Effect of extrusion speeds on the microstructure, texture and mechanical properties of high-speed extrudable MgZnSnMnCa alloy

Vacuum 157 (2018) 180–191

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Effect of extrusion speeds on the microstructure, texture and mechanical properties of high-speed extrudable MgeZneSneMneCa alloy

T

Xing Lua, Guoqun Zhaoa,∗, Jixue Zhoub, Cunsheng Zhanga, Lu Suna a

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, 250061, PR China Shandong Key Laboratory for High Strength Lightweight Metallic Materials, New Materials Research Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250014, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: MgeZneSneMneCa alloy Extrusion speed Microstructure Texture Mechanical properties

A new type of low-cost Mg-3.36Zn-1.06Sn-0.33Mn-0.27Ca (wt.%) (named as ZTMX3100) alloy ingot with good extrudability was prepared by semi-continuous casting. Two-stage homogenization treatment and hot extrusion were performed on the ingot. The effects of extrusion speeds (0.2–10 mm/s) on the extrudability, microstructure and mechanical properties of the alloy were investigated. The excellent extrudability of the alloy is attributed to the high incipient melting temperature of 572 °C resulting from the formation of thermally stable phase CaMgSn. CaMgSn phases distribute along the extrusion direction at each extrusion speed. The average grain size increases gradually and distribution uniformity of grain size firstly increases and then decreases. The as-extruded alloys exhibit textures with (0001) basal planes parallel to extrusion direction. Besides, the alloy exhibits a < 1010 > or < 1010 > − < 112 0 > fiber texture depending on extrusion speed. The alloy extruded at 0.2 mm/s presents the highest mechanical property, which is attributed to the fine grain structure, strengthening effect from second phase particles, and weak basal texture. The alloy extruded at high speed of 10 mm/s (die-exit speed 18 m/min) still maintains a relatively high strength and elongation, which is mainly due to the combined effect of moderate alloying content and thermally stable phase CaMgSn.

1. Introduction Wrought magnesium (Mg) alloys are getting more and more attentions in industrial applications due to their high specific strength and excellent weight reduction. However, the hexagonal close-packed crystal (hcp) structure reduces the formability of Mg alloys, especially the as-extruded Mg alloys, resulting in relatively low extrusion speed of magnesium alloys [1,2]. Low extrusion speed reduces the production efficiency of as-extruded magnesium alloys, which restricts their further applications in industry [3,4]. The extrusion speed of Mg alloy is usually influenced by the alloy composition (alloying element and alloying content) and second phase particle (thermal stability, size and morphology) in the alloy, etc. [5–7]. Generally, higher alloying content leads to lower maximum extrusion speed of the alloy. In MgeAleZn alloy systems, as the Al content increases, more Mg17Al12 particles with a low melting temperature are formed during extrusion, which increases the susceptibility of the alloy to hot cracking and thus decreases its extrudability [8]. For example, AZ31 can be extruded at a die-exit speed of about 15 m/min [9], while AZ91 has the maximum extrusion speed of about 5 m/min [8]. For other high alloying content magnesium

∗

alloys such as ZK60 [10] and Mg-9Gd-3Y-1.5Zn-0.8Zr (wt.%) [11], the increased alloy's susceptibility to hot cracking due to the high alloying content leads to the further reduction of extrusion speed to 0.3–2.5 m/ min. The extrudability of Mg alloy can be improved by lowering the alloying content, such as Mg-0.5Al-0.25Zn-0.1Mn (wt.%) [12], Mg-0.3Al0.21Ca-0.47Mn (wt.%) [7,13], Mg-1.58Zn-0.52Gd (wt.%) [3] and Mg0.21Zn-0.3Ca-0.14Mn (wt.%) [14]. The mentioned above alloys exhibit excellent extrudability by reducing the alloying content such as Al element or Zn element. However, these dilute Mg alloys show relatively low strength due to the insufficiency of alloying contents, which are responsible for bringing various strengthening effects (such as grainrefinement strengthening, solid-solution strengthening and second phase particles strengthening) [8]. Besides, large processing heat generated during high-speed extrusion usually leads to coarse grain structures [3,14,15]. Ultimately, the yield strength of these dilute Mg alloys is generally less than 120 MPa, or even less than 80 MPa [3,4,14,16,17], which is far below than the strength of the AZ or ZK commercial wrought Mg alloys [6]. When the alloy has thermally stable phase with proper morphology

Corresponding author. E-mail address: [email protected] (G. Zhao).

https://doi.org/10.1016/j.vacuum.2018.08.041 Received 23 July 2018; Received in revised form 18 August 2018; Accepted 21 August 2018 Available online 23 August 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.

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continuous direct-chill casting technology. The detailed technological parameters and main features of this technology were described in our previous work [25]. The composition of ZTMX3100 alloy ingot at different positions was measured by inductively coupled plasma emission spectrometer (ICP-AES, Agilent Technology 725-ES). The average composition of the semi-continuous ingot is Mg-3.36Zn-1.06Sn0.33Mn-0.27Ca (wt.%). The average density of the alloy measured by Archimedes principle method is 1.782 g/mm3.

and size, hot cracking can be prevented and grain growth can be impeded effectively during high-speed extrusion, which is beneficial to improve the extrudability and comprehensive mechanical properties of the alloy [15,18,19]. Park et al. [5] indicated that the high-speed extrudability of Mge7Sne1Ale1Zn (wt.%) alloy is attributed to the relatively high incipient melting temperature of the alloy (535 °C), which results from the formation of thermally stable phase Mg2Sn with a melting temperature of 770 °C. However, the average grain size of Mge7Sne1Ale1Zn (wt.%) alloy reaches 65.8 μm at the die-exit speed of 18 m/min, which leads to a relatively low elongation of 13.4%. By comparison, CaMgSn is a more thermally stable phase with a higher melting temperature of 1184 °C [20]. Meanwhile, CaMgSn phase presents strip-like morphology and exhibits more effective pinning effect on the grain boundary during high-speed extrusion, which is conducive to refine the grain size and thus simultaneously improve the strength and elongation of the alloy [20]. Therefore, if the magnesium alloys with moderate alloying content contain thermally stable phase CaMgSn, then excellent comprehensive mechanical properties without obvious grain growth under the condition of high-speed extrusion are likely to be achieved. Recently, MgeZneSn-based alloys have received widespread attention for their low cost and good mechanical properties [21–23]. However, the current researches mainly focus on high-Zn content or high-Sn content alloys. The increased alloy's susceptibility to hot cracking due to the high alloying content results in a low die-exit speed (0.12–2 m/min), which limits the further extensive application of this type of alloy. In our previous work, we have developed a new magnesium alloy with moderate alloying content of Mg-3.0Zn-1.0Sn-0.3Mn0.3Ca (wt.%) (named as ZTMX3100), which contains thermally stable phase CaMgSn and exhibits good comprehensive mechanical properties [24]. However, the investigation of high-speed extrusion on this type of alloy has not been conducted, and the evolution of the microstructure (grain structure, second phase particles), texture, and mechanical properties the alloy extruded at different ram speeds still has not been clarified yet. The extrudability of the alloy and the evolution of the microstructure in the case where the alloy extruded at different ram speeds have a key effect on the extrusion production efficiency and mechanical properties of the alloy, which directly determines whether the alloy has a good industrial application prospects. In this paper, the new type of ZTMX3100 magnesium alloy ingot was prepared by using semi-continuous casting technology. Subsequently, the ZTMX3100 alloy ingot was subjected to thermomechanical processing including two-stage homogenization treatment and hot extrusion at different extrusion speeds (0.2–10 mm/s). The effects of extrusion speeds (0.2–10 mm/s) on the extrudability, microstructure, and mechanical properties were clarified by using optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and mechanical properties test. The evolutions of grain structure (grain size and distribution), second phase particles (size, distribution, and volume fraction), micro-texture and mechanical properties of ZTMX3100 alloys extruded at different ram speeds were studied. The fracture morphology and fracture mode of the alloy extruded at different ram speeds were observed and analyzed. The extrudability of ZTMX3100 magnesium alloy was also investigated. The results show that the alloy containing thermally stable phase CaMgSn still maintains good surface quality and presents a relatively high strength and elongation at high extrusion speed, exhibiting a good prospect in engineering applications.

2.2. Hot extrusion experiment The alloy ingot was machined into the cylindrical billets with the diameter of 118 mm and the height of 200 mm. Then these as-cast billets were homogenized at the condition of two-step homogenization with 340 °C × 12 h + 520 °C × 16 h in the protective atmosphere to achieve a uniform microstructure before extrusion, followed by air cooling. The homogenized billets were kept at 300 °C for 60 min and then the direct extrusion experiments were conducted on an 800 tons extrusion press with the extrusion container diameter of 120 mm. The extrusion temperature was 300 °C and the extrusion ratio was 30. The ram speeds were set as 0.2, 1, 5, and 10 mm/s, respectively, which corresponds to the die-exit speeds of 0.36, 1.8, 9, and 18 m/min. Finally, the extruded bars were obtained after extrusion followed by aircooling. 2.3. Microstructure characterization and mechanical properties test The microstructural characteristics of the as-cast, as-homogenized and as-extruded alloys were observed in an optical microscope (OM, Olympus GX51) and a field emission scanning electron microscope (FESEM, HITACHI SU-70) equipped with an energy dispersive X-ray spectrometer (EDS, HORIBA EX-250). Samples for OM and SEM observations were ground with emery papers up to 2000 grits and polished into mirror-like ones, and then etched with a mixed acid solution (1 g picric acid, 1 mL acetic acid, 20 mL ethanol and 2 mL distilled water) for microstructure observations [26,27]. Grain size was measured by using Image-Pro Plus 6.0 software. More than 1000 grains from different optical micrographs were examined for each condition. Micro-texture tests of the as-extruded alloys were conducted on an electron back scattered diffraction system (EBSD, Oxford InstrumentsHKL) operating at 20 kV, with the field view of 400 μm × 300 μm and the step size of 1 μm. The EBSD data were analyzed with HKL Channel 5 analysis software. The observing surfaces of the as-extruded alloys for OM, SEM, and EBSD tests were the longitudinal section along extrusion direction (ED). The morphology and distribution of the fine second phase particles in the as-extruded alloy were further analyzed by using a high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-TWIN), with an accelerating voltage of 200 kV. TEM foils were prepared by mechanical polishing to less than 50 μm, subsequently punched into 3 mm discs. The final ion-milling was conducted by using a Gatan Precision Ion Polishing System (GATAN691). The thermal properties of the as-cast and as-homogenized ZTMX3100 alloy were studied using a differential scanning calorimetry (DSC, METTLER 1100LF) at the heating rate of 20 K/min under a flowing argon atmosphere. Tensile samples were taken from the central parts of the as-extruded alloy, with their axes corresponded to extrusion direction. According to the testing standard of EN ISO 6892–1 (2009), the dimension of the dog-bone shaped tensile samples is shown in Fig. 1, with the original diameter of 5 mm and original gauge length of 25 mm. The tensile properties of the as-extruded alloy were measured at room temperature by using a MTS CMT5015 universal testing machine with a strain rate of 1.0 × 10−3 s−1. Tensile tests for each extrusion condition were repeated at least three times to ensure the reproducibility of the data. The fracture surfaces of the as-extruded alloys were observed by FE-SEM.

2. Experimental procedures 2.1. Alloy fabrication The new type of ZTMX3100 magnesium alloy ingot with a diameter of 130 mm and a length of 4800 mm was obtained by using semi181

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treatment, the irregular phase is dissolved into the matrix while the thermally stable phase is dissolved from long strip-like into short striplike. According to our previous investigations [24,25], the as-cast ZTMX3100 alloy contains the α-Mg matrix, irregular phase MgZn2, long strip-like phase CaMgSn, and relatively fine phase α-Mn. Based on the EDS results in Fig. 2(e)–(g), it can be preliminarily concluded that Point E is MgZn2, Point F and Point G are CaMgSn. After homogenization treatment, irregular phase MgZn2 is dissolved into the matrix and long strip-like phase CaMgSn distributes more uniformly in the matrix due to partial dissolution.

Fig. 1. The size of the dog-bone shaped tensile sample.

3. Results and discussion 3.2. Extrudability of the alloy 3.1. Microstructure of the as-cast and as-homogenized alloy Fig. 3(a) shows the bar surface of ZTMX3100 alloys extruded at different ram speeds. As can be seen from Fig. 3(a), the extruded bars obtained from different extrusion speeds (0.2–10 mm/s) all present good surface quality without any cracks. Fig. 3(b) gives the DSC curves of the as-cast and as-homogenized ZTMX3100 alloys. According to the DSC curve of the as-cast alloy in Fig. 3(b), two endothermic peaks were directly obtained by the horizontal axis in the as-cast ZTMX3100 alloy, with the onset temperatures of about 338 °C and 573 °C and the peak temperatures of about 345 °C and 585 °C, respectively. The two endothermic peaks correspond to the dissolution of the low-melting-point phase MgZn2 and the thermally stable phase CaMgSn, respectively. As

Fig. 2(a) and (b) show the SEM micrographs of the as-cast and ashomogenized alloys. The average grain sizes (daverage) of the as-cast and as-homogenized alloys are about 68.5 μm and 70.3 μm, respectively. The average grain sizes of the alloy change little before and after homogenization treatment. Fig. 2(c) and (d) give the high-magnification SEM micrographs of Region C and Region D, respectively. Fig. 2(e)–(g) show the EDS results of Point E, Point F and Point G, respectively. According to Fig. 2(c) and (d), the as-cast alloy contains the irregular phase and the strip-like phase. After homogenization

Fig. 2. SEM micrographs of ZTMX3100 alloys and corresponding EDS results: (a) the as-cast, (b) the as-homogenized, (c) the magnified SEM of Region C, (d) the magnified SEM of Region D, (e) EDS results of Point E, (f) EDS results of Point F, (g) EDS results of Point G. 182

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Fig. 3. (a) Bar surfaces of ZTMX3100 alloys extruded at different ram speeds. (b) DSC curves of the as-cast and as-homogenized ZTMX3100 alloys.

the SEM results in Fig. 5. Fig. 7(a) and (d) show the bright field (BF) transmission electron micrograph of the as-extruded alloy extruded at the ram speed of 0.2 mm/s. As shown in Fig. 7(a), there are relatively large-size spherical phase in the alloy. Fig. 7(b) and (c) give the high resolution transmission electron micrograph (HRTEM) of this phase and the corresponding fast Fourier transform (FFT) image, respectively. It can be determined that the interplanar spacings of {112 0} planes are about 0.257 nm, which is consistent with the investigation of Li et al. [28]. Therefore, it can be determined that the relatively large spherical phase is MgZn2. The relatively small spherical phase can also be found in the matrix according to the BF image shown in Fig. 7(d). Fig. 7(e) and (f) show the HRTEM image of the phase and the corresponding FFT image, respectively. The interplanar spacings of {200} planes and {011} planes are determined as 0.445 nm and 0.631 nm, which coincides with the crystallography data of α-Mn provided by International Centre for Diffraction Data [29]. Accordingly, the relatively small spherical phase is conformed to be α-Mn. Compared with the microstructure of the as-cast billet, the grain size of the as-extruded alloys is significantly refined. As can be seen from Fig. 4, the as-extruded alloys almost exhibit uniform and equiaxed grains at different extrusion speeds (0.2–10 mm/s), which indicates that dynamic recrystallization occurs in the range of the above extrusion speeds. The statistical results of grain size at different extrusion speeds are shown in the microstructural characteristics part of Table 1. Table 1 shows that the average grain size of the alloy increases gradually as the extrusion speed increases from 0.2 mm/s to 10 mm/s, with the average grain size of 7.8, 10.6, 13.4, and 16.3 μm, respectively. This is because with the increase of extrusion speed, more heat is generated due to plastic deformation and the friction, thus leading to the increase of billet temperature [5]. Meanwhile, both the dynamic recrystallization and grain growth during extrusion are a kind of thermally activated process, so the driving force for the dynamic recrystallization and grain growth increases [30,31]. As a result, the grain boundary diffusion and migration are enhanced and the grains grow up as the extrusion speed increases. In addition, as the extrusion speed increases, the fraction of thermally stable phase CaMgSn gradually decreases. However, these second phase particles play an important role in pinning grain boundaries and impeding grain growth [24,32]. Finally, the average grain size of the alloy increases gradually as the extrusion speed increases. Therefore, it can be seen that both the extrusion speed and thermally stable phase have an obvious impact on the average grain size of the alloy. The extrusion speed not only affects the average grain size but also influences the mathematical distribution of the grain size. According to references [33–35], the relative grain size dispersion (Δd/dm) can be used to evaluate the uniformity of grain distribution as the standard deviation, where Δd (△d = dmax - dmin) is the absolute grain size range and dm is the average grain size of the alloy. According to Table 1, the minimum grain sizes (dmin) are 1.9, 3.2, 5.1, and 5.5 μm and the maximum grain sizes (dmax) are 20.1, 24.7, 29.0, and 37.4 μm at

can be seen from the DSC curve of the as-homogenized alloy, the endothermic peak in low temperature region disappears, while the endothermic peak in high temperature region still exists with the onset temperature of about 572 °C. The weaker endothermic peak compared to that of the as-cast alloy is mainly due to the partial dissolution of CaMgSn. After proper homogenization, the low-melting-point phase MgZn2 in ZTMX3100 is completely dissolved into the matrix, only partial thermally stable phase CaMgSn remains. The high melting temperature of thermally stable phase CaMgSn leads to a high incipient melting temperature (572 °C) of ZTMX3100 alloy. The relatively high incipient melting temperature improves the extrudability of ZTMX3100 alloy. Thus, ZTMX3100 alloy can be extruded at a high die-exit speed of 18 m/min with good surface quality. 3.3. Effect of extrusion speeds on microstructure Fig. 4 presents the low-magnification and high-magnification optical micrographs of the ZTMX3100 alloys extruded at different ram speeds. Fig. 5(a)–(d) give the magnified SEM micrographs and the corresponding EDS results for particles of ZTMX3100 alloys extruded at different ram speeds. Since the atomic ratios of Ca/Sn are close to 1:1, these micron-sized particles are confirmed to be thermally stable phase CaMgSn. Compared with the relatively random distribution of CaMgSn phases in the as-cast alloy, the as-extruded CaMgSn phases uniformly distribute along the extrusion direction in the form of discontinuous dot-like or chain-like structures at different extrusion speeds, as shown in Figs. 4 and 5. As the extrusion speed increases, the size and fraction of CaMgSn phase gradually decrease, with the volume fraction of 1.89%, 1.12%, 0.91% and 0.72%, respectively. This is mainly due to the fact that as the extrusion speed increases, more heat is generated due to plastic deformation and the friction, thus leading to the increase of billet temperature and more dissolution of second phase particles. In addition, some fine spherical second phase particles mainly distributed on the grain boundaries can also be observed in Fig. 5. In order to further analyze the morphology, size and distribution of the fine second phases in the as-extruded alloy, TEM tests were performed. Fig. 6(a) gives the low-magnification TEM image of the asextruded ZTMX3100 alloy extruded at the ram speed of 0.2 mm/s, indicating that some second phase particles exhibit a relatively uniform distribution in the alloy. Fig. 6(b) and (f) represent the high-magnification TEM images of Region 1 and Region 2 marked in Fig. 6(a), respectively. Fig. 6(c)–(e) show the EDS elemental mappings (Mg, Zn and Mn elements) of Region 3 marked by the red rectangle in Fig. 6(b), respectively. According to Fig. 6(c)–(e), the relatively large spherical phase marked as “A” in Fig. 6(b) can be preliminarily identified as MgZn2, while the relatively small spherical phase marked as “B” can be preliminarily identified as α-Mn. Similarly, Fig. 6(g)–(i) give the EDS elemental mappings (Mg, Sn and Ca elements) of Region 4 marked by the red rectangle in Fig. 6(f), respectively. It can be concluded that the short strip-like phase with larger size marked as “C” in Fig. 6(f) is CaMgSn according to the element distribution, which is consistent with 183

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Fig. 4. Low magnification and high magnification optical micrographs of ZTMX3100 alloys extruded at different ram speeds: (a)–(b) 0.2 mm/s, (c)–(d) 1 mm/s, (e)–(f) 5 mm/s, (g)–(h) 10 mm/s.

increases from 0.2 to 5 mm/s, the uniformity of grain distribution exhibiting a rising trend. As the extrusion speed further increases to 10 mm/s, the uniformity of grain distribution decreases to some extent. This phenomenon is attributed to the change of extrusion temperature and second phase particles (size, distribution, and volume fraction) caused by different extrusion speeds. According to the statistical results shown in Fig. 8, it can be calculated that the maximum value of percentage change of the relative grain size dispersion Δd/dm is 30.9% while that of the average grain size dm is 108.9%. It is obvious that the extrusion speed has a less influence on the grain size distribution than average grain size.

different extrusion speeds (0.2–10 mm/s), respectively. Therefore, the absolute grain size ranges (Δd) are 18.2, 21.5, 23.9, and 31.9 μm and the relative grain size dispersions (Δd/dm) are 2.33, 2.03, 1.78, and 1.95, respectively. Fig. 8 gives the statistical results of the areaweighted grain size distributions, the average grain size (dm) and the relative grain size dispersion (Δd/dm) of the alloys extruded at different ram speeds. The dashed line in the grain size distribution maps presents the average grain size (dm). As shown in Fig. 8, all of the as-extruded alloys at different ram speeds demonstrate the approximate normal distribution with a single peak in spite of various peak values. With the increase of extrusion speed, the absolute grain size ranges (Δd) increase and the relative grain size dispersions (Δd/dm) firstly decreases and then increases gradually. The results show that as the extrusion speed 184

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Fig. 5. Magnified SEM micrographs and corresponding EDS results for particles of ZTMX3100 alloys extruded at different ram speeds: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s.

speeds. According to Figs. 4 and 9, the as-extruded alloys almost exhibit uniform and equiaxed grains and only very few LAGBs exist in the alloys extruded at different ram speeds, which indicates that complete dynamic recrystallization occurs at the extrusion speed of 0.2–10 mm/s investigated in this paper. Fig. 11(a)–(d) show the (0001) pole figures and inverse pole figures (IPFs) referring to the extrusion direction of ZTMX3100 alloys extruded at different ram speeds. ED indicates the extrusion direction and TD indicates the transverse direction. As can be seen from the (0001) pole figures shown in Fig. 11, the as-extruded alloys exhibit textures with (0001) basal planes preferentially parallel to the extrusion direction, which is typical of extruded magnesium alloys [24,36,37]. The maximum basal texture intensities are various at different extrusion speeds. With the increase of extrusion speed, the maximum basal texture intensity of the as-extruded alloy increases gradually from 5.1 to 8.2. Unlike the previous investigations [28], with the increase of extrusion speed, the basal texture is weakened due to the increased volume fraction of dynamic recrystallized grains. As we know, elongated grains usually retain strong fiber texture, which is harmful to the elongation of

3.4. Effect of extrusion speeds on micro-texture Fig. 9 shows the EBSD-derived inverse pole figure (IPF) maps of the ZTMX3100 alloys extruded at different ram speeds. ED is the extrusion direction and different grain colors represent different grain orientations in IPF maps. The thin-white lines and thick-black lines indicate the low angular grain boundaries (LAGBs) and high angular grain boundaries (HAGBs), and the ranges of LAGBs and HAGBs are 2°–15° and 15°–100°, respectively. The misorientation angle of less than 2° is excluded. As can be seen from Fig. 9(a)–(c), the grain size increases gradually and the grain size distribution becomes more uniform with the increase of extrusion speed. Fig. 9(c) and (d) show that the grain size of the alloy further grows as the extrusion speed increases from 5 mm/s to 10 mm/s, and more grains with large size are found in Fig. 9(d), which leading to a less uniform grain distribution of the alloy. The result is consistent with the previous discussion in section 3.3. Fig. 10(a)–(d) give the relative frequency of misorientation angles of ZTMX3100 alloy extruded at different ram speeds. As can be seen from Fig. 10, the fractions of HAGBs are relatively high at different extrusion

Fig. 6. TEM images of as-extruded ZTMX3100 alloy and elemental mappings of the corresponding areas: (a) low-magnification TEM image of the as-extruded alloy, (b) high-magnification TEM image of Region 1, (c) elemental Mg mapping of Region 3, (d) elemental Zn mapping of Region 3, (e) elemental Mn mapping of Region 3, (f) high-magnification TEM image of Region 2, (g) elemental Mg mapping of Region 4, (h) elemental Sn mapping of Region 4, (i) elemental Ca mapping of Region 4. 185

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Fig. 7. TEM images of the as-extruded ZTMX3100 alloy: (a) BF image of the as-extruded alloy; (b) HRTEM of the spherical phase marked by the red rectangle in (a); (c) the corresponding FFT; (d) BF image of the as-extruded alloy; (e) HRTEM of the spherical phase marked by the red rectangle in (d); (f) the corresponding FFT. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

According to the IPFs referring to the extrusion direction shown in Fig. 11, the maximum texture intensities of the as-extruded alloys extruded at different ram speeds locate in different regions on IPFs. At relatively low extrusion speeds (0.2 mm/s and 1 mm/s), the maximum intensities are distributed homogeneously along the arc between the < 1010 > and the < 112 0 > poles, exhibiting a < 1010 > − < 11 2 0 > fiber texture. At relatively high extrusion speeds (5 mm/s and 10 mm/s), the < 112 0 > components vanish gradually and the maximum intensities are mainly centered at the < 1010 > pole, exhibiting a < 1010 > fiber texture. Besides, the < 1010 > component is enhanced with the increase of extrusion speed from 5 to 10 mm/s, which is in accordance with the investigations of Yu et al. [41] and Cheng et al. [10].

the alloy. In this study, the as-extruded alloys exhibit uniform and equiaxed grains with high fractions of HAGBs, and complete dynamic recrystallization occurs at the extrusion speed of 0.2–10 mm/s. It has been reported that the existing second phase particles and grain size have an impact on the grain orientation and texture intensity of the deformed Mg alloys [38,39]. Li et al. [38] pointed out that a large number of second phase particles can provide more randomly oriented nuclei, thereby significantly weakening the basal plane texture of Mg alloys. That means the larger amount of second phase particles, the weaker of basal plane texture. The investigation of Wu et al. [40] showed that basal-oriented grain obtains grain growth advantage over other oriented grains in magnesium alloys, which leads to the strengthening of basal texture with grain growth. Borkar et al. [39] also indicated that the fine grains can provide relatively more random orientations than medium-to-large-sized grains, thus leading to a weaker basal texture. As shown in Fig. 4, Figs. 8 and 9, with the increase of extrusion speed from 0.2 to 10 mm/s, the average grain size of the alloy gradually increases and the fraction of the second phase particles gradually decreases. Therefore, the alloy extruded at higher ram speed presents stronger basal texture.

3.5. Effect of extrusion speeds on mechanical properties The engineering stress-strain curves of ZTMX3100 alloys extruded at different ram speeds obtained from tensile tests are shown in Fig. 12. The corresponding tensile properties such as tensile yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) are further

Table 1 Microstructural characteristics and tensile properties of ZTMX3100 alloys extruded at different ram speeds. Extrusion speed (mm/s)

0.2 1 5 10

Microstructural characteristics

Tensile properties

dm (μm)

dmin (μm)

dmax (μm)

fCaMgSn

YS (MPa)

UTS (MPa)

EL (%)

7.8 10.6 13.4 16.3

1.9 3.2 5.1 5.5

20.1 24.7 29.0 37.4

1.89 1.12 0.91 0.72

191 181 180 177

292 284 277 276

22.3 20.9 19.7 18.0

± ± ± ±

0.9 0.7 0.5 1.1

± ± ± ±

0.6 0.9 0.7 0.6

± ± ± ±

0.5 0.6 0.4 0.4

dm, dmin, dmax and fCaMgSn represent the average grain size, the minimum grain size, the maximum grain size and the fraction of CaMgSn phase in the alloy, respectively. YS, UTS and EL indicate the tensile yield strength, ultimate tensile strength and elongation, respectively. 186

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Fig. 8. Grain size distribution maps of ZTMX3100 alloys extruded at different ram speeds: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s.

motion causing the stress increase. Therefore, the strengthening effect from the second phase particles of the alloy is weakened and thus leading to a reduction in the YS of the as-extruded alloy [5]. According to the previous discussion in section 3.3, the uniformity of grain distribution firstly increases and then decreases, which is not completely consistent with the change in YS value. Therefore, it can be concluded that the YS of the alloy is mainly affected by the average grain size and the second phase particles, while the influence of the grain size

summarized in tensile properties part of Table 1. The YS of the as-extruded alloy decreases with the increase of extrusion speed, which can be attributed to two reasons. For one thing, the average grain size grows up gradually with the increase of extrusion speed. The large grains cause the reduction of YS in accordance with the Hall-Petch relation [42–44]. For another, as the extrusion speed increases, the size and fraction of the second phase particles decrease, as shown in Fig. 4. Sun et al. [32] indicated that the second phases act as barriers to dislocation

Fig. 9. EBSD-derived IPF maps of ZTMX3100 alloys extruded at different ram speeds: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s. ED stands for extrusion direction. 187

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Fig. 10. Misorientation angle distributions of ZTMX3100 alloys extruded at different ram speeds: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s.

intra-grain in finer grain alloys is smaller under the same external stress, which reduces the possibility of crack initiation and extension resulting from local stress concentration. Therefore, the increase of grain size is not conducive to the elongation of the alloy [46].

distribution is relatively weak. As the extrusion speed increases, the elongation of the as-extruded alloy also decreases to some extent. The investigation of Wang et al. [45] shows that the strain difference between the grain boundary and

Fig. 11. The (0001) pole figures and inverse pole figures referring to the extrusion direction of ZTMX3100 alloys extruded at different ram speeds: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s. ED and TD respectively indicate the extrusion direction and transverse direction. 188

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Table 2 YS, UTS and EL of the high-speed extruded magnesium alloys in this work and those reported references.

Fig. 12. The engineering stress–strain curves of ZTMX3100 alloys extruded at different ram speeds.

According to the investigations [47–49], weak basal texture facilitates the operation of basal slip when tension is conducted along the extrusion direction, resulting in an improved elongation of the as-extruded alloy at room temperature. Li et al. [35] also indicated that weaker texture in the Mg alloy increases the work-hardening ability and thus enhances the elongation. As shown in the results of Table 1 and Fig. 11, with the increase of extrusion speed, the average grain size of the asextruded alloy increases from 7.8 μm to 16.3 μm and the maximum basal texture intensity of the as-extruded alloy increases gradually from 5.1 to 8.2. Therefore, the alloy extruded at higher ram speed presents relatively lower elongation. The alloy extruded at the ram speed of 0.2 mm/s presents the highest strength and elongation, with the YS, UTS, and EL of 191 MPa, 292 MPa, and 22.3%, respectively, which is attributed to the combination of fine grain structure, strengthening effect from uniformly distributed second phase particles, and weak basal texture. At high extrusion speed, i.e., at the ram speed of 10 mm/s (die-exit speed of 18 m/min), the as-extruded alloy still maintains a relatively good strength and elongation, with the YS, UTS, and EL of 177 MPa, 276 MPa, and 18.0%, respectively. This is mainly due to the fact that the moderate alloying content alloy contains the thermally stable phase CaMgSn, which exhibits good pinning effect on the grain boundaries of the alloy, and thus can impede the grain growth during high-speed extrusion [20]. The relatively fine grain structure and thermally stable phase CaMgSn help to maintain the good strength and elongation. As a result, the alloy presents a small average grain size and exhibits relatively high comprehensive mechanical properties even at high die-exit speed of 18 m/min. Table 2 gives the mechanical properties of the high-speed extruded magnesium alloys with different compositions in this work and those reported references. As shown in Table 2, though some alloys with low alloying content [3,4,14,16,17,50] can obtain relatively high elongation at high-speed extrusion condition, the YS of these alloys is generally in a low level, which restricts their further industrial applications as structural products. Meanwhile, some alloys with high alloying content [5,51,52] exhibit relatively poor elongation at high extrusion speed, which is not conducive to the subsequent processing. Compared with other Mg alloys, ZTMX3100 alloy still maintains a relatively high yield strength, tensile strength and elongation at high-speed extrusion with the die-exit speed of 18 m/min, which is beneficial to significantly improve the alloy production efficiency and reduce the production cost. The alloy exhibits a good combination of the comprehensive mechanical properties and extrudability, and thus has a good prospect in engineering applications. Fig. 13 presents the high-magnification SEM micrographs of tensile fracture surfaces for ZTMX3100 alloys extruded at different ram speeds and the corresponding EDS results. According to Fig. 13 (a)–(d), a large number of dimples and tearing edges are found in the fracture surface.

Alloy (wt.%)

Die-exit speed YS (MPa) UTS (m/min) (MPa)

EL (%) Reference

Mg-0.2Ce Mge1Mn-1Nd Mg-0.71Zn-0.36Ca-0.07Mn Mg-1.58Zn-0.52Gd Mg-1.58Zn-0.52Gd Mg-1.58Zn-0.52Gd Mg-0.33Al-0.34Ca-0.24Mn Mg-6.81Sn-1.10Al-1.07Zn Mg-6.81Sn-1.10Al-1.07Zn Mge5Sne2Zn Mge5Sne1Zn Mg-3.36Zn-1.06Sn0.33Mn-0.27Ca

15 10 6 6 12 24 30 18 13.5 10 10 18

31 41 37 30.1 28.2 32.9 29 13.4 9.7 15.3 15 18.0

68.6 102 108 117 79 70 136 184 174 132 130 177

170 196 220 213 208 205 203 250 245 235 230 276

[17] [16] [14] [3] [4] [4] [50] [5] [52] [51] [51] This work

With the increase of the extrusion speed, the number of dimples decreases and the dimples become smaller, which is in accordance with the change of elongation shown in Fig. 12. As shown in the red circles marked in Fig. 13 (a)–(d), some micron-sized second phase particles are found in partial fracture dimples at different extrusion speeds. Fig. 13(e) represents the magnified SEM micrograph of the marked Region E shown in Fig. 13(a). EDS analysis is used to examine the element composition of the second phase particle at point F (marked by red-cross). The detailed information from the EDS result of the second phase particle is shown in Fig. 13(f). The EDS result indicates that only Ca, Mg, and Sn elements are detected at point F and the atomic ratio of Ca/Sn is close to 1: 1. Therefore, these particles located in the dimples are demonstrated to be CaMgSn. The cracked second phase particles generally gather at the dimples in the fracture. This is because in the tensile deformation process, the nucleation of micro-crack firstly initiates on the interface due to the unmatched deformation between the second phase particles and the matrix. The further gathering and growup of micro-cracks promote the formation of cracks, eventually leading to the failure of the alloy [24]. Thus, the alloys extruded at different ram speeds exhibit roughly similar fracture mode. 4. Conclusions In this paper, a new type of ZTMX3100 magnesium alloy with good extrudability was developed, and the ingot was produced by using semicontinuous casting technology. Then the ZTMX3100 alloy ingot was subjected to thermo-mechanical processing including two-stage homogenization treatment and hot extrusion at different extrusion speeds (0.2–10 mm/s). The effects of extrusion speeds on the extrudability, microstructure and mechanical properties were systematically investigated. The main conclusions are drawn as follows: (1) ZTMX3100 alloy can be extruded successfully at a high die-exit speed of 18 m/min with good surface quality. The excellent extrudability is attributed to the relatively high incipient melting temperature of 572 °C that results from the formation of thermally stable phase CaMgSn. (2) Complete dynamic recrystallization occurs at the extrusion speed of 0.2–10 mm/s. The grain size is remarkably refined after extrusion. With the increase of extrusion speed, the average grain size increases gradually and the distribution uniformity of grain size firstly increases and then decreases. (3) The as-extruded alloys exhibit textures with (0001) basal planes preferentially parallel to the extrusion direction. As the extrusion speed increases, the maximum basal texture intensity of the as-extruded alloy increases gradually. At relatively low extrusion speeds of 0.2 and 1 mm/s, the maximum intensities are distributed 189

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Fig. 13. SEM micrographs of tensile fracture surfaces of ZTMX3100 alloys extruded at different ram speeds and the corresponding EDS results: (a) 0.2 mm/s, (b) 1 mm/s, (c) 5 mm/s, (d) 10 mm/s, (e) magnified SEM micrograph of Region E, (f) EDS result of point F.

homogeneously along the arc between the < 1010 > and the < 11 2 0 > poles, exhibiting a < 1010 > − < 112 0 > fiber texture. At high extrusion speeds of 5 mm/s and 10 mm/s, the < 112 0 > components vanish gradually and the maximum intensities are mainly centered at the < 1010 > pole, exhibiting a < 1010 > fiber texture. Besides, the < 1010 > component is enhanced with the increase of extrusion speed from 5 to 10 mm/s. (4) The ZTMX3100 alloy extruded at the ram speed of 0.2 mm/s presents the highest strength and elongation, with the YS, UTS, and EL of 191 MPa, 292 MPa, and 22.3%, respectively. And the alloy extruded at high extrusion speed of 10 mm/s (die-exit speed 18 m/ min) still maintains a relatively high strength and elongation due to the relatively fine grain structure and thermally stable phase CaMgSn.

[4]

[5]

[6]

[7]

[8]

[9]

Acknowledgments

[10]

The authors would like to acknowledge the financial support from the Climbing Program for Taishan Scholars of Shandong Province of China (No. 20110804).

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