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Effects of cerium on as-cast microstructure and mechanical properties of Mg–3Sn–2Ca magnesium alloy
Materials Science and Engineering A 512 (2009) 132–138
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of cerium on as-cast microstructure and mechanical properties of Mg–3Sn–2Ca magnesium alloy Mingbo Yang a,b,∗ , Fusheng Pan b , Liang Cheng a , Jia Shen a 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 5 October 2008 Received in revised form 9 January 2009 Accepted 13 February 2009 Keywords: Magnesium alloy Mg–3Sn–2Ca alloy Cerium CaMgSn phase Mechanical properties
a b s t r a c t The Mg–3Sn–2Ca–xCe (x = 0–2.0 wt.%) alloys were prepared by permanent mould casting method, the effects of Ce on the as-cast microstructure and mechanical properties of the alloys were investigated. The results indicated that the volume fraction and size of CaMgSn phase in the Mg–3Sn–2Ca alloy respectively were decreased by adding 0.5–2.0 wt.%Ce, and the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce was relatively smaller. The main phases in the as-cast Mg–3Sn–2Ca alloys with and without adding Ce were ␣-Mg, CaMgSn and Mg2 Ca phases, and Mg12 Ce phase were found in the alloys added more than 1.0 wt.%Ce. The addition of Ce improved the tensile and creep properties of Mg–3Sn–2Ca alloy, and the mechanical properties of Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce were relatively higher. The strengthening mechanism of Ce-containing Mg–3Sn–2Ca alloys was mainly attributed to the refinement of CaMgSn phase. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are the lightest structural alloys commercially available and have great potential for applications in automotive, aerospace industries and others. However, in recent years, improving the elevated temperature properties has become a critical issue for possible application of magnesium alloys in hot component [1,2]. At present, efforts are being made towards developing new corrosion and creep-resistant magnesium alloys based on Mg–Sn–Ca system [3–6], where Sn goes into solid solution with Mg and imparts corrosion resistance while Ca gives creep resistance by forming stable intermetallic particles in the matrix. From recent investigations the Mg–3Sn–2Ca alloy has been identified as one of the most promising magnesium alloys [3,4,7–9]. It has been shown that the Mg–3Sn–2Ca magnesium alloy offers superior creep properties even compared to the creep-resistant magnesium alloy AE42. According to the investigations of Hort et al. [9], the improving of creep properties for Mg–3Sn–2Ca alloy mainly contributes to the CaMgSn and Mg2 Ca phases in the alloy, especially the CaMgSn phase with higher thermal stability. However, it is further reported that the Mg–3Sn–2Ca alloy still do not meet the needs of industrial application due to the poor mechanical properties resulted from the
∗ 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 © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.02.038
coarse CaMgSn phase in the alloy [6]. Therefore, further enhancement in the mechanical properties by alloying and/or microalloying for Mg–3Sn–2Ca alloy needs to be considered. But up to now, the investigations about the effects of alloying and microalloying on the as-cast microstructure and mechanical properties of Mg–3Sn–2Ca alloy are very rare. Due to the above-mentioned reasons, the present work investigates the influence of cerium (Ce), which has been successfully used to improve the mechanical properties of other magnesium alloy systems such as Mg–Al and Mg–Zn [10,11], on the as-cast microstructure and mechanical properties of Mg–3Sn–2Ca alloy. 2. Experimental procedures The Mg–3Sn–2Ca–xCe (x = 0–2.0 wt.%) alloys were prepared from commercially pure Mg and Sn (>99.9 wt.%), the Ca and Ce were added in the form of Mg–19 wt.%Ca and Mg–29 wt.%Ce master alloys, respectively. The experimental alloys were melted in a crucible resistance furnace and protected by a flux addition. After Mg–Ce master alloy was added to the melt at 740 ◦ C, the melt was homogenized by mechanical stirring. After complete mixing, the melt was held at 740 ◦ C for 20 min and then poured into a preheated permanent mould in order to obtain a casting. Furthermore, the samples of experimental alloys were subjected to a solution heat treatment (500 ◦ C/6 h, water cooled) in order to examine the microstructural stability at high temperatures. The specimens whose size has been reported previously [2] were fabricated from
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Table 1 Actual compositions of the as-cast experimental alloys, wt.%. Experimental alloys #
1 2# 3# 4# 5#
Fig. 1. XRD results of the as-cast experimental alloys.
alloy (Mg–3Sn–2Ca) alloy (Mg–3Sn–2Ca–0.5Ce) alloy (Mg–3Sn–2Ca–1.0Ce) alloy (Mg–3Sn–2Ca–1.5Ce) alloy (Mg–3Sn–2Ca–2.0Ce)
Sn
Ca
Ce
Mg
2.92 2.89 2.91 2.93 2.92
1.89 1.90 1.92 1.91 1.90
– 0.45 0.88 1.38 1.81
Bal. Bal. Bal. Bal. Bal.
the casting for tensile and creep tests. The actual chemical compositions of experimental alloys were listed in Table 1. The samples of experimental alloys were etched with an 8% nitric acid distilled water solution, and then examined by using an Olympus Optical microscope and Joel/JSM-6460LV type scanning electron microscope (SEM) equipped with Oxford energy dispersive spectrometer (EDS) with an operating voltage of 20 kV. The phases in the experimental alloys were analyzed by D/Max-1200X type X-ray diffraction (XRD) operated at 40 kV and 30 mA. The tensile properties of experimental alloys at room temperature and 150 ◦ C were determined from a complete stress–strain curve. The ultimate tensile strength (UTS), 0.2% yield strength (YS)
Fig. 2. Optical images of the as-cast experimental alloys: (a) 1# alloy; (b) 2# alloy; (c) 3# alloy; (d) 4# alloy; (e) 5# alloy.
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Fig. 3. SEM images of the as-cast experimental alloys: (a) and (b) 1# alloy; (c) 2# alloy; (d) 3# alloy; (e) 4# alloy; (f) 5# alloy.
and elongation to failure (Elong.) were obtained based on the average of three tests. The constant-load tensile creep tests were performed at 150 ◦ C and 70 MPa for creep extension up to 100 h. The total creep strain and minimum creep rates of experimental alloys were respectively measured from each elongation–time curve and averaged over three tests. 3. Results and discussion 3.1. As-cast microstructure Fig. 1 shows the XRD results of as-cast experimental alloys. At present, two main phases in Mg–3Sn–2Ca alloy have been reported [7–9]. One is identified as CaMgSn phase, and the other one is Mg2 Ca phase. Apparently, adding 0.5 wt.%Ce to Mg–3Sn–2Ca alloy does not cause the formation of any new phases according to the information from Fig. 1. However, the Mg12 Ce new phase is detected in the Mg–3Sn–2Ca alloys added more than 1.0 wt.%Ce. Furthermore, it is found from Fig. 1 that the CaMgSn phases in the 2–5# alloys have lower peak intensities, indicating that the volume fraction of
CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys is lower than that in the Mg–3Sn–2Ca alloy. Figs. 2 and 3 show the optical and SEM images of as-cast experimental alloys, respectively. It is found from Fig. 2 that the experimental alloys are composed of primary ␣-Mg and secondary solidification phases (grey and black precipitates). According to the EDS and XRD results, the grey second phases whose amount is very large are identified as CaMgSn with a typical lamellar morphology (Fig. 3), that the black phases are Mg2 Ca whose typical morphology is shown in Fig. 3b. In addition, the bright CaMgSn phase with a blocky morphology is also identified in the four as-cast experimental alloys, as shown in Fig. 3a. Furthermore, it is found from Figs. 2 and 3 that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy causes an interesting microstructural change. The volume fraction of CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys is lower than that in the Mg–3Sn–2Ca alloy, which is consistent to the XRD results (Fig. 1). In addition, it is also observed from Figs. 2 and 3 that the CaMgSn phases in the 2–5# alloy seem to have smaller size than those in the 1# alloy, indicating that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy can effectively refine the CaMgSn phase in
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Fig. 4. Surface scanning results of the 2# as-cast experimental alloy.
the alloy. Although the exact measurement of size for the CaMgSn phase is difficult because of its complex shape, the average size (in terms of area) of the CaMgSn phase measured before refining is 545 m2 for the 1# alloy and after refining its average size respectively is 64 m2 , 53 m2 , 39 m2 and 35 m2 for the 2# , 3# , 4# and 5# alloys. Based on the above results, it is inferred that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy not only can suppress the formation of CaMgSn phase in the alloy but also can refine the CaMgSn phase in the alloy. At the same time, the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce was relatively smaller. It is well known that the Ce atom has larger atomic radius than the Sn atom (Ce: 0.183 nm; Sn: 0.141 nm). After adding Ce to Mg–3Sn–2Ca alloy, the Ce element is rich in the solid–liquid interface during the solidification process due to the low solid solubility in ␣-Mg matrix. Fig. 4 and Table 2 show the surface scanning and EDS results of the 2# as-cast experimental alloy, respectively. From Fig. 4 it is observed that the Ca, Sn and Ce elements mainly distribute at the grain boundaries, especially the Ce and Sn elements. Moreover, from Table 2 it is found that the concentrations of Sn element in the ␣-Mg matrix and CaMgSn phase at the grain boundaries for the 2# alloy respectively are lower and higher than those for the 1# alloy. Therefore, it is inferred that the above observed low volume fraction and refinement of CaMgSn phase in the Ce-containing
Table 2 EDS results of the 1# and 2# as-cast experimental alloys, wt.%. Elements
1# alloy ␣-Mg
Ca Sn Ce Mg Total
0.99 0.14 – 98.87 100
2# alloy CaMgSn 12.17 23.56 – 64.27 100
Mg2 Ca 14.46 0.13 – 85.41 100
␣-Mg 0.82 0.04 – 99.14 100
CaMgSn 13.26 26.62 0.87 59.25 100
Mg2 Ca 17.65 0.27 0.45 81.63 100
Mg–3Sn–2Ca alloys are possibly related to the Ce enrichment which hinders the Sn atom diffusion and induces the constitution undercooling at the solidification interface front [10]. In addition, it is also inferred that the Mg12 Ce compound which has high melting point and heat resistance, can possibly restrain the growth of CaMgSn phase during the solidification. Consequently, the CaMgSn phases in the 3–5# experimental alloys exhibit smaller size than the 2# experimental alloy. In spite of the above analyses, the exact reason for the observed low volume fraction and refinement of CaMgSn phase in the Ce-containing Mg–3Sn–2Ca alloys are not completely clear. Further investigations need to be considered. 3.2. Mechanical properties The tensile properties including ultimate tensile strength (UTS), 0.2% yield strength (YS), elongation (Elong.) and creep properties of the as-cast experimental alloys, are listed in Table 3. It is observed from Table 3 that the Mg–3Sn–2Ca alloys with and without adding Ce have very high creep-resistant properties at 150 ◦ C and 70 MPa for 100 h. Since the creep-resistant properties of magnesium alloys are mainly related to the structure stability at high temperatures, the solutionized microstructures of 1–4# experimental alloys are examined (Fig. 5). Compared with Figs. 2, 3 and 5, it is found that the solutionized microstructures of experimental alloys seem to be similar to their as-cast microstructures. The CaMgSn phases in the four solutionized alloys are still observed clearly, indicating that the phase does not dissolve into the matrix after solid solution heat treatment. However, compared with Figs. 3b, 5b and 5d, it is found that the Mg2 Ca phases in the experimental alloys seem to have dissolved into the matrix after solid solution treatment. Obviously, the high creep-resistant properties for Mg–3Sn–2Ca alloys with and without adding Ce are mainly attributed to the CaMgSn phase in the alloys due to its high thermal stability. Further, it is found from Table 3 that the tensile and creepresistant properties of the 2–5# alloys are higher than those of the 1# alloy. For example, after adding 2.0 wt.%Ce to Mg–3Sn–2Ca
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Fig. 5. SEM images of the solutionized experimental alloys: (a) and (b) 1# alloy; (c) and (d) 2# alloy; (e) 3# alloy; (f) 4# alloy.
alloy, the ultimate tensile strength, yield strength and elongation of the alloy are increased by 24.4%, 28.6% and 73.7% at room temperature, 22.4%, 28.8% and 56% at 150 ◦ C, separately. At the same time, the minimum creep rate of the alloy at 150 ◦ C and 70 MPa for 100 h decreases from 5.72 × 10−8 s−1 to 2.81 × 10−8 s−1 . The above results indicate that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy can improve the mechanical properties of the alloy. This situation is possibly related to the refinement of CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys. It is well known that the
presence of fine and uniform phases distributed along the grain boundaries is easier to act as an effective straddle to the dislocation motion thus improving the properties of engineering alloys [12]. Apparently, the coarse CaMgSn phases in the 1# alloy would give a detrimental effect on the mechanical properties of the alloy since the cracks can easily nucleate along the interface between CaMgSn particle and ␣-Mg matrix [6]. However, after adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy, the CaMgSn phase in the alloy is refined, then the extending tendency of microcracks will decrease. Accord-
Table 3 Tensile and creep properties of the as-cast experimental alloys. Experimental alloys
Tensile properties
Creep properties 150 ◦ C
Room temperature
#
1 2# 3# 4# 5#
alloy alloy alloy alloy alloy
150 ◦ C and 70 MPa for 100 h
UTS (MPa)
YS (MPa)
Elong. (%)
UTS (MPa)
YS (MPa)
Elong. (%)
Total creep strain (%)
Minimum creep rate (×10−8 s−1 )
127 142 149 157 158
112 128 134 140 144
1.9 3.1 3.2 3.4 3.3
116 132 138 142 142
104 117 127 131 134
9.1 13.4 13.8 14.9 14.2
2.06 1.76 1.29 1.07 1.01
5.72 4.89 3.58 2.97 2.81
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Fig. 6. SEM images of tensile fractographs for the as-cast experimental alloys tested at room temperature: (a) 1# alloy; (b) local magnification of (a); (c) 2# alloy; (d) local magnification of (c); (e) 3# alloy; (f) 4# alloy.
ingly, the mechanical properties of the alloy are improved. Since the CaMgSn phases in the 3–5# experimental alloys have relatively smaller size than the 2# experimental alloy, and the Mg12 Ce phases in the 3–5# experimental alloys are also beneficial to the improvement of creep-resistant properties, consequently the 3–5# experimental alloys exhibit higher mechanical properties than the 2# experimental alloy. In addition, although the volume fraction of CaMgSn phases in the 2–5# experimental alloy is lower than that of the CaMgSn phases in the 1# experimental alloy, which commonly results in the decreasing of strength, the 2–5# experimental alloys still obtain higher tensile and creep-resistant properties than the 1# experimental alloy, especially obtains higher elongation. The reason is also possibly related to the obvious refinement of CaMgSn phases in the 2–5# experimental alloys. 3.3. Fracture behaviour Fig. 6 shows the SEM images of the tensile fracture surfaces for the 1–4# experimental alloys tested at room temperature. As
shown in Fig. 6, an amount of cleavage planes and steps are present and some minute lacerated ridges can also be observed in the local areas of tensile fracture surfaces for the four alloys, which have mixed characteristics of cleavage and quasi-cleavage fractures. However, the fracture surface of Mg–3Sn–2Ca alloy exhibit large cleavage-type facets (arrow ‘A’ in Fig. 6a). On the other hand, in the fracture surfaces of Ce-containing Mg–3Sn–2Ca alloys, only the small cleavage-type facets are observed (arrow ‘B’ in Fig. 6d). This is one of possible reasons for the difference in the tensile properties especially the elongation for the Mg–3Sn–2Ca alloys with and without adding Ce. 4. Conclusions (1) The volume fraction and size of CaMgSn phase in the Mg–3Sn–2Ca alloy respectively are decreased by adding 0.5–2.0 wt.%Ce, and the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce is relatively smaller. The phase compositions of Mg–3Sn–2Ca alloy are ␣-Mg, CaMgSn and Mg2 Ca phases. Adding 0.5 wt.%Ce to Mg–3Sn–2Ca
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alloy does not cause the formation of any new phases in the alloy. However, the Mg12 Ce phase is formed in the Mg–3Sn–2Ca alloys added 1.0–2.0 wt.%Ce. (2) The Ce addition obviously improves the mechanical properties of Mg–3Sn–2Ca alloy at room temperature and 150 ◦ C, and the mechanical properties of Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce are relatively higher. After adding 2.0 wt.%Ce to Mg–3Sn–2Ca alloy, the ultimate tensile strength, yield strength and elongation of the alloy are increased by 24.4%, 28.6% and 73.7% at room temperature, 22.4%, 28.8% and 56% at 150 ◦ C, separately. At the same time, the minimum creep rate of the alloy at 150 ◦ C and 70 MPa for 100 h decreases from 5.72 × 10−8 s−1 to 2.81 × 10−8 s−1 . The strengthening mechanism of Ce-containing Mg–3Sn–2Ca alloys is mainly attributed to the refinement of CaMgSn phase. 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).
References [1] M.B. Yang, F.S. Pan, J. Zhang, J. Zhang, Mater. Sci. Forum 488–489 (2005) 923–926. [2] M.B. Yang, F.S. Pan, R.J. Cheng, J. Shen, Mater. Sci. Eng. A 489 (2008) 413–418. [3] T. Abuleil, N. Hort, H. Dieringa, C. Blawert, Y. Huang, K.U. Kainer, K.P. Rao, Proceedings of the Magnesium Technology in the Global Age, 45th Annual Conference of Metallurgists of CIM, Montreal, Canada, October, 2006, pp. 739–749. [4] K.P. Rao, Y.V.R.K. Prasad, N. Hort, Y. Huang, K.U. Kainer, Key Eng. Mater. 340–341 (2007) 89–94. [5] Y. Huang, N. Hort, T.A. Leil, K.U. Kainer, Y. Liu, Key Eng. Mater. 345–346 (2007) 561–564. [6] T. Abuleil, K.P. Rao, N. Hort, Y. Huang, C. Blawert, H. Dieringa, K.U. Kainer, Proceedings of the Magnesium Technology Symposium (TMS 2007), 136th Annual Meeting and Exhibition of TMS (The Minerals, Metals and Materials Society), The Minerals, Metals & Materials Society (TMS), Orlando, USA, 25 February–1 March, 2007, pp. 257–262. [7] A. Kozlov, M. Ohno, R. Arroyave, Z.K. Liu, R. Schmid-Fetzer, Intermetallics 16 (2008) 299–315. [8] A. Kozlov, M. Ohno, T. Abu Leil, N. Hort, K.U. Kainer, R. Schmid-Fetzer, Intermetallics 16 (2008) 316–321. [9] N. Hort, Y. Huang, T.A. Leil, P. Maier, K.U. Kainer, Adv. Eng. Mater. 8 (2006) 359–364. [10] W.L. Xiao, S.S. Jia, J. Wang, Y.M. Wu, L.M. Wang, Mater. Sci. Eng. A 474 (2008) 317–332. [11] Y. Fan, G.H. Wu, C.Q. Zhai, Mater. Sci. Eng. A 433 (2006) 208–215. [12] N. Balasubramani, U.T.S. Pillai, B.C. Pai, J. Alloys Compd. 457 (2008) 118–123.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of cerium on as-cast microstructure and mechanical properties of Mg–3Sn–2Ca magnesium alloy Mingbo Yang a,b,∗ , Fusheng Pan b , Liang Cheng a , Jia Shen a 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 5 October 2008 Received in revised form 9 January 2009 Accepted 13 February 2009 Keywords: Magnesium alloy Mg–3Sn–2Ca alloy Cerium CaMgSn phase Mechanical properties
a b s t r a c t The Mg–3Sn–2Ca–xCe (x = 0–2.0 wt.%) alloys were prepared by permanent mould casting method, the effects of Ce on the as-cast microstructure and mechanical properties of the alloys were investigated. The results indicated that the volume fraction and size of CaMgSn phase in the Mg–3Sn–2Ca alloy respectively were decreased by adding 0.5–2.0 wt.%Ce, and the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce was relatively smaller. The main phases in the as-cast Mg–3Sn–2Ca alloys with and without adding Ce were ␣-Mg, CaMgSn and Mg2 Ca phases, and Mg12 Ce phase were found in the alloys added more than 1.0 wt.%Ce. The addition of Ce improved the tensile and creep properties of Mg–3Sn–2Ca alloy, and the mechanical properties of Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce were relatively higher. The strengthening mechanism of Ce-containing Mg–3Sn–2Ca alloys was mainly attributed to the refinement of CaMgSn phase. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are the lightest structural alloys commercially available and have great potential for applications in automotive, aerospace industries and others. However, in recent years, improving the elevated temperature properties has become a critical issue for possible application of magnesium alloys in hot component [1,2]. At present, efforts are being made towards developing new corrosion and creep-resistant magnesium alloys based on Mg–Sn–Ca system [3–6], where Sn goes into solid solution with Mg and imparts corrosion resistance while Ca gives creep resistance by forming stable intermetallic particles in the matrix. From recent investigations the Mg–3Sn–2Ca alloy has been identified as one of the most promising magnesium alloys [3,4,7–9]. It has been shown that the Mg–3Sn–2Ca magnesium alloy offers superior creep properties even compared to the creep-resistant magnesium alloy AE42. According to the investigations of Hort et al. [9], the improving of creep properties for Mg–3Sn–2Ca alloy mainly contributes to the CaMgSn and Mg2 Ca phases in the alloy, especially the CaMgSn phase with higher thermal stability. However, it is further reported that the Mg–3Sn–2Ca alloy still do not meet the needs of industrial application due to the poor mechanical properties resulted from the
∗ 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 © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.02.038
coarse CaMgSn phase in the alloy [6]. Therefore, further enhancement in the mechanical properties by alloying and/or microalloying for Mg–3Sn–2Ca alloy needs to be considered. But up to now, the investigations about the effects of alloying and microalloying on the as-cast microstructure and mechanical properties of Mg–3Sn–2Ca alloy are very rare. Due to the above-mentioned reasons, the present work investigates the influence of cerium (Ce), which has been successfully used to improve the mechanical properties of other magnesium alloy systems such as Mg–Al and Mg–Zn [10,11], on the as-cast microstructure and mechanical properties of Mg–3Sn–2Ca alloy. 2. Experimental procedures The Mg–3Sn–2Ca–xCe (x = 0–2.0 wt.%) alloys were prepared from commercially pure Mg and Sn (>99.9 wt.%), the Ca and Ce were added in the form of Mg–19 wt.%Ca and Mg–29 wt.%Ce master alloys, respectively. The experimental alloys were melted in a crucible resistance furnace and protected by a flux addition. After Mg–Ce master alloy was added to the melt at 740 ◦ C, the melt was homogenized by mechanical stirring. After complete mixing, the melt was held at 740 ◦ C for 20 min and then poured into a preheated permanent mould in order to obtain a casting. Furthermore, the samples of experimental alloys were subjected to a solution heat treatment (500 ◦ C/6 h, water cooled) in order to examine the microstructural stability at high temperatures. The specimens whose size has been reported previously [2] were fabricated from
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Table 1 Actual compositions of the as-cast experimental alloys, wt.%. Experimental alloys #
1 2# 3# 4# 5#
Fig. 1. XRD results of the as-cast experimental alloys.
alloy (Mg–3Sn–2Ca) alloy (Mg–3Sn–2Ca–0.5Ce) alloy (Mg–3Sn–2Ca–1.0Ce) alloy (Mg–3Sn–2Ca–1.5Ce) alloy (Mg–3Sn–2Ca–2.0Ce)
Sn
Ca
Ce
Mg
2.92 2.89 2.91 2.93 2.92
1.89 1.90 1.92 1.91 1.90
– 0.45 0.88 1.38 1.81
Bal. Bal. Bal. Bal. Bal.
the casting for tensile and creep tests. The actual chemical compositions of experimental alloys were listed in Table 1. The samples of experimental alloys were etched with an 8% nitric acid distilled water solution, and then examined by using an Olympus Optical microscope and Joel/JSM-6460LV type scanning electron microscope (SEM) equipped with Oxford energy dispersive spectrometer (EDS) with an operating voltage of 20 kV. The phases in the experimental alloys were analyzed by D/Max-1200X type X-ray diffraction (XRD) operated at 40 kV and 30 mA. The tensile properties of experimental alloys at room temperature and 150 ◦ C were determined from a complete stress–strain curve. The ultimate tensile strength (UTS), 0.2% yield strength (YS)
Fig. 2. Optical images of the as-cast experimental alloys: (a) 1# alloy; (b) 2# alloy; (c) 3# alloy; (d) 4# alloy; (e) 5# alloy.
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Fig. 3. SEM images of the as-cast experimental alloys: (a) and (b) 1# alloy; (c) 2# alloy; (d) 3# alloy; (e) 4# alloy; (f) 5# alloy.
and elongation to failure (Elong.) were obtained based on the average of three tests. The constant-load tensile creep tests were performed at 150 ◦ C and 70 MPa for creep extension up to 100 h. The total creep strain and minimum creep rates of experimental alloys were respectively measured from each elongation–time curve and averaged over three tests. 3. Results and discussion 3.1. As-cast microstructure Fig. 1 shows the XRD results of as-cast experimental alloys. At present, two main phases in Mg–3Sn–2Ca alloy have been reported [7–9]. One is identified as CaMgSn phase, and the other one is Mg2 Ca phase. Apparently, adding 0.5 wt.%Ce to Mg–3Sn–2Ca alloy does not cause the formation of any new phases according to the information from Fig. 1. However, the Mg12 Ce new phase is detected in the Mg–3Sn–2Ca alloys added more than 1.0 wt.%Ce. Furthermore, it is found from Fig. 1 that the CaMgSn phases in the 2–5# alloys have lower peak intensities, indicating that the volume fraction of
CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys is lower than that in the Mg–3Sn–2Ca alloy. Figs. 2 and 3 show the optical and SEM images of as-cast experimental alloys, respectively. It is found from Fig. 2 that the experimental alloys are composed of primary ␣-Mg and secondary solidification phases (grey and black precipitates). According to the EDS and XRD results, the grey second phases whose amount is very large are identified as CaMgSn with a typical lamellar morphology (Fig. 3), that the black phases are Mg2 Ca whose typical morphology is shown in Fig. 3b. In addition, the bright CaMgSn phase with a blocky morphology is also identified in the four as-cast experimental alloys, as shown in Fig. 3a. Furthermore, it is found from Figs. 2 and 3 that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy causes an interesting microstructural change. The volume fraction of CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys is lower than that in the Mg–3Sn–2Ca alloy, which is consistent to the XRD results (Fig. 1). In addition, it is also observed from Figs. 2 and 3 that the CaMgSn phases in the 2–5# alloy seem to have smaller size than those in the 1# alloy, indicating that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy can effectively refine the CaMgSn phase in
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Fig. 4. Surface scanning results of the 2# as-cast experimental alloy.
the alloy. Although the exact measurement of size for the CaMgSn phase is difficult because of its complex shape, the average size (in terms of area) of the CaMgSn phase measured before refining is 545 m2 for the 1# alloy and after refining its average size respectively is 64 m2 , 53 m2 , 39 m2 and 35 m2 for the 2# , 3# , 4# and 5# alloys. Based on the above results, it is inferred that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy not only can suppress the formation of CaMgSn phase in the alloy but also can refine the CaMgSn phase in the alloy. At the same time, the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce was relatively smaller. It is well known that the Ce atom has larger atomic radius than the Sn atom (Ce: 0.183 nm; Sn: 0.141 nm). After adding Ce to Mg–3Sn–2Ca alloy, the Ce element is rich in the solid–liquid interface during the solidification process due to the low solid solubility in ␣-Mg matrix. Fig. 4 and Table 2 show the surface scanning and EDS results of the 2# as-cast experimental alloy, respectively. From Fig. 4 it is observed that the Ca, Sn and Ce elements mainly distribute at the grain boundaries, especially the Ce and Sn elements. Moreover, from Table 2 it is found that the concentrations of Sn element in the ␣-Mg matrix and CaMgSn phase at the grain boundaries for the 2# alloy respectively are lower and higher than those for the 1# alloy. Therefore, it is inferred that the above observed low volume fraction and refinement of CaMgSn phase in the Ce-containing
Table 2 EDS results of the 1# and 2# as-cast experimental alloys, wt.%. Elements
1# alloy ␣-Mg
Ca Sn Ce Mg Total
0.99 0.14 – 98.87 100
2# alloy CaMgSn 12.17 23.56 – 64.27 100
Mg2 Ca 14.46 0.13 – 85.41 100
␣-Mg 0.82 0.04 – 99.14 100
CaMgSn 13.26 26.62 0.87 59.25 100
Mg2 Ca 17.65 0.27 0.45 81.63 100
Mg–3Sn–2Ca alloys are possibly related to the Ce enrichment which hinders the Sn atom diffusion and induces the constitution undercooling at the solidification interface front [10]. In addition, it is also inferred that the Mg12 Ce compound which has high melting point and heat resistance, can possibly restrain the growth of CaMgSn phase during the solidification. Consequently, the CaMgSn phases in the 3–5# experimental alloys exhibit smaller size than the 2# experimental alloy. In spite of the above analyses, the exact reason for the observed low volume fraction and refinement of CaMgSn phase in the Ce-containing Mg–3Sn–2Ca alloys are not completely clear. Further investigations need to be considered. 3.2. Mechanical properties The tensile properties including ultimate tensile strength (UTS), 0.2% yield strength (YS), elongation (Elong.) and creep properties of the as-cast experimental alloys, are listed in Table 3. It is observed from Table 3 that the Mg–3Sn–2Ca alloys with and without adding Ce have very high creep-resistant properties at 150 ◦ C and 70 MPa for 100 h. Since the creep-resistant properties of magnesium alloys are mainly related to the structure stability at high temperatures, the solutionized microstructures of 1–4# experimental alloys are examined (Fig. 5). Compared with Figs. 2, 3 and 5, it is found that the solutionized microstructures of experimental alloys seem to be similar to their as-cast microstructures. The CaMgSn phases in the four solutionized alloys are still observed clearly, indicating that the phase does not dissolve into the matrix after solid solution heat treatment. However, compared with Figs. 3b, 5b and 5d, it is found that the Mg2 Ca phases in the experimental alloys seem to have dissolved into the matrix after solid solution treatment. Obviously, the high creep-resistant properties for Mg–3Sn–2Ca alloys with and without adding Ce are mainly attributed to the CaMgSn phase in the alloys due to its high thermal stability. Further, it is found from Table 3 that the tensile and creepresistant properties of the 2–5# alloys are higher than those of the 1# alloy. For example, after adding 2.0 wt.%Ce to Mg–3Sn–2Ca
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Fig. 5. SEM images of the solutionized experimental alloys: (a) and (b) 1# alloy; (c) and (d) 2# alloy; (e) 3# alloy; (f) 4# alloy.
alloy, the ultimate tensile strength, yield strength and elongation of the alloy are increased by 24.4%, 28.6% and 73.7% at room temperature, 22.4%, 28.8% and 56% at 150 ◦ C, separately. At the same time, the minimum creep rate of the alloy at 150 ◦ C and 70 MPa for 100 h decreases from 5.72 × 10−8 s−1 to 2.81 × 10−8 s−1 . The above results indicate that adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy can improve the mechanical properties of the alloy. This situation is possibly related to the refinement of CaMgSn phases in the Ce-containing Mg–3Sn–2Ca alloys. It is well known that the
presence of fine and uniform phases distributed along the grain boundaries is easier to act as an effective straddle to the dislocation motion thus improving the properties of engineering alloys [12]. Apparently, the coarse CaMgSn phases in the 1# alloy would give a detrimental effect on the mechanical properties of the alloy since the cracks can easily nucleate along the interface between CaMgSn particle and ␣-Mg matrix [6]. However, after adding 0.5–2.0 wt.%Ce to Mg–3Sn–2Ca alloy, the CaMgSn phase in the alloy is refined, then the extending tendency of microcracks will decrease. Accord-
Table 3 Tensile and creep properties of the as-cast experimental alloys. Experimental alloys
Tensile properties
Creep properties 150 ◦ C
Room temperature
#
1 2# 3# 4# 5#
alloy alloy alloy alloy alloy
150 ◦ C and 70 MPa for 100 h
UTS (MPa)
YS (MPa)
Elong. (%)
UTS (MPa)
YS (MPa)
Elong. (%)
Total creep strain (%)
Minimum creep rate (×10−8 s−1 )
127 142 149 157 158
112 128 134 140 144
1.9 3.1 3.2 3.4 3.3
116 132 138 142 142
104 117 127 131 134
9.1 13.4 13.8 14.9 14.2
2.06 1.76 1.29 1.07 1.01
5.72 4.89 3.58 2.97 2.81
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Fig. 6. SEM images of tensile fractographs for the as-cast experimental alloys tested at room temperature: (a) 1# alloy; (b) local magnification of (a); (c) 2# alloy; (d) local magnification of (c); (e) 3# alloy; (f) 4# alloy.
ingly, the mechanical properties of the alloy are improved. Since the CaMgSn phases in the 3–5# experimental alloys have relatively smaller size than the 2# experimental alloy, and the Mg12 Ce phases in the 3–5# experimental alloys are also beneficial to the improvement of creep-resistant properties, consequently the 3–5# experimental alloys exhibit higher mechanical properties than the 2# experimental alloy. In addition, although the volume fraction of CaMgSn phases in the 2–5# experimental alloy is lower than that of the CaMgSn phases in the 1# experimental alloy, which commonly results in the decreasing of strength, the 2–5# experimental alloys still obtain higher tensile and creep-resistant properties than the 1# experimental alloy, especially obtains higher elongation. The reason is also possibly related to the obvious refinement of CaMgSn phases in the 2–5# experimental alloys. 3.3. Fracture behaviour Fig. 6 shows the SEM images of the tensile fracture surfaces for the 1–4# experimental alloys tested at room temperature. As
shown in Fig. 6, an amount of cleavage planes and steps are present and some minute lacerated ridges can also be observed in the local areas of tensile fracture surfaces for the four alloys, which have mixed characteristics of cleavage and quasi-cleavage fractures. However, the fracture surface of Mg–3Sn–2Ca alloy exhibit large cleavage-type facets (arrow ‘A’ in Fig. 6a). On the other hand, in the fracture surfaces of Ce-containing Mg–3Sn–2Ca alloys, only the small cleavage-type facets are observed (arrow ‘B’ in Fig. 6d). This is one of possible reasons for the difference in the tensile properties especially the elongation for the Mg–3Sn–2Ca alloys with and without adding Ce. 4. Conclusions (1) The volume fraction and size of CaMgSn phase in the Mg–3Sn–2Ca alloy respectively are decreased by adding 0.5–2.0 wt.%Ce, and the average size of CaMgSn phase in the Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce is relatively smaller. The phase compositions of Mg–3Sn–2Ca alloy are ␣-Mg, CaMgSn and Mg2 Ca phases. Adding 0.5 wt.%Ce to Mg–3Sn–2Ca
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alloy does not cause the formation of any new phases in the alloy. However, the Mg12 Ce phase is formed in the Mg–3Sn–2Ca alloys added 1.0–2.0 wt.%Ce. (2) The Ce addition obviously improves the mechanical properties of Mg–3Sn–2Ca alloy at room temperature and 150 ◦ C, and the mechanical properties of Mg–3Sn–2Ca alloys added 1.5 or 2.0 wt.%Ce are relatively higher. After adding 2.0 wt.%Ce to Mg–3Sn–2Ca alloy, the ultimate tensile strength, yield strength and elongation of the alloy are increased by 24.4%, 28.6% and 73.7% at room temperature, 22.4%, 28.8% and 56% at 150 ◦ C, separately. At the same time, the minimum creep rate of the alloy at 150 ◦ C and 70 MPa for 100 h decreases from 5.72 × 10−8 s−1 to 2.81 × 10−8 s−1 . The strengthening mechanism of Ce-containing Mg–3Sn–2Ca alloys is mainly attributed to the refinement of CaMgSn phase. 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).
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