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2018 International Conference on Computer, Communications and Mechatronics Engineering (CCME 2018) ISBN: 978-1-60595-611-4

Effect of Microalloying on Double-hit Compression

Behavior of AZ31 Alloy

Tong WANG

1,*

, Xiong ZHOU

1

, Tao JIANG

2

and Qi-chi LE

1 1

The Key Laboratory of Electromagnetic Processing of Materials, Northeastern University, Shenyang 110819, China

2

The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China

*Corresponding author

Keywords: Rare earth, Microalloy, Cerium, Magnesium, Double-hit compression.

Abstract. Double-hit compression tests were performed on the AZ31 and AZ31+0.2Ce alloys at 400 °C and 0.1/s. A stress peak was observed in the flow curve at the beginning of the second hit. This hardening phenomenon was attributed to the occurrence of static strain aging due to Al and Ce in solid solution. A stronger strengthening effect was observed in AZ31+0.2Ce due to a greater pinning effect of Ce on moving dislocations. During the second hit, the flow stress first decreased rapidly and then slowly to a certain level. Discontinuous dynamic recrystallization (DRX) occurred in these two alloys at the original grain boundaries. More DRX grains can be observed in the AZ31+0.2Ce alloy. This was because the DRX process was facilitated due to the segregation of Ce at grain boundaries.

Introduction

AZ31 is the most commonly used commercial magnesium alloy [1]. The existence of β-Mg17Al12

phase with a low melting temperature of about 425 °C limits its application at elevated temperatures [2]. Rare earth (RE) elements were utilized to improve the high temperature behavior of magnesium alloys [3]. Due to the high costs of RE, microalloying is considered as an effective method to optimize the AZ31 alloy. Shang et al. [4, 5] studied the formation of second phases and high temperature tensile behavior of RE microalloyed AZ31 alloys. New RE-containing second phases, such as Al-RE and Al-Mn-RE phases, were observed in these alloys and the high temperature tensile properties were also greatly improved. The RE in solid solution was found to have a pinning effect on the dislocations. It can lead to the occurrence of static strain aging (SSA) and dynamic strain aging (DSA) [6, 7] depending on the deformation conditions. Besides, researchers also found that RE atoms trend to segregate at grain boundaries and have a great influence on the dynamic recrystallization (DRX) behavior of magnesium alloys [8-10].

The aim of this work is to investigate the effect of microalloying with Ce on the double-hit

compression behavior of AZ31. Compression tests were performed on the AZ31 and AZ31+0.2Ce

alloys at 400 °C and a strain rate of 0.1/s with an interpass time of 10/540 s. The flow curves were analyzed combined with the observation of deformation microstructure.

Experimental

[image:1.595.84.513.753.799.2]

The chemical compositions of the AZ31 and the AZ31+0.2Ce alloys were analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 8300DV, PerkinElmer) and the result is shown in Table 1.

Table 1. Chemical composition of the AZ31 and AZ31+0.2Ce alloys (wt.%).

Alloy Al Zn Mn Ce Mg

AZ31 3.00 1.06 0.31 -- Balance

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Double-hit compression tests were performed on the cylindrical samples with a diameter of 6 mm and a height of 9 mm at 400 °C and 0.1/s. In one test, the sample was firstly compressed to a true strain of 0.2. At this time, the load was released, and the sample was hold in the furnace at 400 °C for two different interpass times of 10 and 540s, and then further compressed to a total true strain of 0.4 during the second hit. The samples were quenched in water within 5 s after the tests to freeze the deformation microstructure.

Microstructure was observed using Leica DM4M optical microscope (OM). The samples for OM analyses were prepared using standard polishing technique and etched in a solution of 6 g picric acid, 5 mL glacial acetic acid, 100 mL ethanol, and 100 mL distilled water.

[image:2.595.132.466.283.404.2]

Results and Discussion

Fig. 1 shows the optical micrographs of the AZ31 and AZ31+0.2Ce alloys. It can be seen that the grain size of AZ31 decreased with 0.2 wt.% addition of Ce. At the same time, more second phases can also be observed in the AZ31+0.2Ce alloy as shown in Fig. 1(b).

Figure 1. Microstructure of the (a) AZ31 and (b) AZ31+0.2Ce alloys.

Flow curves of the double-hit compression tests of the two alloys are shown in Fig. 2. According to this figure, the flow stress of AZ31+0.2Ce is higher than that of AZ31. It can be attributed to the solid solution of Ce in the matrix and the formation of more second phases in AZ31+0.2Ce. To further investigate the mechanical behavior of these two alloys, in Fig. 2(a), the flow stress at the end of the first hit was designated as σ0. After the first hit and an interpass time of 10 or 540 s, a stress peak σ1

can be observed at the beginning of the second hit. During the second hit, the flow stress decreased

first rapidly and then slowly to a certain level at 0.4 strain, which is designated as σ2. The hardening

effect at the beginning of the second hit was evaluated with the stress increase from σ0 to σ1 and

defined as Δσi (Δσi=σ1-σ0). In a similar manner, the softening effect during the second hit was

evaluated with the stress decrease from σ0 to σ2 and defined as Δσd (Δσd=σ2-σ0). The σ0, σ1, σ2, Δσi and

Δσd measured in these four flow curves are summarized in Table 2. According to Table 2, the stress

increase for AZ31 at the beginning of the second hit is about 5 MPa, while that of AZ31+0.2Ce is higher, about 7~8 MPa.

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[image:3.595.97.496.117.276.2]

Ce-248pm), Al and Ce atoms in solid solution should interact with dislocations and be the reason for the occurrence of the stress peaks. Since the atomic radius of Ce is much larger than Mg and Al, the AZ31+0.2Ce alloy demonstrated greater SSA effect.

Figure 2. Flow curves of the double-hit compression tests of the AZ31 and AZ31+0.2Ce alloys with an interpass time of (a) 10 s and (b) 540 s.

[image:3.595.76.519.555.644.2]

According to Table 2, during the second hit, the stress decrease of AZ31 is lower than that of AZ31+0.2Ce. For the same alloy, the stress decrease during the second hit increased with longer interpass time. The stress decrease for AZ31 with 10 s interpass time is about 1.1 MPa, while it increased to 1.9 MPa with 540 s interpass time. The stress decrease for AZ31+0.2Ce with 10 s interpass time is about 2.1 MPa, which it increased to 5.8 MPa with 540 s interpass time. This softening effect is due to the occurrence of DRX during the second hit of the compression. Fig. 3 shows the microstructure of the double-hit compression tests of AZ31 and AZ31+0.2Ce with 10 and 540 s interpass time. Serrated grain boundaries can be found in these two alloys. Discontinuous DRX took place at the original grain boundaries and presented a “necklacing” microstructure. Comparing Figs. 3(a) and (b), Figs. 3(c) and (d), the original grain boundaries of AZ31+0.2Ce were more serrated and more DRX grains can be observed in this alloy. It indicated that RE additions have a great effect on grain boundaries. Robson [9] investigated the effect of Y and Gd on the DRX behavior of magnesium alloys. Y and Gd atoms were found segregate at grain boundaries and have a strong drag effect on dislocations. As a result, large amounts of dislocations were trapped at the grain boundaries. In this way, the DRX process was facilitated through the addition of RE.

Table 2. The σ0, σ1, σ2, Δσi and Δσd measured in the double-hit compression tests of the AZ31 and AZ31+0.2Ce alloys.

Alloy Interpass

time /s σ0 /Mpa σ1 /Mpa σ2 /Mpa Δσi /Mpa Δσd /Mpa

AZ31 10

57.6 62.6 56.5 5.0 1.1

540 58.2 63.0 56.3 4.8 1.9

AZ31+0.2Ce 10

61.9 69.8 59.8 7.9 2.1

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[image:4.595.118.483.76.350.2]

Figure 3. OM images of the double-hit compression samples: (a) and (b) 10 s interpass time, (c) and (d) 540 s interpass time.

Conclusions

In the present study, the effect of microalloying with Ce on the double-hit compression behavior of AZ31 was investigated at 400 °C and 0.1/s. It was found that adding 0.2 wt.% Ce introduced more second phases and reduced the grain sizes of AZ31. Large Ce atoms in solid solution have a great influence on the mechanical and DRX behavior of the alloy. Stress peaks appeared at the beginning of the second hit due to the occurrence of SSA induced by Al and Ce. This strengthening effect is stronger in the AZ31+0.2Ce alloy because Ce has a larger atomic size. Ce also tended to segregate at grain boundaries and had a drag effect on dislocations. In this way, the addition of Ce also facilitated the DRX process of the AZ31 alloy.

Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities of China (N170903005).

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[5] Shang, L., et al., Effect of microalloying (Ca, Sr, and Ce) on elevated temperature tensile behavior of AZ31 magnesium sheet alloy. Materials Science and Engineering: A, 2011. 528(10): pp. 3761-3770.

[6] Wycliffe, P., Dynamic and static strain aging in substitutional and interstitial alloys. Journal Name: TMS (The Metallurgical Society) Paper Selection; (USA); Journal Volume: 56; Conference: TMS-AIME fall meeting, Detroit, MI (USA), 16-20 Sep 1984. ; None. Medium: X; Size: Pages: 4.

[7] Van Den Beukel, A. and U.F. Kocks, The strain dependence of static and dynamic strain-aging.

Acta Metallurgica, 1982. 30(5): pp. 1027-1034.

[8] Nie, J.F., et al., Solute segregation and precipitation in a creep-resistant Mg–Gd–Zn alloy. Acta

Materialia, 2008. 56(20): pp. 6061-6076.

[9] Robson, J.D., Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The

Role of Grain Boundary Segregation. Metallurgical and Materials Transactions A, 2014. 45(8): pp. 3205-3212.

[10] Nie, J.F., Y.M. Zhu, and N.C. Wilson, Solute Segregation and Aggregation in Mg Alloys. 2015:

Springer International Publishing. 9-9.

[11] Andrade, H.L., M.G. Akben, and J.J. Jonas, Effect of molybdenum, niobium, and vanadium on

static recovery and recrystallization and on solute strengthening in microalloyed steels.

Metallurgical Transactions A, 1983. 14(10): pp. 1967-1977.

[12] Shang, L., Effect of microalloying on microstructure and hot working behavior for AZ31 based

magnesium alloy. 2018.

[13] Hajkazemi, J., et al., Double-hit compression behavior of TWIP steels. Materials Science and

Figure

Table 1. Chemical composition of the AZ31 and AZ31+0.2Ce alloys (wt.%).
Fig. 1 shows the optical micrographs of the AZ31 and AZ31+0.2Ce alloys. It can be seen that the grain size of AZ31 decreased with 0.2 wt.% addition of Ce
Figure 2. Flow curves of the double-hit compression tests of the AZ31 and AZ31+0.2Ce alloys with an interpass time of (a) 10 s and (b) 540 s
Figure 3. OM images of the double-hit compression samples: (a) and (b) 10 s interpass time, (c) and (d) 540 s interpass time

References

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