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Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy
Author’s Accepted Manuscript Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy Xuesong Fu, Yan Yang, Jiawei Hu, Junfei Su, Xueping Zhang, Xiaodong Peng www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)31351-5 https://doi.org/10.1016/j.msea.2017.10.036 MSA35639
To appear in: Materials Science & Engineering A Received date: 22 July 2017 Revised date: 12 October 2017 Accepted date: 12 October 2017 Cite this article as: Xuesong Fu, Yan Yang, Jiawei Hu, Junfei Su, Xueping Zhang and Xiaodong Peng, Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.10.036 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.
Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy Xuesong Fu a, Yan Yang a, b, *, Jiawei Hu a, Junfei Su a, Xueping Zhang a, Xiaodong Peng a, b a. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China b. National Engineering Research Center for Magnesium Alloys, Chongqing 400044, China Abstract: Mg-8Li-1Al-0.5Sn (wt.%) alloy was prepared by casting and deformed by hot extrusion in this study. The microstructure and mechanical properties of as-cast and extruded alloys were studied by OM, SEM, XRD, and the tensile test. The results show that the Mg-8Li-1Al-0.5Sn alloy consists of α-Mg (hcp), β-Li (bcc), LiMgAl2, Mg2Sn, Li2MgSn phases. After hot extrusion, the β-Li phase was refined, and Mg2Sn compounds were distributed uniformly in the matrix. The yield strength and ultimate tensile strength of extruded specimen reach to 240 MPa and 322 MPa respectively at room temperature. At 423 K (150oC), the ultimate tensile strength arrives at 220 MPa for extruded alloy. The strength of Mg-8Li-1Al-0.5Sn alloy is higher than many traditional Mg-Li alloys. Key words: Mg-Li alloys; Sn; microstructure; mechanical properties
1 Introduction Mg-Li alloys have drawn increasing attentions in many areas since they have great potential for use as a structural material for transportation system and electric device components due to the ultra-low density and good cold-formability [1,2]. However, the applications of Mg-Li alloys are limited due to low strength at the room temperature and elevated temperature. Hence, demand for new deformed Mg-Li alloys with good performance in strength is expected in the industry[3]. Alloying is a common way to improve the mechanical properties of materials, and many alloying elements, such as Al, Mn, and rare earth (REs), are frequently used. Various research results are reported that Al is the most common alloying element to improve the tensile strength and hardness due to the solid solution strengthening and intermetallic compound reinforcements for Mg-Li alloys[4-7]. Similarly, Sn is also a kind of common alloying elements for magnesium alloys to improve the mechanical properties[8]. With the addition of Sn, Mg2Sn and Li2MgSn compounds are formed in the matrix, which hinders the dislocation movements to lead to enhancing the mechanical properties. Furthermore, the increasing of compounds with a high melting point have beneficial effects on mechanical behavior at elevated temperatures, such as Mg2Sn[9]. Kim J T et al.[6] have investigated the microstructure and mechanical properties of Mg-11Li-3Al-1Sn-0.4Mn-0.8Ca alloy, and its strength of the extruded alloy arrives at 220 MPa. Yan Y et al.[10] reported the strength of the Mg-10Li-3Al-2.5Sr alloy, and the strength reaches to 170 MPa after the DCCAP processed. Similarly, the Mg-6Sn-5Al-2Si alloy was investigated by Sang G L et al.[11]. According to the report, the strength of the as-cast Mg-6Sn-5Al-2Si alloy reaches to 178 MPa. Kim Y H et al.[9] have studied the mechanical properties of Mg-5Li-3Al-1Sn-0.4Mn alloy and Mg-8Li-3Al-1Sn-0.4Mn alloy. Strengths of the extruded specimens reach to 258 MPa and 240 MPa respectively. Jiang Y et al.[12] have developed the
Corresponding author: Yan Yang; E-mail: [email protected]; Tel: +8623 65102856
Mg-5Sn-0.3Li-1Al alloy with the strength of 291 MPa. Most of the published research shows that the values of the ultimate tensile strength of extruded alloys is less than 300 MPa. However, review of the published literature, there are few reports on the influence of Sn on the duplex structured Mg-Li alloys, especially duplex structured Mg-Li-Al alloys. Thus, the duplex structured Mg-8Li-1Al-0.5Sn quaternary alloy was prepared and discussed in this study. The alloy was cast under the protection of argon atmosphere, followed by extrusion at 553 K (280oC), with an extrusion ratio of 25. The reason why select these particular chemistries was based on the following factors. On the one hand, it is reported that Mg-Li alloys present good performance of strength and ductility in binary phase (5.5 wt.%< Li <11.5 wt.%). On the other hand, excess addition of Al and Sn leads to form coarse block-like compounds to reduce the mechanical properties[13-16]. Hence, 1 wt.% Al and 0.5 wt.% Sn were intended to add into materials to avoid these defects. Various ways, such as optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), were used to observe the microstructure and composition. The tensile properties of as-cast and extruded specimens were tested at room temperature and elevated temperature. The strengthening mechanism of Mg-8Li-1Al-0.5Sn alloy was discussed for the first time.
2 Experimental procedures Pure magnesium (99.9 wt.%), pure tin (99.9 wt.%), pure lithium(99.9 wt.%), and pure aluminum (99.5 wt.%) were used in this study. At the beginning, pure magnesium and pure aluminum were added into induction furnace with a graphite crucible to obtain alloy. The added materials were heated to 993 K (720oC) and held for 1 h under the protection of an argon atmosphere. When all materials were melted, the temperature was reduced to 953 K (680oC), then tin and lithium were added to the melt. Next, keep this temperature for 15 minutes until all elements are dispersed uniformly. Finally, molten metal was poured into a cylinder metal mold (Φ90×300 mm) which was preheated to 473 K (200oC) to obtain as-cast ingots. The as-cast ingots (machined to Φ80×50 mm) was then homogenization treated at 533 K (260oC) for 4 h and extruded into a long rod with a diameter of 16 mm (extrusion ratio was 25:1) at the temperature of 553 K (280oC). The chemical composition of the alloy is shown in Table 1. Table 1 Chemical composition of the studied alloy (wt.%) Designed alloy
Li
Al
Sn
Mg
Mg-8Li-1Al-0.5Sn
7.93
0.96
0.46
Bal.
OM, XRD, and EDS were adopted to analyze the microstructure, morphology, the element composition, and distribution on the surface of the specimen. The SEM images were obtained by secondary electrons (SE) detector which can display microstructural characteristics of materials. Standard tensile specimens with gauge dimensions of Φ5×25 mm were machined from the central regions of as-cast and extruded specimens. The axial direction of the tensile specimens was parallel to the extrusion direction. Tensile tests performance of as-cast and extruded specimens at room temperature were conducted on SANS CMT-5105 tensile tester with a displacement speed of 2 mm/min. The initial strain stress rate is 1.3×10-3s-1 in this study. Scanning electron microscope was used to observe the fracture surface of the alloys.
3 Results and discussions 3.1 Microstructure Optical micrographs of the as-cast Mg-8Li-1Al-0.5Sn alloy are shown in Fig. 1. Fig. 1(a and b) shows that the microstructure is composed of bright area, gray area, and black globular compounds. According to previous research [17], the bright area is identified as α-Mg phase, and the gray area is the β-Li phase. The size of α-Mg phase is about 44μm in diameter. The α-Mg phase shows a coarse block form which is different from previous studies. For instance, as reported in Mg-8Li alloys, the α-Mg phase is lath-like instead of block-like in matrix[18]. Globular black compounds are distributed uniformly in the matrix. It is seen that many small compounds are located along the boundary of β-Li phase. β-Li phases are filled in the region between α-Mg phase, which is separated by boundaries of α-Mg. The microstructure of extruded alloy vertical to the extrusion direction is shown in Fig. 1(c and d). It is easily observed that grains are finer than those of the as-cast alloys. The size of grains decreases from about 44 μm to 6 μm when alloys were extruded from Φ80 mm to Φ16 mm. Both of α-Mg phase and β-Li phase are twisted because of extrusion. Many small grains are found in the β-Li phase which can be explained that the dynamic recrystallization occurs during the extrusion. For the α-Mg phase (hcp), there are only two independent basal slip systems which are fewer than that of β-Li phase (bcc)[19,20]. Therefore, the deformability of α-Mg phase is limited which leads to β-Li phase acting as a mediate of coordinating deform between α-Mg phase and the matrix in the extrusion process. Fig. 1(e and f) shows the microstructure of the extruded alloy parallel to the extrusion direction. As shown in the pictures, the α-Mg phase is elongated to become long strips-like phase due to the hot extrusion. It is found that the α-Mg phase is parallel to the direction of extrusion, and compounds are distributed in the matrix evenly. It is observed that β-Li phase exhibits an equiaxed morphology with a grain size of smaller than 10 μm in diameter.
Fig. 1 Microstructure of Mg-8Li-1Al-0.5Sn alloy: (a)(b) as-cast alloy; (c)(d) extruded alloy, vertical to extruding direction; (e)(f) as-extruded specimen, parallel to extrude direction.
XRD pattern of as-cast Mg-8Li-1Al-0.5Sn is presented in Fig. 2. The peaks in XRD pattern could be identified according to several relevant research [9, 12, 22]. The results show that the alloy consists of α-Mg phase, β-Li phase, LiMgAl2, Mg2Sn and Li2MgSn compounds. The electronegativity difference between Mg and Al is 0.3 while between Mg and Sn is 0.65. Hence, it is easier for Sn than Al to react compounds with Mg. Similarly, the electronegativity difference between Li and Al is 0.63 while between Li and Sn is 0.95, which indicates that Sn is easier than Al to react with Li. Thus Mg2Sn and Li2MgSn are easy to be formed in materials. In order to further confirm the existence of materials, the SEM and EDS were used to analyze the alloys.
Fig. 2 XRD patterns of as-cast Mg-8Li-1Al-0.5Sn alloy
Fig. 3(a and b) shows the SEM images of an as-cast specimen, and the results of EDS are listed in Table 2. It is found that the Mg-8Li-1Al-0.5Sn alloy is composed of α-Mg phase and β-Li phase. The α-Mg phase includes primary α-Mg phase and eutectic α-Mg phase[21]. According to EDS and SEM analysis, the dark area corresponds to primary α-Mg phase while the gray area corresponds to the eutectic structure which consists of the eutectic α-Mg phase and β-Li phase. Meanwhile, there are many small granular compounds shown in gray areas. These particles are enriched in Mg and Al elements, which can be identified as LiMgAl2. It is seen that the tiny particles are about 3μm in diameter. The EDS point analysis shows a high concentration of Mg and Sn in the intermetallic compound area. Even the atom amount ratio of Mg to Sn at the point of A is about 2:1. Compared with point A and C, both of them enrich the Mg and Sn. However, their shapes are different. Jiang et al. [22] reported that the temperature of the formation is different about Mg2Sn and Li2MgSn. The temperature of the Li2MgSn phase is formed more than 609 oC at the melting stage of alloy, while the Mg2Sn phase is precipitated from the solid β-Li phase with a temperature lower than 550 oC. According to the discovery of Jiang, it is inferred that small white particles are Mg2Sn, while small bar-like compounds are Li2MgSn. Fig. 3(c and d) shows the microstructure vertical to the extrusion direction. It is observed that dynamic recrystallization occurred in β-Li phase which leads to grains refinement. The α-Mg phase is surrounded by recrystallized β-Li phase which including many small LiMgAl2 compounds. The dynamic recrystallization not only reduces grain size but also eliminates part of casting defects in materials. The SEM images of as-extruded specimen parallel to the extrusion direction are presented in Fig. 3(e and f). It is seen that the grain size in extruded alloys is approximately 6 μm which is much finer than that of the as-cast alloy. The result is also in agreement with the OM result. There are few bar-like compounds after extrusion, which indicates that the Li2MgSn compounds were crushed. Moreover, Li2MgSn is a kind of metastable phase, which is quickly transformed into Mg2Sn during hot extrusion. These compounds are distributed along grains boundaries which is the favor to second phase strengthening. The α-Mg phase and β-Li phase is alternately distributed in the material.
Fig. 3 SEM images of Mg-8Li-1Al-0.5Sn alloy: (a)(b) as-cast alloy; (c)(d)extruded alloy, vertical to extruding direction; (e)(f) as-extruded specimen, parallel to extrude direction.
Table 2 EDS results at different positions in Fig. 3 Molar fraction/%
Position Mg
Al
Sn
A
68.36
0
31.64
B
60.73
39.27
0
C
85.39
0
14.61
D
79.15
20.85
0
E
77.39
0
22.61
3.2 Mechanical Properties Fig. 4 shows the tensile engineering stress-strain curves of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy at room temperature. Table 3 reveals the values of the yield strength (YS), ultimate tensile strength (UTS) and elongation at different temperatures. The yield strength and ultimate tensile strength of as-cast specimens arrive only at 115 MPa and 152 MPa respectively. The elongation only arrives at approximately 8%. The strength and elongation of the extruded alloy are obviously higher than the as-cast alloy. The ultimate tensile strength (UTS) of extruded Mg-8Li-1Al-0.5Sn specimen even reaches to 322 MPa, which is considerably higher than that of many conventional Mg-Li alloys. The yield strength and elongation arrive at 240 MPa and 11% respectively which are higher than that of the as-cast specimens. The mechanical properties of Mg-8Li-1Al-0.5Sn alloy at elevated temperature was also studied. Fig. 5 shows the tensile engineering stress-strain curves of extruded test alloys at the temperature of 423K (150oC). The results indicate that the specimen still keeps good performance with the ultimate tensile strength (UTS) of 220 MPa. Thus it can be confirmed that addition of Sn can not only enhance the strength of duplex structured Mg-Li-Al alloys at room temperature but also at elevated temperature.
Fig. 4 The nominal tensile stress-strain curve of the Mg-8Li-1Al-0.5Sn alloy at room temperature: (a) as-cast specimen (b) extruded specimen.
Fig. 5 The nominal tensile stress-strain curve of extruded Mg-8Li-1Al-0.5Sn alloy at 423 K (150oC)
Table 3 Mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy at different temperatures Temperature
Room temperature o
423K (150 C)
Yield strength
Ultimate tensile strength
Elongation
(MPa)
(MPa)
(%)
As-cast
115
152
8
As-extruded
240
322
11
As-extruded
180
220
29
State
Compared with the results of previous reports, the extruded Mg-8Li-1Al-0.5Sn alloy in our research has the higher ultimate tensile strength at room temperature and elevated temperature, which can be rationalized on the basis of several factors. As we discussed previously, the fine and uniform dynamic recrystallized grains are obtained during the extrusion, which is beneficial to the improvement of the mechanical properties. According to the Hall-Petch formula, the yield strength of an alloy is inversely proportional to its grain size. After the hot extrusion, dynamic recrystallization leads to grain refinement. The average grain size of the as-cast alloy is about 44 μm, while the average grain size of the extruded alloy is about 6 μm. According to Hall-Petch theory, the yield strength of test alloys is improved a lot due to grain boundaries strengthening. Furthermore, the coarse intermetallic compounds in the as-cast alloys are broken because of hot extrusion. It can reduce the dislocation accumulations around the second phase to avoid the generation and propagation of the crack source, which is good for the improvement of mechanical properties. With the addition of Sn, compounds with a high melting point and hardness are formed in the alloy. Especially, the Mg2Sn phase with the melting pointing of 1051K (778oC) is formed in the test alloys. The crystal structure of Mg2Sn phase belongs to the cubic system. The distribution of Mg2Sn phase is uniform in the extruded alloy. Mg2Sn has the characteristics of good thermal stability and high hardness, which can hind the dislocation slip and lead to second phase strengthening in alloys. In addition, the deformation strengthening would be caused, and the fiber textures would be formed. Texture has a big effect on the mechanical properties of the extruded specimens. As is well known, α-Mg phase has an hcp structure, and the dislocation slips occur mostly on the basal planes (0001) in <11-20> direction, which leads the development of a preferred orientation with the alignment of the c-axis perpendicular to the metal flow direction during extruding[10,23]. After the hot extrusion, it is found that Mg alloys show strong basal plane textures, which is also beneficial to the improvement of the strength of test alloys. During the tensile test of elevated temperature, the grain boundaries are pinned due to the dispersed Li2MgSn and Mg2Sn compounds which can lead to the growth of grains hindered in the matrix. Some compounds distributed along the grain boundaries can act as an effective barrier to hinder the slippage of crystal system. Therefore the sliding of grain boundaries and propagation of cracks are effectively prevented at elevated temperature. The mechanical behavior of Mg-8Li-1Al-0.5Sn alloy at elevated temperature is improved a lot because of the existence of second phase with high melting point and hardness. 3.3 Fracture analysis Tensile fractures of each specimen of Mg-8Li-1Al-0.5Sn alloy are shown in Fig. 6. Fig. 6(a) shows the fracture of the as-cast specimen at room temperature. Many cleavage planes and few dimples are observed on the fracture surface as shown in Fig. 6(a). It indicates that the fracture
mechanism of the as-cast specimen is mainly attributed to brittle fracture and little attribute to ductile fracture. The fracture surface of as-extruded specimens is shown in Fig. 6(b). It is found that the density and size of dimples in the extruded alloy are increased on the fracture surface. Many compounds can be observed in these dimples. It mainly belongs to ductile fracture. The fracture morphology can confirm that extrusion process is regarded as positive effects to enhance the mechanical behavior of test specimens. The tensile fracture morphology obtained at 423 K (150oC) is shown in Fig. 6(c and d). When the conducted temperature reaches to 423 K (150oC), the fracture of Mg-8Li-1Al-0.5Sn alloy consists of many uniform dimples, which is typical of ductile fracture. The fracture morphology shows good ductility of the test alloys, which is consistent with the tensile test results.
Fig. 6 Tensile fracture of Mg-8Li-1Al-0.5Sn alloy: (a) as-cast specimen at room temperature; (b) extruded specimen at room temperature; (c)(d) extruded specimen at 423 K (150oC)
4 Conclusion (1) The duplex structured Mg-8Li-1Al-0.5Sn alloy has been studied in this research. Both as-cast and extruded test specimens consist of α-Mg phase, β-Li phase, MgLiAl2 compounds, Li2MgSn compounds and Mg2Sn compounds. During the extrusion process, the dynamic recrystallization leads to grains refinement. The α-Mg become long strip-like along the extruded direction while the grain size of β-Li phase is refined to approximately 6 μm due to dynamic recrystallization. (2) The extruded Mg-8Li-1Al-0.5Sn specimen with the yield strength of 240 MPa and ultimate tensile strength of 322 MPa, exhibits good mechanical behavior, which is much better than that of the as-cast alloy. The extruded alloy keeps good mechanical performance at 423 K (150oC) with the ultimate tensile strength of 220 MPa due to the existence of the second phase
with high melting point and hardness. The mechanical behavior of Mg-8Li-1Al-0.5Sn alloy is higher than many traditional Mg-Li alloys. (3) Dynamic recrystallization leads to grains refinement of the Mg-8Li-1Al-0.5Sn alloy during extrusion and the distribution of the second phase is improved a lot by the extrusion. The mechanical behavior is significantly improved by extrusion due to grain refinement, dislocation strengthening and second phase strengthening. (4) The tensile fracture mechanism of as-cast Mg-8Li-1Al-0.5Sn alloy belongs to brittle fracture. The fracture surfaces of extruded alloy contain evenly sized dimples, indicating ductile fracture. The tensile fracture is the ductile fracture at the elevated temperature.
5. Acknowledgements The authors would like to acknowledge financial supported by the National Natural Science Foundation (Project No. 51601024), the National Key Research and Development Program of China plan (Project No. 2016YFB0700403 and Project No. 2016YFB0301100), the National Natural Science Foundation (Project No. 51601024), the Chongqing Research Program of Basic Research and Frontier Technology (Project No. cstc2016jcyjA0418) and the Fundamental Research Funds for the Central Universities (Project No. 106112017CDJXY130001).
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PII: DOI: Reference:
S0921-5093(17)31351-5 https://doi.org/10.1016/j.msea.2017.10.036 MSA35639
To appear in: Materials Science & Engineering A Received date: 22 July 2017 Revised date: 12 October 2017 Accepted date: 12 October 2017 Cite this article as: Xuesong Fu, Yan Yang, Jiawei Hu, Junfei Su, Xueping Zhang and Xiaodong Peng, Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.10.036 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.
Microstructure and mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy Xuesong Fu a, Yan Yang a, b, *, Jiawei Hu a, Junfei Su a, Xueping Zhang a, Xiaodong Peng a, b a. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China b. National Engineering Research Center for Magnesium Alloys, Chongqing 400044, China Abstract: Mg-8Li-1Al-0.5Sn (wt.%) alloy was prepared by casting and deformed by hot extrusion in this study. The microstructure and mechanical properties of as-cast and extruded alloys were studied by OM, SEM, XRD, and the tensile test. The results show that the Mg-8Li-1Al-0.5Sn alloy consists of α-Mg (hcp), β-Li (bcc), LiMgAl2, Mg2Sn, Li2MgSn phases. After hot extrusion, the β-Li phase was refined, and Mg2Sn compounds were distributed uniformly in the matrix. The yield strength and ultimate tensile strength of extruded specimen reach to 240 MPa and 322 MPa respectively at room temperature. At 423 K (150oC), the ultimate tensile strength arrives at 220 MPa for extruded alloy. The strength of Mg-8Li-1Al-0.5Sn alloy is higher than many traditional Mg-Li alloys. Key words: Mg-Li alloys; Sn; microstructure; mechanical properties
1 Introduction Mg-Li alloys have drawn increasing attentions in many areas since they have great potential for use as a structural material for transportation system and electric device components due to the ultra-low density and good cold-formability [1,2]. However, the applications of Mg-Li alloys are limited due to low strength at the room temperature and elevated temperature. Hence, demand for new deformed Mg-Li alloys with good performance in strength is expected in the industry[3]. Alloying is a common way to improve the mechanical properties of materials, and many alloying elements, such as Al, Mn, and rare earth (REs), are frequently used. Various research results are reported that Al is the most common alloying element to improve the tensile strength and hardness due to the solid solution strengthening and intermetallic compound reinforcements for Mg-Li alloys[4-7]. Similarly, Sn is also a kind of common alloying elements for magnesium alloys to improve the mechanical properties[8]. With the addition of Sn, Mg2Sn and Li2MgSn compounds are formed in the matrix, which hinders the dislocation movements to lead to enhancing the mechanical properties. Furthermore, the increasing of compounds with a high melting point have beneficial effects on mechanical behavior at elevated temperatures, such as Mg2Sn[9]. Kim J T et al.[6] have investigated the microstructure and mechanical properties of Mg-11Li-3Al-1Sn-0.4Mn-0.8Ca alloy, and its strength of the extruded alloy arrives at 220 MPa. Yan Y et al.[10] reported the strength of the Mg-10Li-3Al-2.5Sr alloy, and the strength reaches to 170 MPa after the DCCAP processed. Similarly, the Mg-6Sn-5Al-2Si alloy was investigated by Sang G L et al.[11]. According to the report, the strength of the as-cast Mg-6Sn-5Al-2Si alloy reaches to 178 MPa. Kim Y H et al.[9] have studied the mechanical properties of Mg-5Li-3Al-1Sn-0.4Mn alloy and Mg-8Li-3Al-1Sn-0.4Mn alloy. Strengths of the extruded specimens reach to 258 MPa and 240 MPa respectively. Jiang Y et al.[12] have developed the
Corresponding author: Yan Yang; E-mail: [email protected]; Tel: +8623 65102856
Mg-5Sn-0.3Li-1Al alloy with the strength of 291 MPa. Most of the published research shows that the values of the ultimate tensile strength of extruded alloys is less than 300 MPa. However, review of the published literature, there are few reports on the influence of Sn on the duplex structured Mg-Li alloys, especially duplex structured Mg-Li-Al alloys. Thus, the duplex structured Mg-8Li-1Al-0.5Sn quaternary alloy was prepared and discussed in this study. The alloy was cast under the protection of argon atmosphere, followed by extrusion at 553 K (280oC), with an extrusion ratio of 25. The reason why select these particular chemistries was based on the following factors. On the one hand, it is reported that Mg-Li alloys present good performance of strength and ductility in binary phase (5.5 wt.%< Li <11.5 wt.%). On the other hand, excess addition of Al and Sn leads to form coarse block-like compounds to reduce the mechanical properties[13-16]. Hence, 1 wt.% Al and 0.5 wt.% Sn were intended to add into materials to avoid these defects. Various ways, such as optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), were used to observe the microstructure and composition. The tensile properties of as-cast and extruded specimens were tested at room temperature and elevated temperature. The strengthening mechanism of Mg-8Li-1Al-0.5Sn alloy was discussed for the first time.
2 Experimental procedures Pure magnesium (99.9 wt.%), pure tin (99.9 wt.%), pure lithium(99.9 wt.%), and pure aluminum (99.5 wt.%) were used in this study. At the beginning, pure magnesium and pure aluminum were added into induction furnace with a graphite crucible to obtain alloy. The added materials were heated to 993 K (720oC) and held for 1 h under the protection of an argon atmosphere. When all materials were melted, the temperature was reduced to 953 K (680oC), then tin and lithium were added to the melt. Next, keep this temperature for 15 minutes until all elements are dispersed uniformly. Finally, molten metal was poured into a cylinder metal mold (Φ90×300 mm) which was preheated to 473 K (200oC) to obtain as-cast ingots. The as-cast ingots (machined to Φ80×50 mm) was then homogenization treated at 533 K (260oC) for 4 h and extruded into a long rod with a diameter of 16 mm (extrusion ratio was 25:1) at the temperature of 553 K (280oC). The chemical composition of the alloy is shown in Table 1. Table 1 Chemical composition of the studied alloy (wt.%) Designed alloy
Li
Al
Sn
Mg
Mg-8Li-1Al-0.5Sn
7.93
0.96
0.46
Bal.
OM, XRD, and EDS were adopted to analyze the microstructure, morphology, the element composition, and distribution on the surface of the specimen. The SEM images were obtained by secondary electrons (SE) detector which can display microstructural characteristics of materials. Standard tensile specimens with gauge dimensions of Φ5×25 mm were machined from the central regions of as-cast and extruded specimens. The axial direction of the tensile specimens was parallel to the extrusion direction. Tensile tests performance of as-cast and extruded specimens at room temperature were conducted on SANS CMT-5105 tensile tester with a displacement speed of 2 mm/min. The initial strain stress rate is 1.3×10-3s-1 in this study. Scanning electron microscope was used to observe the fracture surface of the alloys.
3 Results and discussions 3.1 Microstructure Optical micrographs of the as-cast Mg-8Li-1Al-0.5Sn alloy are shown in Fig. 1. Fig. 1(a and b) shows that the microstructure is composed of bright area, gray area, and black globular compounds. According to previous research [17], the bright area is identified as α-Mg phase, and the gray area is the β-Li phase. The size of α-Mg phase is about 44μm in diameter. The α-Mg phase shows a coarse block form which is different from previous studies. For instance, as reported in Mg-8Li alloys, the α-Mg phase is lath-like instead of block-like in matrix[18]. Globular black compounds are distributed uniformly in the matrix. It is seen that many small compounds are located along the boundary of β-Li phase. β-Li phases are filled in the region between α-Mg phase, which is separated by boundaries of α-Mg. The microstructure of extruded alloy vertical to the extrusion direction is shown in Fig. 1(c and d). It is easily observed that grains are finer than those of the as-cast alloys. The size of grains decreases from about 44 μm to 6 μm when alloys were extruded from Φ80 mm to Φ16 mm. Both of α-Mg phase and β-Li phase are twisted because of extrusion. Many small grains are found in the β-Li phase which can be explained that the dynamic recrystallization occurs during the extrusion. For the α-Mg phase (hcp), there are only two independent basal slip systems which are fewer than that of β-Li phase (bcc)[19,20]. Therefore, the deformability of α-Mg phase is limited which leads to β-Li phase acting as a mediate of coordinating deform between α-Mg phase and the matrix in the extrusion process. Fig. 1(e and f) shows the microstructure of the extruded alloy parallel to the extrusion direction. As shown in the pictures, the α-Mg phase is elongated to become long strips-like phase due to the hot extrusion. It is found that the α-Mg phase is parallel to the direction of extrusion, and compounds are distributed in the matrix evenly. It is observed that β-Li phase exhibits an equiaxed morphology with a grain size of smaller than 10 μm in diameter.
Fig. 1 Microstructure of Mg-8Li-1Al-0.5Sn alloy: (a)(b) as-cast alloy; (c)(d) extruded alloy, vertical to extruding direction; (e)(f) as-extruded specimen, parallel to extrude direction.
XRD pattern of as-cast Mg-8Li-1Al-0.5Sn is presented in Fig. 2. The peaks in XRD pattern could be identified according to several relevant research [9, 12, 22]. The results show that the alloy consists of α-Mg phase, β-Li phase, LiMgAl2, Mg2Sn and Li2MgSn compounds. The electronegativity difference between Mg and Al is 0.3 while between Mg and Sn is 0.65. Hence, it is easier for Sn than Al to react compounds with Mg. Similarly, the electronegativity difference between Li and Al is 0.63 while between Li and Sn is 0.95, which indicates that Sn is easier than Al to react with Li. Thus Mg2Sn and Li2MgSn are easy to be formed in materials. In order to further confirm the existence of materials, the SEM and EDS were used to analyze the alloys.
Fig. 2 XRD patterns of as-cast Mg-8Li-1Al-0.5Sn alloy
Fig. 3(a and b) shows the SEM images of an as-cast specimen, and the results of EDS are listed in Table 2. It is found that the Mg-8Li-1Al-0.5Sn alloy is composed of α-Mg phase and β-Li phase. The α-Mg phase includes primary α-Mg phase and eutectic α-Mg phase[21]. According to EDS and SEM analysis, the dark area corresponds to primary α-Mg phase while the gray area corresponds to the eutectic structure which consists of the eutectic α-Mg phase and β-Li phase. Meanwhile, there are many small granular compounds shown in gray areas. These particles are enriched in Mg and Al elements, which can be identified as LiMgAl2. It is seen that the tiny particles are about 3μm in diameter. The EDS point analysis shows a high concentration of Mg and Sn in the intermetallic compound area. Even the atom amount ratio of Mg to Sn at the point of A is about 2:1. Compared with point A and C, both of them enrich the Mg and Sn. However, their shapes are different. Jiang et al. [22] reported that the temperature of the formation is different about Mg2Sn and Li2MgSn. The temperature of the Li2MgSn phase is formed more than 609 oC at the melting stage of alloy, while the Mg2Sn phase is precipitated from the solid β-Li phase with a temperature lower than 550 oC. According to the discovery of Jiang, it is inferred that small white particles are Mg2Sn, while small bar-like compounds are Li2MgSn. Fig. 3(c and d) shows the microstructure vertical to the extrusion direction. It is observed that dynamic recrystallization occurred in β-Li phase which leads to grains refinement. The α-Mg phase is surrounded by recrystallized β-Li phase which including many small LiMgAl2 compounds. The dynamic recrystallization not only reduces grain size but also eliminates part of casting defects in materials. The SEM images of as-extruded specimen parallel to the extrusion direction are presented in Fig. 3(e and f). It is seen that the grain size in extruded alloys is approximately 6 μm which is much finer than that of the as-cast alloy. The result is also in agreement with the OM result. There are few bar-like compounds after extrusion, which indicates that the Li2MgSn compounds were crushed. Moreover, Li2MgSn is a kind of metastable phase, which is quickly transformed into Mg2Sn during hot extrusion. These compounds are distributed along grains boundaries which is the favor to second phase strengthening. The α-Mg phase and β-Li phase is alternately distributed in the material.
Fig. 3 SEM images of Mg-8Li-1Al-0.5Sn alloy: (a)(b) as-cast alloy; (c)(d)extruded alloy, vertical to extruding direction; (e)(f) as-extruded specimen, parallel to extrude direction.
Table 2 EDS results at different positions in Fig. 3 Molar fraction/%
Position Mg
Al
Sn
A
68.36
0
31.64
B
60.73
39.27
0
C
85.39
0
14.61
D
79.15
20.85
0
E
77.39
0
22.61
3.2 Mechanical Properties Fig. 4 shows the tensile engineering stress-strain curves of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy at room temperature. Table 3 reveals the values of the yield strength (YS), ultimate tensile strength (UTS) and elongation at different temperatures. The yield strength and ultimate tensile strength of as-cast specimens arrive only at 115 MPa and 152 MPa respectively. The elongation only arrives at approximately 8%. The strength and elongation of the extruded alloy are obviously higher than the as-cast alloy. The ultimate tensile strength (UTS) of extruded Mg-8Li-1Al-0.5Sn specimen even reaches to 322 MPa, which is considerably higher than that of many conventional Mg-Li alloys. The yield strength and elongation arrive at 240 MPa and 11% respectively which are higher than that of the as-cast specimens. The mechanical properties of Mg-8Li-1Al-0.5Sn alloy at elevated temperature was also studied. Fig. 5 shows the tensile engineering stress-strain curves of extruded test alloys at the temperature of 423K (150oC). The results indicate that the specimen still keeps good performance with the ultimate tensile strength (UTS) of 220 MPa. Thus it can be confirmed that addition of Sn can not only enhance the strength of duplex structured Mg-Li-Al alloys at room temperature but also at elevated temperature.
Fig. 4 The nominal tensile stress-strain curve of the Mg-8Li-1Al-0.5Sn alloy at room temperature: (a) as-cast specimen (b) extruded specimen.
Fig. 5 The nominal tensile stress-strain curve of extruded Mg-8Li-1Al-0.5Sn alloy at 423 K (150oC)
Table 3 Mechanical properties of as-cast and extruded Mg-8Li-1Al-0.5Sn alloy at different temperatures Temperature
Room temperature o
423K (150 C)
Yield strength
Ultimate tensile strength
Elongation
(MPa)
(MPa)
(%)
As-cast
115
152
8
As-extruded
240
322
11
As-extruded
180
220
29
State
Compared with the results of previous reports, the extruded Mg-8Li-1Al-0.5Sn alloy in our research has the higher ultimate tensile strength at room temperature and elevated temperature, which can be rationalized on the basis of several factors. As we discussed previously, the fine and uniform dynamic recrystallized grains are obtained during the extrusion, which is beneficial to the improvement of the mechanical properties. According to the Hall-Petch formula, the yield strength of an alloy is inversely proportional to its grain size. After the hot extrusion, dynamic recrystallization leads to grain refinement. The average grain size of the as-cast alloy is about 44 μm, while the average grain size of the extruded alloy is about 6 μm. According to Hall-Petch theory, the yield strength of test alloys is improved a lot due to grain boundaries strengthening. Furthermore, the coarse intermetallic compounds in the as-cast alloys are broken because of hot extrusion. It can reduce the dislocation accumulations around the second phase to avoid the generation and propagation of the crack source, which is good for the improvement of mechanical properties. With the addition of Sn, compounds with a high melting point and hardness are formed in the alloy. Especially, the Mg2Sn phase with the melting pointing of 1051K (778oC) is formed in the test alloys. The crystal structure of Mg2Sn phase belongs to the cubic system. The distribution of Mg2Sn phase is uniform in the extruded alloy. Mg2Sn has the characteristics of good thermal stability and high hardness, which can hind the dislocation slip and lead to second phase strengthening in alloys. In addition, the deformation strengthening would be caused, and the fiber textures would be formed. Texture has a big effect on the mechanical properties of the extruded specimens. As is well known, α-Mg phase has an hcp structure, and the dislocation slips occur mostly on the basal planes (0001) in <11-20> direction, which leads the development of a preferred orientation with the alignment of the c-axis perpendicular to the metal flow direction during extruding[10,23]. After the hot extrusion, it is found that Mg alloys show strong basal plane textures, which is also beneficial to the improvement of the strength of test alloys. During the tensile test of elevated temperature, the grain boundaries are pinned due to the dispersed Li2MgSn and Mg2Sn compounds which can lead to the growth of grains hindered in the matrix. Some compounds distributed along the grain boundaries can act as an effective barrier to hinder the slippage of crystal system. Therefore the sliding of grain boundaries and propagation of cracks are effectively prevented at elevated temperature. The mechanical behavior of Mg-8Li-1Al-0.5Sn alloy at elevated temperature is improved a lot because of the existence of second phase with high melting point and hardness. 3.3 Fracture analysis Tensile fractures of each specimen of Mg-8Li-1Al-0.5Sn alloy are shown in Fig. 6. Fig. 6(a) shows the fracture of the as-cast specimen at room temperature. Many cleavage planes and few dimples are observed on the fracture surface as shown in Fig. 6(a). It indicates that the fracture
mechanism of the as-cast specimen is mainly attributed to brittle fracture and little attribute to ductile fracture. The fracture surface of as-extruded specimens is shown in Fig. 6(b). It is found that the density and size of dimples in the extruded alloy are increased on the fracture surface. Many compounds can be observed in these dimples. It mainly belongs to ductile fracture. The fracture morphology can confirm that extrusion process is regarded as positive effects to enhance the mechanical behavior of test specimens. The tensile fracture morphology obtained at 423 K (150oC) is shown in Fig. 6(c and d). When the conducted temperature reaches to 423 K (150oC), the fracture of Mg-8Li-1Al-0.5Sn alloy consists of many uniform dimples, which is typical of ductile fracture. The fracture morphology shows good ductility of the test alloys, which is consistent with the tensile test results.
Fig. 6 Tensile fracture of Mg-8Li-1Al-0.5Sn alloy: (a) as-cast specimen at room temperature; (b) extruded specimen at room temperature; (c)(d) extruded specimen at 423 K (150oC)
4 Conclusion (1) The duplex structured Mg-8Li-1Al-0.5Sn alloy has been studied in this research. Both as-cast and extruded test specimens consist of α-Mg phase, β-Li phase, MgLiAl2 compounds, Li2MgSn compounds and Mg2Sn compounds. During the extrusion process, the dynamic recrystallization leads to grains refinement. The α-Mg become long strip-like along the extruded direction while the grain size of β-Li phase is refined to approximately 6 μm due to dynamic recrystallization. (2) The extruded Mg-8Li-1Al-0.5Sn specimen with the yield strength of 240 MPa and ultimate tensile strength of 322 MPa, exhibits good mechanical behavior, which is much better than that of the as-cast alloy. The extruded alloy keeps good mechanical performance at 423 K (150oC) with the ultimate tensile strength of 220 MPa due to the existence of the second phase
with high melting point and hardness. The mechanical behavior of Mg-8Li-1Al-0.5Sn alloy is higher than many traditional Mg-Li alloys. (3) Dynamic recrystallization leads to grains refinement of the Mg-8Li-1Al-0.5Sn alloy during extrusion and the distribution of the second phase is improved a lot by the extrusion. The mechanical behavior is significantly improved by extrusion due to grain refinement, dislocation strengthening and second phase strengthening. (4) The tensile fracture mechanism of as-cast Mg-8Li-1Al-0.5Sn alloy belongs to brittle fracture. The fracture surfaces of extruded alloy contain evenly sized dimples, indicating ductile fracture. The tensile fracture is the ductile fracture at the elevated temperature.
5. Acknowledgements The authors would like to acknowledge financial supported by the National Natural Science Foundation (Project No. 51601024), the National Key Research and Development Program of China plan (Project No. 2016YFB0700403 and Project No. 2016YFB0301100), the National Natural Science Foundation (Project No. 51601024), the Chongqing Research Program of Basic Research and Frontier Technology (Project No. cstc2016jcyjA0418) and the Fundamental Research Funds for the Central Universities (Project No. 106112017CDJXY130001).
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