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Potential of multi-pass ECAP on improving the mechanical properties of a high-calcium-content Mg-Al-Ca-Mn alloy
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Potential of multi-pass ECAP on improving the mechanical properties of a high-calcium-content Mg-Al-Ca-Mn alloy He Huang a, Huan Liu a,b,∗, Ce Wang a, Jiapeng Sun a,∗, Jing Bai c, Feng Xue c, Jinghua Jiang a, Aibin Ma a a College
of Mechanics and Materials, Hohai University, Nanjing 210000, China and Coastal Engineering Research Institute, Hohai University, Nantong 226300, China c College of Materials Science and Engineering, Southeast University, Nanjing 211189, China
b Ocean
Received 29 January 2019; received in revised form 30 March 2019; accepted 3 April 2019 Available online xxx
Abstract In this study, the multi-pass equal channel angular pressing (ECAP) was employed on a high-calcium-content Mg-Al-Ca-Mn alloy to tailor its microstructure and mechanical properties. The obtained results showed that the network-shaped Mg2 Ca and (Mg, Al)2 Ca eutectic compounds in as-cast alloy were gradually crushed into ultra-fine particles after ECAP, which exhibited a bimodal particle size distribution and most aggregated at original grain boundaries. Dynamic recrystallization (DRX) of α-Mg occurred during hot deformation via a particle stimulated mechanism, and the almost complete DRX with an average grain size around 1.5 μm was obtained after 12p-ECAP. Moreover, abundant nano-sized acicular and spherical precipitates were dynamically precipitated within α-Mg grains during ECAP. Tensile test results indicated that the maximum strength and ductility were acquired for 12p-ECAP alloy with ultimate tensile strength of 372 MPa and fracture elongation of 8%. The enhanced strength of the alloy could be ascribed to fine DRX grains, ultra-fine Ca-containing particles and dynamically precipitated nano-precipitates, while the improved ductility was mainly due to the refined and homogeneous microstructure, and weak texture with high average Schmid factors. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Mg-Al-Ca-Mn alloy; Equal channel angular pressing; Dynamic recrystallization; Precipitates; Mechanical property.
1. Introduction In recent years, magnesium alloys have received increasing attention due to their unique natural advantages of low density, high specific strength, and good biocompatibility, which make them possess great potentials in aircraft, automobiles and biomedical applications [1–4]. However, the absolute strength of pure magnesium is too low to meet the requirements as structural materials [5]. Alloying and plastic deformation are two efficient approaches to improve the mechanical properties of metallic materials [6–12]. At ∗
Corresponding authors. E-mail addresses: [email protected] (H. Liu), [email protected] (J. Sun).
present, the Mg-RE-based alloys with high strength (ultimate tensile strength exceeding 400 MPa) have already been reported, especially for the Mg-RE-Zn based alloys containing long period stacking ordered phases [13–17]. Unfortunately, the cost of RE-containing magnesium alloys is too high for conventional industry applications. Therefore, it is important to develop novel magnesium alloys with no or less rare earth elements but possessing excellent comprehensive mechanical properties. The addition of Ca element in Mg-Al based alloys could increase the strength remarkably, showing great potential as one of the high strength non-RE magnesium alloys [18– 25]. Xu et al. [19] reported that the ultimate tensile strength (UTS) of an extruded Mg-3.5Al-3.3Ca-0.4Mn (wt.%) alloy could reach 420 MPa. Similar results were also reported by
https://doi.org/10.1016/j.jma.2019.04.008 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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Zhang et al. [20] on an extruded Mg-2.7Al-3.5Ca-0.4Mn (wt.%) alloy with UTS up to 457 MPa. The high strengths of these alloys were partly due to the grain refinement caused by dynamic recrystallization during hot processing, and the fine particles or precipitates of Mg2 Ca (C14), Al2 Ca (C15) and (Mg, Al)2 Ca (C36) phases [21–24]. In addition, a strong basal texture was found for the extruded alloys, which further enhanced the strength of the alloys through a texture strengthening mechanism [19,20]. However, the ultra-high strength Mg-Al-Ca-Mn alloys reported so far does not perform well in terms of ductility, with elongations commonly lower than 5% (even lower than 3% for the alloy with the highest strength reported so far) [19,20]. This phenomenon of low ductility was mainly resulted from the uneven grain refinement and the presence of strong basal texture during the extrusion process. Equal channel angular pressing (ECAP) is one of the most popular and effective methods for producing ultra-fine-grained (UFG) metallic materials, which could introduces large deformation strains simply by increasing ECAP passes, together without changing the shape of the samples [26,27]. By now, ECAP has been already conducted on various of Mg alloys, and it showed that multi-pass ECAP was effective to refine the large block-like or network-shaped second phases, such as Mg17 Al12 phase [28], Mg2 Si particles [29], and long period stacking ordered (LPSO) phase [9,30–32]. By refining the hard second phase particles and obtaining a homogeneous microstructure with ultra-fine/fine particles and α-Mg grains, the mechanical properties of the alloys could be remarkably improved, especially for the ductility [9,31]. However, so far there has not been reported ECAP attempts on high strength Mg-Al-Ca-Mn alloys to tailor their microstructure and mechanical properties. Therefore, in this study, we prepared an Mg-Al-Ca-Mn alloy with high volume fraction of Ca-containing second phases, and investigated the evolutions of its microstructure and mechanical properties during ECAP to explore the potential of multi-pass ECAP on enhancing the strength and ductility of Mg-Al-Ca-Mn alloys.
2. Experimental procedure The studied Mg-Al-Ca-Mn alloy was prepared from pure Mg (99.90 wt.%), pure Al (99.95 wt.%), Mg-20 wt.% Ca, and Mg-10 wt.% Mn master alloys. After melting in a low-carbon steel crucible under the protection of a mixed atmosphere of CO2 and SF6 (99:1), the molten metal was poured into a water cooling copper mold with an inner diameter of 60 mm and a length of 250 mm. The chemical composition of the alloy was analyzed by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) and the result is shown in Table 1. The rectangular ECAP specimens with dimensions of 20 mm × 20 mm × 45 mm were cut from the ingot. The ECAP processing was carried out at 350 °C in a rotary-die and the numbers were set as 4, 8, and 12, respectively. The die and the samples were insulated at 350 °C for 10 min every 4 passes of ECAP.
Table 1 Analyzed chemical compositions of the studied Mg-Al-Ca-Mn alloy. Alloy
Chemical compositions (wt.%) Al
Ca
Mn
Mg
Mg-Al-Ca-Mn
4.76
4.47
0.12
Bal.
Fig. 1. Optical images of the as-cast Mg-Al-Ca-Mn alloy.
The microstructure of the as-cast and ECAP alloys were characterized by an Olympus BHM optical metallographic microscope (OM), a Sirion field emission scanning electron microscope (SEM) equipped with a GENESIS 60S X-ray energy spectrometer (EDS), and a Tecnai G2 transmission electron microscope (TEM). The metallographic and SEM samples were prepared by mechanical grinding, polishing and etching (the etchant was 4 wt.% nitric acid in alcoholic solution). The TEM sample was prepared by twin-jet electronpolishing, and the polishing solution was selected from 5 wt.% perchloric acid alcohol solution. Moreover, the ECAP sample was also characterized by the electron back scattered diffraction (EBSD) analysis to evaluate the grain size distribution, texture and Schmid factors. Tensile tests were performed using a CMT-5105 electronic universal testing machine at 0.5 mm/min ram speed at room temperature. Dog-bone tensile samples with gauge of 7.5 mm × 2 mm × 2 mm were machined from the as-cast and ECAP alloys with the loading direction parallel to the ECAP direction. For each alloy state, three parallel specimens were selected. 3. Results and discussion 3.1. Microstructure of the as-cast alloy Fig. 1 shows the optical microstructure of the as-cast alloy, and it is apparent two regions can be distinguished. The α-Mg matrix (gray region) exhibits a typical dendritic structure, and the black second phases are distributed at the grain boundaries, forming a continuous network. The average grain size and the area fraction of the second phase are estimated to be 35 ± 11 μm and 10.3%, respectively. Compared with
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Fig. 2. SEM images of the as-cast Mg-Al-Ca-Mn alloy at (a) low magnification and (b) high magnification.
Fig. 3. TEM images of as-cast Mg-Al-Ca-Mn alloy: (a) (Mg, Al) 2 Ca, (b) Mg2 Ca. (Insets are the corresponding selected area electron diffraction patterns, and locations of SAED are marked by squares).
the reported Mg-Al-Ca-Mn cast alloys [20], the grain size of this alloy is finer, which could be result from the higher Ca content as Ca addition could refine the microstructure of magnesium alloys. The morphology, type and distribution of the second phases in the as-cast alloy were further examined by SEM analysis. As shown in Fig. 2a, the second phase exhibits a typical eutectic structure. Fig. 2b shows the enlargement of an eutectic region, and it can be seen that the second phase exhibits three different morphologies, the straight lamellae (marked by capital letter A), the curly lamellae (marked by B), and some particles (marked by C). To identify these phases, EDS analysis was employed and the results are shown in Table 2. Combining the EDS results and previous reports [19,20], it can be confirmed that the straight lamellae is (Mg, Al)2 Ca (C36) phase, and both the curly lamellae and particles (with higher Ca contents) are Mg2 Ca (C14) phase. Furthermore, Fig. 3 shows the TEM images of the straight lamellae and curly lamellae (particles), as well as their corresponding selected area electron diffraction (SAED) patterns, which further demonstrates the existence of (Mg, Al)2 Ca and Mg2 Ca phases, respectively. With the Ca/Al ratio increases gradually, the second phases formed in Mg-Al-Ca alloys follows the sequence of Mg17 Al12 , Al2 Ca, (Mg, Al)2 Ca and
Table 2 EDS elemental content analysis of the second phases in the as-cast Mg-AlCa-Mn alloys. Area
A B C D
at.%
Phase
Mg
Al
Ca
Mn
78.57 74.59 68.28 98.69
14.18 13.91 19.83 1.09
7.24 11.47 11.85 0.18
0.02 0.03 0.03 0.04
(Mg, Al)2 Ca Mg2 Ca Mg2 Ca α-Mg Matrix
Mg2 Ca [33]. According to the liquid phase projection of the Mg-Al-Ca ternary alloy systems obtained by Suzuki et al. [34], the composition of this alloy locates in the region where (Mg, Al)2 Ca and Mg2 Ca phases are coexisted. Moreover, a few Al-Mn particles were also observed in the as-cast alloy due to the addition of Mn element (which was not shown here). Seen from Table 2, the α-Mg grains (marked by D in Fig. 2b) contain only 0.18 at.% Ca, as Ca has a low solid solubility in Mg. 3.2. Microstructure of the ECAP alloys Fig. 4 shows the SEM images of Mg-Al-Ca-Mn alloys after ECAP for various passes. Seen from Fig. 4 (a, c and
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Fig. 4. SEM images of (a, b) 4p, (c, d) 8p and (e, f) 12p ECAP alloys at (a, c, e) low magnification and (b, d, f) high magnification, respectively. (g) Deformation twins formed at early ECAP; (h) Dispersed precipitates within α-Mg grains.
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Fig. 5. TEM images of the (a, b) refined Mg2 Ca and (Mg, Al)2 Ca particles, (c) plate-like precipitates, and (d) two spherical precipitates with different sizes.
e), the microstructure was gradually refined with increasing ECAP numbers. In early stage of ECAP, the networks of second phase began to break, but they were not thoroughly crushed. As can be seen from the enlargement in Fig. 4b, abundant cracks can be observed on the lamellae, which indicates that the refinement mechanism of the Ca-containing eutectic phases under the thermal processing of ECAP are mechanical disruption. Moreover, deformation twinning was also observed in 4p ECAP alloys (Fig. 4g), suggesting twinning and/or slip operated at the early stage of deformation owing to the poor plasticity of Mg-Al-Ca-Mn alloy system. As the number of ECAP increases, the mechanical crushing process continued and the eutectic phases were further refined (Fig. 4d and 4f). When it comes to 12 passes, the second phases were refined to sub-micron level. In addition, the deformation twining was rarely observed in alloys with high ECAP passes, which could be resulted from the improved plasticity owing to the refined microstructure. However, it should be noticed that most refined second phase particles were still gathered at grain boundaries, but not mixed uniformly within the matrix. This phenomenon has also been reported in other Mg alloy systems, and it must be associated with the shear
strains induced by ECAP [9]. Furthermore, seen from Fig. 4h, plenty of tiny particles (both nano-sized and sub-micronsized) can be found in α-Mg matrix of ECAP alloys. Their sizes are obviously smaller than the refined Ca-containing particles, which might be dynamically precipitated during ECAP. Fig. 5a shows the TEM image of fractured Mg2 Ca particles during ECAP. Apparently, the four particles marked by roman numbers belonged to one original particle (lamella). Under severe plastic deformation, the lamella within the image was crushed into four main segments, and the size of each segment was larger than 1 μm. Moreover, some smaller particles were generated around the four segments, especially at the connection regions of two segments, as shown by the dotted circles in Fig. 5a. These small particles exhibit diameters around 200 nm, which must be peeled off from the severely deformed region with cracks propagation. Seen from Fig. 5b, the number of small particles increased with more ECAP passes, and the refined Mg2 Ca or (Mg, Al)2 Ca particles exhibit a bimodal size distribution in ECAP alloys. Moreover, some nano-sized phases were precipitated within α-Mg grains. Fig. 5c and 5d show the precipitates observed in ECAP
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Fig. 6. (a) Inverse pole figure (IPF) maps and (b) grain size distributions of the 12p-ECAP alloy.
Fig. 7. The distribution of recrystallized, substructure, and deformed grains of the 12p-ECAP alloy.
Fig. 8. TEM micrographs of (a) a sub-grain and (b) DRX grains in ECAP alloys.
alloys with three different morphologies, the plate-like (or acicular) phase, the spherical particles with diameter lower than 10 nm, and the axiolitic particles with size about 100 nm. Xu et al. [19] analyzed the plate-like phase and spherical particles using laser-assisted three-dimensional atom probe (3DAP), and identified them as Al2 Ca phases and Al-Mn compounds, respectively. As for the large particles within α-Mg, the SAED pattern and EDS analysis results (which were not show here) demonstrated they are Mg2 Ca particles. However, the formation of these Mg2 Ca particles cannot be confirmed at the moment. They might be dynamically precipitated from
the matrix under hot deformation, or could be broken particles and mixed within α-Mg grains with more ECAP passes. Considering their relatively large sizes and the short heating period employed in this ECAP processing, we believe the latter one is more reasonable. In addition to the second phase, the α-Mg matrix also undergoes significant changes. Fig. 6 shows the inverse pole figure (IPF) maps and grain size statistics of the 12p-ECAP alloys along the extruded direction. It is obvious that the grain sizes of ECAP alloy displays a normal distribution and reach the peak at about 1.5 μm, demonstrating the effective
Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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Fig. 9. Typical engineering stress-stress curves of Mg-Al-Ca-Mn alloys.
refining effect of multi-pass ECAP. Moreover, there is no obvious deformed regions observed, which is different with the bimodal microstructure that was reported in hot extruded MgAl-Ca-Mn alloys [19,20]. Furthermore, the distribution of the recrystallized, substructure, and deformed grains of the 12pECAP alloy was shown in Fig. 7, which further demonstrates the absence of large deformed grains. The 12p-ECAP alloy exhibits a near complete DRX microstructure, with the area fractions of fine recrystallized grains and substructure grains 71% and 29%, respectively.
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The EBSD results show that the grains of the alloy have undergone significant refinement. Since magnesium is a metal with relatively high stacking fault, dynamic recrystallization is prone to occur during hot processing. The ECAP temperature employed in this work was higher than the recrystallization temperature of magnesium alloys, so the grains mainly undergo dynamic recrystallization under the activation of coupled heat and stress. For magnesium alloys with high volume fraction of hard second phase particles, DRX via the particle-stimulated nucleation (PSN) mechanism was usually proposed [35,36]. Fig. 8a shows the TEM image of an obvious sub-grain interacted with several Mg2 Ca particles in 4p-ECAP alloy. On the one hand, the high-density dislocations gathered at the interfaces of refined Ca-containing particles provide the requirements for nucleation of DRX grains (sub-grain). On the other hand, the fine particles are effective to obstacle the movement of grain boundaries. In addition, Fig. 8b shows the typical morphology and location of DRX grains. Several DRX grains appear near the refined particles region, and they become larger with the distance away from the gathered particles, which further demonstrates the activation of PSN mechanism and the effective pinning effect of these broken particles. 3.3. Mechanical properties Fig. 9a shows the representative stress-strain curves of ascast and ECAP alloys. The as-cast Mg-4.76Al-4.47Ca-0.12Mn alloy exhibits an ultimate tensile strength (UTS) of 179 MPa
Fig. 10. SEM micrographs of the tensile fracture surfaces: (a) as-cast, (b) 4p, (c) 8p, (d) 12p. Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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Fig. 11. (0001), (112¯ 0) and (101¯ 0) pole figures of the (a) 4p, (b) 8p and (c) 12p-ECAP alloys.
and an elongation to failure of 3%. After ECAP the mechanical properties of the alloys were significantly improved, and more numbers of ECAP show more enhancing effect, both in strength and ductility. The 12p-ECAP alloy possesses the UTS and elongation to failure of 372 MPa and 8%, respectively. Fig. 10 shows the fracture surfaces of the as-cast and ECAP alloys after tensile test. It is obvious from Fig. 10a that brittle fracture is the main fracture mode for the ascast alloy, as the fracture surface was smooth and flat, and cracks around large particles were observed (marked by arrows; EDS analysis indicates the composition of Mg-10.36
at.% Al-10.79 at.% Ca-0.09 at.% Mn). After 4 passes of ECAP (Fig. 10b), the surface became rugged, and tear ridges were mainly observed, suggesting the improve of ductility and fracture mode of cleavage fracture. With further increase of ECAP numbers (Fig. 10c and d), although cleavage fracture is still the dominated fracture mode, the features of ductility fracture becomes more distinct, as many dimples were observed on the fracture surface. The above SEM fractograph results demonstrate that the multi-pass ECAP is effective to improve the ductility of high-calcium-content Mg-AlCa-Mn alloy, consistent with the mechanical properties shown in Fig. 9.
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Table 3 Comparison of tensile properties of the as-ECAP Mg-Al-Ca-Mn alloys and others else. Alloy composition(wt.%)
Processing condition
UTS (MPa)
Elongation to failure (%)
Reference
Mg-4.7Al-4.5Ca-0.12Mn
4p-ECAP 8P-ECAP 12p-ECAP Extrusion Extrusion Extrusion Extrusion Extrusion
300 314 372 457 420 333 289 333
6.5 6.3 8 2.5 5.6 5 4 3
This study This study This study [20] [19] [22] [22] [22]
Mg-2.7Al-3.5Ca-0.4Mn Mg-3.5Al-3.3Ca-0.4Mn Mg-3.7Al-3.8Ca Mg- 4.4Al-4.5Ca Mg-4.9Al-5Ca
3.4. Influence of microstructure evolution on mechanical properties of ECAP alloys Tensile results of Fig. 9 show that more numbers of ECAP processing improve both the strength and ductility of the alloys. This could be ascribed to the gradually refined microstructure, including evolution of α-Mg grains and second phase particles. As shown in Fig. 4, the sizes of Mg2 Ca particles decrease with more ECAP numbers, which must be the same for sizes of α-Mg grains. Therefore, the effect of fine grain strengthening and fine particles strengthening is enhanced for the alloy with higher ECAP numbers. When the microstructure is refined and becomes uniform with more passes of ECAP, the ductility could also be improved. In addition, as proved by former microstructure observations, multi-pass RD-ECAP introduces two obvious microstructural evolutions, refining and dynamically precipitating. The refining process involves both the second phase and the α-Mg matrix. Unlike the bimodal grain distribution of extruded alloys, a uniform and refined α-Mg grains were obtained after 12p-ECAP. As for the network-shaped Mg2 Ca and (Mg, Al)2 Ca phases formed in cast alloy, they were effectively refined into fine particles, and the sizes of these particles exhibit a bimodal distribution. One kind of the particles are in the range of 1–2 μm, and the other are near 200 nm. It is reasonable to accept that the mixed particles with two obvious different sizes are effective to obstacle dislocations [37]. However, although both the second phase and α-Mg grains were obviously refined, they were not distributed evenly, and the fine second phase particles were agminated at original grain boundary regions. In case of the precipitates within α-Mg grains, there emerged three different ones, the plate-like Al2 Ca precipitates, the nano-sized (diameter < 10 nm) Al-Mn spherical compounds, and some large spherical particles (diameter ∼ 10 nm). Therefore, considering the remarkably improved tensile strength, it could be attributed to the fine α-Mg grains, refined and bimodal second phase particles, and various nano-sized precipitates in the matrix. Table 3 lists the comparisons of tensile properties of highCa-content Mg-Al-Ca-Mn wrought alloys obtained in this study and in references [19,20,22]. Compared with the three extruded Mg-Al-Ca alloys without Mn content, the ECAP alloy exhibits higher strength and ductility, suggesting the great potential of multi-pass on improving the mechanical properties of this kind of alloys. As for the two extruded
Fig. 12. Schmid factor distributions of {0002} < 112¯ 0 > slip of the 12pECAP alloy.
Mg-Al-Ca-Mn alloys, their strengths are higher than this 12pECAP alloy by 48 MPa and 75 MPa, respectively, but with lower elongation to failure. According to Xu et al. [19] And Zheng et al. [20], the high strength of the extruded Mg-AlCa-Mn alloys were attributed to the ultra-fine DRXed grains pinned by fragmented secondary phases, strong basal texture and dense nano-scale precipitates. Comparing the microstructure of this ECAP alloy with the as-extruded alloys, the main difference is the grain size distribution. Near 30% of the αMg grains in extruded Mg-Al-Ca-Mn alloys were coarse deformed grains with strong basal texture, which significantly strengthened the alloy but impaired the ductility [19,20,38]. Fig. 11 shows the pole figures of the studied ECAP alloys, and it is apparent that with increasing ECAP numbers from 4 to 8, the texture intensity decreases from 26.98 to 9.40, which is consistent with the texture evolutions of magnesium alloys during ECAP [39,40]. The texture intensity of the 12p ECAP alloy is much lower than the extruded alloys. Moreover, Fig. 12 shows the Schmid factor distributions of basal slip for the ECAP alloy. The calculated average Schmid factor is 0.38, which is higher than that for extruded alloys. Based on above discussions, it can be inferred that the texture strengthening plays an important role in strengthening of the wrought Mg-Al-Ca-Mn alloys. Therefore, the weak
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texture of this ECAP alloy results in lower strength and better ductility than extruded alloys. To further improve the strength of this ECAP alloy, subsequent rolling process could be employed to introduce strong texture on the homogeneous ultra-fine grains. Furthermore, it is worth to note that the fractured Ca-containing particles was not uniformly distributed with DRX grains, either in hot extruded alloys or ECAP alloys. The segregation of these particles might be responsible for the poor ductility of wrought Mg-Al-Ca-Mn alloys, as suggested by the SEM fracture surface. Therefore, according to previous experiences, more ECAP will be carried out in our future studies to eliminate the segregation of crushed particles within the ultra-fine grained Mg-Al-Ca-Mn alloys. 4. Conclusions In the present study, the microstructure evolutions and mechanical properties of a high Ca-containing Mg-Al-Ca-Mn alloy during multi-pass ECAP were systematically investigated. The main conclusions can be drawn as follows: (1) The microstructure of the as-cast Mg-4.76Al-4.47Ca0.12Mn alloy was composed of α-Mg grains, two kinds of Ca-containing lamellar eutectic compounds, (Mg, Al)2 Ca and Mg2 Ca phase, and few Al-Mn particles. (2) During multi-pass ECAP, dynamic recrystallization of α-Mg occurred, and the average grain size of DRX grains was around 1.5 μm. The network-shaped second phases were crushed into ultra-fine particles with bimodal size distributions, but were still gathered at original grain boundaries. Moreover, various of nano-sized Al2 Ca and Al-Mn particles were dynamically precipitated within α-Mg grains. (3) Both the strength and ductility of the alloy were improved with increasing ECAP numbers. The 12p-ECAP alloy exhibited optimal mechanical properties with ultimate tensile strength of 372 MPa and elongation to failure of 8%. The enhanced strength of the ECAP alloy could be ascribed to the refined α-Mg grains, ultra-fine and bimodal second phase particles, and nano-sized precipitates, while the improve of ductility was mainly resulted from the refined and uniform microstructure with weak texture and high Schmid factors.
Declaration of Competing Interest None. Acknowledgment This work was supported by the Natural Science Foundation of China (51901068), the Natural Science Foundation of Jiangsu Province of China (BK20160869), the Fundamental Research Funds for the Central Universities (2018B16614) and the Nantong Science and Technology Project.
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Potential of multi-pass ECAP on improving the mechanical properties of a high-calcium-content Mg-Al-Ca-Mn alloy He Huang a, Huan Liu a,b,∗, Ce Wang a, Jiapeng Sun a,∗, Jing Bai c, Feng Xue c, Jinghua Jiang a, Aibin Ma a a College
of Mechanics and Materials, Hohai University, Nanjing 210000, China and Coastal Engineering Research Institute, Hohai University, Nantong 226300, China c College of Materials Science and Engineering, Southeast University, Nanjing 211189, China
b Ocean
Received 29 January 2019; received in revised form 30 March 2019; accepted 3 April 2019 Available online xxx
Abstract In this study, the multi-pass equal channel angular pressing (ECAP) was employed on a high-calcium-content Mg-Al-Ca-Mn alloy to tailor its microstructure and mechanical properties. The obtained results showed that the network-shaped Mg2 Ca and (Mg, Al)2 Ca eutectic compounds in as-cast alloy were gradually crushed into ultra-fine particles after ECAP, which exhibited a bimodal particle size distribution and most aggregated at original grain boundaries. Dynamic recrystallization (DRX) of α-Mg occurred during hot deformation via a particle stimulated mechanism, and the almost complete DRX with an average grain size around 1.5 μm was obtained after 12p-ECAP. Moreover, abundant nano-sized acicular and spherical precipitates were dynamically precipitated within α-Mg grains during ECAP. Tensile test results indicated that the maximum strength and ductility were acquired for 12p-ECAP alloy with ultimate tensile strength of 372 MPa and fracture elongation of 8%. The enhanced strength of the alloy could be ascribed to fine DRX grains, ultra-fine Ca-containing particles and dynamically precipitated nano-precipitates, while the improved ductility was mainly due to the refined and homogeneous microstructure, and weak texture with high average Schmid factors. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Mg-Al-Ca-Mn alloy; Equal channel angular pressing; Dynamic recrystallization; Precipitates; Mechanical property.
1. Introduction In recent years, magnesium alloys have received increasing attention due to their unique natural advantages of low density, high specific strength, and good biocompatibility, which make them possess great potentials in aircraft, automobiles and biomedical applications [1–4]. However, the absolute strength of pure magnesium is too low to meet the requirements as structural materials [5]. Alloying and plastic deformation are two efficient approaches to improve the mechanical properties of metallic materials [6–12]. At ∗
Corresponding authors. E-mail addresses: [email protected] (H. Liu), [email protected] (J. Sun).
present, the Mg-RE-based alloys with high strength (ultimate tensile strength exceeding 400 MPa) have already been reported, especially for the Mg-RE-Zn based alloys containing long period stacking ordered phases [13–17]. Unfortunately, the cost of RE-containing magnesium alloys is too high for conventional industry applications. Therefore, it is important to develop novel magnesium alloys with no or less rare earth elements but possessing excellent comprehensive mechanical properties. The addition of Ca element in Mg-Al based alloys could increase the strength remarkably, showing great potential as one of the high strength non-RE magnesium alloys [18– 25]. Xu et al. [19] reported that the ultimate tensile strength (UTS) of an extruded Mg-3.5Al-3.3Ca-0.4Mn (wt.%) alloy could reach 420 MPa. Similar results were also reported by
https://doi.org/10.1016/j.jma.2019.04.008 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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Zhang et al. [20] on an extruded Mg-2.7Al-3.5Ca-0.4Mn (wt.%) alloy with UTS up to 457 MPa. The high strengths of these alloys were partly due to the grain refinement caused by dynamic recrystallization during hot processing, and the fine particles or precipitates of Mg2 Ca (C14), Al2 Ca (C15) and (Mg, Al)2 Ca (C36) phases [21–24]. In addition, a strong basal texture was found for the extruded alloys, which further enhanced the strength of the alloys through a texture strengthening mechanism [19,20]. However, the ultra-high strength Mg-Al-Ca-Mn alloys reported so far does not perform well in terms of ductility, with elongations commonly lower than 5% (even lower than 3% for the alloy with the highest strength reported so far) [19,20]. This phenomenon of low ductility was mainly resulted from the uneven grain refinement and the presence of strong basal texture during the extrusion process. Equal channel angular pressing (ECAP) is one of the most popular and effective methods for producing ultra-fine-grained (UFG) metallic materials, which could introduces large deformation strains simply by increasing ECAP passes, together without changing the shape of the samples [26,27]. By now, ECAP has been already conducted on various of Mg alloys, and it showed that multi-pass ECAP was effective to refine the large block-like or network-shaped second phases, such as Mg17 Al12 phase [28], Mg2 Si particles [29], and long period stacking ordered (LPSO) phase [9,30–32]. By refining the hard second phase particles and obtaining a homogeneous microstructure with ultra-fine/fine particles and α-Mg grains, the mechanical properties of the alloys could be remarkably improved, especially for the ductility [9,31]. However, so far there has not been reported ECAP attempts on high strength Mg-Al-Ca-Mn alloys to tailor their microstructure and mechanical properties. Therefore, in this study, we prepared an Mg-Al-Ca-Mn alloy with high volume fraction of Ca-containing second phases, and investigated the evolutions of its microstructure and mechanical properties during ECAP to explore the potential of multi-pass ECAP on enhancing the strength and ductility of Mg-Al-Ca-Mn alloys.
2. Experimental procedure The studied Mg-Al-Ca-Mn alloy was prepared from pure Mg (99.90 wt.%), pure Al (99.95 wt.%), Mg-20 wt.% Ca, and Mg-10 wt.% Mn master alloys. After melting in a low-carbon steel crucible under the protection of a mixed atmosphere of CO2 and SF6 (99:1), the molten metal was poured into a water cooling copper mold with an inner diameter of 60 mm and a length of 250 mm. The chemical composition of the alloy was analyzed by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) and the result is shown in Table 1. The rectangular ECAP specimens with dimensions of 20 mm × 20 mm × 45 mm were cut from the ingot. The ECAP processing was carried out at 350 °C in a rotary-die and the numbers were set as 4, 8, and 12, respectively. The die and the samples were insulated at 350 °C for 10 min every 4 passes of ECAP.
Table 1 Analyzed chemical compositions of the studied Mg-Al-Ca-Mn alloy. Alloy
Chemical compositions (wt.%) Al
Ca
Mn
Mg
Mg-Al-Ca-Mn
4.76
4.47
0.12
Bal.
Fig. 1. Optical images of the as-cast Mg-Al-Ca-Mn alloy.
The microstructure of the as-cast and ECAP alloys were characterized by an Olympus BHM optical metallographic microscope (OM), a Sirion field emission scanning electron microscope (SEM) equipped with a GENESIS 60S X-ray energy spectrometer (EDS), and a Tecnai G2 transmission electron microscope (TEM). The metallographic and SEM samples were prepared by mechanical grinding, polishing and etching (the etchant was 4 wt.% nitric acid in alcoholic solution). The TEM sample was prepared by twin-jet electronpolishing, and the polishing solution was selected from 5 wt.% perchloric acid alcohol solution. Moreover, the ECAP sample was also characterized by the electron back scattered diffraction (EBSD) analysis to evaluate the grain size distribution, texture and Schmid factors. Tensile tests were performed using a CMT-5105 electronic universal testing machine at 0.5 mm/min ram speed at room temperature. Dog-bone tensile samples with gauge of 7.5 mm × 2 mm × 2 mm were machined from the as-cast and ECAP alloys with the loading direction parallel to the ECAP direction. For each alloy state, three parallel specimens were selected. 3. Results and discussion 3.1. Microstructure of the as-cast alloy Fig. 1 shows the optical microstructure of the as-cast alloy, and it is apparent two regions can be distinguished. The α-Mg matrix (gray region) exhibits a typical dendritic structure, and the black second phases are distributed at the grain boundaries, forming a continuous network. The average grain size and the area fraction of the second phase are estimated to be 35 ± 11 μm and 10.3%, respectively. Compared with
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Fig. 2. SEM images of the as-cast Mg-Al-Ca-Mn alloy at (a) low magnification and (b) high magnification.
Fig. 3. TEM images of as-cast Mg-Al-Ca-Mn alloy: (a) (Mg, Al) 2 Ca, (b) Mg2 Ca. (Insets are the corresponding selected area electron diffraction patterns, and locations of SAED are marked by squares).
the reported Mg-Al-Ca-Mn cast alloys [20], the grain size of this alloy is finer, which could be result from the higher Ca content as Ca addition could refine the microstructure of magnesium alloys. The morphology, type and distribution of the second phases in the as-cast alloy were further examined by SEM analysis. As shown in Fig. 2a, the second phase exhibits a typical eutectic structure. Fig. 2b shows the enlargement of an eutectic region, and it can be seen that the second phase exhibits three different morphologies, the straight lamellae (marked by capital letter A), the curly lamellae (marked by B), and some particles (marked by C). To identify these phases, EDS analysis was employed and the results are shown in Table 2. Combining the EDS results and previous reports [19,20], it can be confirmed that the straight lamellae is (Mg, Al)2 Ca (C36) phase, and both the curly lamellae and particles (with higher Ca contents) are Mg2 Ca (C14) phase. Furthermore, Fig. 3 shows the TEM images of the straight lamellae and curly lamellae (particles), as well as their corresponding selected area electron diffraction (SAED) patterns, which further demonstrates the existence of (Mg, Al)2 Ca and Mg2 Ca phases, respectively. With the Ca/Al ratio increases gradually, the second phases formed in Mg-Al-Ca alloys follows the sequence of Mg17 Al12 , Al2 Ca, (Mg, Al)2 Ca and
Table 2 EDS elemental content analysis of the second phases in the as-cast Mg-AlCa-Mn alloys. Area
A B C D
at.%
Phase
Mg
Al
Ca
Mn
78.57 74.59 68.28 98.69
14.18 13.91 19.83 1.09
7.24 11.47 11.85 0.18
0.02 0.03 0.03 0.04
(Mg, Al)2 Ca Mg2 Ca Mg2 Ca α-Mg Matrix
Mg2 Ca [33]. According to the liquid phase projection of the Mg-Al-Ca ternary alloy systems obtained by Suzuki et al. [34], the composition of this alloy locates in the region where (Mg, Al)2 Ca and Mg2 Ca phases are coexisted. Moreover, a few Al-Mn particles were also observed in the as-cast alloy due to the addition of Mn element (which was not shown here). Seen from Table 2, the α-Mg grains (marked by D in Fig. 2b) contain only 0.18 at.% Ca, as Ca has a low solid solubility in Mg. 3.2. Microstructure of the ECAP alloys Fig. 4 shows the SEM images of Mg-Al-Ca-Mn alloys after ECAP for various passes. Seen from Fig. 4 (a, c and
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Fig. 4. SEM images of (a, b) 4p, (c, d) 8p and (e, f) 12p ECAP alloys at (a, c, e) low magnification and (b, d, f) high magnification, respectively. (g) Deformation twins formed at early ECAP; (h) Dispersed precipitates within α-Mg grains.
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Fig. 5. TEM images of the (a, b) refined Mg2 Ca and (Mg, Al)2 Ca particles, (c) plate-like precipitates, and (d) two spherical precipitates with different sizes.
e), the microstructure was gradually refined with increasing ECAP numbers. In early stage of ECAP, the networks of second phase began to break, but they were not thoroughly crushed. As can be seen from the enlargement in Fig. 4b, abundant cracks can be observed on the lamellae, which indicates that the refinement mechanism of the Ca-containing eutectic phases under the thermal processing of ECAP are mechanical disruption. Moreover, deformation twinning was also observed in 4p ECAP alloys (Fig. 4g), suggesting twinning and/or slip operated at the early stage of deformation owing to the poor plasticity of Mg-Al-Ca-Mn alloy system. As the number of ECAP increases, the mechanical crushing process continued and the eutectic phases were further refined (Fig. 4d and 4f). When it comes to 12 passes, the second phases were refined to sub-micron level. In addition, the deformation twining was rarely observed in alloys with high ECAP passes, which could be resulted from the improved plasticity owing to the refined microstructure. However, it should be noticed that most refined second phase particles were still gathered at grain boundaries, but not mixed uniformly within the matrix. This phenomenon has also been reported in other Mg alloy systems, and it must be associated with the shear
strains induced by ECAP [9]. Furthermore, seen from Fig. 4h, plenty of tiny particles (both nano-sized and sub-micronsized) can be found in α-Mg matrix of ECAP alloys. Their sizes are obviously smaller than the refined Ca-containing particles, which might be dynamically precipitated during ECAP. Fig. 5a shows the TEM image of fractured Mg2 Ca particles during ECAP. Apparently, the four particles marked by roman numbers belonged to one original particle (lamella). Under severe plastic deformation, the lamella within the image was crushed into four main segments, and the size of each segment was larger than 1 μm. Moreover, some smaller particles were generated around the four segments, especially at the connection regions of two segments, as shown by the dotted circles in Fig. 5a. These small particles exhibit diameters around 200 nm, which must be peeled off from the severely deformed region with cracks propagation. Seen from Fig. 5b, the number of small particles increased with more ECAP passes, and the refined Mg2 Ca or (Mg, Al)2 Ca particles exhibit a bimodal size distribution in ECAP alloys. Moreover, some nano-sized phases were precipitated within α-Mg grains. Fig. 5c and 5d show the precipitates observed in ECAP
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Fig. 6. (a) Inverse pole figure (IPF) maps and (b) grain size distributions of the 12p-ECAP alloy.
Fig. 7. The distribution of recrystallized, substructure, and deformed grains of the 12p-ECAP alloy.
Fig. 8. TEM micrographs of (a) a sub-grain and (b) DRX grains in ECAP alloys.
alloys with three different morphologies, the plate-like (or acicular) phase, the spherical particles with diameter lower than 10 nm, and the axiolitic particles with size about 100 nm. Xu et al. [19] analyzed the plate-like phase and spherical particles using laser-assisted three-dimensional atom probe (3DAP), and identified them as Al2 Ca phases and Al-Mn compounds, respectively. As for the large particles within α-Mg, the SAED pattern and EDS analysis results (which were not show here) demonstrated they are Mg2 Ca particles. However, the formation of these Mg2 Ca particles cannot be confirmed at the moment. They might be dynamically precipitated from
the matrix under hot deformation, or could be broken particles and mixed within α-Mg grains with more ECAP passes. Considering their relatively large sizes and the short heating period employed in this ECAP processing, we believe the latter one is more reasonable. In addition to the second phase, the α-Mg matrix also undergoes significant changes. Fig. 6 shows the inverse pole figure (IPF) maps and grain size statistics of the 12p-ECAP alloys along the extruded direction. It is obvious that the grain sizes of ECAP alloy displays a normal distribution and reach the peak at about 1.5 μm, demonstrating the effective
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Fig. 9. Typical engineering stress-stress curves of Mg-Al-Ca-Mn alloys.
refining effect of multi-pass ECAP. Moreover, there is no obvious deformed regions observed, which is different with the bimodal microstructure that was reported in hot extruded MgAl-Ca-Mn alloys [19,20]. Furthermore, the distribution of the recrystallized, substructure, and deformed grains of the 12pECAP alloy was shown in Fig. 7, which further demonstrates the absence of large deformed grains. The 12p-ECAP alloy exhibits a near complete DRX microstructure, with the area fractions of fine recrystallized grains and substructure grains 71% and 29%, respectively.
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The EBSD results show that the grains of the alloy have undergone significant refinement. Since magnesium is a metal with relatively high stacking fault, dynamic recrystallization is prone to occur during hot processing. The ECAP temperature employed in this work was higher than the recrystallization temperature of magnesium alloys, so the grains mainly undergo dynamic recrystallization under the activation of coupled heat and stress. For magnesium alloys with high volume fraction of hard second phase particles, DRX via the particle-stimulated nucleation (PSN) mechanism was usually proposed [35,36]. Fig. 8a shows the TEM image of an obvious sub-grain interacted with several Mg2 Ca particles in 4p-ECAP alloy. On the one hand, the high-density dislocations gathered at the interfaces of refined Ca-containing particles provide the requirements for nucleation of DRX grains (sub-grain). On the other hand, the fine particles are effective to obstacle the movement of grain boundaries. In addition, Fig. 8b shows the typical morphology and location of DRX grains. Several DRX grains appear near the refined particles region, and they become larger with the distance away from the gathered particles, which further demonstrates the activation of PSN mechanism and the effective pinning effect of these broken particles. 3.3. Mechanical properties Fig. 9a shows the representative stress-strain curves of ascast and ECAP alloys. The as-cast Mg-4.76Al-4.47Ca-0.12Mn alloy exhibits an ultimate tensile strength (UTS) of 179 MPa
Fig. 10. SEM micrographs of the tensile fracture surfaces: (a) as-cast, (b) 4p, (c) 8p, (d) 12p. Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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Fig. 11. (0001), (112¯ 0) and (101¯ 0) pole figures of the (a) 4p, (b) 8p and (c) 12p-ECAP alloys.
and an elongation to failure of 3%. After ECAP the mechanical properties of the alloys were significantly improved, and more numbers of ECAP show more enhancing effect, both in strength and ductility. The 12p-ECAP alloy possesses the UTS and elongation to failure of 372 MPa and 8%, respectively. Fig. 10 shows the fracture surfaces of the as-cast and ECAP alloys after tensile test. It is obvious from Fig. 10a that brittle fracture is the main fracture mode for the ascast alloy, as the fracture surface was smooth and flat, and cracks around large particles were observed (marked by arrows; EDS analysis indicates the composition of Mg-10.36
at.% Al-10.79 at.% Ca-0.09 at.% Mn). After 4 passes of ECAP (Fig. 10b), the surface became rugged, and tear ridges were mainly observed, suggesting the improve of ductility and fracture mode of cleavage fracture. With further increase of ECAP numbers (Fig. 10c and d), although cleavage fracture is still the dominated fracture mode, the features of ductility fracture becomes more distinct, as many dimples were observed on the fracture surface. The above SEM fractograph results demonstrate that the multi-pass ECAP is effective to improve the ductility of high-calcium-content Mg-AlCa-Mn alloy, consistent with the mechanical properties shown in Fig. 9.
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Table 3 Comparison of tensile properties of the as-ECAP Mg-Al-Ca-Mn alloys and others else. Alloy composition(wt.%)
Processing condition
UTS (MPa)
Elongation to failure (%)
Reference
Mg-4.7Al-4.5Ca-0.12Mn
4p-ECAP 8P-ECAP 12p-ECAP Extrusion Extrusion Extrusion Extrusion Extrusion
300 314 372 457 420 333 289 333
6.5 6.3 8 2.5 5.6 5 4 3
This study This study This study [20] [19] [22] [22] [22]
Mg-2.7Al-3.5Ca-0.4Mn Mg-3.5Al-3.3Ca-0.4Mn Mg-3.7Al-3.8Ca Mg- 4.4Al-4.5Ca Mg-4.9Al-5Ca
3.4. Influence of microstructure evolution on mechanical properties of ECAP alloys Tensile results of Fig. 9 show that more numbers of ECAP processing improve both the strength and ductility of the alloys. This could be ascribed to the gradually refined microstructure, including evolution of α-Mg grains and second phase particles. As shown in Fig. 4, the sizes of Mg2 Ca particles decrease with more ECAP numbers, which must be the same for sizes of α-Mg grains. Therefore, the effect of fine grain strengthening and fine particles strengthening is enhanced for the alloy with higher ECAP numbers. When the microstructure is refined and becomes uniform with more passes of ECAP, the ductility could also be improved. In addition, as proved by former microstructure observations, multi-pass RD-ECAP introduces two obvious microstructural evolutions, refining and dynamically precipitating. The refining process involves both the second phase and the α-Mg matrix. Unlike the bimodal grain distribution of extruded alloys, a uniform and refined α-Mg grains were obtained after 12p-ECAP. As for the network-shaped Mg2 Ca and (Mg, Al)2 Ca phases formed in cast alloy, they were effectively refined into fine particles, and the sizes of these particles exhibit a bimodal distribution. One kind of the particles are in the range of 1–2 μm, and the other are near 200 nm. It is reasonable to accept that the mixed particles with two obvious different sizes are effective to obstacle dislocations [37]. However, although both the second phase and α-Mg grains were obviously refined, they were not distributed evenly, and the fine second phase particles were agminated at original grain boundary regions. In case of the precipitates within α-Mg grains, there emerged three different ones, the plate-like Al2 Ca precipitates, the nano-sized (diameter < 10 nm) Al-Mn spherical compounds, and some large spherical particles (diameter ∼ 10 nm). Therefore, considering the remarkably improved tensile strength, it could be attributed to the fine α-Mg grains, refined and bimodal second phase particles, and various nano-sized precipitates in the matrix. Table 3 lists the comparisons of tensile properties of highCa-content Mg-Al-Ca-Mn wrought alloys obtained in this study and in references [19,20,22]. Compared with the three extruded Mg-Al-Ca alloys without Mn content, the ECAP alloy exhibits higher strength and ductility, suggesting the great potential of multi-pass on improving the mechanical properties of this kind of alloys. As for the two extruded
Fig. 12. Schmid factor distributions of {0002} < 112¯ 0 > slip of the 12pECAP alloy.
Mg-Al-Ca-Mn alloys, their strengths are higher than this 12pECAP alloy by 48 MPa and 75 MPa, respectively, but with lower elongation to failure. According to Xu et al. [19] And Zheng et al. [20], the high strength of the extruded Mg-AlCa-Mn alloys were attributed to the ultra-fine DRXed grains pinned by fragmented secondary phases, strong basal texture and dense nano-scale precipitates. Comparing the microstructure of this ECAP alloy with the as-extruded alloys, the main difference is the grain size distribution. Near 30% of the αMg grains in extruded Mg-Al-Ca-Mn alloys were coarse deformed grains with strong basal texture, which significantly strengthened the alloy but impaired the ductility [19,20,38]. Fig. 11 shows the pole figures of the studied ECAP alloys, and it is apparent that with increasing ECAP numbers from 4 to 8, the texture intensity decreases from 26.98 to 9.40, which is consistent with the texture evolutions of magnesium alloys during ECAP [39,40]. The texture intensity of the 12p ECAP alloy is much lower than the extruded alloys. Moreover, Fig. 12 shows the Schmid factor distributions of basal slip for the ECAP alloy. The calculated average Schmid factor is 0.38, which is higher than that for extruded alloys. Based on above discussions, it can be inferred that the texture strengthening plays an important role in strengthening of the wrought Mg-Al-Ca-Mn alloys. Therefore, the weak
Please cite this article as: H. Huang, H. Liu and C. Wang et al., Potential of multi-pass ECAP on improving the mechanical properties of a high-calciumcontent Mg-Al-Ca-Mn alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.04.008
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texture of this ECAP alloy results in lower strength and better ductility than extruded alloys. To further improve the strength of this ECAP alloy, subsequent rolling process could be employed to introduce strong texture on the homogeneous ultra-fine grains. Furthermore, it is worth to note that the fractured Ca-containing particles was not uniformly distributed with DRX grains, either in hot extruded alloys or ECAP alloys. The segregation of these particles might be responsible for the poor ductility of wrought Mg-Al-Ca-Mn alloys, as suggested by the SEM fracture surface. Therefore, according to previous experiences, more ECAP will be carried out in our future studies to eliminate the segregation of crushed particles within the ultra-fine grained Mg-Al-Ca-Mn alloys. 4. Conclusions In the present study, the microstructure evolutions and mechanical properties of a high Ca-containing Mg-Al-Ca-Mn alloy during multi-pass ECAP were systematically investigated. The main conclusions can be drawn as follows: (1) The microstructure of the as-cast Mg-4.76Al-4.47Ca0.12Mn alloy was composed of α-Mg grains, two kinds of Ca-containing lamellar eutectic compounds, (Mg, Al)2 Ca and Mg2 Ca phase, and few Al-Mn particles. (2) During multi-pass ECAP, dynamic recrystallization of α-Mg occurred, and the average grain size of DRX grains was around 1.5 μm. The network-shaped second phases were crushed into ultra-fine particles with bimodal size distributions, but were still gathered at original grain boundaries. Moreover, various of nano-sized Al2 Ca and Al-Mn particles were dynamically precipitated within α-Mg grains. (3) Both the strength and ductility of the alloy were improved with increasing ECAP numbers. The 12p-ECAP alloy exhibited optimal mechanical properties with ultimate tensile strength of 372 MPa and elongation to failure of 8%. The enhanced strength of the ECAP alloy could be ascribed to the refined α-Mg grains, ultra-fine and bimodal second phase particles, and nano-sized precipitates, while the improve of ductility was mainly resulted from the refined and uniform microstructure with weak texture and high Schmid factors.
Declaration of Competing Interest None. Acknowledgment This work was supported by the Natural Science Foundation of China (51901068), the Natural Science Foundation of Jiangsu Province of China (BK20160869), the Fundamental Research Funds for the Central Universities (2018B16614) and the Nantong Science and Technology Project.
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