Nominal alloys

Melt holding time (min)

Types of Mg–10Sr master alloys

DAS (␮m)

AZ31 AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr AZ31 + 0.1 wt% Sr

60 60 60 60 60

Untreatment Original Rolled Aged Remelted

32.64 15.11 11.72 10.34 17.18

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Fig. 4. As-cast microstructures of the AZ31 alloy treated with the various Mg–10Sr master alloys for 0.1% Sr and 60-min melt holding time: (a) original, (b) rolled, (c) aged, and (d) remelted.

the AZ31 alloys treated with the various Mg–10Sr master alloys is possibly related to other mechanism. In general, the grain refinement in industrial applications usually involves adding nucleants and/or solute elements into a melt before casting, and the effect of a solute element may be explained in terms of the growth restriction factor GRF (Eq. (1)) [5,6,12]. In addition, the grain refinement is also related to the heterogeneous nucleation during solidification. One criterion of heterogeneous nucleation is that the disregistry of nucleant planes is less than 6% [16]. According to the above information from Fig. 1, the various Mg–10Sr master alloys are composed of ␣-Mg and Mg17 Sr2 phases. It is well known that the Mg17 Sr2 phase is a hexad crystal structure with a = b = 10.469 nm and c = 10.3 nm [17]. The ␣-Mg phase is a hexagonal close-packed crystal structure with a = 0.320 nm and c = 0.52 nm [18]. Obviously, the lattice disregistry between Mg17 Sr2 and ␣-Mg phases is larger than 6%, indicating that the Mg17 Sr2

phase could not act as heterogenerous nucleus for the ␣-Mg phase. Therefore, the Mg17 Sr2 phases in the various Mg–10Sr master alloys could not directly influence the grain refinement unless the free Sr was obtained by the dissolution of Mg17 Sr2 phases [14]. Fig. 8 shows the XRD result of the as-cast AZ31 alloy treated with the original Mg–10Sr master alloy under 0.1% Sr and 60-min melt holding time. As shown in Fig. 8 and Table 4, in the present investigations any Sr-containing phases are not detected in the AZ31 alloys treated with the various Mg–10Sr master alloys, indicating that the Mg17 Sr2 phases have been completely dissolved into the melt of AZ31 alloy. Since the solid solubility of Sr in magnesium is relatively limited, thus after the Mg17 Sr2 phase is dissolved into the melt of AZ31 alloy, the free Sr will rapidly enrich in the liquid ahead of growing interface and then restrict the grain growth during solidification [5,6]. Therefore, the grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys is believed to be possibly related to the

Fig. 5. Effects of Sr amount and melt holding time on the average grain size of solutionized AZ31 alloy treated with the various Mg–10Sr master alloys: (a) effect of the Sr amount under the 60 min holding time; (b) effect of melt holding time under the Sr amount of 0.1%.

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Fig. 6. Grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys for 0.1% Sr and 60-min melt holding time: (a) without Sr modification, average grain size (AGS) =200 ␮m; (b) original, AGS = 62 ␮m; (c) rolled, AGS = 56 ␮m; (d) aged, AGS = 52 ␮m; (e) remelted, AGS = 65 ␮m. The diameter of each polished section is ∼25 mm.

GRF mechanism. According to the GRF mechanism, larger the value of GRF, higher will be the refinement efficiency. GRF is defined from GRF =



mi c0i (ki − 1)

(1)

i

where mi is the slope of the liquidus line, c0i is the initial concentration of element i and ki is the partition coefficient. Under the experimental condition of this work, i denotes Al, Zn and Sr elements, respectively. According to Ref. [12], mAl = −6.87; mZn = −6.04; mSr = −3.53; kAl = 0.37; kZn = 0.12; kSr = 0.006. The c0Al , c0Zn and c0Sr of the AZ31 alloy are listed in Table 2. Therefore, for

a given Sr amount of 0.1% and 60-min melt holding time, the GRF values of the AZ31 alloys treated with the original, rolled, aged, and remelted Mg–10Sr master alloys are 16.73, 16.79, 16.82 and 16.71 according to Eq. (1), respectively. Similarly, the GRF value of the AZ31 alloy without Sr modification, 16.39, is also calculated out. Based the above results, the grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys is easily explained according to the GRF mechanism. Furthermore, the GRF mechanism may be used to explain the effects of Sr amount and melt holding time on the average grain size of AZ31 alloy treated with the various Mg–10Sr master alloys.

Fig. 7. SEM images of the as-cast AZ31 alloy treated with the various Mg–10Sr master alloys for 0.1% Sr and 60-min melt holding time: (a) original, (b) rolled, (c) aged, and (d) remelted.

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4. Conclusions

Fig. 8. XRD result of the as-cast AZ31 alloy treated by the original Mg–10Sr master alloy for 0.1% Sr and 60-min melt holding time. Table 4 EDS results of as-cast AZ31 alloy treated with the various Mg–10Sr master alloys (at%) Positions

Mg

Al

Zn

Sr

Total (%)

“A” in Fig. 7a “B” in Fig. 7a “C” in Fig. 7a “A” in Fig. 7b “B” in Fig. 7b “C” in Fig. 7b “A” in Fig. 7c “B” in Fig. 7c “C” in Fig. 7c “A” in Fig. 7d “B” in Fig. 7d “C” in Fig. 7d

69.55 67.12 91.07 66.62 65.72 93.57 62.75 65.43 93.15 61.26 66.17 92.11

23.71 25.81 7.54 24.37 26.68 5.13 29.60 29.32 5.47 27.19 25.50 6.52

6.74 7.07 1.26 9.01 7.6 1.07 7.65 5.25 1.12 11.55 8.33 1.23

– – 0.13 – – 0.23 – – 0.26 – – 0.14

100 100 100 100 100 100 100 100 100 100 100 100

For the AZ31 alloy treated with a given Mg–10Sr master alloy for 60-min melt holding time, the amount of Mg17 Sr2 phase in the melt of AZ31 alloy would increase with the Sr amount increasing from 0.01 to 0.1%. Accordingly, the concentration of free Sr and the GRF value would increase. As a result, the refinement efficiency of Mg–10Sr master alloy increases, as shown in Fig. 5a. In addition, for a given Mg–10Sr master alloy and 0.1% Sr, the situation where the average grain size of AZ31 alloy treated for 20-min melt holding time is larger than that of the AZ31 alloy treated for 40, 60 or 80 min (Fig. 5b), is possibly related to the following reason: the Mg17 Sr2 phases in the Mg–10Sr master alloy do not completely dissolved into the melt of AZ31 alloy treated for 20 min thus results in a relatively low concentration of free Sr and a small GRF value.

Results show that the various Mg–10Sr master alloys (original, rolled, aged and remelted) can effectively refine the grains of AZ31 magnesium alloy. Furthermore, for a given 60-min melt holding time, the average grain size of AZ31 alloy treated with the various Mg–10Sr master alloys gradually decreases with the Sr amount increasing from 0.01 to 0.1%. In addition, for a given Mg–10Sr master alloy and 0.1% Sr, the average grain size of AZ31 alloy treated for 20min melt holding time is larger than that of AZ31 alloy treated for 40, 60 or 80 min, and the effect of melt holding time on the average grain size becomes unobvious after 20 min. The grain refinement of AZ31 alloy treated with the various Mg–10Sr master alloys may be explained by the GRF mechanism. Acknowledgements The present work was supported by the National Natural Science Funds for Distinguished Young Scholar in China (No. 50725413), the Major State Basic Research Development Program of China (973) (No. 2007CB613704), the Natural Science Foundation Project of CQ CSTC (No. 2007BB4400), and Chongqing Science and Technology Commission in China (No. 2006AA4012-9-6). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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