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Influence of Zn content on the microstructure and mechanical properties of extruded Mg–5Y–4Gd–0.4Zr alloy
Journal of Alloys and Compounds 481 (2009) 811–818
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Influence of Zn content on the microstructure and mechanical properties of extruded Mg–5Y–4Gd–0.4Zr alloy Ke Liu a,b , Jinghuai Zhang a,b , Guihua Su c , Dingxiang Tang a , L.L. Rokhlin d , F.M. Elkin d , Jian Meng a,∗ a State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate School of the Chinese Academy of Science, Beijing 100049, China c Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, China d Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia
a r t i c l e
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Article history: Received 10 December 2008 Received in revised form 18 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Magnesium alloys Age hardening Tensile strength Mg–Y–Gd–Zn–Zr
a b s t r a c t Microstructures and mechanical properties of the Mg–5Y–4Gd–xZn–0.4Zr alloys have been investigated. These results show that the Mg–5Y–4Gd–0.5Zn–0.4Zr alloy in the peak-aged condition exhibits the highest tensile strength, and the values of the ultimate tensile strength and yield tensile strength are 370 and 300 MPa, respectively. It is suggested that addition of 0.5% Zn has a great effect on age hardening response. The long periodic stacking structure has been found in these Zn-containing alloys, and the volume fraction of this phase increases with increasing Zn addition. This phase plays an important role in improvement of the mechanical properties, especially for the elongations. The  phase precipitates during the ageing process are responsible for the improvement of the mechanical properties of the alloys in the peak-aged condition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloy is the lightest structural material applicable in aerospace and automobile industry. It is because that the magnesium alloys have high specific strength, high stiffness and good damping capacity. Moreover, the resources of the magnesium are abundant and the products of the magnesium alloys are easy to recycle. The widely used magnesium alloys belong to Mg–Al series which have outstanding castability and low costs. However, these alloys have inferior mechanical properties, especially at high temperature. To the best of our knowledge, addition of rare-earth (RE) elements to the magnesium alloys improves the heat resistance and creep properties, e.g. WE54 commercial alloy [1]. The magnesium alloys containing RE show good precipitation hardening during the peak ageing process [2,3]. It is well known that the binary Mg–Gd alloy is an ideal system for precipitation hardening. However, the binary Mg–Gd alloys containing less than 10% Gd exhibit little or no precipitation hardening response during ageing treatment [4]. Although adding more than 20% Gd can increase the hardness and strength, this improves costs and densities of the magnesium alloys.
The Zn is one of the most important elements which can improve the age hardening responses of the Mg–RE alloys. Moreover, the additions of Zn lead to the formation of the long periodic stacking (LPS) structure [5-7]. Suzuki et al. [8] have been reported that addition of trace amounts of Zn to M–Y alloys contributes to the formation of stacking faults which can retard the movement of the dislocations. Recently, Yamasaki et al. [9] have developed a hotextruded Mg–2.3Zn–14Gd (wt.%) alloy which presents a highest tensile proof strength of 345 MPa and a better elongation of 6.3%, because a coherent 14H LPS structure precipitated from a supersaturated ␣-Mg matrix, which has obviously effect on improving mechanical properties. The microstructures and mechanical properties of the Mg–5Y–4Gd–xZn–0.4Zr (x = 0, 0.5, 1.0, 1.5, 2.0 and are identified as A, B, C, D and E, respectively) have been investigated. The LPS structure has been found in the Zn-containing alloys, and the  phase has been observed in the peak-aged alloys. The effect of the LPS structure and  phase coexisting together on the mechanical properties has been investigated in this article. 2. Experimental procedures
∗ Corresponding author. Tel.: +86 431 85262030; fax: +86 431 85698041. E-mail address: [email protected] (J. Meng). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.119
The alloy ingots with nominal compositions of Mg–5Y–4Gd–xZn–0.4Zr were produced from high-purity Mg (99.5%), high-purity Zn (>99.9%), Mg–20Y (wt.%), Mg–25Gd (wt.%) and Mg–35Zr (wt.%) master alloys in an electric resistance furnace at about 750 ◦ C. The mild steel crucible has been filled with a protective atmosphere.
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At about 730 ◦ C, the melts were poured into a water-cooled iron mold with a diameter of 90 mm. Specimens used for as-cast research were cut from the cylinder-shaped ingots. Parts of them were made into rectangular tensile specimens of 15 mm in gauge length, 3 mm in width and 1.5 mm in thickness, and the remaining is used for other investigations. The cylinder-shaped ingots homogenized at 500 ◦ C for 16 h were milled into diameter of 82 mm, and then extruded into rods in an extrusion ratio of 16.8 at 380 ◦ C. Parts of rods were directly aged at 220 ◦ C in order to investigate the age hardening behaviors with time. The tensile specimens of the as-extruded and peak-aged alloys were also machined into the same geometry as the as-cast samples.
Tensile tests were carried out on a uniaxial tensile testing machine at a primary strain rate of 1 mm/min. The tensile axis was aligned parallel to the extrusion direction. Vickers hardness was measured by a hardness tester with a load of 300 g, a dwelling time of 15 s and 20 measurements were collected for each sample. The microstructures of the alloys were observed by an optical microscope (OM), a scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) and a transmission electron microscope (TEM). The grain sizes of the alloys were measured via using an average linear intercept method. The samples were mechanical polishing and etching in a solution of 2.1 g picric acid, 5 ml acetic acid, 35 ml ethanol and 5 ml H2 O. Samples for TEM observations were prepared with a twin-
Fig. 1. Images of the specimens of the as-cast alloys: (a and b) A, (c and d) B, (e and f) D, (g and h) E.
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Fig. 2. X-ray diffraction patterns of the as-cast alloys A and E.
jet technique. Phase analysis was conducted through the use of X-ray diffraction (XRD).
3. Experimental results 3.1. Microstructures of the as-cast alloys The microstructures of the as-cast alloys A, B, D and E, as shown in Fig. 1(a), (c), (e) and (g), are composed of dendrites of magnesium matrix phases separated by eutectic. Fig. 1(b), (d), (f) and (h) are the OM images of these as-cast alloys A, B, D and E at a higher magnification, respectively, in order to observe the second phases. The images at the higher magnification show that a lamellar phase appears with Zn additions and the volume fraction of this phase increases with increasing the content of Zn. Fig. 2 shows the XRD patterns of the as-cast alloys A and E. It can be observed that the as-cast alloy A are mainly composed of ␣-Mg solid solution together with Mg5 (Y, Gd) and Mg24 (Y, Gd) secondary phases where Gd probably substitutes for Y. However, the XRD results of the as-cast alloy E show that phase constituents includes ␣-Mg solid solution, Mg5 (Y, Gd), MgZn2 and Mg12 Zn(Y, Gd) secondary phases. Fig. 3 shows the SEM images and corresponding EDS results of the as-cast alloy E. The EDS analysis result suggests that the composition of the lamellar phase is Mg–8.39 at.% RE–4.6 at.% Zn, as shown in Fig. 3(c), and RE is standing for Gd and Y. The concentration is similar to those in the 18R LPS structure reported by Itoi et al., i.e. Mg–4 at.% Zn–6 at.% Y, which were obtained by EDS spectra [10]. RE and Zn are mainly contained in this phase. Fig. 3(d) shows the EDS result from the point B as indicated in Fig. 3(a). Fig. 3(b) is an image at a higher magnification obtained from the square particle in Fig. 3(a). The result suggests that the composition of this square particle is Mg–19.27 at.% RE–0.94 at.% Zn, where RE stands for Y and Gd. The chemical composition is close to Mg5 (Y, Gd). 3.2. Microstructures of the as-extruded alloys Fig. 4 shows the optical micrographs of the as-extruded alloys A, B, D and E. Both of the dendrites of the matrix and the coarse Mg12 Zn(Y, Gd) were crushed during hot deformation. These intermetallics were located at the grain boundaries. However, the microstructure of the alloy A is different from that of the Zncontaining alloys. The as-extruded alloy A has relatively coarse ␣-Mg grains, with the average grain size of about 16 m. The grain boundaries of the as-extruded alloy A are clear. However, the alloy with Zn addition has fine ␣-Mg grains, and the average grain sizes of the alloys B, D and E are about 10, 9 and 10 m, respectively. It is suggested that the Zn addition has an obvious effect on refining the
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microstructure compared with the alloy A, as shown in Fig. 4(a), (c), (e) and (f). In addition, lots of intermetallic compounds, especially for the lamellar phase, are observed at the grain boundaries in the alloys B, D and E, and the volume fraction of these intermetallics increases with increasing Zn addition. The dynamic recrystallization (DRX) occurred in these alloys during hot extrusion, as shown in Fig. 4(b), (d), (f) and (h). The recrystallized grains formed and began to grow during hot deformation. The average sizes of the recrystallized grains of the alloys A and E in as-extruded condition are about 10 and 3 m, respectively. It is suggested that the intermetallics, especially for the LPS structure distributed around the matrix, play an important role in limiting the growth of the recrystallized grains. In order to investigate the microstructure of this lamellar phase in these Zn-containing alloys in as-extruded condition, the EDS have been carried out. The EDS results of this lamellar phase obtained from the as-extruded alloy E, i.e. Mg–2.95 at.% RE–1.10 at.% Zn, show that the content of both Zn and RE in this phase decreased but the atom ratio of Zn/RE rose, compared with this alloy in the as-cast condition. Note that this lamellar phase is different from the lamellar phase observed in the as-cast condition. It has been reported that the 18R LPS structure formed, located at the grain boundaries which were rich in Y and Zn, in the Mg–Zn–Y RS alloy ribbons or cast alloy ingots as secondary phase [11,12]. Furthermore, it has been reported that the 18R LPS structure was not stable during heat treatment at 500 ◦ C, and transformed to 14H LPS structure when the cast alloys were heat treated for more than 5 h at 500 ◦ C. However, the 14H LPS structures were not developed during casting and ageing at low temperature in the Mg–Zn–Y alloys [10]. The lamellar phase in the as-extruded alloys might be the 14H LPS structure. Fig. 5(a) shows the TEM image of the lamellar phase obtained from the alloy E in the as-extruded condition. Fig. 5(b) shows the SAED pattern of this phase. The extra reflection-spots in the corresponding selected area electron diffraction (SAED) pattern can be indexed as 00n/14 (corresponding to (002)hcp ) and −11n/14. The diffraction pattern suggests a 14H-type LPS structure which is considered to be the same with the 14H LPS structure observed in rapidly solidified (RS) and annealed RS Mg–Zn–Y alloys. As stated above, the lamellar phase in the as-cast alloys is different from that in the as-extruded alloys. The lamellar phase in the as-cast alloys is the 18R LPS structure, but the lamellar phase in the as-extruded alloys is the 14H LPS structure. The 18R LPS structure transforms to a 14H LPS structure via releasing the RE and Zn into the matrix during heat treatment. 3.3. Ageing hardening behaviors of the as-extruded alloys Fig. 6 shows the age hardening behaviors of the as-extruded alloys A, B, C, D and E at 220 ◦ C. The alloys of both A and B exhibit an obvious age hardening response, while the alloys C, D and E display little ageing hardening behaviors (see Fig. 6). The value of the hardness of the as-extruded alloy A is 87 VHN, and the hardness increases obviously after 10 h, and then peak hardness is obtained after 60 h, with a value of 114 VHN. The age hardening curve of the alloy B exhibits an obvious hardening response at the beginning stage after 12 h, and a peak hardness of 113 VHN is obtained after 85 h. The peak hardness of the alloy C is obtained after 95 h, with a value of 98 VHN. Alloys D and E, both of them containing higher content of Zn compared with the alloy B, do not have an obvious increase in hardness. The alloys D and E reach peak hardness after 70 and 60 h, respectively. The value of the peak hardness of the alloy D is 97 VHN, and the value of the peak hardness of the alloy E is 96 VHN. From Fig. 6, it can be suggested that the peak hardness of these alloys decrease with Zn addition. It has been reported that the Zn
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Fig. 3. SEM images and EDS results of the as-cast alloy E: (a) SEM micrograph, (b) Large magnification of the cubic precipitate corresponding to the point B, (c) and (d) EDS results of the points A and B, respectively.
addition has an obvious effect on improving the elongation at the expense of strength [13]. The additions of Zn lead to a formation of the LPS structure, and the volume fraction of this phase increases with increasing Zn addition, as a result of a decrement of the RE content in the supersaturated solution of the matrix. The decrement of the RE concentration leads to little age hardening response of these Mg–Y–Gd alloys which contain higher Zn, i.e. alloys C, D and E. 3.4. Microstructures of the peak-aged alloys In order to investigate precipitation morphologies and discuss the reasons for the ageing response at 220 ◦ C, the TEM images and corresponding SAED patterns from these peak-aged alloys A, D and E are shown in Fig. 7. A great number of particles which are ellipsoidal in morphology are observed in the specimens of the peak-aged alloy A. These particles are distributed throughout the matrix, and the average size of this phase is smaller while the volume fraction is higher. The average size of these particles in alloy A is about 22 nm in thickness and 49 nm in diameter. However, the average size of these particles in alloy B is bigger than that in alloy A, with a thickness of 30 nm and a diameter of 108 nm. The average size of the particles in the alloy E is 27 nm in thickness and 133 nm in diameter. It is suggested that the average size of this phase increases with increasing Zn addition. It has been reported that the main secondary strengthening phase in Mg–RE, Mg–RE–Zn (RE standing for Gd, Y and Nd) alloys is the  phase at peak hardness [14-17]. Based on previous paper and the SAED patterns, these precipitates are identified to be the  phase. This phase with a bco structure plays an important part in age hardening response in theses alloys during ageing at 220 ◦ C. To the best of our knowledge, at the beginning of the stage, the enhancement of hardness is ascribed to the presence of a
metastable phase, i.e.  phase. It has also been reported that  phase transformed to  phase via releasing RE atom in order to keep the structural coherency with matrix at the peak hardness point, and both of them is isomorphic [18,19]. As shows in Fig. 7, the volume fraction of these particles decreases with increasing Zn addition. The volume fraction of this phase in the alloy A is largest. However, the volume fractions of this phase in alloys D and E are smallest, as shown in Fig. 7(b) and (c). Honma et al. [18] found that the number density of the  phase decreased dramatically by the additions of Zn in the Mg–2.1Gd–1.2Y–xZn (x = 0, 0.3 and 1.0) alloys (at.%), and it led to a large reduction in the age hardening response. As stated above, the addition of Zn has an indirect effect on age hardening response, via which removes the RE elements from the matrix reducing the ability of the magnesium matrix to precipitate Mg–RE phase. 3.5. Mechanical properties of alloys A comparison of the typical mechanical properties of all five alloys in different states is listed in Table 1. The strengths of the as-cast alloys tend to decrease while the elongations of these alloys are inclined to increase with increasing the additions of Zn. The as-cast alloy A has higher ultimate tensile strength (UTS) and yield tensile strength (YTS), and the values of the UTS and YTS are 206 and 114 MPa, respectively, with an elongation of 4.2%. However, the alloys B, C, D and E have interior mechanical properties except the elongation. The values of the UTS and YTS of the alloy E are 188 and 104 MPa, respectively, and the elongation is 7.6%. The mechanical properties of these alloys are evidently improved after hot extrusion, especially for the alloy C, as shown in Table 1. The values of UTS and YTS of alloy C in the as-extruded condition are about 340 and 265 MPa, respectively, with a good elongation of 16.3%. Compared with the alloy C in as-cast con-
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Fig. 4. OM images of the specimens of the as-extruded alloys: (a and b) A, (c and d) B, (e and f) D and (g and h) E.
dition, the UTS is improved by 147 MPa, and the increment of YTS is 150 MPa. The extrusion process is beneficial to improve the mechanical properties of the alloys via microstructural refinement. It should be noted that the as-cast alloys exhibit lower mechanical properties due to a coarse microstructure and lack of uniformity
in the distribution of the LPS structure. However, the mechanical properties of the alloys are greatly improved after hot deformation because of refinement of the microstructure. DRX occurred during hot deformation, and it has obviously effect on refining microstructure. The mechanical properties of the as-extruded alloys increase
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K. Liu et al. / Journal of Alloys and Compounds 481 (2009) 811–818 Table 1 Mechanical properties of the alloys in as-cast and as-extruded conditions. Alloys
States
UTS (MPa)
YTS (MPa)
Elongation (%)
A
Cast Extruded
206 (2.3) 274 (2.1)
114 (1.9) 210 (2.2)
4.2 (0.4) 6.3 (0.6)
B
Cast Extruded
181 (2.6) 318 (3.3)
109 (1.7) 253 (3.0)
5.8 (0.5) 12.1 (1.0)
C
Cast Extruded
193 (2.5) 340 (4.1)
115 (1.7) 265 (3.8)
6.4 (0.4) 16.3 (0.9)
D
Cast Extruded
182 (2.0) 330 (3.7)
110 (1.6) 254 (3.4)
6.2 (0.6) 15.4 (1.0)
E
Cast Extruded
188 (2.1) 315 (3.5)
104 (2.3) 251 (3.1)
7.6 (0.8) 14.8 (0.9)
Note: Standard deviation is given in parentheses.
improved, especially for the alloys A and B. The peak-aged alloy B shows the highest mechanical properties at room temperature, and the values of the UTS and YTS are 370 and 300 MPa, respectively. It is found that tensile strength of the alloy B is improved greatly, and the YTS increases by 47 MPa. The peak-aged alloy A also displays a better tensile strength, with a better YTS of 298 MPa. However, the elongation of the alloy A is inferior, and the value of the elongation is 3.5%. It is known that Mg–Gd–Y alloys exhibit a three-stage precipitation sequence during ageing at 200 ◦ C, ␣ -Mg (S.S.S.S) →  (DO19 ) →  (cbco) →  (fcc) [20,21]. However, it is the  phase, with a cbco structure, distributed throughout the ␣-Mg matrix that contributes most to the age hardening response [22]. It is found that the mechanical properties of the peak-aged alloys are improved enormously but the elongation, especially for the alloy A, as shown in Table 2. Decrease of the elongation is mainly ascribed to the presence of the  phase. It is suggested that this phase is good to tensile strength while bad to deformation plasticity. Fig. 5. (a) TEM image of the as-extruded alloy E and corresponding SAED pattern (B//[1 1 2¯ 0]␣ ) from 14H areas in this alloy (b).
with increasing Zn addition due to the increment of the LPS structure, as shown in Table 1. It is suggested that the LPS phase not only enhances the tensile strength, but also improves the deformation plasticity. Table 2 shows the mechanical properties of the peak-aged alloys. The results show that the tensile strengths of the alloys are
4. Discussion The average grain sizes of the as-extruded alloys A, B, D and E have been obtained. It is found that the average grain sizes of these alloys containing Zn are similar. Furthermore, the tensile strength of these alloys does not show great difference among them. According to the Hall–Petch relationship, the increment of the tensile strength associating with refined microstructure is almost the same. However, the elongation of these alloys shows obvious characteristic. It increases with increasing Zn addition. The value of the YTS of the as-extruded alloy B is 253 MPa, with an increment of 43 MPa compared with the as-extruded alloy A. Additions of Zn at levels of 0.5% to the Mg–5Y–4Gd–0.4Zr alloys result in a 20% increase in the YTS. Although the average grain size of the alloy B in as-extruded condition is smaller than that of the alloy A, this increase in tensile strength is not fully accounted for by the refined grain size. The reduction in grain size from 16 to 10 m is expected to yield strength increment of 18 MPa. Provided that the concentration of Zn in the as-extruded alloy B is only about 0.2 at.%, the strengthening contribution from solid solution of Zn in magnesium is expected to Table 2 Mechanical properties of the peak-aged alloys at 220 ◦ C.
Fig. 6. Age hardening responses of the as-extruded alloys A, B, C, D and E during ageing at 220 ◦ C.
Alloys
State
UTS (MPa)
YTS (MPa)
Elongation (%)
A B C D E
T5 T5 T5 T5 T5
361 (3.1) 370 (2.9) 352 (3.4) 339 (3.3) 324 (3.0)
298 (2.8) 300 (3.4) 285 (3.0) 276 (3.1) 263 (3.2)
3.5 (0.6) 6.0(0.8) 12.4(1.1) 14.2(0.9) 13.5(1.0)
Note: T5-peak aged at 220 ◦ C; standard deviation is given in parentheses.
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Fig. 7. TEM images and corresponding SAED (B//[2 1¯ 1¯ 0]␣ ) patterns of specimens of the peak-aged alloys: (a) alloy A, (b) alloy D and (c) alloy E.
be negligible [23]. In order to investigate the solid solution strength of Zn, the hardness of the matrix of these alloys A, B, C, D and E have been obtained, and the values of the hardness of the matrix of the as-extruded alloys A, B, C, D and E are 87, 85, 88, 91 and 92 VHN, respectively. It gives a proof that an addition of 0.5 wt.% Zn to the Mg–5Y–4Gd–0.4Zr alloy, and the addition of Zn increases from 1 to 2 wt.%, do not lead to an obvious increase in hardness. The LPS structure was regarded as the main secondary phase in the Zn-containing alloys, e.g. alloy B. The strengthening contribution from the 14H LPS structure in the as-extruded alloy B is about 25 MPa. In addition, Kawamura et al. [24] developed the highest strength magnesium alloy with yield strength of 600 MPa, and the LPS structure was regarded as a main secondary strengthening phase. The LPS structure played an important role in the improvement of the mechanical properties [9]. The as-extruded alloy C has the highest tensile strength among the as-extruded alloys. However, the tensile strength of the alloys is not greatly improved with further addition of Zn, such as the alloys D and E in the as-extruded condition. The coarse LPS structure lead to a decrease in the tensile strength.
The 18R LPS structure forms in Mg–Zn–Y alloys via rapid solidification, because increasing the cooling rate and adding Y can reduce the energy barrier for forming this phase [25,26]. Moreover, the volume fraction of this LPS structure increases with increasing additions of Zn. As a result, the content of RE (standing for Gd and Y) retained in the matrix decreased. The more Zn is added, the less RE is left in the matrix. So, when these alloys were aged, the volume fraction of the  phase decreased with increasing addition of Zn. In addition, the number density of the  phase in alloy A was so high that the growth of this phase is restricted, see Fig. 7. Conversely, the size of the  phase in the alloy E was larger. It has been reported that the  phase precipitates with three variants within the matrix in the Zn-containing alloys which have a low number density of  phase, and only the variant along a [0 1 1¯ 0]␣ direction remains during coarsening in the peak-aged condition, and it is because that the self-stress orienting phenomenon occurs by elastic interactions of coherency strains of the precipitates [13]. The high number density of the  phase in the Zn free alloy, i.e. alloy A, restrict the growth of the  phase, and this led to an obvious age hardening response.
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From this investigation, it can be concluded that maximum tensile strength in Mg–5Y–4Gd–0.4Zr alloy is attained when an optimal Zn content is added. Maximum tensile strength is due to the contribution of both fine precipitation and small grain size of the alloy. Thus, Zn content should be enough low to remove many rare elements from the magnesium matrix, in such a way that hardening induced by the precipitation of metastable phases during ageing is not prevented, and enough high to refine the grain size of the alloy. 5. Conclusions
Sciences and Jilin Province and Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow. References [1] [2] [3] [4] [5] [6] [7]
The microstructures and the tensile properties of alloys A, B, C, D and E have been investigated by an OM, XRD, SEM and TEM in this article. The investigation can lead to the following conclusions: 1. The 18R LPS structure was observed in the Zn-containing alloys in as-cast condition, and the volume fraction of this phase increases with increasing Zn addition. 2. Hot extrusion processing had an obvious effect on refinement of the microstructure. DRX occurred during hot extrusion and played an important role in refining the microstructure. 3. The 14H LPS structure has been observed in the as-extruded alloys that contain Zn. This phase formed via releasing Zn and RE atoms from the 18R LPS structure, and the ration of Zn/RE of the 14H LPS structure is higher than that of the 18R LPS structure. 4. The peak-aged alloy B exhibited the highest peak hardness and tensile strength, and the values of the UTS and YTS were 370 and 300 MPa, respectively. It was due to the  phase that precipitated within the matrix during ageing at 220 ◦ C. Acknowledgements This project was supported by Hi-Tech Research and Development Program of China (2006AA03Z520), Chinese Academy of
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
I.J. Polmear, Mater. Sci. Technol. 10 (1994) 1. I.A. Anyanwu, S. Kamado, Y. Kojima, Mater. Trans. JIM 42 (2001) 1206. I.A. Anyanwu, S. Kamado, Y. Kojima, Mater. Trans. JIM 42 (2001) 1212. J.F. Nie, X. Gao, S.M. Zhu, Scr. Mater. 53 (2005) 1049. Y.J. Wu, X.Q. Zeng, D.L. Lin, L.M. Peng, W.J. Ding, J. Alloys Compd 477 (2009) 193. D.J. Li, X.Q. Zeng, J. Dong, C.Q. Zhai, W.J. Ding, J. Alloys Compd. 468 (2009) 164. Y. Gao, Q.D. Wang, J.H. Gu, Y. Zhao, Y. Tong, D.D. Yin, J. Alloys Compd 477 (2009) 374. M. Suzuki, T. Kimura, J. Koike, Mater. Sci. Forum 123 (2003) 419. M. Yamasaki, T. Anan, S. Yoshimoto, Y. Kawamura, Scr. Mater. 53 (2005) 799. T. Itoi, T. Seimiya, Y. Kawamura, M. Hirohashi, Scr. Mater. 51 (2004) 107. M. Nishida, Y. Kawamura, T. Yamamuro, Mater. Sci. Eng. A 375–377 (2004) 1217. K. Liu, J.H. Zhang, W. Sun, X. Qiu, H.Y. Lu, D.X. Tang, L.L. Rokhlin, F.M. Elkin, J. Meng, J. Mater. Sci. 44 (2009) 74. T. Honma, T. Ohkubo, S. Kamado, K. Hono, Acta Mater. 55 (2007) 4137. S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloys Compd. 421 (2006) 309. S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloys Compd. 427 (2007) 316. Q.M. Peng, H.W. Dong, L.D. Wang, Y.M. Wu, L.M. Wang, J. Mater. Sci. 42 (2007) 3908. X.B. Liu, R.S. Chen, E.H. Han, J. Alloys Compd. 465 (2008) 232. T. Honma, T. Ohkubo, k. Hono, S. Kamado, Mater. Sci. Eng. A 395 (2005) 301. T. Honma, T. Ohkubo, k. Hono, S. Kamado, Mater. Sci. Eng. A 404 (2005) 330. P. Vostry, B. Smola, I. Stulikova, F.V. Buch, B.L. Mordike, Phys. Stat. Sol. A 175 (1999) 491. B. Smola, I. Stulikova, F.V. Buch, B.L. Mordike, Mater. Sci. Eng. A 324 (2002) 113. T. Kawabata, K. Matsuda, S. Kamado, Y. Kojima, S. Ikeno, Mater. Sci. Forum 303 (2003) 419. C.H. Careress, A. Blake, Phys. Stat. Sol. A 194 (2002) 147. Y. Kawamura, K. Hayashi, A. Inoue, T. Masumoto, Mater. Trans. JIM 42 (2001) 1172. M. Zhang, S.J. Zhu, G.L. Chen, B.F. Zhang, S.K. Guan, Found. Tech. 26 (2005) 983. J.Y. Liu, Z.Z. Zhang, M. Hua, L.Q. Ma, X.D. Shen, Found. Tech. 27 (2006) 258.
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Influence of Zn content on the microstructure and mechanical properties of extruded Mg–5Y–4Gd–0.4Zr alloy Ke Liu a,b , Jinghuai Zhang a,b , Guihua Su c , Dingxiang Tang a , L.L. Rokhlin d , F.M. Elkin d , Jian Meng a,∗ a State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Graduate School of the Chinese Academy of Science, Beijing 100049, China c Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, China d Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russia
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Article history: Received 10 December 2008 Received in revised form 18 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Magnesium alloys Age hardening Tensile strength Mg–Y–Gd–Zn–Zr
a b s t r a c t Microstructures and mechanical properties of the Mg–5Y–4Gd–xZn–0.4Zr alloys have been investigated. These results show that the Mg–5Y–4Gd–0.5Zn–0.4Zr alloy in the peak-aged condition exhibits the highest tensile strength, and the values of the ultimate tensile strength and yield tensile strength are 370 and 300 MPa, respectively. It is suggested that addition of 0.5% Zn has a great effect on age hardening response. The long periodic stacking structure has been found in these Zn-containing alloys, and the volume fraction of this phase increases with increasing Zn addition. This phase plays an important role in improvement of the mechanical properties, especially for the elongations. The  phase precipitates during the ageing process are responsible for the improvement of the mechanical properties of the alloys in the peak-aged condition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloy is the lightest structural material applicable in aerospace and automobile industry. It is because that the magnesium alloys have high specific strength, high stiffness and good damping capacity. Moreover, the resources of the magnesium are abundant and the products of the magnesium alloys are easy to recycle. The widely used magnesium alloys belong to Mg–Al series which have outstanding castability and low costs. However, these alloys have inferior mechanical properties, especially at high temperature. To the best of our knowledge, addition of rare-earth (RE) elements to the magnesium alloys improves the heat resistance and creep properties, e.g. WE54 commercial alloy [1]. The magnesium alloys containing RE show good precipitation hardening during the peak ageing process [2,3]. It is well known that the binary Mg–Gd alloy is an ideal system for precipitation hardening. However, the binary Mg–Gd alloys containing less than 10% Gd exhibit little or no precipitation hardening response during ageing treatment [4]. Although adding more than 20% Gd can increase the hardness and strength, this improves costs and densities of the magnesium alloys.
The Zn is one of the most important elements which can improve the age hardening responses of the Mg–RE alloys. Moreover, the additions of Zn lead to the formation of the long periodic stacking (LPS) structure [5-7]. Suzuki et al. [8] have been reported that addition of trace amounts of Zn to M–Y alloys contributes to the formation of stacking faults which can retard the movement of the dislocations. Recently, Yamasaki et al. [9] have developed a hotextruded Mg–2.3Zn–14Gd (wt.%) alloy which presents a highest tensile proof strength of 345 MPa and a better elongation of 6.3%, because a coherent 14H LPS structure precipitated from a supersaturated ␣-Mg matrix, which has obviously effect on improving mechanical properties. The microstructures and mechanical properties of the Mg–5Y–4Gd–xZn–0.4Zr (x = 0, 0.5, 1.0, 1.5, 2.0 and are identified as A, B, C, D and E, respectively) have been investigated. The LPS structure has been found in the Zn-containing alloys, and the  phase has been observed in the peak-aged alloys. The effect of the LPS structure and  phase coexisting together on the mechanical properties has been investigated in this article. 2. Experimental procedures
∗ Corresponding author. Tel.: +86 431 85262030; fax: +86 431 85698041. E-mail address: [email protected] (J. Meng). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.119
The alloy ingots with nominal compositions of Mg–5Y–4Gd–xZn–0.4Zr were produced from high-purity Mg (99.5%), high-purity Zn (>99.9%), Mg–20Y (wt.%), Mg–25Gd (wt.%) and Mg–35Zr (wt.%) master alloys in an electric resistance furnace at about 750 ◦ C. The mild steel crucible has been filled with a protective atmosphere.
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At about 730 ◦ C, the melts were poured into a water-cooled iron mold with a diameter of 90 mm. Specimens used for as-cast research were cut from the cylinder-shaped ingots. Parts of them were made into rectangular tensile specimens of 15 mm in gauge length, 3 mm in width and 1.5 mm in thickness, and the remaining is used for other investigations. The cylinder-shaped ingots homogenized at 500 ◦ C for 16 h were milled into diameter of 82 mm, and then extruded into rods in an extrusion ratio of 16.8 at 380 ◦ C. Parts of rods were directly aged at 220 ◦ C in order to investigate the age hardening behaviors with time. The tensile specimens of the as-extruded and peak-aged alloys were also machined into the same geometry as the as-cast samples.
Tensile tests were carried out on a uniaxial tensile testing machine at a primary strain rate of 1 mm/min. The tensile axis was aligned parallel to the extrusion direction. Vickers hardness was measured by a hardness tester with a load of 300 g, a dwelling time of 15 s and 20 measurements were collected for each sample. The microstructures of the alloys were observed by an optical microscope (OM), a scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) and a transmission electron microscope (TEM). The grain sizes of the alloys were measured via using an average linear intercept method. The samples were mechanical polishing and etching in a solution of 2.1 g picric acid, 5 ml acetic acid, 35 ml ethanol and 5 ml H2 O. Samples for TEM observations were prepared with a twin-
Fig. 1. Images of the specimens of the as-cast alloys: (a and b) A, (c and d) B, (e and f) D, (g and h) E.
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Fig. 2. X-ray diffraction patterns of the as-cast alloys A and E.
jet technique. Phase analysis was conducted through the use of X-ray diffraction (XRD).
3. Experimental results 3.1. Microstructures of the as-cast alloys The microstructures of the as-cast alloys A, B, D and E, as shown in Fig. 1(a), (c), (e) and (g), are composed of dendrites of magnesium matrix phases separated by eutectic. Fig. 1(b), (d), (f) and (h) are the OM images of these as-cast alloys A, B, D and E at a higher magnification, respectively, in order to observe the second phases. The images at the higher magnification show that a lamellar phase appears with Zn additions and the volume fraction of this phase increases with increasing the content of Zn. Fig. 2 shows the XRD patterns of the as-cast alloys A and E. It can be observed that the as-cast alloy A are mainly composed of ␣-Mg solid solution together with Mg5 (Y, Gd) and Mg24 (Y, Gd) secondary phases where Gd probably substitutes for Y. However, the XRD results of the as-cast alloy E show that phase constituents includes ␣-Mg solid solution, Mg5 (Y, Gd), MgZn2 and Mg12 Zn(Y, Gd) secondary phases. Fig. 3 shows the SEM images and corresponding EDS results of the as-cast alloy E. The EDS analysis result suggests that the composition of the lamellar phase is Mg–8.39 at.% RE–4.6 at.% Zn, as shown in Fig. 3(c), and RE is standing for Gd and Y. The concentration is similar to those in the 18R LPS structure reported by Itoi et al., i.e. Mg–4 at.% Zn–6 at.% Y, which were obtained by EDS spectra [10]. RE and Zn are mainly contained in this phase. Fig. 3(d) shows the EDS result from the point B as indicated in Fig. 3(a). Fig. 3(b) is an image at a higher magnification obtained from the square particle in Fig. 3(a). The result suggests that the composition of this square particle is Mg–19.27 at.% RE–0.94 at.% Zn, where RE stands for Y and Gd. The chemical composition is close to Mg5 (Y, Gd). 3.2. Microstructures of the as-extruded alloys Fig. 4 shows the optical micrographs of the as-extruded alloys A, B, D and E. Both of the dendrites of the matrix and the coarse Mg12 Zn(Y, Gd) were crushed during hot deformation. These intermetallics were located at the grain boundaries. However, the microstructure of the alloy A is different from that of the Zncontaining alloys. The as-extruded alloy A has relatively coarse ␣-Mg grains, with the average grain size of about 16 m. The grain boundaries of the as-extruded alloy A are clear. However, the alloy with Zn addition has fine ␣-Mg grains, and the average grain sizes of the alloys B, D and E are about 10, 9 and 10 m, respectively. It is suggested that the Zn addition has an obvious effect on refining the
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microstructure compared with the alloy A, as shown in Fig. 4(a), (c), (e) and (f). In addition, lots of intermetallic compounds, especially for the lamellar phase, are observed at the grain boundaries in the alloys B, D and E, and the volume fraction of these intermetallics increases with increasing Zn addition. The dynamic recrystallization (DRX) occurred in these alloys during hot extrusion, as shown in Fig. 4(b), (d), (f) and (h). The recrystallized grains formed and began to grow during hot deformation. The average sizes of the recrystallized grains of the alloys A and E in as-extruded condition are about 10 and 3 m, respectively. It is suggested that the intermetallics, especially for the LPS structure distributed around the matrix, play an important role in limiting the growth of the recrystallized grains. In order to investigate the microstructure of this lamellar phase in these Zn-containing alloys in as-extruded condition, the EDS have been carried out. The EDS results of this lamellar phase obtained from the as-extruded alloy E, i.e. Mg–2.95 at.% RE–1.10 at.% Zn, show that the content of both Zn and RE in this phase decreased but the atom ratio of Zn/RE rose, compared with this alloy in the as-cast condition. Note that this lamellar phase is different from the lamellar phase observed in the as-cast condition. It has been reported that the 18R LPS structure formed, located at the grain boundaries which were rich in Y and Zn, in the Mg–Zn–Y RS alloy ribbons or cast alloy ingots as secondary phase [11,12]. Furthermore, it has been reported that the 18R LPS structure was not stable during heat treatment at 500 ◦ C, and transformed to 14H LPS structure when the cast alloys were heat treated for more than 5 h at 500 ◦ C. However, the 14H LPS structures were not developed during casting and ageing at low temperature in the Mg–Zn–Y alloys [10]. The lamellar phase in the as-extruded alloys might be the 14H LPS structure. Fig. 5(a) shows the TEM image of the lamellar phase obtained from the alloy E in the as-extruded condition. Fig. 5(b) shows the SAED pattern of this phase. The extra reflection-spots in the corresponding selected area electron diffraction (SAED) pattern can be indexed as 00n/14 (corresponding to (002)hcp ) and −11n/14. The diffraction pattern suggests a 14H-type LPS structure which is considered to be the same with the 14H LPS structure observed in rapidly solidified (RS) and annealed RS Mg–Zn–Y alloys. As stated above, the lamellar phase in the as-cast alloys is different from that in the as-extruded alloys. The lamellar phase in the as-cast alloys is the 18R LPS structure, but the lamellar phase in the as-extruded alloys is the 14H LPS structure. The 18R LPS structure transforms to a 14H LPS structure via releasing the RE and Zn into the matrix during heat treatment. 3.3. Ageing hardening behaviors of the as-extruded alloys Fig. 6 shows the age hardening behaviors of the as-extruded alloys A, B, C, D and E at 220 ◦ C. The alloys of both A and B exhibit an obvious age hardening response, while the alloys C, D and E display little ageing hardening behaviors (see Fig. 6). The value of the hardness of the as-extruded alloy A is 87 VHN, and the hardness increases obviously after 10 h, and then peak hardness is obtained after 60 h, with a value of 114 VHN. The age hardening curve of the alloy B exhibits an obvious hardening response at the beginning stage after 12 h, and a peak hardness of 113 VHN is obtained after 85 h. The peak hardness of the alloy C is obtained after 95 h, with a value of 98 VHN. Alloys D and E, both of them containing higher content of Zn compared with the alloy B, do not have an obvious increase in hardness. The alloys D and E reach peak hardness after 70 and 60 h, respectively. The value of the peak hardness of the alloy D is 97 VHN, and the value of the peak hardness of the alloy E is 96 VHN. From Fig. 6, it can be suggested that the peak hardness of these alloys decrease with Zn addition. It has been reported that the Zn
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Fig. 3. SEM images and EDS results of the as-cast alloy E: (a) SEM micrograph, (b) Large magnification of the cubic precipitate corresponding to the point B, (c) and (d) EDS results of the points A and B, respectively.
addition has an obvious effect on improving the elongation at the expense of strength [13]. The additions of Zn lead to a formation of the LPS structure, and the volume fraction of this phase increases with increasing Zn addition, as a result of a decrement of the RE content in the supersaturated solution of the matrix. The decrement of the RE concentration leads to little age hardening response of these Mg–Y–Gd alloys which contain higher Zn, i.e. alloys C, D and E. 3.4. Microstructures of the peak-aged alloys In order to investigate precipitation morphologies and discuss the reasons for the ageing response at 220 ◦ C, the TEM images and corresponding SAED patterns from these peak-aged alloys A, D and E are shown in Fig. 7. A great number of particles which are ellipsoidal in morphology are observed in the specimens of the peak-aged alloy A. These particles are distributed throughout the matrix, and the average size of this phase is smaller while the volume fraction is higher. The average size of these particles in alloy A is about 22 nm in thickness and 49 nm in diameter. However, the average size of these particles in alloy B is bigger than that in alloy A, with a thickness of 30 nm and a diameter of 108 nm. The average size of the particles in the alloy E is 27 nm in thickness and 133 nm in diameter. It is suggested that the average size of this phase increases with increasing Zn addition. It has been reported that the main secondary strengthening phase in Mg–RE, Mg–RE–Zn (RE standing for Gd, Y and Nd) alloys is the  phase at peak hardness [14-17]. Based on previous paper and the SAED patterns, these precipitates are identified to be the  phase. This phase with a bco structure plays an important part in age hardening response in theses alloys during ageing at 220 ◦ C. To the best of our knowledge, at the beginning of the stage, the enhancement of hardness is ascribed to the presence of a
metastable phase, i.e.  phase. It has also been reported that  phase transformed to  phase via releasing RE atom in order to keep the structural coherency with matrix at the peak hardness point, and both of them is isomorphic [18,19]. As shows in Fig. 7, the volume fraction of these particles decreases with increasing Zn addition. The volume fraction of this phase in the alloy A is largest. However, the volume fractions of this phase in alloys D and E are smallest, as shown in Fig. 7(b) and (c). Honma et al. [18] found that the number density of the  phase decreased dramatically by the additions of Zn in the Mg–2.1Gd–1.2Y–xZn (x = 0, 0.3 and 1.0) alloys (at.%), and it led to a large reduction in the age hardening response. As stated above, the addition of Zn has an indirect effect on age hardening response, via which removes the RE elements from the matrix reducing the ability of the magnesium matrix to precipitate Mg–RE phase. 3.5. Mechanical properties of alloys A comparison of the typical mechanical properties of all five alloys in different states is listed in Table 1. The strengths of the as-cast alloys tend to decrease while the elongations of these alloys are inclined to increase with increasing the additions of Zn. The as-cast alloy A has higher ultimate tensile strength (UTS) and yield tensile strength (YTS), and the values of the UTS and YTS are 206 and 114 MPa, respectively, with an elongation of 4.2%. However, the alloys B, C, D and E have interior mechanical properties except the elongation. The values of the UTS and YTS of the alloy E are 188 and 104 MPa, respectively, and the elongation is 7.6%. The mechanical properties of these alloys are evidently improved after hot extrusion, especially for the alloy C, as shown in Table 1. The values of UTS and YTS of alloy C in the as-extruded condition are about 340 and 265 MPa, respectively, with a good elongation of 16.3%. Compared with the alloy C in as-cast con-
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Fig. 4. OM images of the specimens of the as-extruded alloys: (a and b) A, (c and d) B, (e and f) D and (g and h) E.
dition, the UTS is improved by 147 MPa, and the increment of YTS is 150 MPa. The extrusion process is beneficial to improve the mechanical properties of the alloys via microstructural refinement. It should be noted that the as-cast alloys exhibit lower mechanical properties due to a coarse microstructure and lack of uniformity
in the distribution of the LPS structure. However, the mechanical properties of the alloys are greatly improved after hot deformation because of refinement of the microstructure. DRX occurred during hot deformation, and it has obviously effect on refining microstructure. The mechanical properties of the as-extruded alloys increase
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K. Liu et al. / Journal of Alloys and Compounds 481 (2009) 811–818 Table 1 Mechanical properties of the alloys in as-cast and as-extruded conditions. Alloys
States
UTS (MPa)
YTS (MPa)
Elongation (%)
A
Cast Extruded
206 (2.3) 274 (2.1)
114 (1.9) 210 (2.2)
4.2 (0.4) 6.3 (0.6)
B
Cast Extruded
181 (2.6) 318 (3.3)
109 (1.7) 253 (3.0)
5.8 (0.5) 12.1 (1.0)
C
Cast Extruded
193 (2.5) 340 (4.1)
115 (1.7) 265 (3.8)
6.4 (0.4) 16.3 (0.9)
D
Cast Extruded
182 (2.0) 330 (3.7)
110 (1.6) 254 (3.4)
6.2 (0.6) 15.4 (1.0)
E
Cast Extruded
188 (2.1) 315 (3.5)
104 (2.3) 251 (3.1)
7.6 (0.8) 14.8 (0.9)
Note: Standard deviation is given in parentheses.
improved, especially for the alloys A and B. The peak-aged alloy B shows the highest mechanical properties at room temperature, and the values of the UTS and YTS are 370 and 300 MPa, respectively. It is found that tensile strength of the alloy B is improved greatly, and the YTS increases by 47 MPa. The peak-aged alloy A also displays a better tensile strength, with a better YTS of 298 MPa. However, the elongation of the alloy A is inferior, and the value of the elongation is 3.5%. It is known that Mg–Gd–Y alloys exhibit a three-stage precipitation sequence during ageing at 200 ◦ C, ␣ -Mg (S.S.S.S) →  (DO19 ) →  (cbco) →  (fcc) [20,21]. However, it is the  phase, with a cbco structure, distributed throughout the ␣-Mg matrix that contributes most to the age hardening response [22]. It is found that the mechanical properties of the peak-aged alloys are improved enormously but the elongation, especially for the alloy A, as shown in Table 2. Decrease of the elongation is mainly ascribed to the presence of the  phase. It is suggested that this phase is good to tensile strength while bad to deformation plasticity. Fig. 5. (a) TEM image of the as-extruded alloy E and corresponding SAED pattern (B//[1 1 2¯ 0]␣ ) from 14H areas in this alloy (b).
with increasing Zn addition due to the increment of the LPS structure, as shown in Table 1. It is suggested that the LPS phase not only enhances the tensile strength, but also improves the deformation plasticity. Table 2 shows the mechanical properties of the peak-aged alloys. The results show that the tensile strengths of the alloys are
4. Discussion The average grain sizes of the as-extruded alloys A, B, D and E have been obtained. It is found that the average grain sizes of these alloys containing Zn are similar. Furthermore, the tensile strength of these alloys does not show great difference among them. According to the Hall–Petch relationship, the increment of the tensile strength associating with refined microstructure is almost the same. However, the elongation of these alloys shows obvious characteristic. It increases with increasing Zn addition. The value of the YTS of the as-extruded alloy B is 253 MPa, with an increment of 43 MPa compared with the as-extruded alloy A. Additions of Zn at levels of 0.5% to the Mg–5Y–4Gd–0.4Zr alloys result in a 20% increase in the YTS. Although the average grain size of the alloy B in as-extruded condition is smaller than that of the alloy A, this increase in tensile strength is not fully accounted for by the refined grain size. The reduction in grain size from 16 to 10 m is expected to yield strength increment of 18 MPa. Provided that the concentration of Zn in the as-extruded alloy B is only about 0.2 at.%, the strengthening contribution from solid solution of Zn in magnesium is expected to Table 2 Mechanical properties of the peak-aged alloys at 220 ◦ C.
Fig. 6. Age hardening responses of the as-extruded alloys A, B, C, D and E during ageing at 220 ◦ C.
Alloys
State
UTS (MPa)
YTS (MPa)
Elongation (%)
A B C D E
T5 T5 T5 T5 T5
361 (3.1) 370 (2.9) 352 (3.4) 339 (3.3) 324 (3.0)
298 (2.8) 300 (3.4) 285 (3.0) 276 (3.1) 263 (3.2)
3.5 (0.6) 6.0(0.8) 12.4(1.1) 14.2(0.9) 13.5(1.0)
Note: T5-peak aged at 220 ◦ C; standard deviation is given in parentheses.
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Fig. 7. TEM images and corresponding SAED (B//[2 1¯ 1¯ 0]␣ ) patterns of specimens of the peak-aged alloys: (a) alloy A, (b) alloy D and (c) alloy E.
be negligible [23]. In order to investigate the solid solution strength of Zn, the hardness of the matrix of these alloys A, B, C, D and E have been obtained, and the values of the hardness of the matrix of the as-extruded alloys A, B, C, D and E are 87, 85, 88, 91 and 92 VHN, respectively. It gives a proof that an addition of 0.5 wt.% Zn to the Mg–5Y–4Gd–0.4Zr alloy, and the addition of Zn increases from 1 to 2 wt.%, do not lead to an obvious increase in hardness. The LPS structure was regarded as the main secondary phase in the Zn-containing alloys, e.g. alloy B. The strengthening contribution from the 14H LPS structure in the as-extruded alloy B is about 25 MPa. In addition, Kawamura et al. [24] developed the highest strength magnesium alloy with yield strength of 600 MPa, and the LPS structure was regarded as a main secondary strengthening phase. The LPS structure played an important role in the improvement of the mechanical properties [9]. The as-extruded alloy C has the highest tensile strength among the as-extruded alloys. However, the tensile strength of the alloys is not greatly improved with further addition of Zn, such as the alloys D and E in the as-extruded condition. The coarse LPS structure lead to a decrease in the tensile strength.
The 18R LPS structure forms in Mg–Zn–Y alloys via rapid solidification, because increasing the cooling rate and adding Y can reduce the energy barrier for forming this phase [25,26]. Moreover, the volume fraction of this LPS structure increases with increasing additions of Zn. As a result, the content of RE (standing for Gd and Y) retained in the matrix decreased. The more Zn is added, the less RE is left in the matrix. So, when these alloys were aged, the volume fraction of the  phase decreased with increasing addition of Zn. In addition, the number density of the  phase in alloy A was so high that the growth of this phase is restricted, see Fig. 7. Conversely, the size of the  phase in the alloy E was larger. It has been reported that the  phase precipitates with three variants within the matrix in the Zn-containing alloys which have a low number density of  phase, and only the variant along a [0 1 1¯ 0]␣ direction remains during coarsening in the peak-aged condition, and it is because that the self-stress orienting phenomenon occurs by elastic interactions of coherency strains of the precipitates [13]. The high number density of the  phase in the Zn free alloy, i.e. alloy A, restrict the growth of the  phase, and this led to an obvious age hardening response.
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From this investigation, it can be concluded that maximum tensile strength in Mg–5Y–4Gd–0.4Zr alloy is attained when an optimal Zn content is added. Maximum tensile strength is due to the contribution of both fine precipitation and small grain size of the alloy. Thus, Zn content should be enough low to remove many rare elements from the magnesium matrix, in such a way that hardening induced by the precipitation of metastable phases during ageing is not prevented, and enough high to refine the grain size of the alloy. 5. Conclusions
Sciences and Jilin Province and Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow. References [1] [2] [3] [4] [5] [6] [7]
The microstructures and the tensile properties of alloys A, B, C, D and E have been investigated by an OM, XRD, SEM and TEM in this article. The investigation can lead to the following conclusions: 1. The 18R LPS structure was observed in the Zn-containing alloys in as-cast condition, and the volume fraction of this phase increases with increasing Zn addition. 2. Hot extrusion processing had an obvious effect on refinement of the microstructure. DRX occurred during hot extrusion and played an important role in refining the microstructure. 3. The 14H LPS structure has been observed in the as-extruded alloys that contain Zn. This phase formed via releasing Zn and RE atoms from the 18R LPS structure, and the ration of Zn/RE of the 14H LPS structure is higher than that of the 18R LPS structure. 4. The peak-aged alloy B exhibited the highest peak hardness and tensile strength, and the values of the UTS and YTS were 370 and 300 MPa, respectively. It was due to the  phase that precipitated within the matrix during ageing at 220 ◦ C. Acknowledgements This project was supported by Hi-Tech Research and Development Program of China (2006AA03Z520), Chinese Academy of
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
I.J. Polmear, Mater. Sci. Technol. 10 (1994) 1. I.A. Anyanwu, S. Kamado, Y. Kojima, Mater. Trans. JIM 42 (2001) 1206. I.A. Anyanwu, S. Kamado, Y. Kojima, Mater. Trans. JIM 42 (2001) 1212. J.F. Nie, X. Gao, S.M. Zhu, Scr. Mater. 53 (2005) 1049. Y.J. Wu, X.Q. Zeng, D.L. Lin, L.M. Peng, W.J. Ding, J. Alloys Compd 477 (2009) 193. D.J. Li, X.Q. Zeng, J. Dong, C.Q. Zhai, W.J. Ding, J. Alloys Compd. 468 (2009) 164. Y. Gao, Q.D. Wang, J.H. Gu, Y. Zhao, Y. Tong, D.D. Yin, J. Alloys Compd 477 (2009) 374. M. Suzuki, T. Kimura, J. Koike, Mater. Sci. Forum 123 (2003) 419. M. Yamasaki, T. Anan, S. Yoshimoto, Y. Kawamura, Scr. Mater. 53 (2005) 799. T. Itoi, T. Seimiya, Y. Kawamura, M. Hirohashi, Scr. Mater. 51 (2004) 107. M. Nishida, Y. Kawamura, T. Yamamuro, Mater. Sci. Eng. A 375–377 (2004) 1217. K. Liu, J.H. Zhang, W. Sun, X. Qiu, H.Y. Lu, D.X. Tang, L.L. Rokhlin, F.M. Elkin, J. Meng, J. Mater. Sci. 44 (2009) 74. T. Honma, T. Ohkubo, S. Kamado, K. Hono, Acta Mater. 55 (2007) 4137. S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloys Compd. 421 (2006) 309. S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloys Compd. 427 (2007) 316. Q.M. Peng, H.W. Dong, L.D. Wang, Y.M. Wu, L.M. Wang, J. Mater. Sci. 42 (2007) 3908. X.B. Liu, R.S. Chen, E.H. Han, J. Alloys Compd. 465 (2008) 232. T. Honma, T. Ohkubo, k. Hono, S. Kamado, Mater. Sci. Eng. A 395 (2005) 301. T. Honma, T. Ohkubo, k. Hono, S. Kamado, Mater. Sci. Eng. A 404 (2005) 330. P. Vostry, B. Smola, I. Stulikova, F.V. Buch, B.L. Mordike, Phys. Stat. Sol. A 175 (1999) 491. B. Smola, I. Stulikova, F.V. Buch, B.L. Mordike, Mater. Sci. Eng. A 324 (2002) 113. T. Kawabata, K. Matsuda, S. Kamado, Y. Kojima, S. Ikeno, Mater. Sci. Forum 303 (2003) 419. C.H. Careress, A. Blake, Phys. Stat. Sol. A 194 (2002) 147. Y. Kawamura, K. Hayashi, A. Inoue, T. Masumoto, Mater. Trans. JIM 42 (2001) 1172. M. Zhang, S.J. Zhu, G.L. Chen, B.F. Zhang, S.K. Guan, Found. Tech. 26 (2005) 983. J.Y. Liu, Z.Z. Zhang, M. Hua, L.Q. Ma, X.D. Shen, Found. Tech. 27 (2006) 258.