Jump to ContentJump to Main Navigation
Show Summary Details
More options …

High Temperature Materials and Processes

Editor-in-Chief: Fukuyama, Hiroyuki

Editorial Board: Waseda, Yoshio / Fecht, Hans-Jörg / Reddy, Ramana G. / Manna, Indranil / Nakajima, Hideo / Nakamura, Takashi / Okabe, Toru / Ostrovski, Oleg / Pericleous, Koulis / Seetharaman, Seshadri / Straumal, Boris / Suzuki, Shigeru / Tanaka, Toshihiro / Terzieff, Peter / Uda, Satoshi / Urban, Knut / Baron, Michel / Besterci, Michael / Byakova, Alexandra V. / Gao, Wei / Glaeser, Andreas / Gzesik, Z. / Hosson, Jeff / Masanori, Iwase / Jacob, Kallarackel Thomas / Kipouros, Georges / Kuznezov, Fedor


IMPACT FACTOR 2018: 0.427
5-year IMPACT FACTOR: 0.471

CiteScore 2018: 0.58

SCImago Journal Rank (SJR) 2018: 0.231
Source Normalized Impact per Paper (SNIP) 2018: 0.377

Open Access
Online
ISSN
2191-0324
See all formats and pricing
More options …
Volume 37, Issue 1

Issues

Effect of Bi on Microstructure and Mechanical Properties of Extruded AZ80-2Sn Magnesium Alloy

Hansong Xue
  • Corresponding author
  • College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
  • National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China
  • Email
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Xinyu Li / Weina Zhang / Zhihui Xing / Jinsong Rao / FuSheng Pan
  • College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
  • National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2017-03-10 | DOI: https://doi.org/10.1515/htmp-2016-0045

Abstract

The effects of Bi on the microstructure and mechanical properties of AZ80-2Sn alloy were investigated. The results show that the addition of Bi within the as-cast AZ80-2Sn alloy promotes the formation of Mg3Bi2 phase, which can refine the grains and make the eutectic phases discontinuous. The addition of 0.5 % Bi within the as-extruded AZ80-2Sn alloy, the average grain size decreases to 12 μm and the fine granular Mg17Al12 and Mg3Bi2 phases are dispersed in the α-Mg matrix. With an increase in Bi content, the Mg17Al12 and Mg3Bi2 phases become coarsened and the grain size increases. The as-extruded AZ80-2Sn-0.5 %Bi alloy has the optimal properties, and the ultimate tensile strength, yield strength and elongation are 379.6 MPa, 247.1 MPa and 14.8 %, respectively.

Keywords: magnesium alloy; bismuth; microstructure; mechanical properties

Introduction

Magnesium alloys, the lightest structural alloys, exhibit a good combination of high specific strength and good castability [1, 2], which make them promising engineering materials for transportation vehicles, aerospace applications and 3C products [3, 4]. To broaden the scope of magnesium alloys applications in commercial, researchers need to develop the alloys with higher strength at a low cost. Among magnesium alloys, the AZ series, which have high room-temperature strength and good corrosion resistance, are widely used as wrought magnesium alloys. Specially, AZ80 alloy has received much attention due to its excellent strength after extrusion process. However, alloys with higher strength must be developed to meet the requirement of more applications. Alloying is an effective method to improve the mechanical properties of magnesium alloys. Rare earth (RE) elements are the important addition elements to magnesium alloys [5, 6], which have a high affinity for Al and can form Al-containing intermetallic compound of Al11RE3 with relatively high melting points [7]. In general, the addition of RE elements to magnesium alloys can refine the grains and effectively improve the mechanical properties [8]. However, the RE-containing alloys are too expensive to be widely used.

Some researches indicated the addition of low-cost Sn could develop the mechanical properties of magnesium alloys. Mahmudi et al. [9] found the as-cast AZ91 alloy with addition of 2 mass % Sn had the highest creep resistance among all tested alloys which was the effects of the formation of Mg2Sn and solid solution hardening of Al and Sn in the Mg matrix. Kim et al. [10] reported that addition of 3–5 mass % Sn into squeeze cast AZ51 alloy could develop twins in the α-Mg matrix and suppress crack growth to improve the tensile and fracture properties. Recently, our study indicated that the addition of 2 mass % Sn into AZ80 alloy can improve the tensile strength, but have little effect on refining grains. Bi is also an important additional element to magnesium alloys due to its potential to improve mechanical properties and cheap price. The addition of Bi into magnesium alloys can form thermally stable Mg3Bi2 phase with high melting temperature of 821 °C [11]. Some researchers had discussed the effect of Bi addition on the microstructure and mechanical properties of magnesium alloys. Wang et al. [12, 13] found the nucleating temperature of primary α-Mg and eutectic phase in AZ80-2 %Bi alloy were decreased during solidification procedure and AZ80-0.5 %Bi alloy had optimum combination of the tensile strength and elongation. Zhou et al. [14] reported that combined addition of minor bismuth and antimony to the AZ91 alloy could refine the β-Mg17Al12 phase and result in the formation of needle-shaped Mg3Bi2 and Mg3Sb2 particles, which were distributed mainly along grain boundaries. Yuan et al. [15] investigated the effect of Bi addition on the microstructure and mechanical properties of AZ91 alloy and the yield strength and creep resistance were increased significantly with the addition of Bi. Therefore, we expected the addition of Bi into AZ80-2Sn alloy could refine the grain and improve the mechanical properties further.

In the present work, Bi had been added into AZ80-2Sn alloy and the effects of Bi on the as-cast and as-extruded microstructures were studied. In addition, the tensile properties of as-extruded AZ80-2Sn alloy with addition of Bi and the strengthening mechanism were investigated.

Experimental

The alloys with nominal composition of AZ80-2Sn-xBi (x=0, 0.5, 1 and 1.5 mass %) were prepared by melting the ingots of commercially pure Mg, Al, Zn, Sn, Bi and Mg-5.0 mass % Mn master alloy in a semi-continuous vacuum induction melting furnace under the protection of SF6 and CO2 mixed gas (volume ratio was 1:100). The metal was held at 720 °C for about 20 min, then poured into a columniform iron mould with internal diameter of 85 mm and cooled rapidly by water. Chemical composition of the investigated alloys was measured with XRF-1800 CCDE sequential X-ray fluorescence spectrometer. The results are listed in Table 1.

Table 1:

Chemical compositions of AZ80-2Sn-xBi alloys.

The ingots for homogenization treatment were held at 420 °C for 12 h followed by air cooling. Then they were extruded at 350 °C immediately. The extrusion parameters are listed in Table 2. Tensile tests were performed on a Sans CMT-5105 electronic universal testing machine with a strain rate of 2 mm/min at room temperature. The tensile directions were parallel to the extrusion direction.

Table 2:

Extrusion parameters of the studied alloys.

Microstructure and morphology were obtained by optical microscopy (OM, NEISS NEOPHOT-30) and scanning electron microscopy (SEM, TESCAN VEGA II) equipped with an Oxford X-ray energy dispersive spectroscopy (EDS) which was employed to determine the phase’s chemical constitution of the alloy. Phase composition was detected by X-ray diffraction (XRD) instrument with Cu Kα and a scanning rate of 0.01 °/s. The samples for OM and SEM observation were etched by 4 % Oxalic Acid.

Results and discussion

Effects of Bi on the as-cast microstructure

Figure 1 is the XRD patterns of the as-cast AZ80–2Sn-x mass %Bi (x=0, 0.5, 1, 1.5) alloys. As shown in Figure 1(a), the as-cast AZ80–2Sn alloy is mainly consisted of α-Mg phase, β-Mg17Al12 phase and Mg2Sn phase, while additional peaks of Mg3Bi2 phase emerge with the addition of Bi (Figure 1(b)–(d)). Figures 2 and 3 show the microstructure of the as-cast AZ80-2Sn alloys with different Bi contents. The as-cast AZ80-2Sn alloy consisted of primary α-Mg matrix, Mg2Sn phase and divorced eutectic β-Mg17Al12 phase, which is semi-continuous and distributes in the form of network at grain boundaries (Figures 2(a) and 3(a)). The average grain size of the as-cast AZ80-2Sn alloy is about 81 μm. However, the amount of semi-continuous eutectic phase decreases and the new fine disperse phase occurs at grain boundaries with addition of 0.5 mass % Bi (Figures 2(b) and 3(c)). XRD and EDS analysis indicate that the new phase is Mg3Bi2 phase. The grains of as-cast AZ80-2Sn-0.5 %Bi alloy get refined and the average grain size decreases to about 72 μm. The grains get further refined with an increase in Bi content, as shown in Figure 2((c) and (d)) and Figure 3((e) and (g)). The average grain size decreases to about 61 μm and more discontinuous β-Mg17Al12 phases occur with addition of 1 mass % Bi. When the contents of Bi increases to 1.5 mass %, the average grain size decreases to about 55 μm, while flaky-shaped Mg3Bi2 phases distribute at grain boundaries and semi-continuous reticulated β-Mg17Al12 phases increase.

The XRD patterns of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.
Figure 1:

The XRD patterns of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

Optical micrographs of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.
Figure 2:

Optical micrographs of the as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

SEM morphology of as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 %Bi; (c) 0.5 %Bi; (e) 1 %Bi; (g) 1.5 %Bi;(b) EDS of A; (d) EDS of B; (f) EDS of C; (h) EDS of D.
Figure 3:

SEM morphology of as-cast AZ80–2Sn alloys with different Bi additions: (a) 0 %Bi; (c) 0.5 %Bi; (e) 1 %Bi; (g) 1.5 %Bi;(b) EDS of A; (d) EDS of B; (f) EDS of C; (h) EDS of D.

The grains of alloys get refined with the addition of Bi. This may be the result of newly formed Mg3Bi2 phase, which has a structure of hexagonal closed-packed (a=0.4666 nm, c=0.7401 nm) [11]. Based on the Bramfitt two-dimensional lattice mismatch degree theory, when the lattice mismatch δ<6 %, it is most effective to promote nucleation as the nuclei during heterogeneous nucleation. The orientation relationship between Mg3Bi2 and α-Mg matrix can be described as [0001] Mg3Bi2//[110] α-Mg and (010) Mg3Bi2//(010) α-Mg. As the lattice mismatch between {010} Mg3Bi2 and {010}α-Mg is only 3 % in the [010] α-Mg direction [11], Mg3Bi2 phase can effectively promote nucleation of α-Mg as the nuclei. Furthermore, Mg3Bi2 has a higher melting point (Tm=821 °C), it can nucleate firstly during solidification and then some of Mg3Bi2 can act as the nucleation sites for α-Mg, stimulating nucleation of α-Mg; while the other Mg3Bi2 may gather around the front of the newly formed α-Mg phase, preventing the further growth of the grain [16]. In this case, the grains of the alloys were refined with addition of Bi.

Effects of Bi on the as-extruded microstructure

The optical micrograph (OM) and back scattered electron (BSE) images of as-extruded AZ80–2Sn alloys with different Bi contents are plotted in Figures 4 and 5. As shown in Figure 4, the grain size of as-extruded alloys is decreased compared with those of as-cast alloys, this is because the grains were broken seriously by the three-dimensional stress and got refined through dynamic recrystallization (DRX). There are some black areas in Figure 4(a), in which some are the broken Mg17Al12 phases and the others are the recrystallized grains (Figure 5(a)). As shown in Figures 4(b) and 5(b), the average grain size of as-extruded AZ80–2Sn alloy with addition of 0.5 mass % Bi decreases from 25 μm to 12 μm and fine granular Mg17Al12 and Mg3Bi2 phases distribute dispersedly in the α-Mg matrix. The effect of 0.5 mass % Bi addition on grain refinement of AZ80–2Sn alloy is obvious. On one hand, Mg3Bi2 phase formed by addition of Bi has good thermal stability, thus can be broke and distribute along grain boundary during extrusion process. The Mg3Bi2 phases along grain boundary are hard to be softened at elevated temperature and can prevent merge of grains and growth. On the other hand, serious distortion around Mg3Bi2 phases can occur during extrusion process, which results in the formation of distorted regions with high dislocation density. The distortion regions can promote the recrystallized grains to nucleate, thus increasing the nucleation rate and refining grains. However, when the content of Bi exceeds 0.5 mass %, the average grain sizes of as-extruded AZ80–2Sn-1 mass %Bi and AZ80–2Sn-1.5 mass %Bi alloys both increase to about 15 μm (Figure 4(c) and 4(d)). In addition, the Mg17Al12 and Mg3Bi2 phases begin to concentrate and grow near grain boundary with addition of 1 mass % Bi (Figure 5(c)), and continue to grow further with addition 1.5 mass % Bi (Figure 5(d)). With an increase in the content of Bi (>0.5 mass %), Mg3Bi2 phases can agglomerate and the dispersion rate decreases.

Optical micrographs of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.
Figure 4:

Optical micrographs of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

SEM back scattered electron images of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.
Figure 5:

SEM back scattered electron images of the as-extruded AZ80–2Sn alloys with different Bi additions: (a) 0 mass %Bi; (b) 0.5 mass %Bi; (c) 1 mass %Bi; (d) 1.5 mass %Bi.

Effects of Bi on the as-extruded mechanical properties

The tensile test results of the as-extruded AZ80–2Sn alloys with different Bi additions are shown in Figure 6. It can be seen that the tensile strength (UTS), yield strength (YS) and elongation of the as-extruded AZ80–2Sn alloys with the addition of Bi are improved. The UTS, YS and elongation of the as-extruded AZ80–2Sn alloy are 349.1 MPa, 229.8 MPa and 9.6 %, respectively. The as-extruded AZ80–2Sn-0.5 mass %Bi alloy has the best combination of UTS, YS and elongation, which are 379.6 MPa, 247.1 MPa and 14.8 %, respectively. Under as-extruded conditions, the UTS, YS and elongation of AZ80–2Sn-0.5 %Bi alloy compared with AZ80–2Sn alloy are improved by 8.7 %, 7.5 % and 54.2 %, respectively, which may be the effect of fine-grain strengthening and dispersion strengthening. According to Hall-Petch equation, σ0 + kd−1/2 [17], the yield strength is in inverse proportion to the square root of grain diameter, so the yield strength of alloys will increase with the refinement of grain. Therefore, the yield strength of as-extruded AZ80–2Sn-0.5 mass % alloy increases with the decrease of grain size. Meanwhile, the fine granular and uniformly distributed Mg17Al12 and Mg3Bi2 phases can act as the obstruction to dislocation slipping, which can also improve the yield strength. When the content of Bi exceeds 0.5 %, the mechanical properties of experimental alloys decrease with further addition of Bi, this is because that the Mg3Bi2 and Mg17Al12 phases become coarsened, which can weaken the effect of dispersion strengthening. Moreover, Mg3Bi2 phase has good thermal stability, so it seems possible that Bi addition can strengthen AZ80–2Sn alloy at elevated temperature. Further investigations are under way.

Tensile properties of the as-extruded AZ80–2Sn alloys with different Bi additions tested at room temperature.
Figure 6:

Tensile properties of the as-extruded AZ80–2Sn alloys with different Bi additions tested at room temperature.

Conclusions

  1. The addition of Bi within AZ80-2Sn alloy, the grain gets refined and a new Mg3Bi2 phase is formed, which has a good thermal stability at elevated temperature. The grain size of as-cast AZ80-2Sn alloy with addition of 1 mass %Bi is decreased from 81 μm to 61 μm and discontinuous β-Mg17Al12 precipitates and fine Mg3Bi2 phases distribute along the grain boundaries. When the content of Bi is 1.5 mass %, the grain size is decreased to 55 μm, while flaky-shaped Mg3Bi2 phases occur at grain boundaries and the amount of discontinuous β-Mg17Al12 phase decreases.

  2. Bi addition also has an obvious effect on refining the grain of as-extruded AZ80–2Sn alloy. With addition of 0.5 % Bi, the average grain size decreases from 25 μm to 12 μm and the fine granular Mg17Al12 and Mg3Bi2 phases are dispersed in the matrix. Further addition of Bi can lead to the increase in grain size and the coarsening of Mg17Al12 and Mg3Bi2 phases.

  3. Under as-extruded condition, Bi addition can improve the tensile properties of AZ80–2Sn alloy. Under the effect of fine-grain strengthening and dispersion strengthening, the as-extruded AZ80–2Sn-0.5 %Bi alloy has the best combined properties and the ultimate tensile strength, yield strength and elongation are 379.6 MPa, 247.1 MPa and 14.8 %, respectively.

References

  • [1]

    I.M. Baghni, Y.S. Wu, J.Q. Li, C.W. Du and W. Zhang, Trans. Nonferrous Met. Soc. China, 13 (2003) 1253–1259. Google Scholar

  • [2]

    B.L. Mordike and T. Ebert, Mater. Sci. Eng. A, 302 (2001) 37–45. CrossrefGoogle Scholar

  • [3]

    H. Alves, U. Koster, E. Aghion and D. Eliezer, Mater. Technol., 16 (2001) 110–126. CrossrefGoogle Scholar

  • [4]

    S. Yao and Y.F. Li, Sci. Total Environ., 44 (2015) 89–96. 

  • [5]

    F. Khomamizadeh, B. Nami and S. Khoshkhooei, Metall. Mater. Trans. A, 36 (2005) 3489–3494. CrossrefGoogle Scholar

  • [6]

    Q.D. Wang, Y.Z. Lu, X.Q. Zeng, W.J. Ding and Y.P. Zhu, Trans. Nonferrous Met. Soc. China, 10 (2000) 235–239. Google Scholar

  • [7]

    C.C. Jain, C.Y. Bai, S.W. Chen and C.H. Koo, Mater. Trans., 48 (2007) 1149–1156. CrossrefGoogle Scholar

  • [8]

    Y.X. Wang, S.K. Guan, X.Q. Zeng and W.J. Dine, Mater. Sci. Eng. A, 416 (2006) 109–118. CrossrefGoogle Scholar

  • [9]

    R. Mahmudi and S. Moeendarbari, Mater. Sci. Eng. A, 566 (2013) 30–39. CrossrefGoogle Scholar

  • [10]

    B. Kim, J. Do, S. Lee and I. Park, Mater. Sci. Eng. A, 527 (2010) 6745–6757. CrossrefGoogle Scholar

  • [11]

    T.T. Sasaki, T. Ohkubo and K. Hono, Scr. Mater., 61 (2009) 72–75. 

  • [12]

    Y.X. Wang, J.W. Fu, J. Wang, T.J. Luo, X.G. Dong and Y.S. Yang, Acta Metall. Sinca, 47 (2011) 410–416. Google Scholar

  • [13]

    Y.X. Wang, J.X. Zhou, J. Wang, T.J. Luo and Y.S. Yang, Trans. Nonferrous Met. Soc. China, 21 (2011) 711–716. CrossrefGoogle Scholar

  • [14]

    W. Zhou, N.N. Aung and Y. Sun, Corros. Sci., 51 (2009) 403–408. CrossrefGoogle Scholar

  • [15]

    G.Y. Yuan, Y.S. Sun and W.J. Ding, Mater. Sci. Eng. A, 308 (2001) 38–44. CrossrefGoogle Scholar

  • [16]

    Y. Dong, X. Lin, J. Ye, T. Zhao and Z. Fan, Mater. Sci. Eng. A, 636 (2015) 600–607. CrossrefGoogle Scholar

  • [17]

    T. Mukai, M. Yamanoi, H. Watanabe, K. Ishikawa and K. Higashi, Mater. Trans., 42 (2001) 1177–1181. CrossrefGoogle Scholar

About the article

Received: 2016-03-05

Accepted: 2016-10-24

Published Online: 2017-03-10

Published in Print: 2018-01-26


This work is financially supported by Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2015jcyjBX0036), the National Natural Science Foundation of China (Grant No. 51571040), National Great Theoretic Research Project (2013CB632200).


Citation Information: High Temperature Materials and Processes, Volume 37, Issue 1, Pages 97–103, ISSN (Online) 2191-0324, ISSN (Print) 0334-6455, DOI: https://doi.org/10.1515/htmp-2016-0045.

Export Citation

© 2018 Walter de Gruyter GmbH, Berlin/Boston.Get Permission

Comments (0)

Please log in or register to comment.
Log in