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Effect of aging on microstructure of Mg-Zn-Er alloys
JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009, p. 1042
Effect of aging on microstructure of Mg-Zn-Er alloys LI Jianhui (ᴢᓎ䕝), DU Wenbo (ᴰ᭛म), LI Shubo (ᴢ⎥⊶), WANG Zhaohui (⥟ᳱ䕝) (School of Materials Science and Engineering, University of Technology Beijing, Beijing 100124, China) Received 4 August 2008; revised 8 September 2008
Abstract: The effect of aging on microstructure of Mg-Zn-Er alloys at 473 K was investigated using hardness measurement, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that both Mg3.8Zn1.5Er and Mg5Zn2.0Er alloys exhibited visible age-hardening effect, especially the latter alloy. Microstructure analysis showed that, after being aged, lots of fine MgZn2 phases with hexagonal structure were found in the Į-Mg matrix. Comparing with Mg3.8Zn1.5Er alloy, the accelerated hardening response of Mg5Zn2.0Er should be contributed to the increase in the content of Zn and Er elements. Keywords: Mg-Zn-Er alloys; aging behavior; microstructure; rare earths
An important class of magnesium alloys is the alloys containing rare earth elements, such as Mg-Al-RE, Mg-Y-RE-Zr and Mg-Zn-RE[1–3], developed for elevated temperature applications. Generally, cerium-based rare earth is added to the magnesium alloys containing aluminum as the dominant alloying element for the improvement of the mechanical properties through the formation of second-phase with high thermally stability[4,5]. The improvement of mechanical properties by adding rare earth with larger solid solubility in magnesium such as Y, Gd and Nd is attributed to solution hardening and precipitation of fine dispersion of intermetallic particles[6–8]. Recently, Mg-ZnRE (RE=Y, Gd) alloys have attracted significant interest because of their good mechanical properties through in situ forming icosahedral quasicrystalline phase (I-phase)[9–11], and I-phase as the strengthening phase is favorable for enhancing mechanical properties of the alloys. Among the rare earth metals, the maximum solubility of erbium in magnesium at 817 K is 32.7 wt.% (6.56 at.%), but decreases to 18.5 wt.% (3.17 at.%) at 573 K, causing great effect of solution and precipitation hardening[12]. The I-phase has been found in Zn-rich alloys Zn-Mg-Er alloys[13,14]. However, up to now, few works are about the effect of erbium on magnesium alloys. Recently, the authors investigated the microstructure of as-cast Mg-Zn-Er alloy and found the I-phase[15]. In the present paper, investigation was carried out on the aging behavior of Mg-Zn-Er alloys at 473 K.
1 Experimental The two test alloys were Mg3.8Zn1.5Er (Alloy 1) and Mg5Zn2.0Er (Alloy 2) with nominal composition in mass fraction. The ingots were prepared by 99.9% Mg, 99.9% Zn and Mg-Er master alloy in a graphite crucible under anti-oxidizing flux in an electric resistance furnace. The melt was poured into a steel mould and then cooled in air. The two ingots were homogenized at 713 K for 8 h before aging. Microstructures were observed by scanning electron microscopy (SEM, FEI Quanta 200) with energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM, JEM-2010). TEM specimens were prepared by electro-polishing and ion beam milling at an incident angle less than 10°, and electro-polishing of the samples was operated in a solution of 20% nitric acid and 80% methanol, using liquid nitrogen cold stage to prevent heating. The homogenized samples were aged at 473 K for 6, 12, 24, 36, 48, and 60 h, respectively. Vickers hardness was carried out on Vickers hardness tester (HXD-1000), and the test load and dwell time was 0.25 N and 15 s, respectively. Each hardness value was the average of at least seven measurements.
2 Results and discussion 2.1 Microstructure Fig. 1 shows SEM micrographs of the two as-cast alloys.
Foundation item: Project supported by National Major Fundamental Research Program of China (2007CB613706) Corresponding author: DU Wenbo (E-mail: [email protected]; Tel.: +86-10-67392917) DOI: 10.1016/S1002-0721(08)60385-3
LI Jianhui et al., Effect of aging on microstructure of Mg-Zn-Er alloys
It indicates that Alloy 2 has smaller average interdendritic distance as compared to Alloy 1, which might be attributed to the refinement of erbium. Moreover, with the increase of alloying elements Zn and Er, more second-phases form in Į-Mg matrix of Alloy 2 and they are mainly distributed along grain boundaries. Both alloys have the same phases. Fig. 2 shows SEM micrographs ofthehomogenized Alloy 1 and Alloy 2. It shows that there is no distinct change observed in microstructure after being homogenized. And it is more clearly observed that the second-phases distributed along the grain boundaries become much coarser with the increase of Zn and Er contact at higher magnification (see Fig. 2(b)). EDS analysis indicates that the precipitate consists of Mg, Zn and Er, that is, Mg-Zn-Er ternary phase. Moreover, The EDS analysis also reveals that some of added Zn went into Į-Mg matrix. 2.2 Age hardening response The microhardness value curves of Alloy 1 and Alloy 2 aged at 473 K for various time intervals are shown in Fig. 3. The microhardness increases before reaching their peak
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values, then gradually decreases for both alloys, indicating noticeable age-hardening response. The peak-hardness value of 63 for Alloy 1 occurs after about 48 h of aging. For Alloy 2, the peak-hardness value of 68 occurs after about 36 h of aging. The higher peak-hardness value obtained with less time for Alloy 2 shows that its aging efficiency is higher than that of Alloy 1. Fig. 4 shows SEM micrographs of Alloy 2 after being aged for 36 h. Comparing the microstructure with the homogenized alloys (see Fig.2 (b)), the grain boundaries become clearer and the intermetallic compounds become more dispersed (see Fig.4 (a)). With the increase in aging time, much finer precipitates appear within the grains. At the peak-aging time, a large number of acicular or granular fine precipitates distribute homogeneously within the grains. The microstructure details of peak-aged Alloy 2 are resolved more clearly under higher magnification (see Fig.4 (b)). The TEM micrographs of the homogenized Alloy 2 and the peak-aged Alloy 2 at 473 K are shown in Fig. 5. TEM observation reveals that the second-phase with an average size of about several microns forms throughout the Į-Mg matrix after homogenization (see Fig. 5(a)), and it is Mg-Zn-
Fig. 1 SEM micrographs of as-cast Mg-Zn-Er alloys (a) Alloy 1; (b) Alloy 2
Fig. 2 SEM micrographs of two alloys after being homogenized at 713 K for 8 h (a) Alloy 1; (b) Alloy 2
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Fig. 3 Aging time vs microhardness of Alloy 1 and Alloy 2 at 473 K
Er ternary phase. And no evidence of fine precipitates is observed within the grain. After being aged at 473 K for 36 h, large amount of rod precipitates can be observed within the Į-Mg matrix (see Fig. 5(b)), which are parallel with each other, and their average size is several hundreds of nanometers in length and about 20 nm in thickness. Selected area electron diffraction (SAED) and EDS analysis indicates that
all these precipitates are MgZn2 with hexagonal structure and lattice constant of a=5.221 nm and c=8.567 nm. According to the age hardening results, the two alloys have visible age hardening effect. The precipitation of rod-shaped hexagonal phase MgZn2 within the grains gives rise to the increase in the hardness during aging up to peak hardness. The peak hardness value of Alloy 2 is higher as compared to that of Alloy 1, and the time when peak hardness occurs is less in Alloy 2. Such change of aging response is attributed to different contents of Zn and Er. That is, high content of Zn and Er accelerates the precipitation and increases the quantity of the precipitates during aging.
3 Conclusions For the response to aging, both Alloy 1 and Alloy 2 exhibited visible age hardening effect and TEM observation revealed that the precipitates in the peak-aged alloy were MgZn2 phase, which resulted in the increase in hardness of
Fig. 4 SEM micrographs of alloys (a) Alloy 2 aged for 36 h; (b) High magnified image of Alloy 2 aged for 36 h
Fig. 5 TEM images of Alloy 2 (a) Homogenized Alloy 2; (b) Alloy 2 aged for 36 h
LI Jianhui et al., Effect of aging on microstructure of Mg-Zn-Er alloys
alloys. Higher peak hardness value with less time of Alloy 2 was due to higher content of Zn and Er.
References: [1] Lv Y Z, Wang Q D, Zeng X Q, Ding W J, Zhai C Q, Zhu Y P. Effects of rare earths on the microstructure, properties and fracture behavior of Mg-Al alloys. Mater. Sci. Eng. A, 2000, 278: 66. [2] Li Q, Wang Q D, Wang Y X, Zeng X Q, Ding W J. Effect of Nd and Y addition on microstructure and mechanical properties of as-cast Mg-Zn-Zr alloy. J. Alloys Compd., 2007, 427: 115. [3] Nakashima K, Iwasaki H, Mori T, Mabuchi M, Nakamura M, Asahina T. Mechanical properties of a powder metallurgically processed Mg-5Y-6Re alloy. Mater. Sci. Eng. A, 2000, 293: 15. [4] Zhu S M, Gibson M A, Nie J F, Easton M A, Abbott T B. Microstructural analysis of the creep resistance of die-cast Mg-4Al-2RE alloy. Scripta Mater., 2008, 58: 477. [5] Moreno I P, Nandy T K, Jones J W, Allison J E, Pollock T M. Microstructural stability and creep of rare-earth containing magnesium alloys. Scripta Mater., 2003, 48: 1029. [6] Wang J G, Hsiung L M, Nieh T G, Mabuchi M. Creep of a heat treated Mg-4Y-3RE alloy. Mater Sci. Eng. A, 2001, 315: 81. [7] Rokhlin L L, Dobatkina T V, Tarytina I E, Timofeev V N, Balakhchi E E. Peculiarities of the phase relations in Mg-rich
1045
alloys of the Mg-Nd-Y system. J. Alloys Compd., 2004, 367: 17. [8] Gao Y, Wang Q D, Gu J H, Zhao Y, Tong Y. Effects of heat treatment on microstructure and mechanical properties of Mg-15Gd-5Y-0.5Er alloy. J. Rare Earths, 2008, 26(2): 298. [9] Singh A, Watanabe M, Kato A, Tsai A P. Microstructure and strength of quasicrystal containing extruded Mg-Zn-Y alloys for elevated temperature application. Mater. Sci. Eng. A, 2004, 385: 382. [10] Bae D H, Lee M H, Kim K T, Kim W T. Application of quasicrystalline particles as a strengthening phase in Mg-Zn-Y alloys. J. Alloys Compd., 2002, 342: 445. [11] Liu Y, Yuan G Y, Ding W J, Lu C. Deformation behavior of Mg-Zn-Gd-based alloys reinforced with quasicrystal and Laves phase at elevated temperatures. J. Alloys Compd., 2007, 427: 160. [12] Rokhlin L L, Nikitina N I, Zolina Z K. Magnesium alloys with erbium. Metal Sci. Heat Treat., 1978, 20(7): 529. [13] Kounis A, Miehe G, Fuess H. Investigation of icosahedral phases in the Zn-Mg-(Y, Er) system by high resolution transmission electron microscopy. Mater. Sci. Eng. A, 2000, 294-296: 323. [14] Tsai A P, Niikura A, Inoue A, Masumoto T. Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE=rare earth metals) systems. Phil. Mag. Lett., 1994, 70(3): 169. [15] Li Jianhui, Du Wenbo, Li Shubo, Wang Zhaohui. Icosahedral quasicrystalline phase in as-cast Mg-Zn-Er alloy. Rare Metals, 2009, 28: 297.
Effect of aging on microstructure of Mg-Zn-Er alloys LI Jianhui (ᴢᓎ䕝), DU Wenbo (ᴰ᭛म), LI Shubo (ᴢ⎥⊶), WANG Zhaohui (⥟ᳱ䕝) (School of Materials Science and Engineering, University of Technology Beijing, Beijing 100124, China) Received 4 August 2008; revised 8 September 2008
Abstract: The effect of aging on microstructure of Mg-Zn-Er alloys at 473 K was investigated using hardness measurement, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that both Mg3.8Zn1.5Er and Mg5Zn2.0Er alloys exhibited visible age-hardening effect, especially the latter alloy. Microstructure analysis showed that, after being aged, lots of fine MgZn2 phases with hexagonal structure were found in the Į-Mg matrix. Comparing with Mg3.8Zn1.5Er alloy, the accelerated hardening response of Mg5Zn2.0Er should be contributed to the increase in the content of Zn and Er elements. Keywords: Mg-Zn-Er alloys; aging behavior; microstructure; rare earths
An important class of magnesium alloys is the alloys containing rare earth elements, such as Mg-Al-RE, Mg-Y-RE-Zr and Mg-Zn-RE[1–3], developed for elevated temperature applications. Generally, cerium-based rare earth is added to the magnesium alloys containing aluminum as the dominant alloying element for the improvement of the mechanical properties through the formation of second-phase with high thermally stability[4,5]. The improvement of mechanical properties by adding rare earth with larger solid solubility in magnesium such as Y, Gd and Nd is attributed to solution hardening and precipitation of fine dispersion of intermetallic particles[6–8]. Recently, Mg-ZnRE (RE=Y, Gd) alloys have attracted significant interest because of their good mechanical properties through in situ forming icosahedral quasicrystalline phase (I-phase)[9–11], and I-phase as the strengthening phase is favorable for enhancing mechanical properties of the alloys. Among the rare earth metals, the maximum solubility of erbium in magnesium at 817 K is 32.7 wt.% (6.56 at.%), but decreases to 18.5 wt.% (3.17 at.%) at 573 K, causing great effect of solution and precipitation hardening[12]. The I-phase has been found in Zn-rich alloys Zn-Mg-Er alloys[13,14]. However, up to now, few works are about the effect of erbium on magnesium alloys. Recently, the authors investigated the microstructure of as-cast Mg-Zn-Er alloy and found the I-phase[15]. In the present paper, investigation was carried out on the aging behavior of Mg-Zn-Er alloys at 473 K.
1 Experimental The two test alloys were Mg3.8Zn1.5Er (Alloy 1) and Mg5Zn2.0Er (Alloy 2) with nominal composition in mass fraction. The ingots were prepared by 99.9% Mg, 99.9% Zn and Mg-Er master alloy in a graphite crucible under anti-oxidizing flux in an electric resistance furnace. The melt was poured into a steel mould and then cooled in air. The two ingots were homogenized at 713 K for 8 h before aging. Microstructures were observed by scanning electron microscopy (SEM, FEI Quanta 200) with energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM, JEM-2010). TEM specimens were prepared by electro-polishing and ion beam milling at an incident angle less than 10°, and electro-polishing of the samples was operated in a solution of 20% nitric acid and 80% methanol, using liquid nitrogen cold stage to prevent heating. The homogenized samples were aged at 473 K for 6, 12, 24, 36, 48, and 60 h, respectively. Vickers hardness was carried out on Vickers hardness tester (HXD-1000), and the test load and dwell time was 0.25 N and 15 s, respectively. Each hardness value was the average of at least seven measurements.
2 Results and discussion 2.1 Microstructure Fig. 1 shows SEM micrographs of the two as-cast alloys.
Foundation item: Project supported by National Major Fundamental Research Program of China (2007CB613706) Corresponding author: DU Wenbo (E-mail: [email protected]; Tel.: +86-10-67392917) DOI: 10.1016/S1002-0721(08)60385-3
LI Jianhui et al., Effect of aging on microstructure of Mg-Zn-Er alloys
It indicates that Alloy 2 has smaller average interdendritic distance as compared to Alloy 1, which might be attributed to the refinement of erbium. Moreover, with the increase of alloying elements Zn and Er, more second-phases form in Į-Mg matrix of Alloy 2 and they are mainly distributed along grain boundaries. Both alloys have the same phases. Fig. 2 shows SEM micrographs ofthehomogenized Alloy 1 and Alloy 2. It shows that there is no distinct change observed in microstructure after being homogenized. And it is more clearly observed that the second-phases distributed along the grain boundaries become much coarser with the increase of Zn and Er contact at higher magnification (see Fig. 2(b)). EDS analysis indicates that the precipitate consists of Mg, Zn and Er, that is, Mg-Zn-Er ternary phase. Moreover, The EDS analysis also reveals that some of added Zn went into Į-Mg matrix. 2.2 Age hardening response The microhardness value curves of Alloy 1 and Alloy 2 aged at 473 K for various time intervals are shown in Fig. 3. The microhardness increases before reaching their peak
1043
values, then gradually decreases for both alloys, indicating noticeable age-hardening response. The peak-hardness value of 63 for Alloy 1 occurs after about 48 h of aging. For Alloy 2, the peak-hardness value of 68 occurs after about 36 h of aging. The higher peak-hardness value obtained with less time for Alloy 2 shows that its aging efficiency is higher than that of Alloy 1. Fig. 4 shows SEM micrographs of Alloy 2 after being aged for 36 h. Comparing the microstructure with the homogenized alloys (see Fig.2 (b)), the grain boundaries become clearer and the intermetallic compounds become more dispersed (see Fig.4 (a)). With the increase in aging time, much finer precipitates appear within the grains. At the peak-aging time, a large number of acicular or granular fine precipitates distribute homogeneously within the grains. The microstructure details of peak-aged Alloy 2 are resolved more clearly under higher magnification (see Fig.4 (b)). The TEM micrographs of the homogenized Alloy 2 and the peak-aged Alloy 2 at 473 K are shown in Fig. 5. TEM observation reveals that the second-phase with an average size of about several microns forms throughout the Į-Mg matrix after homogenization (see Fig. 5(a)), and it is Mg-Zn-
Fig. 1 SEM micrographs of as-cast Mg-Zn-Er alloys (a) Alloy 1; (b) Alloy 2
Fig. 2 SEM micrographs of two alloys after being homogenized at 713 K for 8 h (a) Alloy 1; (b) Alloy 2
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Fig. 3 Aging time vs microhardness of Alloy 1 and Alloy 2 at 473 K
Er ternary phase. And no evidence of fine precipitates is observed within the grain. After being aged at 473 K for 36 h, large amount of rod precipitates can be observed within the Į-Mg matrix (see Fig. 5(b)), which are parallel with each other, and their average size is several hundreds of nanometers in length and about 20 nm in thickness. Selected area electron diffraction (SAED) and EDS analysis indicates that
all these precipitates are MgZn2 with hexagonal structure and lattice constant of a=5.221 nm and c=8.567 nm. According to the age hardening results, the two alloys have visible age hardening effect. The precipitation of rod-shaped hexagonal phase MgZn2 within the grains gives rise to the increase in the hardness during aging up to peak hardness. The peak hardness value of Alloy 2 is higher as compared to that of Alloy 1, and the time when peak hardness occurs is less in Alloy 2. Such change of aging response is attributed to different contents of Zn and Er. That is, high content of Zn and Er accelerates the precipitation and increases the quantity of the precipitates during aging.
3 Conclusions For the response to aging, both Alloy 1 and Alloy 2 exhibited visible age hardening effect and TEM observation revealed that the precipitates in the peak-aged alloy were MgZn2 phase, which resulted in the increase in hardness of
Fig. 4 SEM micrographs of alloys (a) Alloy 2 aged for 36 h; (b) High magnified image of Alloy 2 aged for 36 h
Fig. 5 TEM images of Alloy 2 (a) Homogenized Alloy 2; (b) Alloy 2 aged for 36 h
LI Jianhui et al., Effect of aging on microstructure of Mg-Zn-Er alloys
alloys. Higher peak hardness value with less time of Alloy 2 was due to higher content of Zn and Er.
References: [1] Lv Y Z, Wang Q D, Zeng X Q, Ding W J, Zhai C Q, Zhu Y P. Effects of rare earths on the microstructure, properties and fracture behavior of Mg-Al alloys. Mater. Sci. Eng. A, 2000, 278: 66. [2] Li Q, Wang Q D, Wang Y X, Zeng X Q, Ding W J. Effect of Nd and Y addition on microstructure and mechanical properties of as-cast Mg-Zn-Zr alloy. J. Alloys Compd., 2007, 427: 115. [3] Nakashima K, Iwasaki H, Mori T, Mabuchi M, Nakamura M, Asahina T. Mechanical properties of a powder metallurgically processed Mg-5Y-6Re alloy. Mater. Sci. Eng. A, 2000, 293: 15. [4] Zhu S M, Gibson M A, Nie J F, Easton M A, Abbott T B. Microstructural analysis of the creep resistance of die-cast Mg-4Al-2RE alloy. Scripta Mater., 2008, 58: 477. [5] Moreno I P, Nandy T K, Jones J W, Allison J E, Pollock T M. Microstructural stability and creep of rare-earth containing magnesium alloys. Scripta Mater., 2003, 48: 1029. [6] Wang J G, Hsiung L M, Nieh T G, Mabuchi M. Creep of a heat treated Mg-4Y-3RE alloy. Mater Sci. Eng. A, 2001, 315: 81. [7] Rokhlin L L, Dobatkina T V, Tarytina I E, Timofeev V N, Balakhchi E E. Peculiarities of the phase relations in Mg-rich
1045
alloys of the Mg-Nd-Y system. J. Alloys Compd., 2004, 367: 17. [8] Gao Y, Wang Q D, Gu J H, Zhao Y, Tong Y. Effects of heat treatment on microstructure and mechanical properties of Mg-15Gd-5Y-0.5Er alloy. J. Rare Earths, 2008, 26(2): 298. [9] Singh A, Watanabe M, Kato A, Tsai A P. Microstructure and strength of quasicrystal containing extruded Mg-Zn-Y alloys for elevated temperature application. Mater. Sci. Eng. A, 2004, 385: 382. [10] Bae D H, Lee M H, Kim K T, Kim W T. Application of quasicrystalline particles as a strengthening phase in Mg-Zn-Y alloys. J. Alloys Compd., 2002, 342: 445. [11] Liu Y, Yuan G Y, Ding W J, Lu C. Deformation behavior of Mg-Zn-Gd-based alloys reinforced with quasicrystal and Laves phase at elevated temperatures. J. Alloys Compd., 2007, 427: 160. [12] Rokhlin L L, Nikitina N I, Zolina Z K. Magnesium alloys with erbium. Metal Sci. Heat Treat., 1978, 20(7): 529. [13] Kounis A, Miehe G, Fuess H. Investigation of icosahedral phases in the Zn-Mg-(Y, Er) system by high resolution transmission electron microscopy. Mater. Sci. Eng. A, 2000, 294-296: 323. [14] Tsai A P, Niikura A, Inoue A, Masumoto T. Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE=rare earth metals) systems. Phil. Mag. Lett., 1994, 70(3): 169. [15] Li Jianhui, Du Wenbo, Li Shubo, Wang Zhaohui. Icosahedral quasicrystalline phase in as-cast Mg-Zn-Er alloy. Rare Metals, 2009, 28: 297.