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Effect of microalloying with Ca on the microstructure and mechanical properties of Mg-6 mass%Zn alloys
Materials and Design 98 (2016) 285–293
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Effect of microalloying with Ca on the microstructure and mechanical properties of Mg-6 mass%Zn alloys Y.Z. Du a,b, X.G. Qiao a, M.Y. Zheng a,⁎, D.B. Wang c, K. Wu a, I.S. Golovin d a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, PR China Research and Technology Center, Maanshan Steel Co. LTD. Maanshan 243000, PR China d National University of Science and Technology “MISIS”, Leninsky ave. 4, 119049 Moscow, Russia b c
a r t i c l e
i n f o
Article history: Received 13 November 2015 Received in revised form 4 March 2016 Accepted 5 March 2016 Available online 8 March 2016 Keywords: Mg-Zn-Ca alloy Microalloying Microstructure Precipitates Mechanical properties
a b s t r a c t The Mg-6 mass%Zn alloys microalloyed with different amounts of Ca were cast and extruded. The second phase in the as-cast Mg-6Zn alloy is Mg4Zn7, which is replaced by Ca2Mg6Zn3 with an increase of Ca addition. Microalloying by Ca affects the grain size, dynamic recrystallization, and dynamic precipitation of Mg-6Zn alloys during extrusion. The Ca addition inhibits dynamic recrystallization and grain growth due to the pinning effect of fine precipitates, giving rise to fine dynamic recrystallized grains. A large amount of precipitates is observed in the Ca containing alloys after extrusion. Ca effectively improves strength of the as-extruded Mg-6Zn alloy, which is mainly attributed to refined dynamic recrystallized grains, dense precipitates, and deformed regions with high-density dislocations and strong basal texture. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction For decades, Mg alloys have attracted much interest of engineers and scientists owing to their lightweight. However, applications of Mg alloys are still limited due to their higher cost or lower strength compared to some other alloys, e.g., Al alloys. Many attempts have been made to overcome these disadvantages by means of microalloying, i.e, through trace addition of alloying elements, such as Ca, Zr, Mn, and Si, to Mg alloys. In these elements, Ca appears to be very attractive as it is capable of refining microstructure [1], improving heat-resistant properties [2], reducing flammability [3] and increasing strength of Mg alloys in combination with proper thermomechanical treatments [4–11]. Ca has been added to several alloys systems, such as Mg-Zn-Sn-Al [4], AZ31 [8], Mg-Zn [5], AM60 [9], Mg-Mn [10] and Mg-Al [11]. Among these Mg based alloys, Mg-Zn based alloys have good potential to be high strength wrought magnesium alloys [12] due to their significant solution [13] and precipitation strengthening [14]. Great interests for Mg-Zn-Ca alloys were originally motivated by their superior creep resistant properties [2] due to thermally stable Ca2Mg6Zn3 intermetallic particles [15] and strong ageing response [16–19] caused by the formation of the G.P. zone [17]. Subsequent studies have found that Ca additions in Mg-Zn alloys significantly refine the ⁎ Corresponding author at: School of Materials Science and Engineering, P.O.Box 433, Harbin Institute of Technology, Harbin 150001, PR China. E-mail address: [email protected] (M.Y. Zheng).
http://dx.doi.org/10.1016/j.matdes.2016.03.025 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
microstructure, weaken the basal texture and improve the formability [20–23]. Since Zn and Ca are abundant Earth recourses, they are inexpensive materials, which promotes investigation and commercialization of wrought Mg-Zn-Ca alloys [24,25]. As Mg-Zn-Ca alloys are also considered to be promising low-cost high-strength Mg alloys, many Mg-Zn-Ca based alloys have already been developed, e.g., Mg-6Zn0.2Ca-0.8Zr (all compositions given hereafter relate to weight percentage) [26], Mg-6.15Zn-0.16Ca-0.42Ag-0.57Zr [27], Mg-3Zn-0.25Ca0.5Zr-0.15Mn [28] and Mg-3Zn-0.25Ca-0.5Ag-0.15Mn-0.5Zr [29,30]. The investigation of wrought Mg-Zn-Ca alloys is of great importance for the development of novel low-cost high-strength Mg alloys. Current researches on wrought Mg-Zn-Ca alloys focus on microstructure evolution and properties improvement of Mg-Zn-Ca alloys with specific Ca content through optimizing thermomechanical processing. For example, ultrafine-grained microstructure with the average grain size of 0.7 μm was achieved in a Mg-5.12Zn-0.32Ca alloy through extrusion and equal channel angular pressing (ECAP) [24]; high-strength Mg-4.7Zn-0.5Ca fabricated by hot extrusion at 250 °C exhibited yield strength of 291 MPa and ultimate tensile strength of 329 MPa [31]. Ca content was found to be a significant factor to improve mechanical properties of Mg-Zn-Ca alloys [5,31–33]. However, investigations of mechanisms behind this influence are still rather limited [5]. The objective of the present paper is to investigate the effects of the Ca content at micro-level on the microstructure and the mechanical properties of Mg-Zn alloys.
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2. Experimental procedures Commercially pure Mg, commercially pure Zn, and the Mg-15% Ca master alloys were molten at 750 °C in an electric resistance furnace under a mixed atmosphere of CO2 and SF6. The melts were hold at 720 °C for 10 min, and then poured into a steel mold for solidification. Three alloys were fabricated and they are denoted as Z6, ZX604 and ZX608, respectively. Their chemical compositions are measured with an X-ray spectrometer and listed in Table 1. Rods of 60 mm in diameter and 40 mm in length were cut from the casting ingots and homogenized at 350 °C for 12 h. Afterwards the rods were extruded at 300 °C at the extrusion ratio of 12:1 at ram speed of 5 mm/s. Samples for optical microscopy (OM) and scanning electron microscopy (SEM) observations were ground, mechanically polished and etched in a solution of acetic picral. OM and SEM observations were carried out using an Olympus DP11 optical microscope and a Quatan 200 scanning electron microscope at an operating voltage of 30 kV. Thin foil specimens for transmission electron microscopy (TEM) observations were prepared using a standard combination of mechanical thinning and ion-beam milling techniques. TEM analyses were performed on a Tecnai G2F30 transmission electron microscope operating at 300 kV. The texture of the extruded alloy was characterized by a X'PERT PRO MPD X-ray diffractometer. The (0002), (10 − 10), (10 − 11) and (10 − 12) pole figures were obtained and the texture data were analyzed using PANalytical X'Pert Texture. Micro-texture measurement was carried out on SEM equipped with a TSL-EBSD system. The data acquired with confidence index larger than 0.1 were used to calculate pole figures using OMI Analysis software. All EBSD were carried out at 20 kV, and the step sizes of the as-extruded Z6, ZX604 and ZX608 alloys for EBSD tests are 0.8 μm, 0.5 μm and 0.2 μm, depending on the sizes of DRXed grains. Specimens for EBSD were electro-polished in a solution of 37.5% orthophosphoric acid and 62.5% ethanol at an electric current of 0.1–0.5 A for 10–60 s at a temperature lower than 20 °C. Tensile tests were performed at ambient temperature using an Instron 5569 tensile machine at a crosshead speed of 1 mm/min with a gauge length of 15 mm and a cross sectional area of 6 mm × 2 mm.
Fig. 1. The XRD curves of the as-cast alloys.
alloy exhibits the morphology of a typical divorced eutectic microstructure (Fig. 3a) as the Zn content of Z6 is very close to the maximum solubility [34] of Zn in Mg. The energy-dispersive spectroscopy (EDS) analysis indicates that the second phase contains Mg and Zn (Table 2), thus the second phase is identified as Mg4Zn7 according to the XRD result (Fig. 1). With the addition of 0.4% Ca, the ZX604 alloy shows the divorced eutectic microstructure as well as the lamellar eutectic microstructure at triple junctions of grain boundaries (Fig. 3b). Addition of 0.8% Ca leads to a more lamellar eutectic microstructure formed in the ZX608 (Fig. 3c and d). The EDS results reveal that both the divorced microstructure and the lamellar microstructure contain Ca (Table 2). The second phases in the lamellar microstructure and the divorced microstructure are, according to the XRD results, identified as Ca2Mg6Zn3 and Mg4Zn7 containing Ca (see Section 4.1 for more details), respectively. 3.2. Microstructure of the as-extruded alloys
3. Results 3.1. Microstructure of as-cast alloys Fig. 1 shows the X-ray diffraction (XRD) curves of the three Mg alloys in as-cast condition. The Z6 alloy contains Mg4Zn7 phase and ZX608 alloy contains Ca2Mg6Zn3. The ZX604 alloy with medium addition of Ca contains both Mg4Zn7 and Ca2Mg6Zn3 phase although their peaks are weak. Fig. 2 shows the backscattered electron (BSE) micrographs of Z6, ZX604 and ZX608 alloys in as-cast condition. The amount of the second phase increases with an increase of the Ca content. The distribution of the second phase differs among these alloys. The second phase is discontinuously distributed in the α-Mg matrix of the Z6 alloy (Fig. 2a) and it tends to distribute semi-continuously in the ZX604 alloy (Fig. 2b) and forms a continuous network in the ZX608 alloy (Fig. 2c), respectively. High-magnification SEM micrographs in Fig. 3 show that the morphology of the second phases also varies significantly among three Mg based alloys containing various Ca content. The second phase in Z6 Table 1 Chemical compositions of Mg-Zn-Ca alloys (wt.%). Alloys
Zn
Ca
Mg
Z6 ZX604 ZX608
5.74 6.01 6.02
– 0.36 0.82
Balance Balance Balance
Fig. 4 shows the optical microstructure of three alloys in the extruded condition. The as-extruded Z6 alloy exhibits a fully recrystallized microstructure with second phases oriented along the extrusion direction. The extruded ZX604 and ZX608 alloys exhibit a bimodal microstructure containing both the recrystallized regions and the deformed regions. The areas of the deformed regions increase with an increase of the Ca content, indicating that addition of Ca in the Mg-Zn alloy postpones or inhibits dynamic recrystallization (DRX) during extrusion. The area fractions of the deformed regions of the extruded ZX604 and ZX608 alloys are 4% and 13%, respectively. The undissolved second phases during homogenization treatment are also oriented towards the extrusion direction. The grain sizes of the recrystallized regions of Z6, ZX604 and ZX608 alloy are 14.3 ± 2.1 μm, 8.1 ± 1.4 μm and 3 ± 0.5 μm, respectively. Fig. 5 shows TEM microstructures of the as-extruded alloys. No obvious precipitates are observed in the as-extruded Z6 alloy (Fig. 5a). Fine spherical precipitates as well as the fragmented Ca2Mg6Zn3 (Fig. 5b) particles are observed in the as-extruded ZX604 alloy, which indicates that addition of Ca promotes dynamic precipitation. The fragmented particles at grain boundaries are identified as Ca2Mg6Zn3 by selected area electron diffraction (SAED) (Fig. 6). More precipitates are observed in the matrix as well as at grain boundaries of the as-extruded ZX608 alloy (Fig. 5c and d). The mean diameter of the precipitates of the asextruded ZX604 and ZX608 alloys is measured to be 30 ± 10 nm. The precipitates in the as-extruded ZX608 alloy exhibit both spherical shape and rod-like shape (Fig. 7a). The spherical precipitates at grain boundaries are identified as Ca2Mg6Zn3 by SAED (Fig. 7b) whereas
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Fig. 2. Backscattered electron (BSE) micrographs of the as-cast alloys: (a) Z6, (b) ZX604 and (c) ZX608.
the rod-like precipitates are indexed to be Mg4Zn7 (Fig. 7c). Energy dispersive X-ray spectroscopy (EDX) results reveal that the rod-like precipitates contain small amounts of Ca, which indicates that Ca plays an key role in the dynamic precipitation. The fine spherical precipitates at
grain interiors are identified to be MgZn2 by high-resolution transmission electron microscopy (HRTEM) (Fig. 7e). Fig. 8 shows (0001) pole figures of the as-extruded Mg alloys measured by XRD. The basal poles of the as-extruded Mg-Zn-Ca alloys are
Fig. 3. SEM micrographs of the as-cast alloys: (a) Z6, (b) ZX604, (c) and (d) ZX608.
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4. Discussion
Table 2 EDS results of the as-cast Mg-Zn-Ca alloys (at%). Points
A (Fig. 3a) B (Fig. 3b) C (Fig. 3b) D (Fig. 3d)
Elements Mg
Zn
Ca
74.5 91.3 74.0 72.3
25.5 6.8 19.5 19.3
1.9 6.5 8.4
perpendicular to the extrusion direction, which is known to be a typical texture of Mg alloys after extrusion [35]. The Ca-contained ZX604 and ZX608 alloys exhibit a stronger intensity of basal texture than the binary Z6 alloy. Fig. 9 shows the EBSD maps and corresponding (0001) pole figures of the as-extruded alloys in the DRXed region and the unDRXed region. It can be clearly seen that the unDRXed region in the as-extruded ZX604 and ZX608 alloys exhibits stronger texture intensity than the DRXed region. The DRXed regions of the three alloys also show basal texture. However, Ca containing alloys exhibit weaker texture intensity in the DRXed region compared to the as-extruded Z6 alloys, indicating that Ca addition weakens the basal texture of the recrystallized Mg-Zn-Ca alloys, which is consistent with the previous reports [21,22]. 3.3. Mechanical properties Fig. 10 shows the mechanical properties of the Mg-6Zn-xCa samples. The ultimate tensile stress (UTS), yield stress (YS), elongation-to-failure are summarized in Table 3. The YS, UTS, and the elongation-to-failure of the extruded Z6 are 125 MPa, 276 MPa and 29.7%, respectively. The strength increases while the elongation decreases with the addition of Ca. The YS increases to 169 MPa and 230 MPa for the ZX604 and ZX608 alloys, respectively. The elongation-to-fracture decreases to 21.4% and 15.3% for the as-extruded ZX604 and ZX608 alloys, respectively.
The microstructure and the mechanical properties of Z6, ZX604 and ZX608 are very different in both as-cast and as-extruded condition, although they contain almost the same amount of Zn and subjected to the same thermal processing. These differences are ascribed to the addition of Ca. Trace addition of Ca to the Mg-6Zn alloy causes differences in the formation of intermetallics during casting and extrusion in terms of precipitation, recrystallization, and texture evolution during extrusion, which further causes improved mechanical properties. The effect of addition of Ca is discussed in details as follows. 4.1. Intermetallic particles in as-cast Mg-Zn-Ca alloys A variety of intermetallic phases was reported in Mg-Zn binary alloys system [34,36–40], such as Mg7Zn3, MgZn, Mg4Zn7, MgZn2, Mg7Zn3 and Mg2Zn11. However, the phases in the Mg-Zn binary alloys are still controversial. Mg7Zn3 distributing in the inter-dendrites region and Mg4Zn7 with a globular shape within α-Mg dendrites were observed in the as-cast Mg-8Zn alloy [37]. Presence of Mg4Zn7 and MgZn2 particles were confirmed in the Mg-8Zn alloy during ageing at 200 °C [38]. In this study, Mg4Zn7 phase is detected in the as-cast Z6 alloy by XRD analysis, which was also previously observed in the ascast Mg-4Zn alloy [41]. Addition of Ca to the Mg-6Zn alloy causes a gradual replacement of the Mg-Zn binary compound with Ca2Mg6Zn3 ternary compound in the ZX604 and ZX608 alloys. The second phases in the Mg-Zn-Ca alloy are commonly Mg2Ca or Ca2Mg6Zn3, which depends on the atomic ratio of Zn/Ca. Ca2Mg6Zn3 predominates when the Zn/Ca atomic ratio is higher than 1.23 [42]. Mg2Ca was not detected in the present Ca containing alloys since the atomic ratio of Zn/Ca of ZX604 and ZX608 is much higher than the critical value of 1.23 [42]. The eutectic morphology of Mg alloys during solidification may be fibrous/lamellar or fully or partially divorced [43]. The Z6 alloy exhibits partially divorced eutectic morphologies, whereas the ZX604 and ZX608 alloys show eutectic characteristics in the as-cast condition. The formation of fully or partially divorced eutectic morphologies is
Fig. 4. Optical images of the as-extruded alloys: (a) Z6, (b) ZX604 and (c) ZX608.
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Fig. 5. The bright field TEM images of the as-extruded alloys: (a) Z6, (b) ZX604, (c) and (d) ZX608.
typical for Mg-Al or Mg-Zn binary alloys, e.g. Mg-9Al alloy [44] and Mg9Zn alloy [45], which is because large amount of primary α-Mg restricts the eutectic reaction to a very small and isolated interdendritic region, and it further results in the solidification of the eutectic liquid outside coupled eutectic growth [43]. The morphology of the eutectic microstructure is lamellar or rod-like shaped in Ca containing alloys, which was also observed in the as-cast Mg-4.5Zn-1.13Ca alloy [32]. Addition of Ca causes intensive constitutional undercooling ahead of solid/liquid interface in the liquid layer [46], promoting the primary α-Mg phase solidification and result in the increase of Ca and Zn concentration in the liquid. As a result, the composition ahead of the solid/liquid interface
reaches the eutectic composition and falls within a zone of the coupled eutectic growth. Therefore, addition of Ca promotes to form a lamellar morphology of Ca2Mg6Zn3 and α-Mg. 4.2. Effects of Ca addition on microstructure of as-extruded Mg-Zn alloys A large volume fraction of fine precipitates including Ca2Mg6Zn3 and Mg4Zn7 are detected in Ca containing alloys (Fig. 5b and d) after extrusion, though such densely-dispersed precipitates are not observed in the as-extruded Z6 alloy (Fig. 5a), indicating that Ca promotes dynamic precipitation of Z6 alloy during extrusion.
Fig. 6. TEM analysis of the as-extruded ZX604 alloy: (a) bright field image and (b) SAED of the second phase A in (a).
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Fig. 7. TEM analysis of the as-extruded ZX608 alloy: (a) bright field image, SAED of second phases arrowed as (b) A and (c) B in (a), (d) EDX of the second phase arrowed as B in (a), (e) HRTEM of the fine spherical precipitates arrowed as C in (a).
The alloys were homogenized at 350 °C for 12 h before extrusion. The second phase of Mg4Zn7 was almost dissolved into the matrix, while Ca2Mg6Zn3 was partially dissolved. As a result, the ashomogenized Ca containing alloys exhibits supersaturation of Ca and Zn in the matrix, whereas the Z6 alloy is supersaturated with Zn. Therefore, dynamic precipitation in the Mg-Zn based alloys is possible during hot extrusion or subsequent cooling. However, no precipitates were detected in the as-extruded Z6 alloy but large amount of precipitates were observed in the Ca containing alloys. The dynamic precipitation during extrusion in Mg-6Zn base alloys depends on the microalloying elements as well as extrusion temperatures [26,47,48]. A large number of fine
precipitates were observed in a Mg-6Zn alloy when extruded at lower temperature (210 °C) [47], but not detected when extruded at a higher temperature (350 °C) [48]. Such phenomenon is related to the poor thermal stability of the Mg-Zn binary compounds [37]. Microalloying elements addition in Mg alloys, such as Zr, Mn and Ag, were used to promote dynamic precipitation [27,48,49]. Ca and Zn are likely to form CaZn clusters due to larger negative enthalpy of mixing between Ca-Zn than those of Mg-Zn and Mg-Ca [16]. Such segregation is energetically favorable for nucleation of dynamic precipitation during deformation. Ca is detected in the fine precipitates Mg4Zn7 by EDX (Fig. 9d), suggesting that Ca plays a critical role in the formation of the precipitates during
Fig. 8. Pole figures of the extruded alloys: (a) Z6, (b) ZX604 and (c) ZX608.
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Fig. 9. The EBSD maps and corresponding (0001) pole figures of DRXed and unDRXed regions of the as-extruded alloys.
extrusion. Additionally, ZX604 and ZX608 alloys contain undissolved bulk Ca2Mg6Zn3 phase, which results in the heterogeneously deformation and further causes increased amount of crystal defects during hot extrusion. Such crystal defects provide energetically favorable nucleation sites for the precipitates. The existence of Ca in the precipitates might improve the thermal stabilities and increase the temperature of the dynamic precipitation. Therefore, Ca addition results in the dynamic precipitation of Mg-6Zn alloy during extrusion at 300 °C. Fine Ca2Mg6Zn3 precipitates were observed at grain boundaries in the ZX604 and ZX608 alloys. The previous investigations suggested that Gd with larger atomic radius compared with Mg is segregated to grain boundaries [50] or twinning boundaries [51]. It was also confirmed that Ca segregated to grain boundaries in Mg-0.3Zn-0.1Ca alloy [52]. Therefore, it is expected that Ca was preferentially distributed to grain boundaries in the Ca containing alloys in the present study. As a result, fine Ca2Mg6Zn3 is preferred to precipitate around grain boundaries during extrusion for the Ca containing alloys. Such fine Ca2Mg6Zn3 dispersed at grain boundaries was also reported in the as-extruded Mg-
5.99Zn-1.76Ca-0.35Mn [33], Mg-3Zn-0.5Ag-0.25Ca-0.15Mn [30] and Mg-5Zn-0.25Ca-0.3Zr [28] alloys. Ca addition into Mg-6Zn alloy leads to grain refinement of dynamic recrystallization and retention of deformed region after extrusion, as shown in Fig. 4. This indicates that Ca addition retards and postpones recrystallization and grain growth of the Mg-6Zn alloy. The second phase strongly influences the microstructure evolution during deformation, depending on the size and the distribution of the second phase. Generally, coarse particles result in deformation zone and promote nucleation of recrystallization, known as particle stimulated nucleation (PSN) [53], whereas fine precipitates inhibit dynamic recrystallization during deformation due to their pinning effect on the dislocation mobility [54–56]. A large amount of coarse Ca2Mg6Zn3 phases are not dissolved into the matrix during homogenization (Fig. 4b and c) and coarse second phases surrounded by fine DRXed grains are observed in Fig. 4b and c, indicating that the coarse Ca2Mg6Zn3 particles stimulate dynamic recrystallization during extrusion. Meanwhile, the undissolved Ca2Mg6Zn3 is fragmented during extrusion and distributed around grain boundaries (Fig. 6a), which hinders grain growth. In addition, fine Ca2Mg6Zn3 particles precipitating along grain boundaries during extrusion (Fig. 7b) effectively inhibit grain growth. Both these two factors are beneficial for grain refinement. The amount of Ca2Mg6Zn3 is proportional to the Ca content in Mg-Zn alloys. Therefore, the DRXed grain is refined with an increase of Ca addition for the alloys with a high level of Ca contents. The deformed region retained in the Ca containing alloys might be related to the pinning effects from fine precipitates and solute segregation to grain boundaries. Dislocation walls were observed in the unDRXed region for the as-extruded ZX608 alloy (Fig. 11). Meanwhile, fine precipitates were also observed (Fig. 11), which would inhibit the mobility of dislocations. Consequently, the recrystallization process relying on the rearrangement of dislocations was prohibited. Additionally, Ca solute segregation to grain boundaries was expected to provide a
Table 3 Tensile properties of the as-extruded Mg-Zn-Ca alloys.
Fig. 10. Typical stress-strain curves of the as-extruded Mg-Zn-Ca alloys.
alloys
YS (MPa)
UTS (MPa)
Elongation to failure (%)
Z6 ZX604 ZX608
125 ± 5 169 ± 4 230 ± 8
276 ± 1 276 ± 3 304 ± 1
29.7 ± 2.7 21.4 ± 1.1 15.3 ± 1.6
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Fig. 11. The bight field TEM image in the deformed region of the as-extruded ZX608 alloy showing the dislocations.
with a similar size. The volume fractions of the precipitates in the asextruded ZX608 alloy was higher than that of ZX604 alloy, indicating that the contribution of precipitates strengthening was increased with an increase of Ca addition. Dislocation strengthening was determined by dislocation density [62]. The dislocation was consumed by dynamic recrystallization, resulting in low dislocation density in the DRXed region. While the deformed regions contain high-density dislocations due to the existence of large strain. The deformed region was increased with an increase of Ca for the as-extruded Mg-Zn-Ca alloys, which was beneficial for further improvement of strength. Additionally, the texture affects the strength significantly in Mg alloys [63]. The basal texture with a basal plane parallel to extrusion direction is always observed in Mg alloys after extrusion, which limited the activity of the basal slip when tensile along extrusion direction, thus increasing the yield strength. For the as-extruded Mg-Zn-Ca alloy, Ca containing alloys showed stronger basal texture intensity, which should be another contributor for the strength improvement. 5. Conclusions
strong drag force suppressing DRX, which would further prohibit the DRX process. Ca containing alloys exhibited stronger texture intensity after extrusion compared to the as-extruded Z6 alloy (Fig. 9). From the microstructure observation, it can be seen that Ca-based alloys contained deformed regions, which always exhibited strong basal texture [57]. However, the texture intensity of the recrystallization region in Ca containing alloys was weaker compared with the as-extruded Z6 alloy. It indicated that Ca addition weakened the recrystallization texture. The texture weakening effect in Ca containing Mg alloys are commonly observed. However, the weakening mechanisms are still unclear. It is believed that the weakening of recrystallization texture after Ca addition in the present study was related to particle stimulated nucleation (PSN) and pinning effects from fine particles or solute segregation to grain boundaries. The undissolved Ca2Mg6Zn3 phase during homogenization stimulated the nucleation of dynamic recrystallization during extrusion (Figs. 2e and 3a), which could randomize the orientation of dynamic recrystallized grains [58]. Particle pinning to grain boundaries was suggested to give rise to texture weakening in AZ31-xCa (x = 0.4 and 0.8 wt%) alloys [59]. Therefore, the fine precipitates in Ca containing alloys might be another factor weakening the recrystallization texture. Additionally, Ca segregation to grain boundaries could effectively prohibit the mobility of high energy grain boundaries [52], affecting the preferred grain growth and resulting in the weakening of recrystallization texture. Though the Ca segregation to grain boundaries was not observed directly in the present study, it is still expected that Ca segregation should be of significance to weaken the texture after recrystallization. 4.3. Strengthening mechanisms Ca addition effectively improves the strength of Mg-Zn alloy after extrusion, but decreases the ductility (Fig. 10). Compared with the asextruded Z6 alloy, the as-extruded Mg-Zn-Ca ternary alloys exhibited refined microstructure, fine dynamic precipitates, and elongated deformed region. Therefore, fine grains in the DRXed region, a large number of fine precipitates, and high dislocation density in the deformed region should be contributed to the strength improvement. The strength improvement due to grain refinement was increased with a decrease of grain size according to the Hall-Petch relationship [60]. The average DRXed grain size of the as-extruded Z6 alloy was 14.3 μm, while the grain size was decreased to be 8.1 μm and 3 μm for the as-extruded ZX604 and ZX608 alloys, respectively. Therefore, the grain refinement contribution to the strength would increase with an increase of Ca addition. Precipitation strengthening was related to the size, distribution, morphology and the density of precipitates [61]. The as-extruded ZX604 and ZX608 alloys contained spherical precipitates
Mg-6Zn alloys microalloyed with different content of calcium were cast and extruded, and the microstructure and mechanical properties of the alloys were investigated. The main conclusions are as following: 1. The second phase in the as-cast Mg-6Zn alloy is divorced Mg4Zn7, which is gradually replaced by lamellar Ca2Mg6Zn3 phase distributing around grain boundaries with the increase of Ca addition. 2. Ca addition stabilizes the precipitates and gives rise to much more defects during extrusion, promoting dynamic precipitation. 3. The densely distributed precipitates in Ca containing alloys postpones dynamic recrystallization and restricts grain growth, resulting in a bimodal microstructure with fine DRXed grains and deformed region. 4. The yield strength is improved by increasing Ca content, attributing to fine DRXed grains, high density precipitates and deformed region with high-density dislocation and strong basal texture. Acknowledgements This study was supported by National Key Basic Research Program of China (No. 2013CB632200), National Natural Science Foundation of China (No. 51271063 and 51571068) and MISiS K3 Project (No. К3-2015-005). XGQ thanks funding from the Fundamental Research Funds for the Central Universities under Grant No. HIT. NSRIF.2015003, the General Financial Grant from the China Postdoctoral Science Foundation under Grant No 2013M531034, Specialized Research Fund for the Doctoral Program of Higher Education under Grant No. 20132302120001, Hei Long Jiang Postdoctoral Foundation (No. LBH-Z13088). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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Effect of microalloying with Ca on the microstructure and mechanical properties of Mg-6 mass%Zn alloys Y.Z. Du a,b, X.G. Qiao a, M.Y. Zheng a,⁎, D.B. Wang c, K. Wu a, I.S. Golovin d a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, PR China Research and Technology Center, Maanshan Steel Co. LTD. Maanshan 243000, PR China d National University of Science and Technology “MISIS”, Leninsky ave. 4, 119049 Moscow, Russia b c
a r t i c l e
i n f o
Article history: Received 13 November 2015 Received in revised form 4 March 2016 Accepted 5 March 2016 Available online 8 March 2016 Keywords: Mg-Zn-Ca alloy Microalloying Microstructure Precipitates Mechanical properties
a b s t r a c t The Mg-6 mass%Zn alloys microalloyed with different amounts of Ca were cast and extruded. The second phase in the as-cast Mg-6Zn alloy is Mg4Zn7, which is replaced by Ca2Mg6Zn3 with an increase of Ca addition. Microalloying by Ca affects the grain size, dynamic recrystallization, and dynamic precipitation of Mg-6Zn alloys during extrusion. The Ca addition inhibits dynamic recrystallization and grain growth due to the pinning effect of fine precipitates, giving rise to fine dynamic recrystallized grains. A large amount of precipitates is observed in the Ca containing alloys after extrusion. Ca effectively improves strength of the as-extruded Mg-6Zn alloy, which is mainly attributed to refined dynamic recrystallized grains, dense precipitates, and deformed regions with high-density dislocations and strong basal texture. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction For decades, Mg alloys have attracted much interest of engineers and scientists owing to their lightweight. However, applications of Mg alloys are still limited due to their higher cost or lower strength compared to some other alloys, e.g., Al alloys. Many attempts have been made to overcome these disadvantages by means of microalloying, i.e, through trace addition of alloying elements, such as Ca, Zr, Mn, and Si, to Mg alloys. In these elements, Ca appears to be very attractive as it is capable of refining microstructure [1], improving heat-resistant properties [2], reducing flammability [3] and increasing strength of Mg alloys in combination with proper thermomechanical treatments [4–11]. Ca has been added to several alloys systems, such as Mg-Zn-Sn-Al [4], AZ31 [8], Mg-Zn [5], AM60 [9], Mg-Mn [10] and Mg-Al [11]. Among these Mg based alloys, Mg-Zn based alloys have good potential to be high strength wrought magnesium alloys [12] due to their significant solution [13] and precipitation strengthening [14]. Great interests for Mg-Zn-Ca alloys were originally motivated by their superior creep resistant properties [2] due to thermally stable Ca2Mg6Zn3 intermetallic particles [15] and strong ageing response [16–19] caused by the formation of the G.P. zone [17]. Subsequent studies have found that Ca additions in Mg-Zn alloys significantly refine the ⁎ Corresponding author at: School of Materials Science and Engineering, P.O.Box 433, Harbin Institute of Technology, Harbin 150001, PR China. E-mail address: [email protected] (M.Y. Zheng).
http://dx.doi.org/10.1016/j.matdes.2016.03.025 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
microstructure, weaken the basal texture and improve the formability [20–23]. Since Zn and Ca are abundant Earth recourses, they are inexpensive materials, which promotes investigation and commercialization of wrought Mg-Zn-Ca alloys [24,25]. As Mg-Zn-Ca alloys are also considered to be promising low-cost high-strength Mg alloys, many Mg-Zn-Ca based alloys have already been developed, e.g., Mg-6Zn0.2Ca-0.8Zr (all compositions given hereafter relate to weight percentage) [26], Mg-6.15Zn-0.16Ca-0.42Ag-0.57Zr [27], Mg-3Zn-0.25Ca0.5Zr-0.15Mn [28] and Mg-3Zn-0.25Ca-0.5Ag-0.15Mn-0.5Zr [29,30]. The investigation of wrought Mg-Zn-Ca alloys is of great importance for the development of novel low-cost high-strength Mg alloys. Current researches on wrought Mg-Zn-Ca alloys focus on microstructure evolution and properties improvement of Mg-Zn-Ca alloys with specific Ca content through optimizing thermomechanical processing. For example, ultrafine-grained microstructure with the average grain size of 0.7 μm was achieved in a Mg-5.12Zn-0.32Ca alloy through extrusion and equal channel angular pressing (ECAP) [24]; high-strength Mg-4.7Zn-0.5Ca fabricated by hot extrusion at 250 °C exhibited yield strength of 291 MPa and ultimate tensile strength of 329 MPa [31]. Ca content was found to be a significant factor to improve mechanical properties of Mg-Zn-Ca alloys [5,31–33]. However, investigations of mechanisms behind this influence are still rather limited [5]. The objective of the present paper is to investigate the effects of the Ca content at micro-level on the microstructure and the mechanical properties of Mg-Zn alloys.
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2. Experimental procedures Commercially pure Mg, commercially pure Zn, and the Mg-15% Ca master alloys were molten at 750 °C in an electric resistance furnace under a mixed atmosphere of CO2 and SF6. The melts were hold at 720 °C for 10 min, and then poured into a steel mold for solidification. Three alloys were fabricated and they are denoted as Z6, ZX604 and ZX608, respectively. Their chemical compositions are measured with an X-ray spectrometer and listed in Table 1. Rods of 60 mm in diameter and 40 mm in length were cut from the casting ingots and homogenized at 350 °C for 12 h. Afterwards the rods were extruded at 300 °C at the extrusion ratio of 12:1 at ram speed of 5 mm/s. Samples for optical microscopy (OM) and scanning electron microscopy (SEM) observations were ground, mechanically polished and etched in a solution of acetic picral. OM and SEM observations were carried out using an Olympus DP11 optical microscope and a Quatan 200 scanning electron microscope at an operating voltage of 30 kV. Thin foil specimens for transmission electron microscopy (TEM) observations were prepared using a standard combination of mechanical thinning and ion-beam milling techniques. TEM analyses were performed on a Tecnai G2F30 transmission electron microscope operating at 300 kV. The texture of the extruded alloy was characterized by a X'PERT PRO MPD X-ray diffractometer. The (0002), (10 − 10), (10 − 11) and (10 − 12) pole figures were obtained and the texture data were analyzed using PANalytical X'Pert Texture. Micro-texture measurement was carried out on SEM equipped with a TSL-EBSD system. The data acquired with confidence index larger than 0.1 were used to calculate pole figures using OMI Analysis software. All EBSD were carried out at 20 kV, and the step sizes of the as-extruded Z6, ZX604 and ZX608 alloys for EBSD tests are 0.8 μm, 0.5 μm and 0.2 μm, depending on the sizes of DRXed grains. Specimens for EBSD were electro-polished in a solution of 37.5% orthophosphoric acid and 62.5% ethanol at an electric current of 0.1–0.5 A for 10–60 s at a temperature lower than 20 °C. Tensile tests were performed at ambient temperature using an Instron 5569 tensile machine at a crosshead speed of 1 mm/min with a gauge length of 15 mm and a cross sectional area of 6 mm × 2 mm.
Fig. 1. The XRD curves of the as-cast alloys.
alloy exhibits the morphology of a typical divorced eutectic microstructure (Fig. 3a) as the Zn content of Z6 is very close to the maximum solubility [34] of Zn in Mg. The energy-dispersive spectroscopy (EDS) analysis indicates that the second phase contains Mg and Zn (Table 2), thus the second phase is identified as Mg4Zn7 according to the XRD result (Fig. 1). With the addition of 0.4% Ca, the ZX604 alloy shows the divorced eutectic microstructure as well as the lamellar eutectic microstructure at triple junctions of grain boundaries (Fig. 3b). Addition of 0.8% Ca leads to a more lamellar eutectic microstructure formed in the ZX608 (Fig. 3c and d). The EDS results reveal that both the divorced microstructure and the lamellar microstructure contain Ca (Table 2). The second phases in the lamellar microstructure and the divorced microstructure are, according to the XRD results, identified as Ca2Mg6Zn3 and Mg4Zn7 containing Ca (see Section 4.1 for more details), respectively. 3.2. Microstructure of the as-extruded alloys
3. Results 3.1. Microstructure of as-cast alloys Fig. 1 shows the X-ray diffraction (XRD) curves of the three Mg alloys in as-cast condition. The Z6 alloy contains Mg4Zn7 phase and ZX608 alloy contains Ca2Mg6Zn3. The ZX604 alloy with medium addition of Ca contains both Mg4Zn7 and Ca2Mg6Zn3 phase although their peaks are weak. Fig. 2 shows the backscattered electron (BSE) micrographs of Z6, ZX604 and ZX608 alloys in as-cast condition. The amount of the second phase increases with an increase of the Ca content. The distribution of the second phase differs among these alloys. The second phase is discontinuously distributed in the α-Mg matrix of the Z6 alloy (Fig. 2a) and it tends to distribute semi-continuously in the ZX604 alloy (Fig. 2b) and forms a continuous network in the ZX608 alloy (Fig. 2c), respectively. High-magnification SEM micrographs in Fig. 3 show that the morphology of the second phases also varies significantly among three Mg based alloys containing various Ca content. The second phase in Z6 Table 1 Chemical compositions of Mg-Zn-Ca alloys (wt.%). Alloys
Zn
Ca
Mg
Z6 ZX604 ZX608
5.74 6.01 6.02
– 0.36 0.82
Balance Balance Balance
Fig. 4 shows the optical microstructure of three alloys in the extruded condition. The as-extruded Z6 alloy exhibits a fully recrystallized microstructure with second phases oriented along the extrusion direction. The extruded ZX604 and ZX608 alloys exhibit a bimodal microstructure containing both the recrystallized regions and the deformed regions. The areas of the deformed regions increase with an increase of the Ca content, indicating that addition of Ca in the Mg-Zn alloy postpones or inhibits dynamic recrystallization (DRX) during extrusion. The area fractions of the deformed regions of the extruded ZX604 and ZX608 alloys are 4% and 13%, respectively. The undissolved second phases during homogenization treatment are also oriented towards the extrusion direction. The grain sizes of the recrystallized regions of Z6, ZX604 and ZX608 alloy are 14.3 ± 2.1 μm, 8.1 ± 1.4 μm and 3 ± 0.5 μm, respectively. Fig. 5 shows TEM microstructures of the as-extruded alloys. No obvious precipitates are observed in the as-extruded Z6 alloy (Fig. 5a). Fine spherical precipitates as well as the fragmented Ca2Mg6Zn3 (Fig. 5b) particles are observed in the as-extruded ZX604 alloy, which indicates that addition of Ca promotes dynamic precipitation. The fragmented particles at grain boundaries are identified as Ca2Mg6Zn3 by selected area electron diffraction (SAED) (Fig. 6). More precipitates are observed in the matrix as well as at grain boundaries of the as-extruded ZX608 alloy (Fig. 5c and d). The mean diameter of the precipitates of the asextruded ZX604 and ZX608 alloys is measured to be 30 ± 10 nm. The precipitates in the as-extruded ZX608 alloy exhibit both spherical shape and rod-like shape (Fig. 7a). The spherical precipitates at grain boundaries are identified as Ca2Mg6Zn3 by SAED (Fig. 7b) whereas
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Fig. 2. Backscattered electron (BSE) micrographs of the as-cast alloys: (a) Z6, (b) ZX604 and (c) ZX608.
the rod-like precipitates are indexed to be Mg4Zn7 (Fig. 7c). Energy dispersive X-ray spectroscopy (EDX) results reveal that the rod-like precipitates contain small amounts of Ca, which indicates that Ca plays an key role in the dynamic precipitation. The fine spherical precipitates at
grain interiors are identified to be MgZn2 by high-resolution transmission electron microscopy (HRTEM) (Fig. 7e). Fig. 8 shows (0001) pole figures of the as-extruded Mg alloys measured by XRD. The basal poles of the as-extruded Mg-Zn-Ca alloys are
Fig. 3. SEM micrographs of the as-cast alloys: (a) Z6, (b) ZX604, (c) and (d) ZX608.
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4. Discussion
Table 2 EDS results of the as-cast Mg-Zn-Ca alloys (at%). Points
A (Fig. 3a) B (Fig. 3b) C (Fig. 3b) D (Fig. 3d)
Elements Mg
Zn
Ca
74.5 91.3 74.0 72.3
25.5 6.8 19.5 19.3
1.9 6.5 8.4
perpendicular to the extrusion direction, which is known to be a typical texture of Mg alloys after extrusion [35]. The Ca-contained ZX604 and ZX608 alloys exhibit a stronger intensity of basal texture than the binary Z6 alloy. Fig. 9 shows the EBSD maps and corresponding (0001) pole figures of the as-extruded alloys in the DRXed region and the unDRXed region. It can be clearly seen that the unDRXed region in the as-extruded ZX604 and ZX608 alloys exhibits stronger texture intensity than the DRXed region. The DRXed regions of the three alloys also show basal texture. However, Ca containing alloys exhibit weaker texture intensity in the DRXed region compared to the as-extruded Z6 alloys, indicating that Ca addition weakens the basal texture of the recrystallized Mg-Zn-Ca alloys, which is consistent with the previous reports [21,22]. 3.3. Mechanical properties Fig. 10 shows the mechanical properties of the Mg-6Zn-xCa samples. The ultimate tensile stress (UTS), yield stress (YS), elongation-to-failure are summarized in Table 3. The YS, UTS, and the elongation-to-failure of the extruded Z6 are 125 MPa, 276 MPa and 29.7%, respectively. The strength increases while the elongation decreases with the addition of Ca. The YS increases to 169 MPa and 230 MPa for the ZX604 and ZX608 alloys, respectively. The elongation-to-fracture decreases to 21.4% and 15.3% for the as-extruded ZX604 and ZX608 alloys, respectively.
The microstructure and the mechanical properties of Z6, ZX604 and ZX608 are very different in both as-cast and as-extruded condition, although they contain almost the same amount of Zn and subjected to the same thermal processing. These differences are ascribed to the addition of Ca. Trace addition of Ca to the Mg-6Zn alloy causes differences in the formation of intermetallics during casting and extrusion in terms of precipitation, recrystallization, and texture evolution during extrusion, which further causes improved mechanical properties. The effect of addition of Ca is discussed in details as follows. 4.1. Intermetallic particles in as-cast Mg-Zn-Ca alloys A variety of intermetallic phases was reported in Mg-Zn binary alloys system [34,36–40], such as Mg7Zn3, MgZn, Mg4Zn7, MgZn2, Mg7Zn3 and Mg2Zn11. However, the phases in the Mg-Zn binary alloys are still controversial. Mg7Zn3 distributing in the inter-dendrites region and Mg4Zn7 with a globular shape within α-Mg dendrites were observed in the as-cast Mg-8Zn alloy [37]. Presence of Mg4Zn7 and MgZn2 particles were confirmed in the Mg-8Zn alloy during ageing at 200 °C [38]. In this study, Mg4Zn7 phase is detected in the as-cast Z6 alloy by XRD analysis, which was also previously observed in the ascast Mg-4Zn alloy [41]. Addition of Ca to the Mg-6Zn alloy causes a gradual replacement of the Mg-Zn binary compound with Ca2Mg6Zn3 ternary compound in the ZX604 and ZX608 alloys. The second phases in the Mg-Zn-Ca alloy are commonly Mg2Ca or Ca2Mg6Zn3, which depends on the atomic ratio of Zn/Ca. Ca2Mg6Zn3 predominates when the Zn/Ca atomic ratio is higher than 1.23 [42]. Mg2Ca was not detected in the present Ca containing alloys since the atomic ratio of Zn/Ca of ZX604 and ZX608 is much higher than the critical value of 1.23 [42]. The eutectic morphology of Mg alloys during solidification may be fibrous/lamellar or fully or partially divorced [43]. The Z6 alloy exhibits partially divorced eutectic morphologies, whereas the ZX604 and ZX608 alloys show eutectic characteristics in the as-cast condition. The formation of fully or partially divorced eutectic morphologies is
Fig. 4. Optical images of the as-extruded alloys: (a) Z6, (b) ZX604 and (c) ZX608.
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Fig. 5. The bright field TEM images of the as-extruded alloys: (a) Z6, (b) ZX604, (c) and (d) ZX608.
typical for Mg-Al or Mg-Zn binary alloys, e.g. Mg-9Al alloy [44] and Mg9Zn alloy [45], which is because large amount of primary α-Mg restricts the eutectic reaction to a very small and isolated interdendritic region, and it further results in the solidification of the eutectic liquid outside coupled eutectic growth [43]. The morphology of the eutectic microstructure is lamellar or rod-like shaped in Ca containing alloys, which was also observed in the as-cast Mg-4.5Zn-1.13Ca alloy [32]. Addition of Ca causes intensive constitutional undercooling ahead of solid/liquid interface in the liquid layer [46], promoting the primary α-Mg phase solidification and result in the increase of Ca and Zn concentration in the liquid. As a result, the composition ahead of the solid/liquid interface
reaches the eutectic composition and falls within a zone of the coupled eutectic growth. Therefore, addition of Ca promotes to form a lamellar morphology of Ca2Mg6Zn3 and α-Mg. 4.2. Effects of Ca addition on microstructure of as-extruded Mg-Zn alloys A large volume fraction of fine precipitates including Ca2Mg6Zn3 and Mg4Zn7 are detected in Ca containing alloys (Fig. 5b and d) after extrusion, though such densely-dispersed precipitates are not observed in the as-extruded Z6 alloy (Fig. 5a), indicating that Ca promotes dynamic precipitation of Z6 alloy during extrusion.
Fig. 6. TEM analysis of the as-extruded ZX604 alloy: (a) bright field image and (b) SAED of the second phase A in (a).
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Fig. 7. TEM analysis of the as-extruded ZX608 alloy: (a) bright field image, SAED of second phases arrowed as (b) A and (c) B in (a), (d) EDX of the second phase arrowed as B in (a), (e) HRTEM of the fine spherical precipitates arrowed as C in (a).
The alloys were homogenized at 350 °C for 12 h before extrusion. The second phase of Mg4Zn7 was almost dissolved into the matrix, while Ca2Mg6Zn3 was partially dissolved. As a result, the ashomogenized Ca containing alloys exhibits supersaturation of Ca and Zn in the matrix, whereas the Z6 alloy is supersaturated with Zn. Therefore, dynamic precipitation in the Mg-Zn based alloys is possible during hot extrusion or subsequent cooling. However, no precipitates were detected in the as-extruded Z6 alloy but large amount of precipitates were observed in the Ca containing alloys. The dynamic precipitation during extrusion in Mg-6Zn base alloys depends on the microalloying elements as well as extrusion temperatures [26,47,48]. A large number of fine
precipitates were observed in a Mg-6Zn alloy when extruded at lower temperature (210 °C) [47], but not detected when extruded at a higher temperature (350 °C) [48]. Such phenomenon is related to the poor thermal stability of the Mg-Zn binary compounds [37]. Microalloying elements addition in Mg alloys, such as Zr, Mn and Ag, were used to promote dynamic precipitation [27,48,49]. Ca and Zn are likely to form CaZn clusters due to larger negative enthalpy of mixing between Ca-Zn than those of Mg-Zn and Mg-Ca [16]. Such segregation is energetically favorable for nucleation of dynamic precipitation during deformation. Ca is detected in the fine precipitates Mg4Zn7 by EDX (Fig. 9d), suggesting that Ca plays a critical role in the formation of the precipitates during
Fig. 8. Pole figures of the extruded alloys: (a) Z6, (b) ZX604 and (c) ZX608.
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Fig. 9. The EBSD maps and corresponding (0001) pole figures of DRXed and unDRXed regions of the as-extruded alloys.
extrusion. Additionally, ZX604 and ZX608 alloys contain undissolved bulk Ca2Mg6Zn3 phase, which results in the heterogeneously deformation and further causes increased amount of crystal defects during hot extrusion. Such crystal defects provide energetically favorable nucleation sites for the precipitates. The existence of Ca in the precipitates might improve the thermal stabilities and increase the temperature of the dynamic precipitation. Therefore, Ca addition results in the dynamic precipitation of Mg-6Zn alloy during extrusion at 300 °C. Fine Ca2Mg6Zn3 precipitates were observed at grain boundaries in the ZX604 and ZX608 alloys. The previous investigations suggested that Gd with larger atomic radius compared with Mg is segregated to grain boundaries [50] or twinning boundaries [51]. It was also confirmed that Ca segregated to grain boundaries in Mg-0.3Zn-0.1Ca alloy [52]. Therefore, it is expected that Ca was preferentially distributed to grain boundaries in the Ca containing alloys in the present study. As a result, fine Ca2Mg6Zn3 is preferred to precipitate around grain boundaries during extrusion for the Ca containing alloys. Such fine Ca2Mg6Zn3 dispersed at grain boundaries was also reported in the as-extruded Mg-
5.99Zn-1.76Ca-0.35Mn [33], Mg-3Zn-0.5Ag-0.25Ca-0.15Mn [30] and Mg-5Zn-0.25Ca-0.3Zr [28] alloys. Ca addition into Mg-6Zn alloy leads to grain refinement of dynamic recrystallization and retention of deformed region after extrusion, as shown in Fig. 4. This indicates that Ca addition retards and postpones recrystallization and grain growth of the Mg-6Zn alloy. The second phase strongly influences the microstructure evolution during deformation, depending on the size and the distribution of the second phase. Generally, coarse particles result in deformation zone and promote nucleation of recrystallization, known as particle stimulated nucleation (PSN) [53], whereas fine precipitates inhibit dynamic recrystallization during deformation due to their pinning effect on the dislocation mobility [54–56]. A large amount of coarse Ca2Mg6Zn3 phases are not dissolved into the matrix during homogenization (Fig. 4b and c) and coarse second phases surrounded by fine DRXed grains are observed in Fig. 4b and c, indicating that the coarse Ca2Mg6Zn3 particles stimulate dynamic recrystallization during extrusion. Meanwhile, the undissolved Ca2Mg6Zn3 is fragmented during extrusion and distributed around grain boundaries (Fig. 6a), which hinders grain growth. In addition, fine Ca2Mg6Zn3 particles precipitating along grain boundaries during extrusion (Fig. 7b) effectively inhibit grain growth. Both these two factors are beneficial for grain refinement. The amount of Ca2Mg6Zn3 is proportional to the Ca content in Mg-Zn alloys. Therefore, the DRXed grain is refined with an increase of Ca addition for the alloys with a high level of Ca contents. The deformed region retained in the Ca containing alloys might be related to the pinning effects from fine precipitates and solute segregation to grain boundaries. Dislocation walls were observed in the unDRXed region for the as-extruded ZX608 alloy (Fig. 11). Meanwhile, fine precipitates were also observed (Fig. 11), which would inhibit the mobility of dislocations. Consequently, the recrystallization process relying on the rearrangement of dislocations was prohibited. Additionally, Ca solute segregation to grain boundaries was expected to provide a
Table 3 Tensile properties of the as-extruded Mg-Zn-Ca alloys.
Fig. 10. Typical stress-strain curves of the as-extruded Mg-Zn-Ca alloys.
alloys
YS (MPa)
UTS (MPa)
Elongation to failure (%)
Z6 ZX604 ZX608
125 ± 5 169 ± 4 230 ± 8
276 ± 1 276 ± 3 304 ± 1
29.7 ± 2.7 21.4 ± 1.1 15.3 ± 1.6
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Fig. 11. The bight field TEM image in the deformed region of the as-extruded ZX608 alloy showing the dislocations.
with a similar size. The volume fractions of the precipitates in the asextruded ZX608 alloy was higher than that of ZX604 alloy, indicating that the contribution of precipitates strengthening was increased with an increase of Ca addition. Dislocation strengthening was determined by dislocation density [62]. The dislocation was consumed by dynamic recrystallization, resulting in low dislocation density in the DRXed region. While the deformed regions contain high-density dislocations due to the existence of large strain. The deformed region was increased with an increase of Ca for the as-extruded Mg-Zn-Ca alloys, which was beneficial for further improvement of strength. Additionally, the texture affects the strength significantly in Mg alloys [63]. The basal texture with a basal plane parallel to extrusion direction is always observed in Mg alloys after extrusion, which limited the activity of the basal slip when tensile along extrusion direction, thus increasing the yield strength. For the as-extruded Mg-Zn-Ca alloy, Ca containing alloys showed stronger basal texture intensity, which should be another contributor for the strength improvement. 5. Conclusions
strong drag force suppressing DRX, which would further prohibit the DRX process. Ca containing alloys exhibited stronger texture intensity after extrusion compared to the as-extruded Z6 alloy (Fig. 9). From the microstructure observation, it can be seen that Ca-based alloys contained deformed regions, which always exhibited strong basal texture [57]. However, the texture intensity of the recrystallization region in Ca containing alloys was weaker compared with the as-extruded Z6 alloy. It indicated that Ca addition weakened the recrystallization texture. The texture weakening effect in Ca containing Mg alloys are commonly observed. However, the weakening mechanisms are still unclear. It is believed that the weakening of recrystallization texture after Ca addition in the present study was related to particle stimulated nucleation (PSN) and pinning effects from fine particles or solute segregation to grain boundaries. The undissolved Ca2Mg6Zn3 phase during homogenization stimulated the nucleation of dynamic recrystallization during extrusion (Figs. 2e and 3a), which could randomize the orientation of dynamic recrystallized grains [58]. Particle pinning to grain boundaries was suggested to give rise to texture weakening in AZ31-xCa (x = 0.4 and 0.8 wt%) alloys [59]. Therefore, the fine precipitates in Ca containing alloys might be another factor weakening the recrystallization texture. Additionally, Ca segregation to grain boundaries could effectively prohibit the mobility of high energy grain boundaries [52], affecting the preferred grain growth and resulting in the weakening of recrystallization texture. Though the Ca segregation to grain boundaries was not observed directly in the present study, it is still expected that Ca segregation should be of significance to weaken the texture after recrystallization. 4.3. Strengthening mechanisms Ca addition effectively improves the strength of Mg-Zn alloy after extrusion, but decreases the ductility (Fig. 10). Compared with the asextruded Z6 alloy, the as-extruded Mg-Zn-Ca ternary alloys exhibited refined microstructure, fine dynamic precipitates, and elongated deformed region. Therefore, fine grains in the DRXed region, a large number of fine precipitates, and high dislocation density in the deformed region should be contributed to the strength improvement. The strength improvement due to grain refinement was increased with a decrease of grain size according to the Hall-Petch relationship [60]. The average DRXed grain size of the as-extruded Z6 alloy was 14.3 μm, while the grain size was decreased to be 8.1 μm and 3 μm for the as-extruded ZX604 and ZX608 alloys, respectively. Therefore, the grain refinement contribution to the strength would increase with an increase of Ca addition. Precipitation strengthening was related to the size, distribution, morphology and the density of precipitates [61]. The as-extruded ZX604 and ZX608 alloys contained spherical precipitates
Mg-6Zn alloys microalloyed with different content of calcium were cast and extruded, and the microstructure and mechanical properties of the alloys were investigated. The main conclusions are as following: 1. The second phase in the as-cast Mg-6Zn alloy is divorced Mg4Zn7, which is gradually replaced by lamellar Ca2Mg6Zn3 phase distributing around grain boundaries with the increase of Ca addition. 2. Ca addition stabilizes the precipitates and gives rise to much more defects during extrusion, promoting dynamic precipitation. 3. The densely distributed precipitates in Ca containing alloys postpones dynamic recrystallization and restricts grain growth, resulting in a bimodal microstructure with fine DRXed grains and deformed region. 4. The yield strength is improved by increasing Ca content, attributing to fine DRXed grains, high density precipitates and deformed region with high-density dislocation and strong basal texture. Acknowledgements This study was supported by National Key Basic Research Program of China (No. 2013CB632200), National Natural Science Foundation of China (No. 51271063 and 51571068) and MISiS K3 Project (No. К3-2015-005). XGQ thanks funding from the Fundamental Research Funds for the Central Universities under Grant No. HIT. NSRIF.2015003, the General Financial Grant from the China Postdoctoral Science Foundation under Grant No 2013M531034, Specialized Research Fund for the Doctoral Program of Higher Education under Grant No. 20132302120001, Hei Long Jiang Postdoctoral Foundation (No. LBH-Z13088). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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