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Development of dilute Mg–Zn–Ca–Mn alloy with high performance via extrusion
Journal of Alloys and Compounds 668 (2016) 13e21
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Development of dilute MgeZneCaeMn alloy with high performance via extrusion M.G. Jiang a, b, C. Xu c, T. Nakata c, H. Yan a, *, R.S. Chen a, **, S. Kamado c a
The Group of Magnesium Alloys and Their Applications, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China b University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China c Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2015 Received in revised form 21 January 2016 Accepted 25 January 2016 Available online 28 January 2016
Dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. Excellent balance of strength and ductility was achieved via extrusion process by grain refinement and texture modification. The dilute alloy extruded at 300 C exhibited a tensile yield strength of 307 MPa and an elongation of 20.6% due to a bimodal microstructure consisting of fine dynamically recrystallized (DRXed) grains of ~2.3 mm and strongly textured coarse unDRXed grains. After extrusion at 350 C, almost fully DRXed microstructure (DRX fraction of ~0.96) and significantly weakened basal texture (maximum intensity of 3.2) with rare-earth texture component were achieved, consequently resulting in a high compression/tension yield ratio of ~0.8 with tensile elongation of 30.0% and tensile yield strength of 164 MPa. Besides, fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth. © 2016 Elsevier B.V. All rights reserved.
Keywords: MgeZneCaeMn alloy Extrusion Microstructure Texture Mechanical properties
1. Introduction Lightweight magnesium (Mg) alloys have attracted considerable attention due to their potential applications for weight reduction in automotive and aerospace industries. However, the insufficient mechanical properties and high cost are main challenges for their widespread applications. The extrusion process is an industrially available and effective method to improve the mechanical properties of Mg alloys. Various new Mg alloys have been extruded, including rare-earth (RE)containing and RE-free alloys, and these alloys have revealed a wide range of mechanical properties. The extruded RE-containing alloys showed much higher strengths, e.g. Mge3.9Zne1.6Gd alloy (wt.%) with tensile yield strength (TYS) of 395 MPa and ultimate tensile strength (UTS) of 417 MPa [1] and Mge10.0 Gd-5.7Y-1.6Zne0.6Zr alloy (wt.%) with TYS of 419 MPa and UTS of 461 MPa [2]. Due to the cost issues, RE-free Mg alloys are much more competitive for commercial utilizations. Recently, MgeZneCa system alloys have
* Corresponding authors. ** Corresponding authors. E-mail addresses: [email protected] (H. Yan), [email protected] (R.S. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.01.195 0925-8388/© 2016 Elsevier B.V. All rights reserved.
received progressive interest due to their low cost, good precipitation hardenability [3,4] and creep resistance [5,6], as well as good biodegradability [7,8]. Du et al. [9] achieved a high strength with TYS of 292 MPa and UTS of 305 MPa in Mge4.5Zne1.1Ca (wt.%) alloy after extrusion at 300 C, which was ascribed to fine dynamically recrystallized (DRXed) grains and strong basal texture. Recently, Li et al. [10] reported a high ductility of 36.7% in Mge3.0Zne0.2Ca (wt. %) alloy after extrusion at 300 C by obtaining weakened basal texture. On the other hand, recent study [11] on Mge1.0Zne0.3Ca (wt.%) alloy with lower Zn content exhibited an excellent balance of strength and ductility with TYS of 238 MPa and elongation of 31% after extrusion at 300 C. Zhang et al. [12] have previously developed a similar Mge1.0Zne0.5Ca (wt.%) alloy, and obtained a high strength of 300 MPa after extrusion at 310 C due to refined grains and strong basal texture and a high ductility of 44% after extrusion at 370 C due to weakened extrusion texture, and their following study [13] further confirmed that the addition of Ca weakened the extrusion texture, thus leading to an improved ductility. Such a texture weakening effect of Ca element has been previously reported in a Ca-containing AZ31 alloy after extrusion by Laser et al. [14]. Furthermore, Stanford et al. [15] studied the effect of Ca concentration on the texture of extruded MgeMneCa alloys, and
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reported that the addition of Ca weakened the extrusion texture in a similar manner to RE elements and introduced a new texture peak at around <11e21 > parallel to the extrusion direction (ED), which is commonly referred to as RE texture in the literature [16e20]. Recently, typical RE texture was obtained in binary Mge0.5 wt.%Ca alloy after extrusion at 300 C and 350 C [21] and Ca addition (0.5 wt.%) to pure Mg and Mge2.0 wt.%Zn resulted in a modified non-basal texture with the basal poles highly tilted toward ED in the extruded sheets [22]. These investigations suggest that excellent strength and ductility can be expected in MgeZneCa alloys using grain refinement and texture modification via extrusion process despite their alloy content. Besides, the addition of Mn to MgeZneCa alloys has proven to be effective method to refine the grain size [23] and facilitate the formation of fine precipitates [24] during extrusion. In this study, therefore, we developed an extremely dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy as low-cost and lowdensity wrought Mg alloy, and apply hot extrusion to the dilute alloy to achieve excellent balance of strength and ductility, which can be comparable to other MgeZneCa system alloys with much higher alloy content, by grain refinement and texture modification.
2. Experimental procedures Alloy ingot with a composition of Mge0.21Zne0.30Cae0.14Mn in wt.% (hereafter, dilute ZXM alloy) was prepared by resistance melting high purity Mg, Zn, Mn and Mge30 wt.% Ca master alloy in a steel crucible under the protection of a gas mixture of SF6 (1 Vol.%) and CO2 (99 Vol.%) and casting into a preheated steel mold with a dimension of 75 mm 180 mm 200 mm. Billets of 43 mm in diameter and 35 mm in height were machined from the ingot and homogenized at 400 C for 12 h followed by 450 C for 12 h in an argon atmosphere, and finally quenched into water. Then these homogenized billets were extruded at 300 C, 350 C and 400 C using indirect extrusion method under an extrusion ratio of 20 and a die-exit speed of 1.2 m/min. Hereafter, the dilute alloys extruded at 300 C, 350 C and 400 C are denoted as dilute ZXM-300, ZXM350 and ZXM-400, respectively. For comparison, typical commercial AZ31 (Mge3.18Ale1.00Zne0.06Mn, wt.%) Mg alloy was homogenized at 415 C for 16 h and extruded at 350 C using the same extrusion process. The microstructures were observed using optical microscope (OM, Olympus BX51M), field-emission scanning electron microscope (FE-SEM, JEOL JSM-7000F) equipped with wave-lengthdispersive X-ray spectroscopy (WDX), electron back-scatter diffraction (EBSD) and transmission electron microscope (TEM, JEOL JEM-2100F) equipped with energy-dispersive X-ray spectroscopy (EDX) operating at 200 kV. EBSD observation was conducted on a JEOL JSM-7000F FE-SEM equipped with an EDAX-TSL EBSD system operating at 15 kV with a step size of 0.5e1.5 mm. Grain sizes and textures of the as-extruded samples were characterized by EBSD method and the data were analyzed with OIM Analysis software. To ensure statistical rigor, more than 2000 grains were examined for each condition. Thin foil specimens for the TEM observations were prepared by punching 3 mm in diameter discs, mechanical polishing and ion milling using a Gatan Precision Ion Polishing System (GATAN691). The tensile and compressive tests were performed using a Shimadzu Autograph AG-I (50 kN) at room temperature under an initial strain rate of 1.0 10 3 s 1 with the stress direction parallel to the ED. The tensile specimens were 4 mm in diameter and 22 mm in gauge length, and the compressive specimens were 4 mm in diameter and 10 mm in height.
3. Results and discussion 3.1. Microstructures of the as-cast and as-homogenized dilute ZXM alloy Fig. 1aeh shows the optical microstructures and WDX analysis results of the as-cast dilute ZXM alloy. The grain size was ~250 mm and many second phase particles distributed both at the grain boundaries (GBs) and within the grain interiors (Fig. 1a and b). According to the WDX elemental mappings (Fig. 1ceg), the second phases both at GBs (indicated by black arrows in Fig. 1c) and within grain interiors (indicated by white arrows in Fig. 1c) consisted mainly of Ca and Mg elements. The corresponding point analysis results (Fig. 1h) reveal a relatively small amount of Zn element in these particles, which was much lower than Ca element, and Zn content in the particles within grain interiors was even the same with that in Mg matrix (0.16 at%). Nie et al. [3] investigated the ascast microstructure of Mge1Zne1Ca (wt.%) alloy using TEM and confirmed that Mg2Ca was the primary intermetallic phase despite the detected small amount of Zn element. In MgeZneCa alloys with higher Zn and Ca content with Zn/Ca (wt.%) ratio 2, Ca2Mg6Zn3 phase, as well as Mg2Ca, was commonly observed [25,26]. Therefore in this case, it is believed that the second phase particles at GBs and within the grain interiors were expected to be Mg2Ca rather than Ca2Mg6Zn3 phase in the as-cast dilute ZXM alloy, and no MgeZn phases were detected in this study. After homogenization treatment (Fig. 1i and j), the second phase particles formed upon casting were dramatically dissolved into the Mg matrix and the grain size increased up to ~400 mm. 3.2. Microstructures of the as-extruded dilute ZXM alloy Fig. 2 shows the optical microstructures at the central part of the dilute ZXM and AZ31 extruded bars. The dilute ZXM-300 exhibited a typical bimodal grain structure consisting of fine equixial dynamically recrystallized (DRXed) grain regions and coarse unDRXed regions elongated along the ED (indicated by black arrows in Fig. 2). With increasing extrusion temperature, DRX fraction of the dilute ZXM alloy increased from 0.60 (dilute ZXM-300) to 0.98 (dilute ZXM-400), finally leading to an almost fully DRXed microstructure. For AZ31 alloy, the DRX fraction was 0.99. In this study, DRX fraction was determined by an Image-Pro Plus 6.0 software using low magnified optical microstructures taken at five different locations from the surface to the center in the extruded bar. In the dilute ZXM alloy (Fig. 2aef), grain size gradually increased with increasing extrusion temperature, and many second phases uniformly precipitated in DRXed and unDRXed regions. Compared to the dilute ZXM alloy, no second phases were observed in the AZ31 alloy after extrusion (Fig. 2g and h). In the inverse pole figure map (Fig. 3a), the dilute ZXM-300 exhibited fine DRXed grains of ~2.3 mm with relatively random orientations, and coarse unDRXed grains with basal planes parallel to the ED (highlighted by three-dimensional hexagons in the rectangular region). Due to the high DRX fraction, unDRXed grains were not detected in the dilute ZXM-400 and as-extruded AZ31 alloys (Fig. 3c and d). It is important to note that the DRXed grain size of the dilute ZXM alloy (~7.3 mm) was much finer than that of the AZ31 alloy (~16.2 mm) after extrusion at the same temperature of 350 C. The microstructure of the dilute ZXM-300 was further examined using TEM and the results are presented in Fig. 4. TEM bright field (BF) images (Fig. 4a and b) show fine DRXed grains of ~1 mm and many spherical second phases. From the high-angle annular darkfield (HAADF) image (Fig. 4c, magnified rectangular region in Fig. 4a), it is evident that spherical second phases distributed at DRXed GBs and within grain interiors and the particle size varied
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Fig. 1. (a, b) Optical microstructures and (ceh) WDX analysis results of the as-cast dilute ZXM alloy: (c) SEM image and (deg) corresponding WDX elemental mappings of Mg, Zn, Ca and Mn and (h) point analysis results, and (i, j) optical microstructures of the as-homogenized dilute ZXM alloy.
Fig. 2. Optical microstructures of the as-extruded (aef) dilute ZXM and (g, h) AZ31 alloys. Black arrows indicate the unDRXed regions.
from 10 nm to 100 nm. The corresponding EDX analysis results (Fig. 4deg) reveal two types of second phases. The fine spherical particles of ~10 nm were found to be enriched with Mn, and the relatively large spherical ones of 20e100 nm were found to be enriched mainly with Ca. In the selected area electron diffraction (SAED) pattern of the Ca containing spherical precipitate (Fig. 4h), the extra spots were not clear except for the spots of Mg matrix, which is consistent with the result observed by TEM in binary Mge0.3 wt.% Ca alloy [4]. According to the above EDX analysis results, the precipitates in this study were expected to be the Mg2Ca and a-Mn phases. Our result is different from that reported in Mge4.5Zne1.1Ca [9] and Mge6.0Zne1.8Cae0.4Mn [27] (wt.%) alloys with higher Zn and Ca content, in which the dynamic
precipitates during extrusion were Ca2Mg6Zn3 phase. From the above discussion, therefore, it can be concluded that fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion due to low Zn content in the dilute ZXM alloy and acted as effective pinning obstacles against DRXed grain growth, contributing to grain refinement. 3.3. Textures of the as-extruded dilute ZXM alloy Fig. 5 shows (0001) pole figures of the as-extruded dilute ZXM and AZ31 alloys. For bimodal grain structures, the textures of unDRXed and DRXed regions were separately analyzed by EBSD, as presented in Fig. 5a and b. It is apparent that the unDRXed regions
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Fig. 3. Inverse pole figure maps of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys.
were strongly textured, while the fine DRXed grain regions exhibited a weakened basal texture. Due to the low DRX fraction (0.60), the dilute ZXM-300 exhibited a relatively strong basal texture with maximum intensity of 6.4 (Fig. 5a). With increasing extrusion temperature from 300 C to 400 C (Fig. 5aec), the maximum basal texture intensity decreased from 6.4 to 2.9 due to the dramatic decrease of unDRXed regions. Meanwhile, the basal poles showed an increasing angle distribution (from ±30 to ± 45 ) away from transverse direction (TD) to the ED, as highlighted in red in Fig. 5. Here, TD indicates the direction perpendicular to the ED. Especially for the dilute ZXM-400, weaker basal texture with broader orientation distributions (±45 ) was detected compared to AZ31 alloy despite nearly the same DRX fraction and grain size. From the inverse pole figures as shown in Fig. 6, it can be seen that the dilute ZXM-300 exhibited a typical <10e10 > fiber texture with [10e10] orientation parallel to the ED, which is typical for extruded Mg alloys. With increasing extrusion temperature from 300 C to 400 C (Fig. 6aec) [10e10], fiber component was weakened from 14.0 to 2.1, and a new component between [2e1e12] and [2e1e11] became visible and strengthened as highlighted by blue circles in Fig. 6b and c. Compared to the dilute ZXM-400 alloy, it is interesting that such a component was not observed in the as-extruded AZ31 even after experiencing a full DRX process. Upon the basis of the previous studies on the effect of Ca on the extrusion texture [12e15,21,22], as stated earlier, it is reasonable to believe that the weakened basal texture with RE texture component between [2e1e12] and [2e1e11] can be ascribed to the addition of Ca in the dilute ZXM alloy. It is well known that commercial Mg alloys usually develop strong basal textures during hot extrusion, which accounts for their poor formability and severe anisotropy. One of the most promising approaches to solve this problem is to obtain the RE texture in Mg extrusions and the effect of RE texture on the ductility has been clearly established [16e18]. The RE texture component provides more grains that are favorable for basal slip and extension twinning, leading to an increased tensile ductility and improved yield asymmetry in the alloys [16]. With respect to the RE texture
formation mechanism, PSN was first reported in RE-containing Mg alloys to explain this phenomenon during DRX [28,29]. Recent studies on dilute MgeRE solid solution alloys [19,30], however, have provided direct evidence of the segregation and clustering of RE elements at GBs, which could greatly contribute to retarding the mobility of dislocations and GBs, and subsequently altering and weakening the overall texture. Like RE elements, it is believed that Ca in Mg alloy could also result in a strong solute drag effect due to the large atomic radius difference between Mg and Ca. The study on the extrusion of binary Mge0.5 wt.%Ca alloy [21] reported that the formation of RE texture components was enhanced by extruding under dynamic strain aging conditions, which implied, on the other hand, that the solute drag of Ca may be responsible for the introduction of RE texture component. However to date, no direct and convincing evidence was provided to explain the texture weakening mechanism in Ca-containing Mg alloys. Thus, a more comprehensive study is certainly needed in the future. 3.4. Mechanical properties of the as-extruded dilute ZXM alloy Fig. 7 shows the tensile and compressive stressestrain curves of the as-homogenized and as-extruded ZXM and AZ31 alloys and the summary view of the mechanical properties, including the values of TYS, compressive yield strength (CYS), UTS, elongation and CYS/ TYS (TYS/CYS). As-homogenized dilute ZXM alloy showed TYS, UTS and elongation of 36 MPa, 134 MPa and 7.3%, respectively, with a high TYS/CYS ratio of 0.92 due to the almost random orientations. After extrusion, it is obvious that both strength and ductility improved synchronously. The dilute ZXM-300 exhibited an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), but a high yield asymmetry with CYS/TYS ratio of 0.54. With increasing extrusion temperature, tensile strength decreased gradually with significantly improved ductility and yield asymmetry. The dilute ZXM-350 showed better tensile strengtheductility combination (TYS of 164 MPa and elongation of 30.0%) with lower yield asymmetry (CYS/TYS ratio of 0.77) than AZ31 alloy (TYS of 154 MPa, elongation of 26.0% and CYS/TYS ratio
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Fig. 4. TEM observation results of the dilute ZXM alloy extruded at 300 C: (a, b) TEM BF images, (b) HAADF image of the rectangular region shown in (a), (deg) corresponding EDX elemental mappings of the region shown in (c), and (h) corresponding SAED pattern of the Ca containing spherical precipitate taken from the beam direction of [11e20]Mg.
Fig. 5. (0001) pole figures of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys.
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Fig. 6. Inverse pole figures of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys. Inverse pole figures refer to the ED.
Fig. 7. (a) Tensile and (b) compressive stressestrain curves of the as-homogenized and as-extruded ZXM and AZ31 alloys, and (c) summary view of the mechanical properties, including the values of TYS, CYS, UTS, elongation and CYS/TYS (TYS/CYS).
of 0.68) extruded at the same temperature. In this study, the high performance of the dilute ZXM alloy after extrusion can be mainly attributed to the refined grains from HallePetch relation and basal texture through its effect on the activation of slip system. It is evident that grain refinement resulted in an obvious increase of both TYS and CYS. Since basal slip and {10e12} extension twin are dominant deformation modes in Mg
alloys at room temperature, the average Schmid factors for (0001) <11e20 > basal slip when the tensile stress is along the ED and for (10e12)<10e1e1> extension twin when the compressive stress is along the ED were analyzed by EBSD, as presented in Fig. 8. As mentioned above, the dilute ZXM-300 exhibited an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), which can be ascribed to the fine DRXed grains of ~2.3 mm
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Fig. 8. Schmid factor distribution histograms of the as-extruded dilute ZXM and AZ31 alloys: (a) (0001)<11e20 > basal slip when the tensile stress is along the ED, and (b) (10e12) <10e1e1> extension twin when the compressive stress is along the ED.
and strongly textured coarse unDRXed grains with extremely low Schmid factor for basal slip (0.06). The average Schmid factor for basal slip along the ED was only 0.16 (Fig. 8a), suggesting the difficulty of basal slip activation. Besides, {10e12} extension twinning is not favored due to the strongly textured grain orientations. In this case, prismatic slip of dislocation with high critical resolved shear stress (CRSS, 39.2 MPa [31]) is deduced to govern the macroscopic yield at room temperature [32,33], thereby leading to high TYS. On the other hand, the average Schmid factor for extension twin was 0.44 when compressive deformation is along the ED (Fig. 8b), presenting a severe concave-shaped compressive curve (Fig. 7b), which is a typical feature of {10e12} extension twinning dominated deformation [34,35]. Thus due to the extensive CRSS differences between prismatic slip and {10e12} extension twin (CRSS, 2.0e2.8 MPa [31]), quite high yield asymmetry of the dilute ZXM-300 appeared during tension and compression along the ED. With increasing extrusion temperature, it can be seen in Fig. 8 that the average Schmid factor for basal slip gradually increased from 0.16 to 0.27 with the decreased average Schmid factor for extension twin from 0.44 to 0.35 due to the formation of weakened basal texture with RE texture component, giving rise to the improved ductility and yield asymmetry. Compared to the AZ31 alloy extruded at the same temperature, the dilute ZXM-350 exhibited higher Schmid factor for basal slip and lower Schmid factor for extension twin and finer DRXed grain size of ~7.3 mm, consequently resulting in a better tensile strengtheductility combination and lower yield asymmetry with CYS/TYS ratio of 0.77. As for the dilute ZXM-400, it had the same DRXed grain size of ~16.2 mm with AZ31 alloy extruded at 350 C (Fig. 3), but exhibited much lower TYS and higher elongation. This is because weaker basal texture with RE texture component of the dilute ZXM-400 resulted in a higher Schmid factor for basal slip (0.27). Besides, it is known that solid solution strengthening is strongly dependent on atomic solute concentration. In this case, dilute ZXM alloy contained much lower alloy content (0.32 at%) than AZ31 alloy (3.30 at%) and many Mg2Ca and a-Mn phases, as described above, precipitated during extrusion, thereby leading to a deficiency of solid solution strengthening. This is probably another important reason for the low strength of the extruded ZXM alloy despite the same grain size. On the other hand, it is noteworthy that the formation of fine spherical Mg2Ca and a-Mn phases instead of large brittle particles was favorable for the ductility improvement [36]. Fig. 9 shows relationships between tensile yield strength and elongation, and between CYS/TYS ratio and tensile elongation of various extruded Mg alloys, including commercial Mg alloys [37e39], representative MgeZneCa system alloys [9e13,23,24,27,40e45], and other RE-free Mg alloys [46e49], as well as RE-containing alloys [1,2,50e54]. It can be seen that the
dilute ZXM-300 in this study shows an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), which is better than commercially available Mg alloys and even comparable to representative MgeZneCa system alloys with higher alloy content, such as Mge6.0Zne1.8Cae0.4Mn (TYS of 289 MPa and elongation of 16%) [27], Mge5.7Zne0.2Cae0.8Zr (TYS of 310 MPa and elongation of 18%) [42] and Mge6.1Zne0.40Age0.2Cae0.6Zr (TYS of 289 MPa and elongation of 17%) [44] (wt.%) alloys. Besides, the tensile ductility of the dilute ZXM-350 and ZXM-400 is found to be superior to that of other various Mg alloys, expect for Mge3.0Zne0.2Ca [10], Mge1.0Zne0.2Ca [13] and Mge1.0Zne0.5 Ca [12] (wt.%) alloys. On the other hand, the dilute ZXM-300 exhibits higher yield asymmetry (CYS/TYS ratio of 0.54) than other MgeZneCa series alloys with higher alloy content, such as Mge6.0Zne1.8Cae0.4Mn (CYS/ TYS ratio of 0.89) [27], Mge5.7Zne0.2Cae0.8Zr (CYS/TYS ratio of 0.84) [42] and Mge6.1Zne0.40Age0.2Cae0.6Zr (CYS/TYS ratio of 0.85) [44] (wt.%) alloys, which was mainly ascribed to low DRX fraction and limited number of fine precipitates resulting from the dilute alloy content. However, the yield asymmetry of dilute ZXM alloy extruded at higher temperature was significantly improved (CYS/TYS ratio of ~0.8) due to the weakened basal texture with RE texture component, which can be comparable to that of other extruded Mg alloys. Therefore, these preceding results point to the fact that newly developed dilute ZXM alloy in this study could provide a various combination of strength and ductility, as well as yield asymmetry, after extrusion at different temperatures, which could be a potential candidate as low-cost and low-density wrought Mg alloy for various demands in industrial applications. 4. Conclusions In summary, dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. The dilute alloy extruded at 300 C exhibited a bimodal microstructure consisting of fine DRXed grains of ~2.3 mm and strongly textured coarse unDRXed grains, consequently resulting in an excellent balance of strength and ductility (TYS of 307 MPa and elongation of 20.6%), which is better than commercially available Mg alloys and even comparable to representative MgeZneCa system alloys with higher alloy content. After extrusion at 350 C, low yield asymmetry (CYS/TYS ratio of ~0.8) with TYS of 164 MPa and elongation of 30.0% was obtained due to the almost fully DRXed microstructure (DRX fraction of ~0.96) and significantly weakened basal texture (maximum intensity of 3.2) with RE texture component. Besides, fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth. Thus, the newly developed dilute
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Fig. 9. Relationships (a) between tensile yield strength and elongation, and (b) between CYS/TYS ratio and tensile elongation of various extruded Mg alloys, including commercial Mg alloys [37e39], representative MgeZneCa system alloys [9e13,23,24,27,40e45], and other RE-free Mg alloys [46e49], as well as RE-containing alloys [1,2,50e54].
Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy could be a potential candidate as low-cost and low-density wrought Mg alloy with excellent balance of strength and ductility, as well as low yield asymmetry.
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Acknowledgments [17]
The authors gratefully acknowledge the financial supports from the State Key Program of National Natural Science of China (No. 51531002), National Basic Research Program of China (973 Program, No. 2013CB632202) and National Natural Science Foundation of China (NSFC, No. 51301173).
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M.G. Jiang et al. / Journal of Alloys and Compounds 668 (2016) 13e21 Mater. 66 (2012) 1061e1064. [36] H. Yan, S. Xu, R. Chen, S. Kamado, T. Honma, E. Han, Activation of {10-12} twinning and basal slip in a high ductile Mg-2.0 Zn-0.8 Gd rolled sheet with a non-basal texture during tensile deformation at room temperature, J. Alloys Compd. 566 (2013) 98e107. [37] S. Kamado, H. Ohara, Y. Kojiam, Advanced Manufacturing Technologies of Magnesium Alloys, CMCbooks, Japan, 2005. [38] Z. Zachariah, S.S.V. Tatiparti, S. Mishra, N. Ramakrishnan, U. Ramamurty, Tensionecompression asymmetry in an extruded Mg alloy AM30: Temperature and strain rate effects, Mater. Sci. Eng. A 572 (2013) 8e18. [39] L. Chen, C. Wang, W. Wu, Z. Liu, G.M. Stoica, L. Wu, P.K. Liaw, Low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy, Metall. Mater. Trans. A 38 (2007) 2235e2241. [40] A. H€ anzi, A. Sologubenko, P. Gunde, M. Schinhammer, P. Uggowitzer, Design considerations for achieving simultaneously high-strength and highly ductile magnesium alloys, Philos. Mag. Lett. 92 (2012) 417e427. [41] H. Somekawa, T. Mukai, High strength and fracture toughness balance on the extruded MgeCaeZn alloy, Mater. Sci. Eng. A 459 (2007) 366e370. [42] T. Homma, C.L. Mendis, K. Hono, S. Kamado, Effect of Zr addition on the mechanical properties of as-extruded MgeZneCaeZr alloys, Mater. Sci. Eng. A 527 (2010) 2356e2362. [43] A. H€ anzi, F. Dalla Torre, A. Sologubenko, P. Gunde, R. Schmid-Fetzer, € ffler, P. Uggowitzer, Design strategy for microalloyed ultraM. Kuehlein, J. Lo ductile magnesium alloys, Philos. Mag. Lett. 89 (2009) 377e390. [44] C.L. Mendis, K. Oh-ishi, Y. Kawamura, T. Honma, S. Kamado, K. Hono, Precipitation-hardenable Mge2.4Zne0.1Age0.1Cae0.16Zr (at.%) wrought magnesium alloy, Acta Mater. 57 (2009) 749e760. [45] J. Geng, J.F. Nie, Microstructure and mechanical properties of extruded Mge1Cae1Zne0.6Zr alloy, Mater. Sci. Eng. A 653 (2016) 27e34.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Development of dilute MgeZneCaeMn alloy with high performance via extrusion M.G. Jiang a, b, C. Xu c, T. Nakata c, H. Yan a, *, R.S. Chen a, **, S. Kamado c a
The Group of Magnesium Alloys and Their Applications, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China b University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China c Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2015 Received in revised form 21 January 2016 Accepted 25 January 2016 Available online 28 January 2016
Dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. Excellent balance of strength and ductility was achieved via extrusion process by grain refinement and texture modification. The dilute alloy extruded at 300 C exhibited a tensile yield strength of 307 MPa and an elongation of 20.6% due to a bimodal microstructure consisting of fine dynamically recrystallized (DRXed) grains of ~2.3 mm and strongly textured coarse unDRXed grains. After extrusion at 350 C, almost fully DRXed microstructure (DRX fraction of ~0.96) and significantly weakened basal texture (maximum intensity of 3.2) with rare-earth texture component were achieved, consequently resulting in a high compression/tension yield ratio of ~0.8 with tensile elongation of 30.0% and tensile yield strength of 164 MPa. Besides, fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth. © 2016 Elsevier B.V. All rights reserved.
Keywords: MgeZneCaeMn alloy Extrusion Microstructure Texture Mechanical properties
1. Introduction Lightweight magnesium (Mg) alloys have attracted considerable attention due to their potential applications for weight reduction in automotive and aerospace industries. However, the insufficient mechanical properties and high cost are main challenges for their widespread applications. The extrusion process is an industrially available and effective method to improve the mechanical properties of Mg alloys. Various new Mg alloys have been extruded, including rare-earth (RE)containing and RE-free alloys, and these alloys have revealed a wide range of mechanical properties. The extruded RE-containing alloys showed much higher strengths, e.g. Mge3.9Zne1.6Gd alloy (wt.%) with tensile yield strength (TYS) of 395 MPa and ultimate tensile strength (UTS) of 417 MPa [1] and Mge10.0 Gd-5.7Y-1.6Zne0.6Zr alloy (wt.%) with TYS of 419 MPa and UTS of 461 MPa [2]. Due to the cost issues, RE-free Mg alloys are much more competitive for commercial utilizations. Recently, MgeZneCa system alloys have
* Corresponding authors. ** Corresponding authors. E-mail addresses: [email protected] (H. Yan), [email protected] (R.S. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.01.195 0925-8388/© 2016 Elsevier B.V. All rights reserved.
received progressive interest due to their low cost, good precipitation hardenability [3,4] and creep resistance [5,6], as well as good biodegradability [7,8]. Du et al. [9] achieved a high strength with TYS of 292 MPa and UTS of 305 MPa in Mge4.5Zne1.1Ca (wt.%) alloy after extrusion at 300 C, which was ascribed to fine dynamically recrystallized (DRXed) grains and strong basal texture. Recently, Li et al. [10] reported a high ductility of 36.7% in Mge3.0Zne0.2Ca (wt. %) alloy after extrusion at 300 C by obtaining weakened basal texture. On the other hand, recent study [11] on Mge1.0Zne0.3Ca (wt.%) alloy with lower Zn content exhibited an excellent balance of strength and ductility with TYS of 238 MPa and elongation of 31% after extrusion at 300 C. Zhang et al. [12] have previously developed a similar Mge1.0Zne0.5Ca (wt.%) alloy, and obtained a high strength of 300 MPa after extrusion at 310 C due to refined grains and strong basal texture and a high ductility of 44% after extrusion at 370 C due to weakened extrusion texture, and their following study [13] further confirmed that the addition of Ca weakened the extrusion texture, thus leading to an improved ductility. Such a texture weakening effect of Ca element has been previously reported in a Ca-containing AZ31 alloy after extrusion by Laser et al. [14]. Furthermore, Stanford et al. [15] studied the effect of Ca concentration on the texture of extruded MgeMneCa alloys, and
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reported that the addition of Ca weakened the extrusion texture in a similar manner to RE elements and introduced a new texture peak at around <11e21 > parallel to the extrusion direction (ED), which is commonly referred to as RE texture in the literature [16e20]. Recently, typical RE texture was obtained in binary Mge0.5 wt.%Ca alloy after extrusion at 300 C and 350 C [21] and Ca addition (0.5 wt.%) to pure Mg and Mge2.0 wt.%Zn resulted in a modified non-basal texture with the basal poles highly tilted toward ED in the extruded sheets [22]. These investigations suggest that excellent strength and ductility can be expected in MgeZneCa alloys using grain refinement and texture modification via extrusion process despite their alloy content. Besides, the addition of Mn to MgeZneCa alloys has proven to be effective method to refine the grain size [23] and facilitate the formation of fine precipitates [24] during extrusion. In this study, therefore, we developed an extremely dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy as low-cost and lowdensity wrought Mg alloy, and apply hot extrusion to the dilute alloy to achieve excellent balance of strength and ductility, which can be comparable to other MgeZneCa system alloys with much higher alloy content, by grain refinement and texture modification.
2. Experimental procedures Alloy ingot with a composition of Mge0.21Zne0.30Cae0.14Mn in wt.% (hereafter, dilute ZXM alloy) was prepared by resistance melting high purity Mg, Zn, Mn and Mge30 wt.% Ca master alloy in a steel crucible under the protection of a gas mixture of SF6 (1 Vol.%) and CO2 (99 Vol.%) and casting into a preheated steel mold with a dimension of 75 mm 180 mm 200 mm. Billets of 43 mm in diameter and 35 mm in height were machined from the ingot and homogenized at 400 C for 12 h followed by 450 C for 12 h in an argon atmosphere, and finally quenched into water. Then these homogenized billets were extruded at 300 C, 350 C and 400 C using indirect extrusion method under an extrusion ratio of 20 and a die-exit speed of 1.2 m/min. Hereafter, the dilute alloys extruded at 300 C, 350 C and 400 C are denoted as dilute ZXM-300, ZXM350 and ZXM-400, respectively. For comparison, typical commercial AZ31 (Mge3.18Ale1.00Zne0.06Mn, wt.%) Mg alloy was homogenized at 415 C for 16 h and extruded at 350 C using the same extrusion process. The microstructures were observed using optical microscope (OM, Olympus BX51M), field-emission scanning electron microscope (FE-SEM, JEOL JSM-7000F) equipped with wave-lengthdispersive X-ray spectroscopy (WDX), electron back-scatter diffraction (EBSD) and transmission electron microscope (TEM, JEOL JEM-2100F) equipped with energy-dispersive X-ray spectroscopy (EDX) operating at 200 kV. EBSD observation was conducted on a JEOL JSM-7000F FE-SEM equipped with an EDAX-TSL EBSD system operating at 15 kV with a step size of 0.5e1.5 mm. Grain sizes and textures of the as-extruded samples were characterized by EBSD method and the data were analyzed with OIM Analysis software. To ensure statistical rigor, more than 2000 grains were examined for each condition. Thin foil specimens for the TEM observations were prepared by punching 3 mm in diameter discs, mechanical polishing and ion milling using a Gatan Precision Ion Polishing System (GATAN691). The tensile and compressive tests were performed using a Shimadzu Autograph AG-I (50 kN) at room temperature under an initial strain rate of 1.0 10 3 s 1 with the stress direction parallel to the ED. The tensile specimens were 4 mm in diameter and 22 mm in gauge length, and the compressive specimens were 4 mm in diameter and 10 mm in height.
3. Results and discussion 3.1. Microstructures of the as-cast and as-homogenized dilute ZXM alloy Fig. 1aeh shows the optical microstructures and WDX analysis results of the as-cast dilute ZXM alloy. The grain size was ~250 mm and many second phase particles distributed both at the grain boundaries (GBs) and within the grain interiors (Fig. 1a and b). According to the WDX elemental mappings (Fig. 1ceg), the second phases both at GBs (indicated by black arrows in Fig. 1c) and within grain interiors (indicated by white arrows in Fig. 1c) consisted mainly of Ca and Mg elements. The corresponding point analysis results (Fig. 1h) reveal a relatively small amount of Zn element in these particles, which was much lower than Ca element, and Zn content in the particles within grain interiors was even the same with that in Mg matrix (0.16 at%). Nie et al. [3] investigated the ascast microstructure of Mge1Zne1Ca (wt.%) alloy using TEM and confirmed that Mg2Ca was the primary intermetallic phase despite the detected small amount of Zn element. In MgeZneCa alloys with higher Zn and Ca content with Zn/Ca (wt.%) ratio 2, Ca2Mg6Zn3 phase, as well as Mg2Ca, was commonly observed [25,26]. Therefore in this case, it is believed that the second phase particles at GBs and within the grain interiors were expected to be Mg2Ca rather than Ca2Mg6Zn3 phase in the as-cast dilute ZXM alloy, and no MgeZn phases were detected in this study. After homogenization treatment (Fig. 1i and j), the second phase particles formed upon casting were dramatically dissolved into the Mg matrix and the grain size increased up to ~400 mm. 3.2. Microstructures of the as-extruded dilute ZXM alloy Fig. 2 shows the optical microstructures at the central part of the dilute ZXM and AZ31 extruded bars. The dilute ZXM-300 exhibited a typical bimodal grain structure consisting of fine equixial dynamically recrystallized (DRXed) grain regions and coarse unDRXed regions elongated along the ED (indicated by black arrows in Fig. 2). With increasing extrusion temperature, DRX fraction of the dilute ZXM alloy increased from 0.60 (dilute ZXM-300) to 0.98 (dilute ZXM-400), finally leading to an almost fully DRXed microstructure. For AZ31 alloy, the DRX fraction was 0.99. In this study, DRX fraction was determined by an Image-Pro Plus 6.0 software using low magnified optical microstructures taken at five different locations from the surface to the center in the extruded bar. In the dilute ZXM alloy (Fig. 2aef), grain size gradually increased with increasing extrusion temperature, and many second phases uniformly precipitated in DRXed and unDRXed regions. Compared to the dilute ZXM alloy, no second phases were observed in the AZ31 alloy after extrusion (Fig. 2g and h). In the inverse pole figure map (Fig. 3a), the dilute ZXM-300 exhibited fine DRXed grains of ~2.3 mm with relatively random orientations, and coarse unDRXed grains with basal planes parallel to the ED (highlighted by three-dimensional hexagons in the rectangular region). Due to the high DRX fraction, unDRXed grains were not detected in the dilute ZXM-400 and as-extruded AZ31 alloys (Fig. 3c and d). It is important to note that the DRXed grain size of the dilute ZXM alloy (~7.3 mm) was much finer than that of the AZ31 alloy (~16.2 mm) after extrusion at the same temperature of 350 C. The microstructure of the dilute ZXM-300 was further examined using TEM and the results are presented in Fig. 4. TEM bright field (BF) images (Fig. 4a and b) show fine DRXed grains of ~1 mm and many spherical second phases. From the high-angle annular darkfield (HAADF) image (Fig. 4c, magnified rectangular region in Fig. 4a), it is evident that spherical second phases distributed at DRXed GBs and within grain interiors and the particle size varied
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Fig. 1. (a, b) Optical microstructures and (ceh) WDX analysis results of the as-cast dilute ZXM alloy: (c) SEM image and (deg) corresponding WDX elemental mappings of Mg, Zn, Ca and Mn and (h) point analysis results, and (i, j) optical microstructures of the as-homogenized dilute ZXM alloy.
Fig. 2. Optical microstructures of the as-extruded (aef) dilute ZXM and (g, h) AZ31 alloys. Black arrows indicate the unDRXed regions.
from 10 nm to 100 nm. The corresponding EDX analysis results (Fig. 4deg) reveal two types of second phases. The fine spherical particles of ~10 nm were found to be enriched with Mn, and the relatively large spherical ones of 20e100 nm were found to be enriched mainly with Ca. In the selected area electron diffraction (SAED) pattern of the Ca containing spherical precipitate (Fig. 4h), the extra spots were not clear except for the spots of Mg matrix, which is consistent with the result observed by TEM in binary Mge0.3 wt.% Ca alloy [4]. According to the above EDX analysis results, the precipitates in this study were expected to be the Mg2Ca and a-Mn phases. Our result is different from that reported in Mge4.5Zne1.1Ca [9] and Mge6.0Zne1.8Cae0.4Mn [27] (wt.%) alloys with higher Zn and Ca content, in which the dynamic
precipitates during extrusion were Ca2Mg6Zn3 phase. From the above discussion, therefore, it can be concluded that fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion due to low Zn content in the dilute ZXM alloy and acted as effective pinning obstacles against DRXed grain growth, contributing to grain refinement. 3.3. Textures of the as-extruded dilute ZXM alloy Fig. 5 shows (0001) pole figures of the as-extruded dilute ZXM and AZ31 alloys. For bimodal grain structures, the textures of unDRXed and DRXed regions were separately analyzed by EBSD, as presented in Fig. 5a and b. It is apparent that the unDRXed regions
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Fig. 3. Inverse pole figure maps of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys.
were strongly textured, while the fine DRXed grain regions exhibited a weakened basal texture. Due to the low DRX fraction (0.60), the dilute ZXM-300 exhibited a relatively strong basal texture with maximum intensity of 6.4 (Fig. 5a). With increasing extrusion temperature from 300 C to 400 C (Fig. 5aec), the maximum basal texture intensity decreased from 6.4 to 2.9 due to the dramatic decrease of unDRXed regions. Meanwhile, the basal poles showed an increasing angle distribution (from ±30 to ± 45 ) away from transverse direction (TD) to the ED, as highlighted in red in Fig. 5. Here, TD indicates the direction perpendicular to the ED. Especially for the dilute ZXM-400, weaker basal texture with broader orientation distributions (±45 ) was detected compared to AZ31 alloy despite nearly the same DRX fraction and grain size. From the inverse pole figures as shown in Fig. 6, it can be seen that the dilute ZXM-300 exhibited a typical <10e10 > fiber texture with [10e10] orientation parallel to the ED, which is typical for extruded Mg alloys. With increasing extrusion temperature from 300 C to 400 C (Fig. 6aec) [10e10], fiber component was weakened from 14.0 to 2.1, and a new component between [2e1e12] and [2e1e11] became visible and strengthened as highlighted by blue circles in Fig. 6b and c. Compared to the dilute ZXM-400 alloy, it is interesting that such a component was not observed in the as-extruded AZ31 even after experiencing a full DRX process. Upon the basis of the previous studies on the effect of Ca on the extrusion texture [12e15,21,22], as stated earlier, it is reasonable to believe that the weakened basal texture with RE texture component between [2e1e12] and [2e1e11] can be ascribed to the addition of Ca in the dilute ZXM alloy. It is well known that commercial Mg alloys usually develop strong basal textures during hot extrusion, which accounts for their poor formability and severe anisotropy. One of the most promising approaches to solve this problem is to obtain the RE texture in Mg extrusions and the effect of RE texture on the ductility has been clearly established [16e18]. The RE texture component provides more grains that are favorable for basal slip and extension twinning, leading to an increased tensile ductility and improved yield asymmetry in the alloys [16]. With respect to the RE texture
formation mechanism, PSN was first reported in RE-containing Mg alloys to explain this phenomenon during DRX [28,29]. Recent studies on dilute MgeRE solid solution alloys [19,30], however, have provided direct evidence of the segregation and clustering of RE elements at GBs, which could greatly contribute to retarding the mobility of dislocations and GBs, and subsequently altering and weakening the overall texture. Like RE elements, it is believed that Ca in Mg alloy could also result in a strong solute drag effect due to the large atomic radius difference between Mg and Ca. The study on the extrusion of binary Mge0.5 wt.%Ca alloy [21] reported that the formation of RE texture components was enhanced by extruding under dynamic strain aging conditions, which implied, on the other hand, that the solute drag of Ca may be responsible for the introduction of RE texture component. However to date, no direct and convincing evidence was provided to explain the texture weakening mechanism in Ca-containing Mg alloys. Thus, a more comprehensive study is certainly needed in the future. 3.4. Mechanical properties of the as-extruded dilute ZXM alloy Fig. 7 shows the tensile and compressive stressestrain curves of the as-homogenized and as-extruded ZXM and AZ31 alloys and the summary view of the mechanical properties, including the values of TYS, compressive yield strength (CYS), UTS, elongation and CYS/ TYS (TYS/CYS). As-homogenized dilute ZXM alloy showed TYS, UTS and elongation of 36 MPa, 134 MPa and 7.3%, respectively, with a high TYS/CYS ratio of 0.92 due to the almost random orientations. After extrusion, it is obvious that both strength and ductility improved synchronously. The dilute ZXM-300 exhibited an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), but a high yield asymmetry with CYS/TYS ratio of 0.54. With increasing extrusion temperature, tensile strength decreased gradually with significantly improved ductility and yield asymmetry. The dilute ZXM-350 showed better tensile strengtheductility combination (TYS of 164 MPa and elongation of 30.0%) with lower yield asymmetry (CYS/TYS ratio of 0.77) than AZ31 alloy (TYS of 154 MPa, elongation of 26.0% and CYS/TYS ratio
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Fig. 4. TEM observation results of the dilute ZXM alloy extruded at 300 C: (a, b) TEM BF images, (b) HAADF image of the rectangular region shown in (a), (deg) corresponding EDX elemental mappings of the region shown in (c), and (h) corresponding SAED pattern of the Ca containing spherical precipitate taken from the beam direction of [11e20]Mg.
Fig. 5. (0001) pole figures of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys.
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Fig. 6. Inverse pole figures of the as-extruded (aec) dilute ZXM and (d) AZ31 alloys. Inverse pole figures refer to the ED.
Fig. 7. (a) Tensile and (b) compressive stressestrain curves of the as-homogenized and as-extruded ZXM and AZ31 alloys, and (c) summary view of the mechanical properties, including the values of TYS, CYS, UTS, elongation and CYS/TYS (TYS/CYS).
of 0.68) extruded at the same temperature. In this study, the high performance of the dilute ZXM alloy after extrusion can be mainly attributed to the refined grains from HallePetch relation and basal texture through its effect on the activation of slip system. It is evident that grain refinement resulted in an obvious increase of both TYS and CYS. Since basal slip and {10e12} extension twin are dominant deformation modes in Mg
alloys at room temperature, the average Schmid factors for (0001) <11e20 > basal slip when the tensile stress is along the ED and for (10e12)<10e1e1> extension twin when the compressive stress is along the ED were analyzed by EBSD, as presented in Fig. 8. As mentioned above, the dilute ZXM-300 exhibited an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), which can be ascribed to the fine DRXed grains of ~2.3 mm
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Fig. 8. Schmid factor distribution histograms of the as-extruded dilute ZXM and AZ31 alloys: (a) (0001)<11e20 > basal slip when the tensile stress is along the ED, and (b) (10e12) <10e1e1> extension twin when the compressive stress is along the ED.
and strongly textured coarse unDRXed grains with extremely low Schmid factor for basal slip (0.06). The average Schmid factor for basal slip along the ED was only 0.16 (Fig. 8a), suggesting the difficulty of basal slip activation. Besides, {10e12} extension twinning is not favored due to the strongly textured grain orientations. In this case, prismatic slip of dislocation with high critical resolved shear stress (CRSS, 39.2 MPa [31]) is deduced to govern the macroscopic yield at room temperature [32,33], thereby leading to high TYS. On the other hand, the average Schmid factor for extension twin was 0.44 when compressive deformation is along the ED (Fig. 8b), presenting a severe concave-shaped compressive curve (Fig. 7b), which is a typical feature of {10e12} extension twinning dominated deformation [34,35]. Thus due to the extensive CRSS differences between prismatic slip and {10e12} extension twin (CRSS, 2.0e2.8 MPa [31]), quite high yield asymmetry of the dilute ZXM-300 appeared during tension and compression along the ED. With increasing extrusion temperature, it can be seen in Fig. 8 that the average Schmid factor for basal slip gradually increased from 0.16 to 0.27 with the decreased average Schmid factor for extension twin from 0.44 to 0.35 due to the formation of weakened basal texture with RE texture component, giving rise to the improved ductility and yield asymmetry. Compared to the AZ31 alloy extruded at the same temperature, the dilute ZXM-350 exhibited higher Schmid factor for basal slip and lower Schmid factor for extension twin and finer DRXed grain size of ~7.3 mm, consequently resulting in a better tensile strengtheductility combination and lower yield asymmetry with CYS/TYS ratio of 0.77. As for the dilute ZXM-400, it had the same DRXed grain size of ~16.2 mm with AZ31 alloy extruded at 350 C (Fig. 3), but exhibited much lower TYS and higher elongation. This is because weaker basal texture with RE texture component of the dilute ZXM-400 resulted in a higher Schmid factor for basal slip (0.27). Besides, it is known that solid solution strengthening is strongly dependent on atomic solute concentration. In this case, dilute ZXM alloy contained much lower alloy content (0.32 at%) than AZ31 alloy (3.30 at%) and many Mg2Ca and a-Mn phases, as described above, precipitated during extrusion, thereby leading to a deficiency of solid solution strengthening. This is probably another important reason for the low strength of the extruded ZXM alloy despite the same grain size. On the other hand, it is noteworthy that the formation of fine spherical Mg2Ca and a-Mn phases instead of large brittle particles was favorable for the ductility improvement [36]. Fig. 9 shows relationships between tensile yield strength and elongation, and between CYS/TYS ratio and tensile elongation of various extruded Mg alloys, including commercial Mg alloys [37e39], representative MgeZneCa system alloys [9e13,23,24,27,40e45], and other RE-free Mg alloys [46e49], as well as RE-containing alloys [1,2,50e54]. It can be seen that the
dilute ZXM-300 in this study shows an excellent balance of strength and ductility (YTS of 307 MPa and elongation of 20.6%), which is better than commercially available Mg alloys and even comparable to representative MgeZneCa system alloys with higher alloy content, such as Mge6.0Zne1.8Cae0.4Mn (TYS of 289 MPa and elongation of 16%) [27], Mge5.7Zne0.2Cae0.8Zr (TYS of 310 MPa and elongation of 18%) [42] and Mge6.1Zne0.40Age0.2Cae0.6Zr (TYS of 289 MPa and elongation of 17%) [44] (wt.%) alloys. Besides, the tensile ductility of the dilute ZXM-350 and ZXM-400 is found to be superior to that of other various Mg alloys, expect for Mge3.0Zne0.2Ca [10], Mge1.0Zne0.2Ca [13] and Mge1.0Zne0.5 Ca [12] (wt.%) alloys. On the other hand, the dilute ZXM-300 exhibits higher yield asymmetry (CYS/TYS ratio of 0.54) than other MgeZneCa series alloys with higher alloy content, such as Mge6.0Zne1.8Cae0.4Mn (CYS/ TYS ratio of 0.89) [27], Mge5.7Zne0.2Cae0.8Zr (CYS/TYS ratio of 0.84) [42] and Mge6.1Zne0.40Age0.2Cae0.6Zr (CYS/TYS ratio of 0.85) [44] (wt.%) alloys, which was mainly ascribed to low DRX fraction and limited number of fine precipitates resulting from the dilute alloy content. However, the yield asymmetry of dilute ZXM alloy extruded at higher temperature was significantly improved (CYS/TYS ratio of ~0.8) due to the weakened basal texture with RE texture component, which can be comparable to that of other extruded Mg alloys. Therefore, these preceding results point to the fact that newly developed dilute ZXM alloy in this study could provide a various combination of strength and ductility, as well as yield asymmetry, after extrusion at different temperatures, which could be a potential candidate as low-cost and low-density wrought Mg alloy for various demands in industrial applications. 4. Conclusions In summary, dilute Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. The dilute alloy extruded at 300 C exhibited a bimodal microstructure consisting of fine DRXed grains of ~2.3 mm and strongly textured coarse unDRXed grains, consequently resulting in an excellent balance of strength and ductility (TYS of 307 MPa and elongation of 20.6%), which is better than commercially available Mg alloys and even comparable to representative MgeZneCa system alloys with higher alloy content. After extrusion at 350 C, low yield asymmetry (CYS/TYS ratio of ~0.8) with TYS of 164 MPa and elongation of 30.0% was obtained due to the almost fully DRXed microstructure (DRX fraction of ~0.96) and significantly weakened basal texture (maximum intensity of 3.2) with RE texture component. Besides, fine spherical Mg2Ca and a-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth. Thus, the newly developed dilute
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M.G. Jiang et al. / Journal of Alloys and Compounds 668 (2016) 13e21
Fig. 9. Relationships (a) between tensile yield strength and elongation, and (b) between CYS/TYS ratio and tensile elongation of various extruded Mg alloys, including commercial Mg alloys [37e39], representative MgeZneCa system alloys [9e13,23,24,27,40e45], and other RE-free Mg alloys [46e49], as well as RE-containing alloys [1,2,50e54].
Mge0.21Zne0.30Cae0.14Mn (wt.%) alloy could be a potential candidate as low-cost and low-density wrought Mg alloy with excellent balance of strength and ductility, as well as low yield asymmetry.
[15]
[16]
Acknowledgments [17]
The authors gratefully acknowledge the financial supports from the State Key Program of National Natural Science of China (No. 51531002), National Basic Research Program of China (973 Program, No. 2013CB632202) and National Natural Science Foundation of China (NSFC, No. 51301173).
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