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Microstructures and properties of reciprocatingly extruded Mg-6.4Zn-1.1Y alloys
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Acta Metall. Sin.(Engl. Lett.) V01.21 No.1 pp37-42 Feb. 2008
ACTA METALLURGICA SINICA (ENGLISH LETTERS) www.ams.org.cn
MICROSTRUCTURES AND PROPERTIES OF RECIPROCATINGLY EXTRUDED Mg-6.4Zn-1.1Y ALLOYS Z.M. Zhang', C.J. Xu and X.F. Guo School of Materials Science 8z Engineering, Xi'an University of Technology, Xi'an 710048, China Manuscript received 6 April 2007
A n icosahedml Mg3 YZ% quasicrystalline phase can be pmduced in Mg-Zn- Y system alloys when a proper amount of Z n and Y is contained, and it is feasible to prepare the quasicrystal phase-reinforced low-density magnesium alloy. I n this article, phase constituents and the eflect of reciprocating extrusion on microstructures and properties of the as-cast Mg-6.4Zn-1.1 Y alloy are analyzed. The microstructure of the as-cast Mg-6.4Zn-1.1 Y alloy consists of the a-Mg solid solution, iwsahedml Mg3 1Z l c , quasicrystal, and Mg3 YzZn3 and MgZn2 compounds. After the alloy was reciprocatingly extruded for four passes, grains were refined, Mg3 Y2 Zn3 and MgZn2 phases dissolved into the matrix, whereas, Mg3 YZmj precipitated and distributed uniformly. The alloy possesses the best performance at this state; the tensile strength, yield strength, and elongation are 323.4 MPa, 258.2 MPa, and 19.7%, respectively. I n comparison with that of the as-cast alloy, the tensile strength, yield strength, and elongation of the reciprocatingly extruded alloy increase b y 258.3%, 397.5%) and 18 times, respectively. It is concluded that reciprocating extrusion can substantially improve the properties of the as-cast Mg-6.4Zn-1.1 Y alloy, particularly for elongation. The high performance of the Mg-6.4Zn-1.1 Y alloy after reciprocating extrusion can be attributed to dispersion strengthening and p i n - r e f i n e d microstructures. KEY WORDS Reciprocating extrusion; Mg-Zn- Y alloy; Icosahedml quasicrystal
1. Introduction
Magnesium alloys possess a lot of merits, such as low density, high strength to density ratio, high damping capacity, good workability, and high thermal conductivity. Moreover, they have low cost and can be recycled without pollution. Magnesium alloys have been used in numerous applications in many fields, such as, automobile, communication, and aerospace industries. It is expected that magnesium alloys will be one of the most competitive low-density, high-strength structural materials in the future. Mg-Zn system alloys have proper properties at room temperature. Recently, Mg-Zn-Y alloys have attracted more attention because of their excellent mechanical properties after rapid solidification and extrusion['-4]. The previous studies show that Y can improve the ambient and elevated temperature properties simultaneously 151. Icosahedral quasicrystall Mg3YZng can form in Mg-Zn-Y ternary alloys, when appropriate amount of Zn and Y are included in the alloys, from which, the preparation of quasicrystal reinforced low-density magnesium alloys is feasible [6p71.As a novel extrusion technology, the reciprocating extru*Corresponding author. Tel: +86 29 82312009 ext. 802; Fax: +86 29 82312009 ext. 808. E-mail address: zmzhangQxant.edu.cn;zmzhang1Qhotmail.com (Z.M. Zhang)
. 38 sion can effectively improve mechanical properties of metallic alloys by refining grains and secondary phases. Moreover, the distribution of secondary phases becomes more uniform after extrusion ~ 9 3 . In this article, t.he as-cast Mg-6.4Zn-1.lY alloys were reciprocatingly extruded up to 12 passes. The dependence of microstructure and properties of the alloy on extrusion passes was studied and the mechanism for microstructural evolution and variation of properties was analyzed. 2. Experimental
The nominal chemical composition of the Mg-6.4Zn-1.1Y alloy is 6.4 at. pct Zn and 1.1 at. pct Y. The alloy has been prepared from high purity magnesium (>99.9%), zinc (>99.9%): and the Mg-40 wt pct Y master alloy. The fusion procedure is as follows. First, the surface of pure metals and master alloys was cleaned to remove any oxide and dirt. Then a batch of charges was weighted by using a physical balance, according to the composition of the alloy. Second, a proper amount of complex salts (50%NaC1+50%KCl) was melted in a graphite crucible, t.hen purity magnesium, zinc, and Mg-Y alloy were added into the furnace in an orderly fashion. Finally, the molten metal was poured into a columniform graphite mold after the melt was degassed and deslagged. The melt was protected from oxidation by using a mixture of CO:! with 0.3 vol. pct SF6, during melting and pouring. After the melt was solidified in the graphite mold, the ingot bar was machined into a rod of 50 mm in diameter and 75 mm in length. The rod was heat treated at 350°C for 4 h before extrusion. The die for reciprocating extrusion was made of H13 steel. A hydraulic press used in this work could provide an extrusion force up to 300 tons. When the die was heated to 350"C, the rod was reciprocatingly extruded for 2, 4, 8, and 12 passes: respectively. and then squeezed into a rod with a diameter of 12 mm by direct extrusion at 300°C. The extrusion ratios for reciprocating and direct extrusion were 12.8 and 17.4, respectively. A further detailed description on the extrusion procedure can be found elsewhere !loj. The specimen was prepared based on the standard metallographic approach and etched in an alcoholic solution with 4% nitric acid. A Nikon Epiphot metallographic microscope and a JEOL JSM-6700F scanning electron microscope were used to observe the microstructure, and a Rigaku D/rnax-SC X-ray diffractometer was used to detmined the phase constituents. Regarding the mechanical properties of the alloy, the specimen was machined according to the Chinese Standard GB6397-86: with the gauge length of 36 mm and the gauge diameter of 6 mm. The tensile test was conducted in a WDW3100 electromechanical universal materials testing machine and the stretch rate was 1 mm/min.
3. Results 3.1 hlicrostrulctur-e Fig. 1 shows the microstructures of the as-cast Mg-6.4Zn-l.lY alloy, consisting of coarse
equiaxed deiidrites and interdendritic reticular phases. The maximum primary dendrite arm length is nearly 150 pm. and the maximum secondary dendrite arm length is around
. 39 .
Fig.1 Microstructures of conventionally solidified Mg-6.4Zn-1.1Y alloys.
100 pm. It can be seen clearly that the interdendritic phases show a lamellar and granular morphology, and the lamellar spacing is about 1 pm. Fig.2 shows the X-ray spectrum of the alloy. It indicates that the structure of the alloy contains four phases, that is, the magnesium solid solution w M g , icosahedral MggYZng quasicrystal, ternary compound phase Mg3Y2Zn3, and binary com28deg. pound MgZn2. a-Mg is a primary phase and it is dendritic; other phases form later and Fig2 XRD spectrum of conventionally solidified Mg-6.4Zn-1.1Y alloys. are distributed between the dendrites ill]. In the case of reciprocating extrusion, the metallographic specimen is prepared parallel to the extrusion direction. Fig.3 shows the microstructures of the Mg-6.4Zn-1.1Y alloy after reciprocating extrusion for 2, 4,8, and 12 passes, respectively. The microstructure morphology changes greatly compared with that of the as-cast state, and the fibrous microstructure that distributes along the extrusion direction dominates. Coarse dendrite
Fig.3 Microstructures of Mg-6.4Zn-1.1Y alloy after reciprocating extrusion for 2 (a), 4(b), 8 (c) and 12 (d) passes.
. 40
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0-hlg and interdendritic reticular phases cannot be observed any longer. Fine secondary phases distribute in the matrix uniforrnly. It can be observed that the size of the secondary phases increases slightly as the extrusion pass increases. Fig.4 shows the matrix morphology of the alloy. Grains are equiaxed and their average size is about 5 pm. In comparison with Fig.1. it can be F i g 4 Matrix morphology of as-cast Mg-6.4Znreadily seen that grains are refined substan1.1Y alloy reciprocatingly extruded for 4 tially passes. The X-ray diffraction profile of the Mgti.4Zn-1 . l Y alloy, after reciprocating extrusion for four passes, is shown in Fig.5. It, can be observed that there are only two phases, that is, 0-hIg and icosahedral h'Ig3YZn~quasicrystal, in the alloy. W-hen comparing Fig.5 with Fig.2, it can be seen that the diffraction peaks of the hIg3YZns phases are higher than those in t,ht as-cast state, indicating that Mg3Y2Zii3 and MgZn2 may decompose or dissolve into the matrix, thus producing more RlgJYZnG phases. It can be con- Fig.5 XRD spectrum of Mg-6.4Zn-1.1Y alloy recip rocatingly extruded for 4 passes. cluded that the fine secondary granules in the microstructure of the RIg-6.4Zn-1.1Y alloy shown in Figs.3 and 4 are all Mg3YZn6 phases, and the microstructure of the four-pass reciprocatingly extruded R~lg-6.4Zn-l.lYalloy consists of ct-Mg and icosahedral Mg3YZn6 qunsicryst al.
3.2 Properties The properties of the hlg-6.4Zn-l.lY alloy after reciprocating extrusion for various passes are shown in Table 1. For comparison, the counterparts of the as-cast Mg-6.4Zn-1.1Y alloy are also included in the list. It can be seen that the properties of the Mg-6.4Zn-1.1Y alloy have been improved remarkably after reciprocating extrusion. The tensile strength arid yield strength of the alloy, after reciprocating extrusion for 2 passes, has increased to 312.5 hlPa and 117.6 MPa, respectively, and the elongation increases more significantly from 1.176 at the as-cast state to 20%. After the alloy is processed by the reciprocating ext,rrision for 4 passes, elongation remains almost unchanged, whereas, tensile strength and yield strength increase further and reach 323.4 MPa and 258.2 MPa, respectively. In c-omparisoii with t.hat obtained in the as-cast state, the tensile strength, yield strength, a ~ i delongation of the alloy, after four-pass reciprocating extrusion, increase by 258.3%, 3Yi.554. arid 18 times, respectively. More reciprocating extrusion cycles can lower the tensile strclngth and yield strength, and slightly increase the elongation. It can be concluded that reciprocating extrusion is an effective technology for improving the tensile strength and yield strength of the hIg-6.4Zn-l.lY alloy, especially with a remarkable increase in
. 41 plasticity. Table 1 Mechanical properties of as-cast and reciprocatingly extruded Mg-6.4Zn-1.1Y alloys Extrusion pass 0 (as-cast) 2 4 8 12
Tensile strength/MPa 90.5 312.5 323.4 327.2 215.0
Yield strength/MPa 51.9 117.6 258.2 202.4 178.0
Elongation/% 1.1 20.0 19.7 18.3 23.1
After comparing the properties of commonly used deformable magnesium alloys, such as, MB15 and MB21 [12],it can be seen that the Mg-6.4Zn-1.1Y alloy, after four-pass reciprocating extrusion, possesses the most excellent properties. 4. Discussion
4.1 Microstructure evolution In the as-cast Mg-6.4Zn-1.1Y alloy, four phases, that is, a-Mg, M&YZng, MgsYzZn3, and MgZn2 can be identified. After being reciprocatingly extruded for 4 passes, Mg3Y2Zn3 and MgZn2 compounds disappear, and the alloy has only two phases, that is, w M g and Mg3YZng. MgZn2 may decompose and/or dissolve into the matrix during the long-time extrusion process. Meanwhile, the Mg3Y2Zn3 compound becomes unstable at high temperatures. The ratio of Zn to Y in the Mg3Y2Zn3 phase will accommodate, on account of atom diffusion, and eventually transform into Mg3YZng. The MggYZng phase possesses an icosahedral quasicrystalline structure. It is hard and brittle at room temperature, whereas, it breaks into small pieces under extrusion pressure. It has been revealed that the Mg3YZng phase can precipitate from a-Mg matrix, during hot extrusion [131. Therefore, the amount of quasicrystal MggYZng phases may increase as the extrusion pass increases. The original and precipitated MggYZng quasicrystal can coarsen during further hot processing, making the size of the MgsYZng phases increase slightly with an increase in the extrusion pass. Dynamic recrystallization can occur in the alloy at the extrusion temperature; thus an equiaxed dynamic recrystallized microstructure can be yielded after reciprocating extrusion. Furthermore, the fine and uniformly distributed MgsYZng phases can impede the growth of the grains, and thus, a finegrained structure can be obtained after reciprocating extrusion.
4.2 Properties The mechanical properties of the reciprocatingly extruded Mg-6.4Zn-i. 1Y alloy are much higher than that of the as-cast alloy. The reason for the strengthening can be analyzed as follows. The coarse dendrites, reticular interdendritic phases, and shrinkage porosity in the cast Mg-6.4Zn-1.1Y alloy will dissever the matrix and impose a negative effect on the alloy properties. Reciprocating extrusion technology integrates the ordinary extrusion with mushrooming deformation at one pass. Shrinkage porosity can be flattened under extrusion pressure, and the inner surface of the porosity may contact and join each other. Thus the alloy gets denser. Hot extrusion deformation can break up coarse dendrites and speed up atom diffusion, thus decrease segregation and improve'the properties of the alloy.
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42
More icosahedral hIg3YZns qua4crystal phases form and distribute uniformly during reciprocating extrusion. The TLlggYZng phases obtained under the as-cast state are fragmciitized and thus become finer. Note that MgjYZng phases are of significant importance in improving the properties of the alloy efficiently, by means of the dispersion strengthening mechanism. Moreover. the grain refinement resulting from the reciprocating extrusion deforination and dynamic recrystallization plays a prominent role in increasing the strength of the Mg-6.4Zn-l.lY alloy. According to the Hall-Petch relationship, the smaller the grain sizes in a crystalline material, the higher the yield strength that can be obtained. Moreover, the fitting parameter in the Hall-Petch relationship for magnesium is high [141 and thus grain refinement can substantially improve the properties of the Mg-6.4Zn-1.1Y alloy. 5 . Conclusions
The as-cast microstructure of the Mg-6.4Zn-1.1Y alloy consists of magnesium solid solution, icosahedral hlgsYZn6 quasicrystal, hlg~Y2Zn3,and MgZn2 compound. After the alloy is reciprocatingly extruded for 4 passes, RIg3Y2Zn3 and MgZn2 disappear, whereas, more hiIg3YZnG phases are yielded and the original Mg3YZn6 phases are refined. By using the reciprocating extrusion technique, the mechanical properties of the Mg-6.4Zn-1.1Y alloy can be reniarkably improved, especially for plasticity. The alloy shows the best performance after reciprocating extrusion for 4 passes, that is, the tensile strength, yield strength, and elongation of the alloy are 323.4 MPa, 258.2 MPa, and 19.7%, respectively. In comparison with that obtained under the as-cast state, the tensile strength and yield strength of the alloy increase by 258.3% and 397.5%, and the elongation increases 18 times. Reciprocating extrusion is helpful in precipitating new Mg3YZng phases and thus refining the matrix grains and Mg3YZn6 phases. Dispersion strengthening and grain refinement strengthening are two primary mechanisnis for the improvement of reciprocatingly extruded Mg-6.4Zn1.lY alloys. Acknow1edgement.r--- This work UIUS supported by the National Natural Science Foundation of China (Grant N o . 502'71 054). Shaanxi Provincial Nature Scientific Research Project (Grant No. 2003El 11). and S R F f o r ROCS, SEM (101-220325).
REFERENCES [ l ] X.F. Guo and D. Shechtnian. Glass Phys. Chem. 31(1) (2005) 44.
121 I1.H. Rae, M.H. Lee. K.7'. Kim. W.T. Kim and D.H. Kim, J . Alloy. Compd. 342(1-2) (2002) 445. 131 Y. Kawamura. I(.Hayashi. A. Inoue and T. hlasumoto, Mater. h n s . 42 (2001) 1172. j4j Z.P. Luo. D.Y. Song and S.Q. Zhang. J. Alloy. Compd. 230(2) (1995) 109. [5j M. Matsuda, S. Ii. . ' 1 Kawamura, Y. Ikuhara and M. Nishida, Mater. Sci. Eng. A 3 9 3 ( 1 - 2 ) (2005) 269. [S] F. Shi. X.F. (:uo and Z.M. Zhang, Ch,in. J. N o n f e m u s Met. 141 (2004) 112. [7! S . Yi, E.S. Park. J.B. Ok.1V.T. Kim and D.H.Kim,Micron 33 (2002) 565. [8! H.S. Chi]. K.S.Liu and J.W. Yeh. Mater. Sci. Eng. A 2 7 7 ( 1 - 2 ) (2000) 25. [9] S.W. Lee. H.Y. b'ang. Y.L. Chen, J . W . Yeh and C.F. Yang, Adv. Eng. Mater. 6 ( 1 2 ) (2004) 948. 101 L. Liu. C.J. Xu. Z.M. Zhang and X.F. Guo, 7hn.s. Mater. Heat Pmatment 27(3) (2006) 50. 111 J.N. Feng. X . F . Guo. C.J. Xu and Z.h-1. Zhang, J. Chin. Rare Earth SOC.2 4 (2006) 86. ,121 Z.H. Clien. DeJornicsble Magnesium Alloys (Beijing: Chemical Industry Press, 2005) p.353. 131 I..J. Kim. D.H. Bat. and D.H. Kim, Mater. Sci. Eng. A 3 5 9 ( 1 - 2 ) (2003) 313. 141 J. Zhang arid Z.H. Zhang, Magnesium Alloys and their Applications (Beijing: Chemical Industry Press, '2004) p.78.
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ScienceDirect
Acta Metall. Sin.(Engl. Lett.) V01.21 No.1 pp37-42 Feb. 2008
ACTA METALLURGICA SINICA (ENGLISH LETTERS) www.ams.org.cn
MICROSTRUCTURES AND PROPERTIES OF RECIPROCATINGLY EXTRUDED Mg-6.4Zn-1.1Y ALLOYS Z.M. Zhang', C.J. Xu and X.F. Guo School of Materials Science 8z Engineering, Xi'an University of Technology, Xi'an 710048, China Manuscript received 6 April 2007
A n icosahedml Mg3 YZ% quasicrystalline phase can be pmduced in Mg-Zn- Y system alloys when a proper amount of Z n and Y is contained, and it is feasible to prepare the quasicrystal phase-reinforced low-density magnesium alloy. I n this article, phase constituents and the eflect of reciprocating extrusion on microstructures and properties of the as-cast Mg-6.4Zn-1.1 Y alloy are analyzed. The microstructure of the as-cast Mg-6.4Zn-1.1 Y alloy consists of the a-Mg solid solution, iwsahedml Mg3 1Z l c , quasicrystal, and Mg3 YzZn3 and MgZn2 compounds. After the alloy was reciprocatingly extruded for four passes, grains were refined, Mg3 Y2 Zn3 and MgZn2 phases dissolved into the matrix, whereas, Mg3 YZmj precipitated and distributed uniformly. The alloy possesses the best performance at this state; the tensile strength, yield strength, and elongation are 323.4 MPa, 258.2 MPa, and 19.7%, respectively. I n comparison with that of the as-cast alloy, the tensile strength, yield strength, and elongation of the reciprocatingly extruded alloy increase b y 258.3%, 397.5%) and 18 times, respectively. It is concluded that reciprocating extrusion can substantially improve the properties of the as-cast Mg-6.4Zn-1.1 Y alloy, particularly for elongation. The high performance of the Mg-6.4Zn-1.1 Y alloy after reciprocating extrusion can be attributed to dispersion strengthening and p i n - r e f i n e d microstructures. KEY WORDS Reciprocating extrusion; Mg-Zn- Y alloy; Icosahedml quasicrystal
1. Introduction
Magnesium alloys possess a lot of merits, such as low density, high strength to density ratio, high damping capacity, good workability, and high thermal conductivity. Moreover, they have low cost and can be recycled without pollution. Magnesium alloys have been used in numerous applications in many fields, such as, automobile, communication, and aerospace industries. It is expected that magnesium alloys will be one of the most competitive low-density, high-strength structural materials in the future. Mg-Zn system alloys have proper properties at room temperature. Recently, Mg-Zn-Y alloys have attracted more attention because of their excellent mechanical properties after rapid solidification and extrusion['-4]. The previous studies show that Y can improve the ambient and elevated temperature properties simultaneously 151. Icosahedral quasicrystall Mg3YZng can form in Mg-Zn-Y ternary alloys, when appropriate amount of Zn and Y are included in the alloys, from which, the preparation of quasicrystal reinforced low-density magnesium alloys is feasible [6p71.As a novel extrusion technology, the reciprocating extru*Corresponding author. Tel: +86 29 82312009 ext. 802; Fax: +86 29 82312009 ext. 808. E-mail address: zmzhangQxant.edu.cn;zmzhang1Qhotmail.com (Z.M. Zhang)
. 38 sion can effectively improve mechanical properties of metallic alloys by refining grains and secondary phases. Moreover, the distribution of secondary phases becomes more uniform after extrusion ~ 9 3 . In this article, t.he as-cast Mg-6.4Zn-1.lY alloys were reciprocatingly extruded up to 12 passes. The dependence of microstructure and properties of the alloy on extrusion passes was studied and the mechanism for microstructural evolution and variation of properties was analyzed. 2. Experimental
The nominal chemical composition of the Mg-6.4Zn-1.1Y alloy is 6.4 at. pct Zn and 1.1 at. pct Y. The alloy has been prepared from high purity magnesium (>99.9%), zinc (>99.9%): and the Mg-40 wt pct Y master alloy. The fusion procedure is as follows. First, the surface of pure metals and master alloys was cleaned to remove any oxide and dirt. Then a batch of charges was weighted by using a physical balance, according to the composition of the alloy. Second, a proper amount of complex salts (50%NaC1+50%KCl) was melted in a graphite crucible, t.hen purity magnesium, zinc, and Mg-Y alloy were added into the furnace in an orderly fashion. Finally, the molten metal was poured into a columniform graphite mold after the melt was degassed and deslagged. The melt was protected from oxidation by using a mixture of CO:! with 0.3 vol. pct SF6, during melting and pouring. After the melt was solidified in the graphite mold, the ingot bar was machined into a rod of 50 mm in diameter and 75 mm in length. The rod was heat treated at 350°C for 4 h before extrusion. The die for reciprocating extrusion was made of H13 steel. A hydraulic press used in this work could provide an extrusion force up to 300 tons. When the die was heated to 350"C, the rod was reciprocatingly extruded for 2, 4, 8, and 12 passes: respectively. and then squeezed into a rod with a diameter of 12 mm by direct extrusion at 300°C. The extrusion ratios for reciprocating and direct extrusion were 12.8 and 17.4, respectively. A further detailed description on the extrusion procedure can be found elsewhere !loj. The specimen was prepared based on the standard metallographic approach and etched in an alcoholic solution with 4% nitric acid. A Nikon Epiphot metallographic microscope and a JEOL JSM-6700F scanning electron microscope were used to observe the microstructure, and a Rigaku D/rnax-SC X-ray diffractometer was used to detmined the phase constituents. Regarding the mechanical properties of the alloy, the specimen was machined according to the Chinese Standard GB6397-86: with the gauge length of 36 mm and the gauge diameter of 6 mm. The tensile test was conducted in a WDW3100 electromechanical universal materials testing machine and the stretch rate was 1 mm/min.
3. Results 3.1 hlicrostrulctur-e Fig. 1 shows the microstructures of the as-cast Mg-6.4Zn-l.lY alloy, consisting of coarse
equiaxed deiidrites and interdendritic reticular phases. The maximum primary dendrite arm length is nearly 150 pm. and the maximum secondary dendrite arm length is around
. 39 .
Fig.1 Microstructures of conventionally solidified Mg-6.4Zn-1.1Y alloys.
100 pm. It can be seen clearly that the interdendritic phases show a lamellar and granular morphology, and the lamellar spacing is about 1 pm. Fig.2 shows the X-ray spectrum of the alloy. It indicates that the structure of the alloy contains four phases, that is, the magnesium solid solution w M g , icosahedral MggYZng quasicrystal, ternary compound phase Mg3Y2Zn3, and binary com28deg. pound MgZn2. a-Mg is a primary phase and it is dendritic; other phases form later and Fig2 XRD spectrum of conventionally solidified Mg-6.4Zn-1.1Y alloys. are distributed between the dendrites ill]. In the case of reciprocating extrusion, the metallographic specimen is prepared parallel to the extrusion direction. Fig.3 shows the microstructures of the Mg-6.4Zn-1.1Y alloy after reciprocating extrusion for 2, 4,8, and 12 passes, respectively. The microstructure morphology changes greatly compared with that of the as-cast state, and the fibrous microstructure that distributes along the extrusion direction dominates. Coarse dendrite
Fig.3 Microstructures of Mg-6.4Zn-1.1Y alloy after reciprocating extrusion for 2 (a), 4(b), 8 (c) and 12 (d) passes.
. 40
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0-hlg and interdendritic reticular phases cannot be observed any longer. Fine secondary phases distribute in the matrix uniforrnly. It can be observed that the size of the secondary phases increases slightly as the extrusion pass increases. Fig.4 shows the matrix morphology of the alloy. Grains are equiaxed and their average size is about 5 pm. In comparison with Fig.1. it can be F i g 4 Matrix morphology of as-cast Mg-6.4Znreadily seen that grains are refined substan1.1Y alloy reciprocatingly extruded for 4 tially passes. The X-ray diffraction profile of the Mgti.4Zn-1 . l Y alloy, after reciprocating extrusion for four passes, is shown in Fig.5. It, can be observed that there are only two phases, that is, 0-hIg and icosahedral h'Ig3YZn~quasicrystal, in the alloy. W-hen comparing Fig.5 with Fig.2, it can be seen that the diffraction peaks of the hIg3YZns phases are higher than those in t,ht as-cast state, indicating that Mg3Y2Zii3 and MgZn2 may decompose or dissolve into the matrix, thus producing more RlgJYZnG phases. It can be con- Fig.5 XRD spectrum of Mg-6.4Zn-1.1Y alloy recip rocatingly extruded for 4 passes. cluded that the fine secondary granules in the microstructure of the RIg-6.4Zn-1.1Y alloy shown in Figs.3 and 4 are all Mg3YZn6 phases, and the microstructure of the four-pass reciprocatingly extruded R~lg-6.4Zn-l.lYalloy consists of ct-Mg and icosahedral Mg3YZn6 qunsicryst al.
3.2 Properties The properties of the hlg-6.4Zn-l.lY alloy after reciprocating extrusion for various passes are shown in Table 1. For comparison, the counterparts of the as-cast Mg-6.4Zn-1.1Y alloy are also included in the list. It can be seen that the properties of the Mg-6.4Zn-1.1Y alloy have been improved remarkably after reciprocating extrusion. The tensile strength arid yield strength of the alloy, after reciprocating extrusion for 2 passes, has increased to 312.5 hlPa and 117.6 MPa, respectively, and the elongation increases more significantly from 1.176 at the as-cast state to 20%. After the alloy is processed by the reciprocating ext,rrision for 4 passes, elongation remains almost unchanged, whereas, tensile strength and yield strength increase further and reach 323.4 MPa and 258.2 MPa, respectively. In c-omparisoii with t.hat obtained in the as-cast state, the tensile strength, yield strength, a ~ i delongation of the alloy, after four-pass reciprocating extrusion, increase by 258.3%, 3Yi.554. arid 18 times, respectively. More reciprocating extrusion cycles can lower the tensile strclngth and yield strength, and slightly increase the elongation. It can be concluded that reciprocating extrusion is an effective technology for improving the tensile strength and yield strength of the hIg-6.4Zn-l.lY alloy, especially with a remarkable increase in
. 41 plasticity. Table 1 Mechanical properties of as-cast and reciprocatingly extruded Mg-6.4Zn-1.1Y alloys Extrusion pass 0 (as-cast) 2 4 8 12
Tensile strength/MPa 90.5 312.5 323.4 327.2 215.0
Yield strength/MPa 51.9 117.6 258.2 202.4 178.0
Elongation/% 1.1 20.0 19.7 18.3 23.1
After comparing the properties of commonly used deformable magnesium alloys, such as, MB15 and MB21 [12],it can be seen that the Mg-6.4Zn-1.1Y alloy, after four-pass reciprocating extrusion, possesses the most excellent properties. 4. Discussion
4.1 Microstructure evolution In the as-cast Mg-6.4Zn-1.1Y alloy, four phases, that is, a-Mg, M&YZng, MgsYzZn3, and MgZn2 can be identified. After being reciprocatingly extruded for 4 passes, Mg3Y2Zn3 and MgZn2 compounds disappear, and the alloy has only two phases, that is, w M g and Mg3YZng. MgZn2 may decompose and/or dissolve into the matrix during the long-time extrusion process. Meanwhile, the Mg3Y2Zn3 compound becomes unstable at high temperatures. The ratio of Zn to Y in the Mg3Y2Zn3 phase will accommodate, on account of atom diffusion, and eventually transform into Mg3YZng. The MggYZng phase possesses an icosahedral quasicrystalline structure. It is hard and brittle at room temperature, whereas, it breaks into small pieces under extrusion pressure. It has been revealed that the Mg3YZng phase can precipitate from a-Mg matrix, during hot extrusion [131. Therefore, the amount of quasicrystal MggYZng phases may increase as the extrusion pass increases. The original and precipitated MggYZng quasicrystal can coarsen during further hot processing, making the size of the MgsYZng phases increase slightly with an increase in the extrusion pass. Dynamic recrystallization can occur in the alloy at the extrusion temperature; thus an equiaxed dynamic recrystallized microstructure can be yielded after reciprocating extrusion. Furthermore, the fine and uniformly distributed MgsYZng phases can impede the growth of the grains, and thus, a finegrained structure can be obtained after reciprocating extrusion.
4.2 Properties The mechanical properties of the reciprocatingly extruded Mg-6.4Zn-i. 1Y alloy are much higher than that of the as-cast alloy. The reason for the strengthening can be analyzed as follows. The coarse dendrites, reticular interdendritic phases, and shrinkage porosity in the cast Mg-6.4Zn-1.1Y alloy will dissever the matrix and impose a negative effect on the alloy properties. Reciprocating extrusion technology integrates the ordinary extrusion with mushrooming deformation at one pass. Shrinkage porosity can be flattened under extrusion pressure, and the inner surface of the porosity may contact and join each other. Thus the alloy gets denser. Hot extrusion deformation can break up coarse dendrites and speed up atom diffusion, thus decrease segregation and improve'the properties of the alloy.
.
42
More icosahedral hIg3YZns qua4crystal phases form and distribute uniformly during reciprocating extrusion. The TLlggYZng phases obtained under the as-cast state are fragmciitized and thus become finer. Note that MgjYZng phases are of significant importance in improving the properties of the alloy efficiently, by means of the dispersion strengthening mechanism. Moreover. the grain refinement resulting from the reciprocating extrusion deforination and dynamic recrystallization plays a prominent role in increasing the strength of the Mg-6.4Zn-l.lY alloy. According to the Hall-Petch relationship, the smaller the grain sizes in a crystalline material, the higher the yield strength that can be obtained. Moreover, the fitting parameter in the Hall-Petch relationship for magnesium is high [141 and thus grain refinement can substantially improve the properties of the Mg-6.4Zn-1.1Y alloy. 5 . Conclusions
The as-cast microstructure of the Mg-6.4Zn-1.1Y alloy consists of magnesium solid solution, icosahedral hlgsYZn6 quasicrystal, hlg~Y2Zn3,and MgZn2 compound. After the alloy is reciprocatingly extruded for 4 passes, RIg3Y2Zn3 and MgZn2 disappear, whereas, more hiIg3YZnG phases are yielded and the original Mg3YZn6 phases are refined. By using the reciprocating extrusion technique, the mechanical properties of the Mg-6.4Zn-1.1Y alloy can be reniarkably improved, especially for plasticity. The alloy shows the best performance after reciprocating extrusion for 4 passes, that is, the tensile strength, yield strength, and elongation of the alloy are 323.4 MPa, 258.2 MPa, and 19.7%, respectively. In comparison with that obtained under the as-cast state, the tensile strength and yield strength of the alloy increase by 258.3% and 397.5%, and the elongation increases 18 times. Reciprocating extrusion is helpful in precipitating new Mg3YZng phases and thus refining the matrix grains and Mg3YZn6 phases. Dispersion strengthening and grain refinement strengthening are two primary mechanisnis for the improvement of reciprocatingly extruded Mg-6.4Zn1.lY alloys. Acknow1edgement.r--- This work UIUS supported by the National Natural Science Foundation of China (Grant N o . 502'71 054). Shaanxi Provincial Nature Scientific Research Project (Grant No. 2003El 11). and S R F f o r ROCS, SEM (101-220325).
REFERENCES [ l ] X.F. Guo and D. Shechtnian. Glass Phys. Chem. 31(1) (2005) 44.
121 I1.H. Rae, M.H. Lee. K.7'. Kim. W.T. Kim and D.H. Kim, J . Alloy. Compd. 342(1-2) (2002) 445. 131 Y. Kawamura. I(.Hayashi. A. Inoue and T. hlasumoto, Mater. h n s . 42 (2001) 1172. j4j Z.P. Luo. D.Y. Song and S.Q. Zhang. J. Alloy. Compd. 230(2) (1995) 109. [5j M. Matsuda, S. Ii. . ' 1 Kawamura, Y. Ikuhara and M. Nishida, Mater. Sci. Eng. A 3 9 3 ( 1 - 2 ) (2005) 269. [S] F. Shi. X.F. (:uo and Z.M. Zhang, Ch,in. J. N o n f e m u s Met. 141 (2004) 112. [7! S . Yi, E.S. Park. J.B. Ok.1V.T. Kim and D.H.Kim,Micron 33 (2002) 565. [8! H.S. Chi]. K.S.Liu and J.W. Yeh. Mater. Sci. Eng. A 2 7 7 ( 1 - 2 ) (2000) 25. [9] S.W. Lee. H.Y. b'ang. Y.L. Chen, J . W . Yeh and C.F. Yang, Adv. Eng. Mater. 6 ( 1 2 ) (2004) 948. 101 L. Liu. C.J. Xu. Z.M. Zhang and X.F. Guo, 7hn.s. Mater. Heat Pmatment 27(3) (2006) 50. 111 J.N. Feng. X . F . Guo. C.J. Xu and Z.h-1. Zhang, J. Chin. Rare Earth SOC.2 4 (2006) 86. ,121 Z.H. Clien. DeJornicsble Magnesium Alloys (Beijing: Chemical Industry Press, 2005) p.353. 131 I..J. Kim. D.H. Bat. and D.H. Kim, Mater. Sci. Eng. A 3 5 9 ( 1 - 2 ) (2003) 313. 141 J. Zhang arid Z.H. Zhang, Magnesium Alloys and their Applications (Beijing: Chemical Industry Press, '2004) p.78.