- Email: [email protected]
Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys
Author’s Accepted Manuscript Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys Shineng Sun, Yuping Ren, Liqing Wang, Bo Yang, Hongxiao Li, Gaowu Qin www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30802-X http://dx.doi.org/10.1016/j.msea.2017.06.037 MSA35175
To appear in: Materials Science & Engineering A Received date: 30 November 2016 Revised date: 16 May 2017 Accepted date: 10 June 2017 Cite this article as: Shineng Sun, Yuping Ren, Liqing Wang, Bo Yang, Hongxiao Li and Gaowu Qin, Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.037 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 galley proof before it is published in its final citable 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.
Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys Shineng Sun, Yuping Ren*, Liqing Wang, Bo Yang, Hongxiao Li, Gaowu Qin* Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
[email protected] [email protected] *Corresponding Author:
1
Abstract: An unusual effect of Mn on the microstructures and mechanical properties of as-extruded Zn alloys has been found in our paper. Although the grain of Zn matrix is remarkably refined, and the number of MnZn13 compounds increases with the increasing of Mn contents, the volume fraction of the tensile twins obviously decreases. As a result, the tensile strength slightly decreases, and the elongation remarkably increases in as-extruded Zn-Mn binary alloys. This phenomenon is largely different from as-extruded Zn-based binary alloys containing Mg or Al. It means that the tensile twin largely signifies the comprehensive mechanical properties of as-extruded Zn-Mn alloys. Keywords: Zn-Mn alloys; Extrusion; Electron backscattering diffraction (EBSD); Twin; Grain refining.
1. Introduction Recently, Zn and Zn-based alloys have been extensively investigated with interest as the newest biodegradable metallic materials, due to their excellent biological compatibility and moderate bio-corrosion rate [1-3]. However, the mechanical properties of as-cast or deformed pure Zn is too poor to satisfy the requirement of implant devices [4-6]. Although the tensile strength of as-cast Zn-Mg binary alloys was improved, the plasticity remarkably decreased [7, 8]. Therefore, as-wrought Zn-based alloys containing Al, Mg, Cu, Sr or Ca have been developed in order to improve the combined properties [9-12]. Although the comprehensive mechanical properties of the Zn based binary alloys containing slight Al contents can 2
be improved by hot extrusion due to the grain refinement [13, 14], the Al element is a harmful element in human nutrition so that the Zn-Al-based Zn alloys are unsuitable for biodegradable implant materials [15]. As one of necessary trace element in human body, Mg has been in detail investigated as a new alloying element in as-wrought Zn-based binary alloys [16, 17]. The ultimate tensile strength of as-extruded Zn-Mg binary alloys increased from 250 to 340 MPa, but the elongation dramatically decreased from 22 to 6 % with increasing Mg contain from 0.15 to 1 wt.%, which is attributed to the collective role of grain refinement and second phase (Mg2Zn11) [18]. However, the effect of Ca or Sr contents on the mechanical properties of as-wrought Zn-based binary alloy is still not adequately investigated. On the other hand, Ca, Sr or Mn was selected to further alloying of as-extruded or rolled Zn-1Mg alloys in order to improving the mechanical properties of Zn-based alloys without losing the good biocompatibility. However, the effects of Ca or Sr the mechanical properties of as-wrought Zn-1Mg alloys were not obvious. It is noted that the ultimate tensile strength was improved from 210 to 300 MPa, and elongation from 8 to 25 %, respectively, when the as-rolled Zn-1Mg alloy was added by 0.1 wt.% Mn under the similar forming process [9, 19]. Unfortunately, the microstructure mechanism for the improvement in the comprehensive properties is not clearly interpreted in as-rolled Zn-1Mg-0.1Mn alloys. Moreover, although the superplasticity of as-extruded Zn-Mn alloys at 280~320 ℃ has been investigated, the effect of Mn additions on the room temperature mechanical properties of as-wrought Zn-based alloys is little noticed [20, 21]. Therefore, in the present work, the Zn-Mn binary 3
alloys with different Mn contents were prepared by indirect extrusion method. And then, the influence of Mn contents on the microstructure and tensile properties has been investigated in order to understand the relationship between the microstructure and property in as-extruded Zn-Mn binary alloys. Finally, the valuable data are to be provided for the composition design and process optimization of as-wrought Zn-based alloys containing Mn. 2. Experimental procedures High pure Zn (99.99 wt. %) and Zn-10 Mn (wt. %) master alloys were used to prepare four experimental alloys Zn-X Mn (X=0, 0.2, 0.4, 0.6 wt. %). The molten alloy at 500 ℃ was casted into a cone-shaped steel mould and cooled down in air. As-cast ingots were held at 300 ℃ for 4h, and then air cooled. These homogeneous ingots were extruded at 200 ℃ with an extrusion ratio of 16, and then air cooled to room temperature. The mechanical properties of as-extruded Zn-Mn alloys were determined by room temperature tensile tests at a strain rate of 10-3 s-1 using an AG-X universal testing machine. Dog-bone shaped specimens were 5 mm in diameter with 25 mm gauge length. The specimens were cut from as-extruded Zn-Mn alloys where their tensile axis is parallel to the extrusion direction. For each composition, three experiments were at least conducted to check the repeatability of the results. Fracture morphology of the specimens after the tensile test was examined using scanning electron microscope (SEM) in order to determine the fracture modes. The microstructure along the extrusion direction of as-extruded Zn-Mn alloys 4
was observed, and the samples were ground with SiC abrasive papers from 400 to 2000 # and polished by diamond pates with 2.5 μm particles. Grain size and twin feature were obtained by electron backscatter diffraction (EBSD) technique and Channel 5 analysis system (Oxford HKL) in combination with a JEM 7001F field emission scanning electron microscope operated at an accelerating voltage of 20 kV. The phase constituents of as-extruded Zn-Mn alloys were determined by X-ray diffraction (XRD) using copper Ka X-ray radiation. 3. Results The engineering stress-strain curves of as-extruded Zn-Mn alloys with different Mn contents are shown in Fig.1 (a). The ultimate tensile strength and elongation as-extruded pure Zn are 117 MPa and 14 %, respectively, which are close to the previously reported values the tensile properties of extruded pure Zn [9]. Both ultimate tensile strength and elongation of Zn-Mn alloys increase due to minor Mn additions. However with the increasing of Mn content from 0.2 to 0.6 wt. %, the ultimate tensile strength and the yield strength of as-extruded Zn-Mn alloys decrease from 220 to 182 MPa and from 132 to 118 MPa, respectively, while the elongation remarkably increases from 48 to 71 %. It shows that the plasticity of as-wrought Zn-based alloys can be clearly improved when a small amount of Mn is added. The tensile strength and elongation of as-extruded Zn-Mn alloys are illustrated in Fig. 1 (b), the results obtained in other as-wrought Zn-based binary alloys are shown for comparison [9, 18]. As the Mn content increases, the ultimate tensile strength decreases and the elongation increases. However, this is not the case in other reported 5
Zn-based alloys containing Mg and Al elements. As the Mg or Al content increases, the ultimate tensile strength increases and the elongation decreases. The ultimate tensile strength of as-extruded Zn-Mg alloys increases from 250 to 400 MPa, while the elongation remarkably decreases from 22 to 1 % with the increasing of Mg content from 0.15 to 3 wt. %. It is believed that the mechanical properties of Zn-Mg alloys attributed to the existence of the hard and brittle Mg2Zn11 intermetallic compounds [18]. In fact, the Mg2Zn11 particles are large and distributed in the shape of continuous net at the grain boundaries of Zn matrix with the Mg additions. Moreover, its volume fraction also increased. Finally, the plasticity of as-extruded Zn-Mg alloys remarkably decreased. The ultimate tensile strength of as-extruded Zn-Al alloys increase from 203 to 223 MPa, while the elongation dramatically drops from 33 to 24 % with Al contents from 0.5 to 1 wt.%. Unlike Zn alloys containing Mg, no second phase formed in Zn-Al alloys. It is believed that solid solution strengthening is also responsible for improved mechanical properties [18]. In general, the solute atom can make the strength improve, and the plasticity reduces, such as the role of solute atom in the Fe, Mg or Zr-based alloys [22-24]. Obviously, the role of Mn is remarkably different from that of other elements such as Mg or Al in as-wrought Zn-based alloys [18]. Therefore, it shows that Mn has totally different effect on the mechanical properties in as-extruded Zn-based alloys. The Zn-Mn alloys show enhanced strength and ductility at low content (0.2 wt.%), decrease strength and increase ductility at high contents (0.4 and 0.6 wt.%). The fracture surfaces of as-extruded Zn-Mn alloys after tensile tests are observed, 6
as shown in Fig.2. The intragranular cracks, cleavage steps and cleavage planes are surrounded by tearing ridges in pure Zn (Fig.2 (a)), which indicates typical cleavage fracture mode. As-extruded pure Zn exhibited typical cleavage fracture which is consistent with its low elongation to fracture. At the same time, the obvious neck can be observed in Zn-Mn alloys, as seen in the macro-photo inserted in Fig. 1(a). The fracture surface consists of dimples (Fig.2 (b)-(d)), indicating the ductile fracture mode in as-extruded Zn-Mn binary alloys. Moreover, the number of fine dimples of as-extruded Zn-Mn alloys increases with the increasing of Mn contents from 0.2 to 0.6 wt. % (Fig.2 (b)-(d)). This is in good accordance with that of the elongation values obtained in tensile tests. It means that the brittle fracture of pure Zn can be transformed into the ductile fracture by Mn additions. However, the cleavage fracture remains no change in as-extruded Zn-Mg alloys with different Mg contents, while the fracture surface of Zn-Al alloys with many uniform and fine dimple. It can be inferred that the effect of Mn on the fracture mode of as-wrought Zn-based alloys is different from that of Mg, and similar to that of Al in as-wrought Zn-based alloys [18]. But the effect of alloying elements on the fracture mechanism of as-wrought Zn-based alloys is still further investigated based on the systematical microstructure close to the fracture. Fig.3 shows the microstructures as-extruded Zn-Mn alloys obtained by Electron Back-Scattered Diffraction (EBSD). Obviously, the complete recrystallization followed by, the equiaxed grain formation occurs in all as-extruded Zn-Mn alloys. The grain of Zn matrix is remarkably refined due to Mn additions (Fig.3). The 7
average grain size of as-extruded Zn-Mn alloys decreases from 4 to 2 μm with the increasing of Mn contents from 0.2 to 0.6 wt.%, as listed in Table 1. This is far smaller than that of as-extruded pure Zn (82 μm). At the same time, the {10-12} tensile twin is also observed in as-extruded Zn-Mn alloys. The tensile twins with a rotation angle of 86.5° around a <11-20> axis are marked with red lines in Fig.3 (a, c, e, g). The volume fraction of the tensile twin is only 1.4 % in as-extruded pure Zn. The amount of the tensile twin increases to 14.2 % when 0.2 wt.% Mn is added. Subsequently, its volume fraction decreases with the increasing of Mn contents, as listed in Table 1. Besides, most of grains show (0001) orientation in all as-extruded Zn-Mn alloys Fig.3 (b, d, f, h). It means that the texture is independent on the contents of Mn. The XRD patterns of as-extruded Zn-Mn alloys are shown in Fig. 4. The results show that there is only the Zn-rich phase in as-extruded Zn-0.2 Mn alloy. The MnZn13 intermetallic compound is identified when the Mn content is more than 0.4 wt. % (Fig.4). Generally, the tensile strength of most alloys can be enhanced, and their elongation decreases with the increasing of solute contents. However, the role of Mn in as-extruded Zn-Mn alloys is not the case. Although there is the solubility of Mn, refining grain and the increasing amount of MnZn13 phases with the increasing of Mn contents from 0.2 to 0.6 wt. %, the tensile strength is not increased, while the elongation remarkably increases in as-extruded Zn-Mn alloys. The solubility of Mn in Zn matrix is about 0.2% at 200 ℃ according to the Zn-Mn binary phase diagram 8
[25]. It means that the Mn solubility is basically the same in three Zn-Mn alloys indirectly extruded at 200 ℃ with different Mn contents. Therefore, the solid solution strength effect may remain the same in as-extruded Zn-Mn alloys. On the other hand, the average grain size decreases from 4 to 2 μm in as-extruded Zn-Mn alloys with the increasing of Mn contents from 0.2 to 0.6 wt.%. It is well know that grain refinement can simultaneously improve strength and plasticity. It means that both strength and plasticity of as-extruded Zn-Mn alloys should be improved with the increasing of Mn content. Therefore, it is inevitable that there are other factors affecting the mechanical properties of as-extruded Zn-Mn alloys. The twin is a kind of deformation mechanism in HCP metal. Liu et al. have reported that the difference in ultimate tensile strength is very small between the two Mg-3Al-1Zn alloys with distinct grain size. With the grain refinement from 30 to 9 μm in Mg-based alloys, the volume fraction of twins drops from 15 to 10% [26]. It was believed that the twins act as barriers to the movement of dislocations, leading to an increase in flow stress. It also implies that the attribution of 5% twins to the mechanical properties is equivalent to that of grain refining from 30 to 9 μm in Mg-based alloys. The EBSD results show that a large number of tensile twins form in as-extruded Zn-0.2 Mn alloy, and its volume fraction reaches about 14.2 %. Then, the volume fraction of twins decreases to 2.3 % when the Mn contents increases to 0.6 wt. %. Moreover, the phenomenon is also found in the Zn-Mn alloy coats [27]. Unfortunately, no twins were observed in Zn-based alloys containing Mg or Al. On the other hand, the formation mechanism is not still clear and further investigated in the Zn-Mn alloys. So, the effect of Mn 9
additions on the mechanical properties of as-extruded Zn-Mn alloy is the combined role of solid solution, refining grain and twin. Therefore, it can be also inferred that the solid solution, refining grain and twin together affect the mechanical properties in as-extruded Zn-0.2 Mn alloy, while the solution, refining grain and MgZn13 compound in as-extruded Zn-0.6 Mn alloys. Besides, the texture feature has no change with the increasing of Mn contents, which implies the mechanical properties of as-extruded Zn-Mn alloys are not influenced by the texture. Therefore, it can be concluded that the twin plays an important role in determining the mechanical properties of as-extruded Zn-Mn alloys. To sum up, the grain size is refined from 4 to 2 m, and the volume fraction of the tensile twin decreases from 14.2 to 2.3 % with the increasing of Mn contents from 0.2 to 0.6 wt. %. Although the number of the MnZn13 compounds also increases, the tensile strength of as-extruded Zn-Mn alloys decreases from 220 to 182 MPa, while the elongation increases from 48 to 71 %. It means that the tensile twin is a key factor affecting the mechanical properties of as-extruded Zn-Mn alloys. This is largely different from the results obtained in as-extruded binary Zn-based alloys containing Mg or Al. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.51525101, No.51371046), the program for New Century Excellent Talents in University (Grant No. NECT-12-0109), the Fundamental Research Foundation for the Central Universities (Grant No.N130610002，N141008001). 10
References:
[1] P. K. Bowen, J. Drelich, J. Goldman, Adv. Mater. 25 (2013) 2577-2582. [2] N.S. Murni, M.S. Dambatta, S.K. Yeap, G.R.A. Froemming, H. Hermawan, Mater. Sci. Eng. C 49 (2015) 560-566. [3] X.W. Liu, J.K. Sun, Y.H. Yang, Z.J. Pu, Y.F. Zheng, Mater. Lett. 161 (2015) 53-56. [4] J. Cheng, B. Liu, Y.H. Wu, Y.F. Zheng, J. Mater. Sci. Technol. 29 (2013) 619-627. [5] Z.L. Liu, R.Q. Li, R.P. Jiang, X.Q. Li, M.X. Zhang, J. Alloys Compd. 687 (2016) 885-892. [6] C.Z. Yao, Z.C. Wang, S.L. Tay, T.P. Zhu, W. G, J. Alloys Compd. 602 (2014) 101-107. [7] D. Vojtěch, J. Kubásek, J. Šerák, P. Novák, Acta Biomater. 7 (2011) 3515-3522. [8] J. Kubásek, D. Vojtěch, Proc. Metal 5 (2012) 23–25. [9] H.F. Li, X.H. Xie, Y.F. Zheng, Y. Cong, F.Y. Zhou, K.J. Qiu, X. Wang, S.H. Chen, L. Huang, L. Tian, L. Qin, Sci. Rep. 5 (2015) 10719. [10] J.L. Niu, Z.B. Tang, H. Huang, J.Pei, H. Zhang, G.Y. Yang, W.J. Ding, Mater. Sci. Eng. C 69 (2016) 407-413. [11] H.B. Gong, K. Wang, R. Strich, J.G. Zhou, J. Biomed. Mater. Res. B Appl. Biomater. 103 (2015) 1632-1640. [12] H.F. Li, H.T. Yang, Y.F. Zheng, F.Y. Zhou, K.J. Qiu, X. Wang, Mater. Des. 83 (2015) 95-102. 11
[13] Y.M. Zhang, L.J. Yang, X.D. Zeng, B.Z. Zheng, Z.L. Song, Mater. Des. 50 (2013) 223-229. [14] M. T. Abou EI-khair, A. Daoud, A. Ismail, Mater. Lett. 58 (2004) 1754-1760. [15] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater.Sci. 12 (2008) 63-72. [16] J. Kubasek, D. Vojtech, E. Jablonska, I. Pospisilova, J. Lipov, T. Ruml, Mater. Sci. Eng. C 58 (2016) 24-35. [17] X.W. Liu, J.K. Sun, Y.H. Yang, F.Y. Zhou, Z.J. Pu, L. Li, Y.F. Zheng, Mater. Lett. 162 (2016) 242-245. [18] E. Mostaed, M. Sikor-Jasinska, A. Mostaed, S. Loffredo, A.G. Demir, B. Previtli, D. Mantovani, R. Beanland, M. Vedani, J. Mech. Behav. Biomed. Mater. 60 (2016) 581-602. [19] X.W. Liu, J.K. Sun, F.Y. Zhou, Y.H. Yang, Z.J. Pu, L. Li, Y.F. Zheng, Mater. Des. 94 (2016) 95-104. [20] L. Martin, D. Nikolai, G. Stoyko, J. Mater. Eng. Perform. 25 (2016) 3838-3844. [21] K. Kostov, N. Dulgerov, V. Angelova, H. Argirov, J. Univ. Chem. Technol. Metall. 41 (2006) 361-364. [22] E. Koç, M. Bobby Kannan, M. Ünal, E. Candan, J. Alloys Comp. 648 (2015) 291-296. [23] Z.G. Zhang, Z.H. Feng, X.J. Jiang, X.Y. Zhang, M.Z. Ma, R.P. Liu, Mater. Sci. Eng. A 652 (2016) 77-83. [24] S.L. Li, Y.L. Wang, X.T. Wang, Mater. Sci. Eng. A 639 (2015) 640-646. 12
[25] D.D. Huang, S.H. Liu, H.H. Xu, Y. Du, J. Alloys Comp. 688 (2016) 1115-1124. [26] G.D. Liu, R.L. Xin, F.Y. Liu, Q. Liu, Mater. Des. 107 (2016) 503-510. [27] Y.B. Wang, J.M. Zeng, Mater. Des. 69 (2015) 64-69.
13
Fig.1 (a) The engineering stress-strain curves of as-extruded Zn-Mn alloys with different Mn contents and macroscopic images of the samples after tensile tests. (b) Summary of the mechanical properties of as-wrought Zn-based binary alloys in the literatures and this study.
14
Fig. 2 Fracture morphology of as-extruded Zn-Mn alloys with different Mn contents. (a) 0; (b) 0.2; (c) 0.4; (d) 0.6.
15
Fig.3 EBSD maps of as-extruded Zn-Mn alloys with different Mn additions (a) and (b) 16
pure Zn; (c) and (d) Zn-0.2Mn alloy; (e) and (f) Zn-0.4Mn alloy; (g) and (f) Zn-0.6Mn alloy; The {10-12} tensile twins are shown in red (86.5 ° <11-20> ± 5 °).
Fig.4 XRD patterns of as-extruded Zn-Mn alloys with different Mn contents.
17
Table 1 The grain size and volume fraction of twins in as-extruded Zn-Mn alloys Alloy
Grain size
Volume fraction of twins
Zn-0.2 Mn
4 μm
14.2 %
Zn-0.4 Mn
3 μm
11.7 %
Zn-0.6 Mn
2 μm
2.3 %
18
PII: DOI: Reference:
S0921-5093(17)30802-X http://dx.doi.org/10.1016/j.msea.2017.06.037 MSA35175
To appear in: Materials Science & Engineering A Received date: 30 November 2016 Revised date: 16 May 2017 Accepted date: 10 June 2017 Cite this article as: Shineng Sun, Yuping Ren, Liqing Wang, Bo Yang, Hongxiao Li and Gaowu Qin, Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.037 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 galley proof before it is published in its final citable 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.
Abnormal effect of Mn addition on the mechanical properties of as-extruded Zn alloys Shineng Sun, Yuping Ren*, Liqing Wang, Bo Yang, Hongxiao Li, Gaowu Qin* Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
[email protected] [email protected] *Corresponding Author:
1
Abstract: An unusual effect of Mn on the microstructures and mechanical properties of as-extruded Zn alloys has been found in our paper. Although the grain of Zn matrix is remarkably refined, and the number of MnZn13 compounds increases with the increasing of Mn contents, the volume fraction of the tensile twins obviously decreases. As a result, the tensile strength slightly decreases, and the elongation remarkably increases in as-extruded Zn-Mn binary alloys. This phenomenon is largely different from as-extruded Zn-based binary alloys containing Mg or Al. It means that the tensile twin largely signifies the comprehensive mechanical properties of as-extruded Zn-Mn alloys. Keywords: Zn-Mn alloys; Extrusion; Electron backscattering diffraction (EBSD); Twin; Grain refining.
1. Introduction Recently, Zn and Zn-based alloys have been extensively investigated with interest as the newest biodegradable metallic materials, due to their excellent biological compatibility and moderate bio-corrosion rate [1-3]. However, the mechanical properties of as-cast or deformed pure Zn is too poor to satisfy the requirement of implant devices [4-6]. Although the tensile strength of as-cast Zn-Mg binary alloys was improved, the plasticity remarkably decreased [7, 8]. Therefore, as-wrought Zn-based alloys containing Al, Mg, Cu, Sr or Ca have been developed in order to improve the combined properties [9-12]. Although the comprehensive mechanical properties of the Zn based binary alloys containing slight Al contents can 2
be improved by hot extrusion due to the grain refinement [13, 14], the Al element is a harmful element in human nutrition so that the Zn-Al-based Zn alloys are unsuitable for biodegradable implant materials [15]. As one of necessary trace element in human body, Mg has been in detail investigated as a new alloying element in as-wrought Zn-based binary alloys [16, 17]. The ultimate tensile strength of as-extruded Zn-Mg binary alloys increased from 250 to 340 MPa, but the elongation dramatically decreased from 22 to 6 % with increasing Mg contain from 0.15 to 1 wt.%, which is attributed to the collective role of grain refinement and second phase (Mg2Zn11) [18]. However, the effect of Ca or Sr contents on the mechanical properties of as-wrought Zn-based binary alloy is still not adequately investigated. On the other hand, Ca, Sr or Mn was selected to further alloying of as-extruded or rolled Zn-1Mg alloys in order to improving the mechanical properties of Zn-based alloys without losing the good biocompatibility. However, the effects of Ca or Sr the mechanical properties of as-wrought Zn-1Mg alloys were not obvious. It is noted that the ultimate tensile strength was improved from 210 to 300 MPa, and elongation from 8 to 25 %, respectively, when the as-rolled Zn-1Mg alloy was added by 0.1 wt.% Mn under the similar forming process [9, 19]. Unfortunately, the microstructure mechanism for the improvement in the comprehensive properties is not clearly interpreted in as-rolled Zn-1Mg-0.1Mn alloys. Moreover, although the superplasticity of as-extruded Zn-Mn alloys at 280~320 ℃ has been investigated, the effect of Mn additions on the room temperature mechanical properties of as-wrought Zn-based alloys is little noticed [20, 21]. Therefore, in the present work, the Zn-Mn binary 3
alloys with different Mn contents were prepared by indirect extrusion method. And then, the influence of Mn contents on the microstructure and tensile properties has been investigated in order to understand the relationship between the microstructure and property in as-extruded Zn-Mn binary alloys. Finally, the valuable data are to be provided for the composition design and process optimization of as-wrought Zn-based alloys containing Mn. 2. Experimental procedures High pure Zn (99.99 wt. %) and Zn-10 Mn (wt. %) master alloys were used to prepare four experimental alloys Zn-X Mn (X=0, 0.2, 0.4, 0.6 wt. %). The molten alloy at 500 ℃ was casted into a cone-shaped steel mould and cooled down in air. As-cast ingots were held at 300 ℃ for 4h, and then air cooled. These homogeneous ingots were extruded at 200 ℃ with an extrusion ratio of 16, and then air cooled to room temperature. The mechanical properties of as-extruded Zn-Mn alloys were determined by room temperature tensile tests at a strain rate of 10-3 s-1 using an AG-X universal testing machine. Dog-bone shaped specimens were 5 mm in diameter with 25 mm gauge length. The specimens were cut from as-extruded Zn-Mn alloys where their tensile axis is parallel to the extrusion direction. For each composition, three experiments were at least conducted to check the repeatability of the results. Fracture morphology of the specimens after the tensile test was examined using scanning electron microscope (SEM) in order to determine the fracture modes. The microstructure along the extrusion direction of as-extruded Zn-Mn alloys 4
was observed, and the samples were ground with SiC abrasive papers from 400 to 2000 # and polished by diamond pates with 2.5 μm particles. Grain size and twin feature were obtained by electron backscatter diffraction (EBSD) technique and Channel 5 analysis system (Oxford HKL) in combination with a JEM 7001F field emission scanning electron microscope operated at an accelerating voltage of 20 kV. The phase constituents of as-extruded Zn-Mn alloys were determined by X-ray diffraction (XRD) using copper Ka X-ray radiation. 3. Results The engineering stress-strain curves of as-extruded Zn-Mn alloys with different Mn contents are shown in Fig.1 (a). The ultimate tensile strength and elongation as-extruded pure Zn are 117 MPa and 14 %, respectively, which are close to the previously reported values the tensile properties of extruded pure Zn [9]. Both ultimate tensile strength and elongation of Zn-Mn alloys increase due to minor Mn additions. However with the increasing of Mn content from 0.2 to 0.6 wt. %, the ultimate tensile strength and the yield strength of as-extruded Zn-Mn alloys decrease from 220 to 182 MPa and from 132 to 118 MPa, respectively, while the elongation remarkably increases from 48 to 71 %. It shows that the plasticity of as-wrought Zn-based alloys can be clearly improved when a small amount of Mn is added. The tensile strength and elongation of as-extruded Zn-Mn alloys are illustrated in Fig. 1 (b), the results obtained in other as-wrought Zn-based binary alloys are shown for comparison [9, 18]. As the Mn content increases, the ultimate tensile strength decreases and the elongation increases. However, this is not the case in other reported 5
Zn-based alloys containing Mg and Al elements. As the Mg or Al content increases, the ultimate tensile strength increases and the elongation decreases. The ultimate tensile strength of as-extruded Zn-Mg alloys increases from 250 to 400 MPa, while the elongation remarkably decreases from 22 to 1 % with the increasing of Mg content from 0.15 to 3 wt. %. It is believed that the mechanical properties of Zn-Mg alloys attributed to the existence of the hard and brittle Mg2Zn11 intermetallic compounds [18]. In fact, the Mg2Zn11 particles are large and distributed in the shape of continuous net at the grain boundaries of Zn matrix with the Mg additions. Moreover, its volume fraction also increased. Finally, the plasticity of as-extruded Zn-Mg alloys remarkably decreased. The ultimate tensile strength of as-extruded Zn-Al alloys increase from 203 to 223 MPa, while the elongation dramatically drops from 33 to 24 % with Al contents from 0.5 to 1 wt.%. Unlike Zn alloys containing Mg, no second phase formed in Zn-Al alloys. It is believed that solid solution strengthening is also responsible for improved mechanical properties [18]. In general, the solute atom can make the strength improve, and the plasticity reduces, such as the role of solute atom in the Fe, Mg or Zr-based alloys [22-24]. Obviously, the role of Mn is remarkably different from that of other elements such as Mg or Al in as-wrought Zn-based alloys [18]. Therefore, it shows that Mn has totally different effect on the mechanical properties in as-extruded Zn-based alloys. The Zn-Mn alloys show enhanced strength and ductility at low content (0.2 wt.%), decrease strength and increase ductility at high contents (0.4 and 0.6 wt.%). The fracture surfaces of as-extruded Zn-Mn alloys after tensile tests are observed, 6
as shown in Fig.2. The intragranular cracks, cleavage steps and cleavage planes are surrounded by tearing ridges in pure Zn (Fig.2 (a)), which indicates typical cleavage fracture mode. As-extruded pure Zn exhibited typical cleavage fracture which is consistent with its low elongation to fracture. At the same time, the obvious neck can be observed in Zn-Mn alloys, as seen in the macro-photo inserted in Fig. 1(a). The fracture surface consists of dimples (Fig.2 (b)-(d)), indicating the ductile fracture mode in as-extruded Zn-Mn binary alloys. Moreover, the number of fine dimples of as-extruded Zn-Mn alloys increases with the increasing of Mn contents from 0.2 to 0.6 wt. % (Fig.2 (b)-(d)). This is in good accordance with that of the elongation values obtained in tensile tests. It means that the brittle fracture of pure Zn can be transformed into the ductile fracture by Mn additions. However, the cleavage fracture remains no change in as-extruded Zn-Mg alloys with different Mg contents, while the fracture surface of Zn-Al alloys with many uniform and fine dimple. It can be inferred that the effect of Mn on the fracture mode of as-wrought Zn-based alloys is different from that of Mg, and similar to that of Al in as-wrought Zn-based alloys [18]. But the effect of alloying elements on the fracture mechanism of as-wrought Zn-based alloys is still further investigated based on the systematical microstructure close to the fracture. Fig.3 shows the microstructures as-extruded Zn-Mn alloys obtained by Electron Back-Scattered Diffraction (EBSD). Obviously, the complete recrystallization followed by, the equiaxed grain formation occurs in all as-extruded Zn-Mn alloys. The grain of Zn matrix is remarkably refined due to Mn additions (Fig.3). The 7
average grain size of as-extruded Zn-Mn alloys decreases from 4 to 2 μm with the increasing of Mn contents from 0.2 to 0.6 wt.%, as listed in Table 1. This is far smaller than that of as-extruded pure Zn (82 μm). At the same time, the {10-12} tensile twin is also observed in as-extruded Zn-Mn alloys. The tensile twins with a rotation angle of 86.5° around a <11-20> axis are marked with red lines in Fig.3 (a, c, e, g). The volume fraction of the tensile twin is only 1.4 % in as-extruded pure Zn. The amount of the tensile twin increases to 14.2 % when 0.2 wt.% Mn is added. Subsequently, its volume fraction decreases with the increasing of Mn contents, as listed in Table 1. Besides, most of grains show (0001) orientation in all as-extruded Zn-Mn alloys Fig.3 (b, d, f, h). It means that the texture is independent on the contents of Mn. The XRD patterns of as-extruded Zn-Mn alloys are shown in Fig. 4. The results show that there is only the Zn-rich phase in as-extruded Zn-0.2 Mn alloy. The MnZn13 intermetallic compound is identified when the Mn content is more than 0.4 wt. % (Fig.4). Generally, the tensile strength of most alloys can be enhanced, and their elongation decreases with the increasing of solute contents. However, the role of Mn in as-extruded Zn-Mn alloys is not the case. Although there is the solubility of Mn, refining grain and the increasing amount of MnZn13 phases with the increasing of Mn contents from 0.2 to 0.6 wt. %, the tensile strength is not increased, while the elongation remarkably increases in as-extruded Zn-Mn alloys. The solubility of Mn in Zn matrix is about 0.2% at 200 ℃ according to the Zn-Mn binary phase diagram 8
[25]. It means that the Mn solubility is basically the same in three Zn-Mn alloys indirectly extruded at 200 ℃ with different Mn contents. Therefore, the solid solution strength effect may remain the same in as-extruded Zn-Mn alloys. On the other hand, the average grain size decreases from 4 to 2 μm in as-extruded Zn-Mn alloys with the increasing of Mn contents from 0.2 to 0.6 wt.%. It is well know that grain refinement can simultaneously improve strength and plasticity. It means that both strength and plasticity of as-extruded Zn-Mn alloys should be improved with the increasing of Mn content. Therefore, it is inevitable that there are other factors affecting the mechanical properties of as-extruded Zn-Mn alloys. The twin is a kind of deformation mechanism in HCP metal. Liu et al. have reported that the difference in ultimate tensile strength is very small between the two Mg-3Al-1Zn alloys with distinct grain size. With the grain refinement from 30 to 9 μm in Mg-based alloys, the volume fraction of twins drops from 15 to 10% [26]. It was believed that the twins act as barriers to the movement of dislocations, leading to an increase in flow stress. It also implies that the attribution of 5% twins to the mechanical properties is equivalent to that of grain refining from 30 to 9 μm in Mg-based alloys. The EBSD results show that a large number of tensile twins form in as-extruded Zn-0.2 Mn alloy, and its volume fraction reaches about 14.2 %. Then, the volume fraction of twins decreases to 2.3 % when the Mn contents increases to 0.6 wt. %. Moreover, the phenomenon is also found in the Zn-Mn alloy coats [27]. Unfortunately, no twins were observed in Zn-based alloys containing Mg or Al. On the other hand, the formation mechanism is not still clear and further investigated in the Zn-Mn alloys. So, the effect of Mn 9
additions on the mechanical properties of as-extruded Zn-Mn alloy is the combined role of solid solution, refining grain and twin. Therefore, it can be also inferred that the solid solution, refining grain and twin together affect the mechanical properties in as-extruded Zn-0.2 Mn alloy, while the solution, refining grain and MgZn13 compound in as-extruded Zn-0.6 Mn alloys. Besides, the texture feature has no change with the increasing of Mn contents, which implies the mechanical properties of as-extruded Zn-Mn alloys are not influenced by the texture. Therefore, it can be concluded that the twin plays an important role in determining the mechanical properties of as-extruded Zn-Mn alloys. To sum up, the grain size is refined from 4 to 2 m, and the volume fraction of the tensile twin decreases from 14.2 to 2.3 % with the increasing of Mn contents from 0.2 to 0.6 wt. %. Although the number of the MnZn13 compounds also increases, the tensile strength of as-extruded Zn-Mn alloys decreases from 220 to 182 MPa, while the elongation increases from 48 to 71 %. It means that the tensile twin is a key factor affecting the mechanical properties of as-extruded Zn-Mn alloys. This is largely different from the results obtained in as-extruded binary Zn-based alloys containing Mg or Al. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No.51525101, No.51371046), the program for New Century Excellent Talents in University (Grant No. NECT-12-0109), the Fundamental Research Foundation for the Central Universities (Grant No.N130610002，N141008001). 10
References:
[1] P. K. Bowen, J. Drelich, J. Goldman, Adv. Mater. 25 (2013) 2577-2582. [2] N.S. Murni, M.S. Dambatta, S.K. Yeap, G.R.A. Froemming, H. Hermawan, Mater. Sci. Eng. C 49 (2015) 560-566. [3] X.W. Liu, J.K. Sun, Y.H. Yang, Z.J. Pu, Y.F. Zheng, Mater. Lett. 161 (2015) 53-56. [4] J. Cheng, B. Liu, Y.H. Wu, Y.F. Zheng, J. Mater. Sci. Technol. 29 (2013) 619-627. [5] Z.L. Liu, R.Q. Li, R.P. Jiang, X.Q. Li, M.X. Zhang, J. Alloys Compd. 687 (2016) 885-892. [6] C.Z. Yao, Z.C. Wang, S.L. Tay, T.P. Zhu, W. G, J. Alloys Compd. 602 (2014) 101-107. [7] D. Vojtěch, J. Kubásek, J. Šerák, P. Novák, Acta Biomater. 7 (2011) 3515-3522. [8] J. Kubásek, D. Vojtěch, Proc. Metal 5 (2012) 23–25. [9] H.F. Li, X.H. Xie, Y.F. Zheng, Y. Cong, F.Y. Zhou, K.J. Qiu, X. Wang, S.H. Chen, L. Huang, L. Tian, L. Qin, Sci. Rep. 5 (2015) 10719. [10] J.L. Niu, Z.B. Tang, H. Huang, J.Pei, H. Zhang, G.Y. Yang, W.J. Ding, Mater. Sci. Eng. C 69 (2016) 407-413. [11] H.B. Gong, K. Wang, R. Strich, J.G. Zhou, J. Biomed. Mater. Res. B Appl. Biomater. 103 (2015) 1632-1640. [12] H.F. Li, H.T. Yang, Y.F. Zheng, F.Y. Zhou, K.J. Qiu, X. Wang, Mater. Des. 83 (2015) 95-102. 11
[13] Y.M. Zhang, L.J. Yang, X.D. Zeng, B.Z. Zheng, Z.L. Song, Mater. Des. 50 (2013) 223-229. [14] M. T. Abou EI-khair, A. Daoud, A. Ismail, Mater. Lett. 58 (2004) 1754-1760. [15] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater.Sci. 12 (2008) 63-72. [16] J. Kubasek, D. Vojtech, E. Jablonska, I. Pospisilova, J. Lipov, T. Ruml, Mater. Sci. Eng. C 58 (2016) 24-35. [17] X.W. Liu, J.K. Sun, Y.H. Yang, F.Y. Zhou, Z.J. Pu, L. Li, Y.F. Zheng, Mater. Lett. 162 (2016) 242-245. [18] E. Mostaed, M. Sikor-Jasinska, A. Mostaed, S. Loffredo, A.G. Demir, B. Previtli, D. Mantovani, R. Beanland, M. Vedani, J. Mech. Behav. Biomed. Mater. 60 (2016) 581-602. [19] X.W. Liu, J.K. Sun, F.Y. Zhou, Y.H. Yang, Z.J. Pu, L. Li, Y.F. Zheng, Mater. Des. 94 (2016) 95-104. [20] L. Martin, D. Nikolai, G. Stoyko, J. Mater. Eng. Perform. 25 (2016) 3838-3844. [21] K. Kostov, N. Dulgerov, V. Angelova, H. Argirov, J. Univ. Chem. Technol. Metall. 41 (2006) 361-364. [22] E. Koç, M. Bobby Kannan, M. Ünal, E. Candan, J. Alloys Comp. 648 (2015) 291-296. [23] Z.G. Zhang, Z.H. Feng, X.J. Jiang, X.Y. Zhang, M.Z. Ma, R.P. Liu, Mater. Sci. Eng. A 652 (2016) 77-83. [24] S.L. Li, Y.L. Wang, X.T. Wang, Mater. Sci. Eng. A 639 (2015) 640-646. 12
[25] D.D. Huang, S.H. Liu, H.H. Xu, Y. Du, J. Alloys Comp. 688 (2016) 1115-1124. [26] G.D. Liu, R.L. Xin, F.Y. Liu, Q. Liu, Mater. Des. 107 (2016) 503-510. [27] Y.B. Wang, J.M. Zeng, Mater. Des. 69 (2015) 64-69.
13
Fig.1 (a) The engineering stress-strain curves of as-extruded Zn-Mn alloys with different Mn contents and macroscopic images of the samples after tensile tests. (b) Summary of the mechanical properties of as-wrought Zn-based binary alloys in the literatures and this study.
14
Fig. 2 Fracture morphology of as-extruded Zn-Mn alloys with different Mn contents. (a) 0; (b) 0.2; (c) 0.4; (d) 0.6.
15
Fig.3 EBSD maps of as-extruded Zn-Mn alloys with different Mn additions (a) and (b) 16
pure Zn; (c) and (d) Zn-0.2Mn alloy; (e) and (f) Zn-0.4Mn alloy; (g) and (f) Zn-0.6Mn alloy; The {10-12} tensile twins are shown in red (86.5 ° <11-20> ± 5 °).
Fig.4 XRD patterns of as-extruded Zn-Mn alloys with different Mn contents.
17
Table 1 The grain size and volume fraction of twins in as-extruded Zn-Mn alloys Alloy
Grain size
Volume fraction of twins
Zn-0.2 Mn
4 μm
14.2 %
Zn-0.4 Mn
3 μm
11.7 %
Zn-0.6 Mn
2 μm
2.3 %
18