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Effect of friction stir processing on microstructure and tensile properties of as-cast Mg–8Li–3Al–2Sn (wt.%) alloy
Journal Pre-proof Effect of friction stir processing on microstructure and tensile properties of as-cast Mg-8Li-3Al-2Sn (wt.%) alloy Xiaoyang Chen, Yang Zhang, Mengqi Cong PII:
S0042-207X(20)30129-9
DOI:
https://doi.org/10.1016/j.vacuum.2020.109292
Reference:
VAC 109292
To appear in:
Vacuum
Received Date: 13 January 2020 Revised Date:
11 February 2020
Accepted Date: 21 February 2020
Please cite this article as: Chen X, Zhang Y, Cong M, Effect of friction stir processing on microstructure and tensile properties of as-cast Mg-8Li-3Al-2Sn (wt.%) alloy, Vacuum, https://doi.org/10.1016/ j.vacuum.2020.109292. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Elsevier Ltd. All rights reserved.
Effect of friction stir processing on microstructure and tensile properties of ascast Mg-8Li-3Al-2Sn (wt.%) alloy Xiaoyang Chen, Yang Zhang *, Mengqi Cong Key Laboratory of Advanced Materials Design and Additive Manufacturing of Jiangsu Province, Jiangsu University of Technology, 213001, Changzhou, China
Abstract: A new Mg-8Li-3Al-2Sn (LAT832, wt.%) alloy was cast by vacuum induction melting and processed by friction stir processing (FSP). Microstructure and tensile properties of as-cast and FSP LAT832 samples were studied comparatively. It is found that, as-cast LAT832 sample consists of α-Mg+β-Li duplex-phase matrix and MgLiAl2 and Li2MgSn intermetallic phases. After FSP, an inverted-trapezoidal stir zone (SZ) is formed and onion-ring structure occurs at the center of SZ. Complete dynamic recrystallization (DRX) occurs in SZ of FSP LAT832 sample and α-Mg grains are significantly refined, but the microstructure is inhomogeneous. Li2MgSn phase is crushed into coarse particles while MgLiAl2 phase is decomposed into fine AlLi particles. Tensile properties of as-cast LAT832 sample are significantly enhanced by FSP, which benefits from strengthening effect of grain refinement of αMg phase and broken Li2MgSn particles. Key words: Mg-Li alloy; Friction stir processing; Microstructure; Tensile properties
Corresponding
author:
[email protected]
Yang
Zhang,
Tel:
+86-519-8695-3280,
E-mail:
The application of lightweight materials in aerospace field can lead to a significant increase in effective load and reduction in cost. Mg alloys are a kind of typical lightweight structural materials, which have significant advantages in weight reduction compared with steel or Al alloys [1-4]. As Li is the lightest metallic element, Li element can further reduce the density of Mg alloys and the resulting Mg-Li alloys are called ultra-light alloy. In addition, as the crystal structure of Li is body-centered cubic (BCC), the deformation ability is also significantly improved. In recent years, Mg-Li alloys have become a research hotspot among Mg alloys [5-7]. The weakness in strength is a main obstacle to the large-scale application of MgLi alloys. Conventional plastic processing techniques, including extrusion and rolling, are often used to improve the mechanical properties [8, 9]. Friction stir processing (FSP), known as an effective severe plastic deformation technique, can achieve obvious grain refinement and improved mechanical properties [10, 11]. G. Liu et al. [12] reported that, FSP induced grain refinement and improved tensile properties of Mg-9Li-1Zn alloy. F.C. Liu et al. [13] found that, Mg-Li alloy exhibited superplasticity at different temperatures after FSP. Up to now, the existing researches on FSP of Mg-Li alloys have only been done on several simple alloy systems. The evolution of intermetallic phases in Mg-Li alloys during FSP is also scarce. In this paper, a new Mg-8Li-3Al-2Sn (LAT832, wt.%) alloy was cast by vacuum induction melting. The as-cast LAT832 sample was processed by FSP. Microstructure and tensile properties of LAT832 alloy were studied in both as-cast and FSP states. Mechanisms of microstructure evolution and enhancement in tensile properties were analyzed. As-cast LAT832 ingots were prepared by vacuum induction melting with commercial pure Mg, Li, Al and Sn (99.9%) as raw materials. Plates were cut by wire cut electrical discharge machining for following FSP. Before FSP, oxides on the plate surface were cleaned via manual grinding. FSP process was conducted with a friction stir welding machine (FSW-LM-BM16, FSW Technology, China). The diameter of the stirring shoulder was 15 mm. A conical threaded pin was used. The length of the pin was 8.0 mm and its diameter at the base and the tip was 6.5 mm and 4.3 mm respectively. The selection of suitable processing parameters is important for FSP. In this paper, the travel speed was set as 60 mm/min and the rotation speed was set as 1000 rpm.
Microstructure specimens were cut from as-cast and FSP LAT832 samples (in the transverse-normal direction (TD-ND) plane for FSP sample). Optical microscope (OM, Primotech, Zeiss, Germany) and scanning electron microscope (SEM, Simga 500 VP, Zeiss, Germany) were used for microstructure characterization. Quantitative metallographic analysis was done with Image Pro Plus 6.0. Tensile test was conducted by a universal testing machine (CMT-5205, Wance, China). Tensile test was conducted according to ISO6892-1:2009 and the strain rate was set as 1.67×10−3 s−1. The gauge size of the tensile test specimens was 10×3.5×2 mm. Tensile test specimens of FSP LAT832 sample were cut along the processing direction (PD) of FSP and the gauge was completely in SZ. At least three specimens were repeatedly tested for each sample. Morphology of tensile tested fracture surface was also observed by SEM. Fig. 1(a) presents the XRD results of as-cast LAT832 sample. According to the XRD results, as-cast LAT832 sample consists of four phases, i.e. α-Mg, β-Li, Li2MgSn and MgLiAl2. As seen in Fig. 1(b), as-cast LAT832 sample exhibits a typical morphology of duplex-phase matrix, which matches Mg-Li phase diagram. The white, coarse, dendritic grains are α-Mg phase while the surrounding gray grains are β-Li phase. The volume fraction of α-Mg is much higher than β-Li. According to quantitative metallographic analysis results, α-Mg content is ~59.2 vol.% while β-Li content is only ~30.8 vol.%. There are two different kinds of intermetallic phases in as-cast LAT832 sample (shown in BSE-SEM image in Fig. 1(c)). According to the literatures [14, 15], the coarse, bright, feathered intermetallic phases are Li2MgSn phase and the fine, gray, granular intermetallic phases which distributed in β-Li phase are MgLiAl2 phase. The difference in contrast of two kinds of intermetallic phases also indicates their difference in chemical composition. Bright parts usually correspond to elements with high atomic number, while gray parts correspond to elements with low atomic number. The atomic number of Sn element in Li2MgSn phase is 50, which is significantly higher than other elements. Therefore, it confirms that the bright intermetallic phase in the BSE-SEM image is Li2MgSn phase and the gray intermetallic phase is MgLiAl2 phase.
Fig.1. XRD results and microstructure of as-cast LAT832 sample: (a) XRD results of as-cast LAT832 sample; (b) microstructure of as-cast LAT832 sample observed by OM and (c) microstructure of as-cast LAT832 sample observed by BSE-SEM. Fig.2 shows the cross-sectional macrostructure of FSP LAT832 sample. After FSP, an inverted-trapezoidal SZ is formed. The height of SZ is ~10.9 mm and the width in the middle of SZ is ~11.7 mm. It is obvious that, the microstructure of SZ in FSP LAT832 sample is inhomogeneous. The transition from base metal (BM) to SZ is sharp in advancing side (AS) while it is smooth in retreating side (RS). During FSP, the material is driven by the rotating force of the stirring pin. As the flow direction of the material in AS is the same as the rotating direction of the stirring pin, the transition region is narrow. When the material flows from AS to RS, the material is squeezed by the stirring pin thread, which makes the material flow complicated and leads to a wide transition region [16]. Moreover, onion-ring structure is found in the center of SZ. The presence of so-called onion-ring structure was also reported in other FSP studies [17, 18]. As the stirring pin rotates rapidly during FSP, the cylindrical sheets of processed metal are extruded in each rotation. When the microstructure characterization was done at the cross-section, such onion-ring structure is presented.
Fig. 2. Cross-sectional macrostructure of FSP LAT832 sample. Fig. 3 shows the microstructure of five representative regions (region A~E in Fig. 2) in SZ of FSP LAT832 sample observed by OM and SEM. Complete dynamic crystallization (DRX) occurs in SZ of FSP LAT832 sample. The inhomogeneity of microstructure in different SZ regions is observed. Average grain size of DRXed αMg grains in region A~E is ~3.6, 4.8, 1.8, 4.4 and 1.9 µm respectively. The DRXed αMg grains in region A, B and D are relatively coarse while the DRXed α-Mg grains in region C and E are fine. In the vertical direction, the DRXed α-Mg grains in the upper and central regions of SZ are coarse, while those in the bottom region are fine. This is because the friction between the shoulder and the plate surface generates a large amount of heat during FSP, resulting in an obvious temperature gradient in SZ, and the temperature in the upper is significantly higher than that in the bottom. Han et al. [19] found that, during FSP of Mg-Nd-Zn-Zr alloy, the peak temperature appeared on the upper surface of the processed sample and the peak temperature exceeded 590℃. Therefore, the upper and central regions are obviously softened by heat. The deformation energy accumulated for DRX is limited. Moreover, the DRXed grains are easy to coarsen at elevated temperature. In the horizontal direction, the severe plastic deformation on the advancing side contributes to the formation of fine DRXed grains, while the retreating side has a larger DRXed grain size because its degree of plastic deformation is less than the advancing side. As shown in Fig. 3, the intermetallic phases exhibit different morphology evolution after FSP. Firstly, the coarse feathered Li2MgSn phase is crushed into coarse particles. Secondly, the granular MgLiAl2 phase transfers into fine particles. The size of the particles decreases from micron-
sized to submicron order. Here, the difference in thermal stability of the intermetallic phases should be taken into consideration. Unlike Li2MgSn phase with high thermal stability, the thermal stability of MgLiAl2 phase is poor and it would transfer into AlLi phase even at room temperature. Therefore, it is reasonable to conclude that, due to the significant friction heat, MgLiAl2 phase decomposes and forms AlLi phase during FSP and subsequent cooling process. Since the distribution of MgLiAl2 phase in as-cast LAT832 sample is inhomogeneous and it aggregates inside β-Li phase, the new AlLi phase in FSP LAT832 sample also distributes uneven and aggregates inside β-Li phase, seen in Fig. 3(b).
Fig. 3. Microstructure of five representative regions (region A~E in Fig. 2) in SZ of FSP LAT832 sample observed by OM and SEM: (a) OM image of A; (b) SEM image of A; (c) OM image of B; (d) SEM image of B; (e) OM image of C; (f) SEM image of C; (g) OM image of D; (h) SEM image of D; (i) OM image of E and (j) SEM image of E. Fig. 4(a) presents tensile test results of as-cast and FSP LAT832 samples and Fig. 4(b) shows typical tensile stress-strain curves. Tensile test results confirm that, FSP improves the tensile properties of LAT832 alloy significantly. Yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of as-cast LAT832 sample are 153MPa, 215MPa and 17%. After FSP, YS, UTS and EL of FSP LAT832 sample reach 218MPa, 259MPa and 19%. In other words, YS, UTS and EL of FSP LAT832 sample are increased by 42%, 20% and 12%, compared to as-cast LAT832 sample. Fig. 4(c, d) presents fracture morphology of tensile-tested samples observed by SEM. Fracture morphology of as-cast LAT832 sample exhibits a mixture of cleavage and dimple fracture mode. The cleavage fracture is mainly induced by the deformation and fracture of coarse dendritic α-Mg grains in as-cast LAT832 sample, while the dimples are formed in β-Li phase. Fracture morphology of FSP LAT832 sample is totally different from as-cast LAT832 sample. No cleavage fracture is observed but only dimples are found in the fracture surface. Moreover, coarse broken particles of
Li2MgSn phase are found in the dimples (as circled in Fig. 4(d)), which may act as crack initiation during tensile deformation.
Fig. 4. Tensile test results and fracture morphology of as-cast and FSP LAT832 samples: (a) tensile test results; (b) typical tensile stress-strain curves; (c) fracture morphology of as-cast LAT832 sample and (d) fracture morphology of FSP LAT832 sample. In this study, FSP exhibits good capability in improving tensile properties of LAT832 alloy. Firstly, according to Hall-Petch equation, YS would increase as the average grain size decreases. In the duplex-phase matrix, α-Mg phase is the hard phase and mainly bears the deformation force. After FSP, the size of α-Mg grains is decreased from several hundred microns to several microns, which contributes to the strength enhancement significantly. Secondly, although the volume fraction of Li2MgSn phase in LAT832 alloy is relatively high, its strengthening effect in as-cast LAT832 sample is limited, since it is coarse and aggregates at the grain boundaries. However, after FSP, Li2MgSn phase is broken into particles and dispersed in the matrix, which provides effective obstacles to the slip of dislocation and thereby strengthens the matrix [20]. As the strengthening effect of AlLi phase is poor, its effect on strength is ignored [21]. Contrary to YS, work hardening amplitude of FSP sample is higher than as-cast sample, which may be attributed to the influence of
texture formed in α-Mg phase after FSP [22]. In terms of ductility, because of the existence of duplex-phase matrix, LAT832 alloy exhibits good ductility in as-cast state. After FSP, although the grain refinement is beneficial to the further improvement in ductility, a large number of broken Li2MgSn particles would also increase the possibility of crack initiation and thereby deteriorate the ductility, which is also confirmed by the observation of fracture surface morphology of FSP LAT832 sample (seen in Fig. 4(d)). In this study, LAT832 alloy was cast by vacuum induction melting and processed by FSP. Microstructure and tensile properties of LAT832 alloy were studied in both as-cast and FSP states. (1) As-cast LAT832 sample consists of αMg+β-Li duplex-phase matrix and MgLiAl2 and Li2MgSn intermetallic phases. An inverted-trapezoidal SZ is formed after FSP. Transition between BM and SZ is sharp in AS while it is smooth in RS. Onion-ring structure occurs at the center of SZ. (2) Complete DRX occurs in SZ of FSP LAT832 sample and α-Mg grains are significantly refined, but the microstructure is inhomogeneous. The DRXed α-Mg grains in the bottom and AS regions of SZ are finer than those in the upper, central and RS regions. The coarse feathered Li2MgSn phase is broken into coarse particles while MgLiAl2 phase is decomposed into fine AlLi particles. (3) FSP significantly improves the tensile properties of LAT832 alloy. YS, UTS and EL of FSP LAT832 sample are increased by 42%, 20% and 12%, compared to as-cast LAT832 sample. The increase of strength mainly comes from the strengthening effect of grain refinement of α-Mg phase and broken Li2MgSn phase. Acknowledgement The authors are grateful for the financial support from National Natural Science Foundation of China (No. 51601076); China Postdoctoral Science Foundation (No. 2019M650096) and Natural Science Fund for Colleges and Universities in Jiangsu Province (No. 17KJA430005). References [1] X. Heng, Y. Zhang, W. Rong, Y. Wu, L .Peng, A super high-strength Mg-Gd-YZn-Mn alloy fabricated by hot extrusion and strain aging, Mater. Des. 169 (2019) 107666 https://doi.org/10.1016/j.matdes.2019.107666. [2] G.H. Huang, D.D. Yin, J.W. Lu, H. Zhou, Y. Zeng, G.F. Quan, Q.D. Wang, Microstructure, texture and mechanical properties evolution of extruded fine-grained
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Onion-ring structure occurs at the center of stir zone. Dynamic recrystallization occurs in stir zone of friction stir processing sample. Li2MgSn phase is crushed into coarse particles during friction stir processing. MgLiAl2 phase is decomposed into fine AlLi particles during friction stir processing. The tensile properties are significantly enhanced by friction stir processing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0042-207X(20)30129-9
DOI:
https://doi.org/10.1016/j.vacuum.2020.109292
Reference:
VAC 109292
To appear in:
Vacuum
Received Date: 13 January 2020 Revised Date:
11 February 2020
Accepted Date: 21 February 2020
Please cite this article as: Chen X, Zhang Y, Cong M, Effect of friction stir processing on microstructure and tensile properties of as-cast Mg-8Li-3Al-2Sn (wt.%) alloy, Vacuum, https://doi.org/10.1016/ j.vacuum.2020.109292. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Elsevier Ltd. All rights reserved.
Effect of friction stir processing on microstructure and tensile properties of ascast Mg-8Li-3Al-2Sn (wt.%) alloy Xiaoyang Chen, Yang Zhang *, Mengqi Cong Key Laboratory of Advanced Materials Design and Additive Manufacturing of Jiangsu Province, Jiangsu University of Technology, 213001, Changzhou, China
Abstract: A new Mg-8Li-3Al-2Sn (LAT832, wt.%) alloy was cast by vacuum induction melting and processed by friction stir processing (FSP). Microstructure and tensile properties of as-cast and FSP LAT832 samples were studied comparatively. It is found that, as-cast LAT832 sample consists of α-Mg+β-Li duplex-phase matrix and MgLiAl2 and Li2MgSn intermetallic phases. After FSP, an inverted-trapezoidal stir zone (SZ) is formed and onion-ring structure occurs at the center of SZ. Complete dynamic recrystallization (DRX) occurs in SZ of FSP LAT832 sample and α-Mg grains are significantly refined, but the microstructure is inhomogeneous. Li2MgSn phase is crushed into coarse particles while MgLiAl2 phase is decomposed into fine AlLi particles. Tensile properties of as-cast LAT832 sample are significantly enhanced by FSP, which benefits from strengthening effect of grain refinement of αMg phase and broken Li2MgSn particles. Key words: Mg-Li alloy; Friction stir processing; Microstructure; Tensile properties
Corresponding
author:
[email protected]
Yang
Zhang,
Tel:
+86-519-8695-3280,
E-mail:
The application of lightweight materials in aerospace field can lead to a significant increase in effective load and reduction in cost. Mg alloys are a kind of typical lightweight structural materials, which have significant advantages in weight reduction compared with steel or Al alloys [1-4]. As Li is the lightest metallic element, Li element can further reduce the density of Mg alloys and the resulting Mg-Li alloys are called ultra-light alloy. In addition, as the crystal structure of Li is body-centered cubic (BCC), the deformation ability is also significantly improved. In recent years, Mg-Li alloys have become a research hotspot among Mg alloys [5-7]. The weakness in strength is a main obstacle to the large-scale application of MgLi alloys. Conventional plastic processing techniques, including extrusion and rolling, are often used to improve the mechanical properties [8, 9]. Friction stir processing (FSP), known as an effective severe plastic deformation technique, can achieve obvious grain refinement and improved mechanical properties [10, 11]. G. Liu et al. [12] reported that, FSP induced grain refinement and improved tensile properties of Mg-9Li-1Zn alloy. F.C. Liu et al. [13] found that, Mg-Li alloy exhibited superplasticity at different temperatures after FSP. Up to now, the existing researches on FSP of Mg-Li alloys have only been done on several simple alloy systems. The evolution of intermetallic phases in Mg-Li alloys during FSP is also scarce. In this paper, a new Mg-8Li-3Al-2Sn (LAT832, wt.%) alloy was cast by vacuum induction melting. The as-cast LAT832 sample was processed by FSP. Microstructure and tensile properties of LAT832 alloy were studied in both as-cast and FSP states. Mechanisms of microstructure evolution and enhancement in tensile properties were analyzed. As-cast LAT832 ingots were prepared by vacuum induction melting with commercial pure Mg, Li, Al and Sn (99.9%) as raw materials. Plates were cut by wire cut electrical discharge machining for following FSP. Before FSP, oxides on the plate surface were cleaned via manual grinding. FSP process was conducted with a friction stir welding machine (FSW-LM-BM16, FSW Technology, China). The diameter of the stirring shoulder was 15 mm. A conical threaded pin was used. The length of the pin was 8.0 mm and its diameter at the base and the tip was 6.5 mm and 4.3 mm respectively. The selection of suitable processing parameters is important for FSP. In this paper, the travel speed was set as 60 mm/min and the rotation speed was set as 1000 rpm.
Microstructure specimens were cut from as-cast and FSP LAT832 samples (in the transverse-normal direction (TD-ND) plane for FSP sample). Optical microscope (OM, Primotech, Zeiss, Germany) and scanning electron microscope (SEM, Simga 500 VP, Zeiss, Germany) were used for microstructure characterization. Quantitative metallographic analysis was done with Image Pro Plus 6.0. Tensile test was conducted by a universal testing machine (CMT-5205, Wance, China). Tensile test was conducted according to ISO6892-1:2009 and the strain rate was set as 1.67×10−3 s−1. The gauge size of the tensile test specimens was 10×3.5×2 mm. Tensile test specimens of FSP LAT832 sample were cut along the processing direction (PD) of FSP and the gauge was completely in SZ. At least three specimens were repeatedly tested for each sample. Morphology of tensile tested fracture surface was also observed by SEM. Fig. 1(a) presents the XRD results of as-cast LAT832 sample. According to the XRD results, as-cast LAT832 sample consists of four phases, i.e. α-Mg, β-Li, Li2MgSn and MgLiAl2. As seen in Fig. 1(b), as-cast LAT832 sample exhibits a typical morphology of duplex-phase matrix, which matches Mg-Li phase diagram. The white, coarse, dendritic grains are α-Mg phase while the surrounding gray grains are β-Li phase. The volume fraction of α-Mg is much higher than β-Li. According to quantitative metallographic analysis results, α-Mg content is ~59.2 vol.% while β-Li content is only ~30.8 vol.%. There are two different kinds of intermetallic phases in as-cast LAT832 sample (shown in BSE-SEM image in Fig. 1(c)). According to the literatures [14, 15], the coarse, bright, feathered intermetallic phases are Li2MgSn phase and the fine, gray, granular intermetallic phases which distributed in β-Li phase are MgLiAl2 phase. The difference in contrast of two kinds of intermetallic phases also indicates their difference in chemical composition. Bright parts usually correspond to elements with high atomic number, while gray parts correspond to elements with low atomic number. The atomic number of Sn element in Li2MgSn phase is 50, which is significantly higher than other elements. Therefore, it confirms that the bright intermetallic phase in the BSE-SEM image is Li2MgSn phase and the gray intermetallic phase is MgLiAl2 phase.
Fig.1. XRD results and microstructure of as-cast LAT832 sample: (a) XRD results of as-cast LAT832 sample; (b) microstructure of as-cast LAT832 sample observed by OM and (c) microstructure of as-cast LAT832 sample observed by BSE-SEM. Fig.2 shows the cross-sectional macrostructure of FSP LAT832 sample. After FSP, an inverted-trapezoidal SZ is formed. The height of SZ is ~10.9 mm and the width in the middle of SZ is ~11.7 mm. It is obvious that, the microstructure of SZ in FSP LAT832 sample is inhomogeneous. The transition from base metal (BM) to SZ is sharp in advancing side (AS) while it is smooth in retreating side (RS). During FSP, the material is driven by the rotating force of the stirring pin. As the flow direction of the material in AS is the same as the rotating direction of the stirring pin, the transition region is narrow. When the material flows from AS to RS, the material is squeezed by the stirring pin thread, which makes the material flow complicated and leads to a wide transition region [16]. Moreover, onion-ring structure is found in the center of SZ. The presence of so-called onion-ring structure was also reported in other FSP studies [17, 18]. As the stirring pin rotates rapidly during FSP, the cylindrical sheets of processed metal are extruded in each rotation. When the microstructure characterization was done at the cross-section, such onion-ring structure is presented.
Fig. 2. Cross-sectional macrostructure of FSP LAT832 sample. Fig. 3 shows the microstructure of five representative regions (region A~E in Fig. 2) in SZ of FSP LAT832 sample observed by OM and SEM. Complete dynamic crystallization (DRX) occurs in SZ of FSP LAT832 sample. The inhomogeneity of microstructure in different SZ regions is observed. Average grain size of DRXed αMg grains in region A~E is ~3.6, 4.8, 1.8, 4.4 and 1.9 µm respectively. The DRXed αMg grains in region A, B and D are relatively coarse while the DRXed α-Mg grains in region C and E are fine. In the vertical direction, the DRXed α-Mg grains in the upper and central regions of SZ are coarse, while those in the bottom region are fine. This is because the friction between the shoulder and the plate surface generates a large amount of heat during FSP, resulting in an obvious temperature gradient in SZ, and the temperature in the upper is significantly higher than that in the bottom. Han et al. [19] found that, during FSP of Mg-Nd-Zn-Zr alloy, the peak temperature appeared on the upper surface of the processed sample and the peak temperature exceeded 590℃. Therefore, the upper and central regions are obviously softened by heat. The deformation energy accumulated for DRX is limited. Moreover, the DRXed grains are easy to coarsen at elevated temperature. In the horizontal direction, the severe plastic deformation on the advancing side contributes to the formation of fine DRXed grains, while the retreating side has a larger DRXed grain size because its degree of plastic deformation is less than the advancing side. As shown in Fig. 3, the intermetallic phases exhibit different morphology evolution after FSP. Firstly, the coarse feathered Li2MgSn phase is crushed into coarse particles. Secondly, the granular MgLiAl2 phase transfers into fine particles. The size of the particles decreases from micron-
sized to submicron order. Here, the difference in thermal stability of the intermetallic phases should be taken into consideration. Unlike Li2MgSn phase with high thermal stability, the thermal stability of MgLiAl2 phase is poor and it would transfer into AlLi phase even at room temperature. Therefore, it is reasonable to conclude that, due to the significant friction heat, MgLiAl2 phase decomposes and forms AlLi phase during FSP and subsequent cooling process. Since the distribution of MgLiAl2 phase in as-cast LAT832 sample is inhomogeneous and it aggregates inside β-Li phase, the new AlLi phase in FSP LAT832 sample also distributes uneven and aggregates inside β-Li phase, seen in Fig. 3(b).
Fig. 3. Microstructure of five representative regions (region A~E in Fig. 2) in SZ of FSP LAT832 sample observed by OM and SEM: (a) OM image of A; (b) SEM image of A; (c) OM image of B; (d) SEM image of B; (e) OM image of C; (f) SEM image of C; (g) OM image of D; (h) SEM image of D; (i) OM image of E and (j) SEM image of E. Fig. 4(a) presents tensile test results of as-cast and FSP LAT832 samples and Fig. 4(b) shows typical tensile stress-strain curves. Tensile test results confirm that, FSP improves the tensile properties of LAT832 alloy significantly. Yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of as-cast LAT832 sample are 153MPa, 215MPa and 17%. After FSP, YS, UTS and EL of FSP LAT832 sample reach 218MPa, 259MPa and 19%. In other words, YS, UTS and EL of FSP LAT832 sample are increased by 42%, 20% and 12%, compared to as-cast LAT832 sample. Fig. 4(c, d) presents fracture morphology of tensile-tested samples observed by SEM. Fracture morphology of as-cast LAT832 sample exhibits a mixture of cleavage and dimple fracture mode. The cleavage fracture is mainly induced by the deformation and fracture of coarse dendritic α-Mg grains in as-cast LAT832 sample, while the dimples are formed in β-Li phase. Fracture morphology of FSP LAT832 sample is totally different from as-cast LAT832 sample. No cleavage fracture is observed but only dimples are found in the fracture surface. Moreover, coarse broken particles of
Li2MgSn phase are found in the dimples (as circled in Fig. 4(d)), which may act as crack initiation during tensile deformation.
Fig. 4. Tensile test results and fracture morphology of as-cast and FSP LAT832 samples: (a) tensile test results; (b) typical tensile stress-strain curves; (c) fracture morphology of as-cast LAT832 sample and (d) fracture morphology of FSP LAT832 sample. In this study, FSP exhibits good capability in improving tensile properties of LAT832 alloy. Firstly, according to Hall-Petch equation, YS would increase as the average grain size decreases. In the duplex-phase matrix, α-Mg phase is the hard phase and mainly bears the deformation force. After FSP, the size of α-Mg grains is decreased from several hundred microns to several microns, which contributes to the strength enhancement significantly. Secondly, although the volume fraction of Li2MgSn phase in LAT832 alloy is relatively high, its strengthening effect in as-cast LAT832 sample is limited, since it is coarse and aggregates at the grain boundaries. However, after FSP, Li2MgSn phase is broken into particles and dispersed in the matrix, which provides effective obstacles to the slip of dislocation and thereby strengthens the matrix [20]. As the strengthening effect of AlLi phase is poor, its effect on strength is ignored [21]. Contrary to YS, work hardening amplitude of FSP sample is higher than as-cast sample, which may be attributed to the influence of
texture formed in α-Mg phase after FSP [22]. In terms of ductility, because of the existence of duplex-phase matrix, LAT832 alloy exhibits good ductility in as-cast state. After FSP, although the grain refinement is beneficial to the further improvement in ductility, a large number of broken Li2MgSn particles would also increase the possibility of crack initiation and thereby deteriorate the ductility, which is also confirmed by the observation of fracture surface morphology of FSP LAT832 sample (seen in Fig. 4(d)). In this study, LAT832 alloy was cast by vacuum induction melting and processed by FSP. Microstructure and tensile properties of LAT832 alloy were studied in both as-cast and FSP states. (1) As-cast LAT832 sample consists of αMg+β-Li duplex-phase matrix and MgLiAl2 and Li2MgSn intermetallic phases. An inverted-trapezoidal SZ is formed after FSP. Transition between BM and SZ is sharp in AS while it is smooth in RS. Onion-ring structure occurs at the center of SZ. (2) Complete DRX occurs in SZ of FSP LAT832 sample and α-Mg grains are significantly refined, but the microstructure is inhomogeneous. The DRXed α-Mg grains in the bottom and AS regions of SZ are finer than those in the upper, central and RS regions. The coarse feathered Li2MgSn phase is broken into coarse particles while MgLiAl2 phase is decomposed into fine AlLi particles. (3) FSP significantly improves the tensile properties of LAT832 alloy. YS, UTS and EL of FSP LAT832 sample are increased by 42%, 20% and 12%, compared to as-cast LAT832 sample. The increase of strength mainly comes from the strengthening effect of grain refinement of α-Mg phase and broken Li2MgSn phase. Acknowledgement The authors are grateful for the financial support from National Natural Science Foundation of China (No. 51601076); China Postdoctoral Science Foundation (No. 2019M650096) and Natural Science Fund for Colleges and Universities in Jiangsu Province (No. 17KJA430005). References [1] X. Heng, Y. Zhang, W. Rong, Y. Wu, L .Peng, A super high-strength Mg-Gd-YZn-Mn alloy fabricated by hot extrusion and strain aging, Mater. Des. 169 (2019) 107666 https://doi.org/10.1016/j.matdes.2019.107666. [2] G.H. Huang, D.D. Yin, J.W. Lu, H. Zhou, Y. Zeng, G.F. Quan, Q.D. Wang, Microstructure, texture and mechanical properties evolution of extruded fine-grained
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Onion-ring structure occurs at the center of stir zone. Dynamic recrystallization occurs in stir zone of friction stir processing sample. Li2MgSn phase is crushed into coarse particles during friction stir processing. MgLiAl2 phase is decomposed into fine AlLi particles during friction stir processing. The tensile properties are significantly enhanced by friction stir processing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: