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Novel structures observed in Mg–Gd–Y–Zr during isothermal ageing by atomic-scale HAADF-STEM
Materials Letters 152 (2015) 287–289
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Novel structures observed in Mg–Gd–Y–Zr during isothermal ageing by atomic-scale HAADF-STEM Jingxu Zheng, Xuesong Xu, Kunyi Zhang, Bin Chen n Shanghai Jiao Tong University, Frontier Research Center for Materials Structure, Shanghai, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2015 Accepted 29 March 2015 Available online 6 April 2015
Mg–Gd–Y–Zr alloy has been widely studied due to its excellent properties. Its ageing precipitation sequence was identified as a four-stage precipitation including S.S.S.S.—β″–β0 –β1–β. In this paper, the precipitate phases of Mg–Gd–Y–Zr during isothermal ageing at 225 1C are investigated by atomic resolution HAADF-STEM and, as a result, novel structures are observed. Viewed from [0 0 0 1]Mg axis, the general morphology of rare earth (RE) atoms is “main bodies” linked by “bridges”. In bridge areas, some RE atoms form separated chains of hexagons, which has not been clearly reported previously. Furthermore, a defect in the precipitation process, which is the interface between two encountered β0 parts, is characterized. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Electron microscopy Microstructure Precipitate
1. Introduction The need for ever-lighter materials in industry indicates that Mg based alloys, with their low density, are promising materials for many structural applications. However, the application of Mg is limited by its unsatisfactory mechanical property. Therefore, a large number of researches are conducted to remedy the drawbacks. Particularly, Mg–RE alloys achieve extremely high strength and elongation during precipitation hardening [1–4]. In order to gain better understanding of the age strengthening mechanism and thus design more efficient age strategies, efforts have been made to investigate the precipitation during age treatment [5,6]. The precipitation sequence of Mg–Gd series alloy is previously identified basically as S.S.S.S.—β″(Mg3Gd)–β0 (Mg7Gd)–β1(Mg3Gd)–β(Mg5Gd) [7]. However, there are still many controversies over the phase transformations in the precipitation [8]. In this paper, detailed morphology of the precipitates, which cannot be fully observed by HRTEM, are elucidated by atomicscaled HAADF-STEM.
2. Experimental procedures The Mg–Gd–Y–Zr alloy in this research was prepared from pure Mg and Mg–30Gd, Mg–25Y, Mg–25Zr master alloys (wt%) by melting in an electrical resistance furnace under the protection of mixture gas of SF6 and CO2. The ingot was solution treated at n
Corresponding author. Tel: þ 86 21 5474 5404. E-mail address: [email protected] (B. Chen).
http://dx.doi.org/10.1016/j.matlet.2015.03.145 0167-577X/& 2015 Elsevier B.V. All rights reserved.
773 K in a sulfur atmosphere for 6 h followed by quenching in water. The chemical composition is determined by inductively coupled plasma optical emission spectrometer (ICP-OES) analysis and is given in Table 1. The alloy is age treated in silicon oil at constant temperature of 798 K for 81 h, cooled in room temperature atmosphere. Disks with a diameter of 3 mm and a thickness of 50 μm are punched from manually ground slices cut from the aged bulks. The disks are twin jet electro-polished at 238 K in a solution of 4%HClO4– C2H5OH solution. Atomic scale characterization was acquired using the probe aberration-corrected JEOL ARM200F equipped with a 200 kV cold field-emission gun. All images were obtained with a 28-mradprobe convergence angle. The HAADF (Z contrast images) were acquired using a 90-mrad inner-detector angle.
3. Results and discussion Fig. 1 shows the overall morphology of precipitate in lowmagnification HAADF-STEM obtained by a beam parallel to the [0 0 0 1]Mg axis of α-Mg matrix. Obviously, the morphology comprises main body areas and “bridge” areas with a smaller width, which link two main bodies. Main body areas consist of atoms with β0 phase [9] arrangement. Meanwhile, it is notable that bridge areas show a combined structure of more than one atom arrangement. Within the bridge area, there are one arrangement that looks like β0 arrangement with its linear zigzag RE atoms projection and a new sort of arrangement with its hexagon RE atoms projection and Mg atoms between two chains of RE
288
J. Zheng et al. / Materials Letters 152 (2015) 287–289
hexagons. For simplicity, this structure of separated hexagon chains is called βM hereafter in this paper. Fig. 2(a) and (b) show the details of bridge areas which present us the clear atomic arrangement within the areas. Fig. 2(a) is the magnified image of the selected area in Fig. 1. The difference between the projected structures of β″ [10,11] and βM is that in β″ the hexagons link to each other compactly while in βM the RE hexagons are separated by Mg atoms in one orientation but in another orientation the hexagons still link together. As a result, the projection of RE atoms in β″ form compact hexagons areas and RE atoms in βM form separated “chains of RE hexagons”. In addition, it seems like bridge areas are composed of βM and β0 structures, but actually the linear zigzag structure is different from the previously reported β0 . Specifically, as is shown in Fig. 2(b), in Table 1 Chemical composition of Mg–Gd–Y–Zr alloy. Element
Gd
Y
Zr
Mg
Chemical Composition (wt%)
9.28
2.93
0.35
Balance
the main body area, A,B and C have mirror symmetry, of which the mirror plane is (1 1 2 0), with their adjacencies, while in the bridge areas, D and E do not conform to mirror symmetry, instead D and E are symmetric under translation of one layer perpendicular to the (1 1 2 0) plane. When considering one chain of hexagons in βM as two separated zigzag lines as F in Fig. 2(b), structures in bridge areas, for example, D,E,F and G all have translational symmetry perpendicular to (1120) plane with their adjacencies. However, the distance between two zigzag lines in main body areas and bridge areas are both 0.6 nm. The interface between the bridge and the upper main body in Fig. 2(b) is noteworthy. The distance between C and D is 0.9 nm, which is larger than the distance between two zigzag lines within main bodies or bridges, and there is no mirror symmetry or translational symmetry between them, but C and D are parallel. However, the bridge in Fig. 2(b) is coherent with the lower main body, because H and I are coherent both in distance and symmetry. It suggests that the bridge grows from the lower main body and then attempts to connect to the upper main body. Fig. 3(a) shows a defect area observed, which is similar to a stacking fault, and Fig. 3(b) shows the detailed atom arrangement within the defect. Conspicuously, J and K are common β0 arrangement and, when considering L and M as separated zigzag lines, the zigzag lines on the right and N conform β0 arrangement. The hexagonal structures in L and M are, actually the overlap of two β0 parts, which are in the process of moving the fault in either direction. This defect is the interface of two encountered β0 parts when growing.
4. Conclusions In the Mg–Gd–Y–Zr alloy, which is isothermal treated at 798 K for 81 h, the following conclusions about the precipitates can be drawn on the basis of the atomic-scale HAADF-STEM observation: Viewed from [0 0 0 1]Mg axis,
Fig. 1. Low magnification HAADF-STEM image beamed from [0 0 0 1]Mg axis.
1. The overall morphology of the precipitates is “main bodies” linked by “bridges”. 2. Previously unobserved structures, such as βM, exist in bridge areas. 3. The interface between two β0 parts forms a defect which results in hexagonal RE columns that are not fully occupied.
Fig. 2. (a) and (b) HAADF-STEM images beamed from [0 0 0 1]Mg axis focused on bridge areas.
J. Zheng et al. / Materials Letters 152 (2015) 287–289
289
Fig. 3. (a) and (b) HAADF-STEM image beamed from [0 0 0 1]Mg axis focused on a defect area.
Acknowledgements The authors gratefully acknowledge the help of Yunwen Chen (Zhejiang University), DongyueXie (Shanghai Jiao Tong University), Xinyun Zhang (Shenzhen Middle School), Xingji Zheng and Laijin Luo (Shenzhen Foreign Languages School). This paper is financially supported by the National Natural Science Foundation of China (Grant no. 51171107). References [1] Anyanwu IA, Kamado S, Kojima Y. Platform Science and Technology for Advanced Magnesium Alloys. Aging Characteristics and High Temperature Tensile Properties of Mg–Gd–Y–Zr Alloys. Mater Trans 2001;42:1206–11. [2] Drits M, Sviderskaya Z, Rokhlin L, Nikitina N. Effect of alloying on the properties of Mg Gd alloys. Met Sci Heat Treat 1979;21:887–9.
[3] Peng Q, Huang Y, Kainer KU, Hort N. High ductile as-cast Mg–RE based alloys at room temperature. Mater Lett 2012;83:209–12. [4] Wang J, Song P, Huang S, Pan F. High-strength and good-ductility Mg–RE–Zn– Mn magnesium alloy with long-period stacking ordered phase. Mater Lett 2013;93:415–8. [5] He S, Zeng XQ, Peng L, Gao X, Nie J, Ding W. Precipitation in a Mg–10Gd–3Y– 0.4 Zr (wt%) alloy during isothermal ageing at 250 C. J Alloys Compd 2006;421:309–13. [6] He S, Zeng XQ, Peng L, Gao X, Nie J, Ding W. Microstructure and strengthening mechanism of high strength Mg–10Gd–2Y–0.5 Zr alloy. J Alloys Compd 2007;427:316–23. [7] Nie J, Muddle B. Characterisation of strengthening precipitate phases in a Mg– Y–Nd alloy. Acta Mater 2000;48:1691–703. [8] Tang Y-j, Zhang Z-y, Jin L, Dong J, Ding W. Research progress on ageing precipitation of Mg–Gd alloys. Chin J Nonferrous Met 2014;24:8–24. [9] Nishijima M, Hiraga K, Yamasaki M, Kawamura Y. Characterization of BETA0 phase precipitates in an Mg–5 at% Gd alloy aged in a peak hardness condition, studied by high-angle annular detector dark-field scanning transmission electron microscopy. Mater Trans 2006;47:2109–12. [10] Apps P, Karimzadeh H, King J, Lorimer G. Precipitation reactions in magnesium-rare earth alloys containing yttrium, gadolinium or dysprosium. Scr Mater 2003;48:1023–8. [11] Nishijima M, Hiraga K. Structural changes of precipitates in an Mg–5 at% Gd alloy studied by transmission electron microscopy. Mater Trans 2007;48:10–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Novel structures observed in Mg–Gd–Y–Zr during isothermal ageing by atomic-scale HAADF-STEM Jingxu Zheng, Xuesong Xu, Kunyi Zhang, Bin Chen n Shanghai Jiao Tong University, Frontier Research Center for Materials Structure, Shanghai, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2015 Accepted 29 March 2015 Available online 6 April 2015
Mg–Gd–Y–Zr alloy has been widely studied due to its excellent properties. Its ageing precipitation sequence was identified as a four-stage precipitation including S.S.S.S.—β″–β0 –β1–β. In this paper, the precipitate phases of Mg–Gd–Y–Zr during isothermal ageing at 225 1C are investigated by atomic resolution HAADF-STEM and, as a result, novel structures are observed. Viewed from [0 0 0 1]Mg axis, the general morphology of rare earth (RE) atoms is “main bodies” linked by “bridges”. In bridge areas, some RE atoms form separated chains of hexagons, which has not been clearly reported previously. Furthermore, a defect in the precipitation process, which is the interface between two encountered β0 parts, is characterized. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Electron microscopy Microstructure Precipitate
1. Introduction The need for ever-lighter materials in industry indicates that Mg based alloys, with their low density, are promising materials for many structural applications. However, the application of Mg is limited by its unsatisfactory mechanical property. Therefore, a large number of researches are conducted to remedy the drawbacks. Particularly, Mg–RE alloys achieve extremely high strength and elongation during precipitation hardening [1–4]. In order to gain better understanding of the age strengthening mechanism and thus design more efficient age strategies, efforts have been made to investigate the precipitation during age treatment [5,6]. The precipitation sequence of Mg–Gd series alloy is previously identified basically as S.S.S.S.—β″(Mg3Gd)–β0 (Mg7Gd)–β1(Mg3Gd)–β(Mg5Gd) [7]. However, there are still many controversies over the phase transformations in the precipitation [8]. In this paper, detailed morphology of the precipitates, which cannot be fully observed by HRTEM, are elucidated by atomicscaled HAADF-STEM.
2. Experimental procedures The Mg–Gd–Y–Zr alloy in this research was prepared from pure Mg and Mg–30Gd, Mg–25Y, Mg–25Zr master alloys (wt%) by melting in an electrical resistance furnace under the protection of mixture gas of SF6 and CO2. The ingot was solution treated at n
Corresponding author. Tel: þ 86 21 5474 5404. E-mail address: [email protected] (B. Chen).
http://dx.doi.org/10.1016/j.matlet.2015.03.145 0167-577X/& 2015 Elsevier B.V. All rights reserved.
773 K in a sulfur atmosphere for 6 h followed by quenching in water. The chemical composition is determined by inductively coupled plasma optical emission spectrometer (ICP-OES) analysis and is given in Table 1. The alloy is age treated in silicon oil at constant temperature of 798 K for 81 h, cooled in room temperature atmosphere. Disks with a diameter of 3 mm and a thickness of 50 μm are punched from manually ground slices cut from the aged bulks. The disks are twin jet electro-polished at 238 K in a solution of 4%HClO4– C2H5OH solution. Atomic scale characterization was acquired using the probe aberration-corrected JEOL ARM200F equipped with a 200 kV cold field-emission gun. All images were obtained with a 28-mradprobe convergence angle. The HAADF (Z contrast images) were acquired using a 90-mrad inner-detector angle.
3. Results and discussion Fig. 1 shows the overall morphology of precipitate in lowmagnification HAADF-STEM obtained by a beam parallel to the [0 0 0 1]Mg axis of α-Mg matrix. Obviously, the morphology comprises main body areas and “bridge” areas with a smaller width, which link two main bodies. Main body areas consist of atoms with β0 phase [9] arrangement. Meanwhile, it is notable that bridge areas show a combined structure of more than one atom arrangement. Within the bridge area, there are one arrangement that looks like β0 arrangement with its linear zigzag RE atoms projection and a new sort of arrangement with its hexagon RE atoms projection and Mg atoms between two chains of RE
288
J. Zheng et al. / Materials Letters 152 (2015) 287–289
hexagons. For simplicity, this structure of separated hexagon chains is called βM hereafter in this paper. Fig. 2(a) and (b) show the details of bridge areas which present us the clear atomic arrangement within the areas. Fig. 2(a) is the magnified image of the selected area in Fig. 1. The difference between the projected structures of β″ [10,11] and βM is that in β″ the hexagons link to each other compactly while in βM the RE hexagons are separated by Mg atoms in one orientation but in another orientation the hexagons still link together. As a result, the projection of RE atoms in β″ form compact hexagons areas and RE atoms in βM form separated “chains of RE hexagons”. In addition, it seems like bridge areas are composed of βM and β0 structures, but actually the linear zigzag structure is different from the previously reported β0 . Specifically, as is shown in Fig. 2(b), in Table 1 Chemical composition of Mg–Gd–Y–Zr alloy. Element
Gd
Y
Zr
Mg
Chemical Composition (wt%)
9.28
2.93
0.35
Balance
the main body area, A,B and C have mirror symmetry, of which the mirror plane is (1 1 2 0), with their adjacencies, while in the bridge areas, D and E do not conform to mirror symmetry, instead D and E are symmetric under translation of one layer perpendicular to the (1 1 2 0) plane. When considering one chain of hexagons in βM as two separated zigzag lines as F in Fig. 2(b), structures in bridge areas, for example, D,E,F and G all have translational symmetry perpendicular to (1120) plane with their adjacencies. However, the distance between two zigzag lines in main body areas and bridge areas are both 0.6 nm. The interface between the bridge and the upper main body in Fig. 2(b) is noteworthy. The distance between C and D is 0.9 nm, which is larger than the distance between two zigzag lines within main bodies or bridges, and there is no mirror symmetry or translational symmetry between them, but C and D are parallel. However, the bridge in Fig. 2(b) is coherent with the lower main body, because H and I are coherent both in distance and symmetry. It suggests that the bridge grows from the lower main body and then attempts to connect to the upper main body. Fig. 3(a) shows a defect area observed, which is similar to a stacking fault, and Fig. 3(b) shows the detailed atom arrangement within the defect. Conspicuously, J and K are common β0 arrangement and, when considering L and M as separated zigzag lines, the zigzag lines on the right and N conform β0 arrangement. The hexagonal structures in L and M are, actually the overlap of two β0 parts, which are in the process of moving the fault in either direction. This defect is the interface of two encountered β0 parts when growing.
4. Conclusions In the Mg–Gd–Y–Zr alloy, which is isothermal treated at 798 K for 81 h, the following conclusions about the precipitates can be drawn on the basis of the atomic-scale HAADF-STEM observation: Viewed from [0 0 0 1]Mg axis,
Fig. 1. Low magnification HAADF-STEM image beamed from [0 0 0 1]Mg axis.
1. The overall morphology of the precipitates is “main bodies” linked by “bridges”. 2. Previously unobserved structures, such as βM, exist in bridge areas. 3. The interface between two β0 parts forms a defect which results in hexagonal RE columns that are not fully occupied.
Fig. 2. (a) and (b) HAADF-STEM images beamed from [0 0 0 1]Mg axis focused on bridge areas.
J. Zheng et al. / Materials Letters 152 (2015) 287–289
289
Fig. 3. (a) and (b) HAADF-STEM image beamed from [0 0 0 1]Mg axis focused on a defect area.
Acknowledgements The authors gratefully acknowledge the help of Yunwen Chen (Zhejiang University), DongyueXie (Shanghai Jiao Tong University), Xinyun Zhang (Shenzhen Middle School), Xingji Zheng and Laijin Luo (Shenzhen Foreign Languages School). This paper is financially supported by the National Natural Science Foundation of China (Grant no. 51171107). References [1] Anyanwu IA, Kamado S, Kojima Y. Platform Science and Technology for Advanced Magnesium Alloys. Aging Characteristics and High Temperature Tensile Properties of Mg–Gd–Y–Zr Alloys. Mater Trans 2001;42:1206–11. [2] Drits M, Sviderskaya Z, Rokhlin L, Nikitina N. Effect of alloying on the properties of Mg Gd alloys. Met Sci Heat Treat 1979;21:887–9.
[3] Peng Q, Huang Y, Kainer KU, Hort N. High ductile as-cast Mg–RE based alloys at room temperature. Mater Lett 2012;83:209–12. [4] Wang J, Song P, Huang S, Pan F. High-strength and good-ductility Mg–RE–Zn– Mn magnesium alloy with long-period stacking ordered phase. Mater Lett 2013;93:415–8. [5] He S, Zeng XQ, Peng L, Gao X, Nie J, Ding W. Precipitation in a Mg–10Gd–3Y– 0.4 Zr (wt%) alloy during isothermal ageing at 250 C. J Alloys Compd 2006;421:309–13. [6] He S, Zeng XQ, Peng L, Gao X, Nie J, Ding W. Microstructure and strengthening mechanism of high strength Mg–10Gd–2Y–0.5 Zr alloy. J Alloys Compd 2007;427:316–23. [7] Nie J, Muddle B. Characterisation of strengthening precipitate phases in a Mg– Y–Nd alloy. Acta Mater 2000;48:1691–703. [8] Tang Y-j, Zhang Z-y, Jin L, Dong J, Ding W. Research progress on ageing precipitation of Mg–Gd alloys. Chin J Nonferrous Met 2014;24:8–24. [9] Nishijima M, Hiraga K, Yamasaki M, Kawamura Y. Characterization of BETA0 phase precipitates in an Mg–5 at% Gd alloy aged in a peak hardness condition, studied by high-angle annular detector dark-field scanning transmission electron microscopy. Mater Trans 2006;47:2109–12. [10] Apps P, Karimzadeh H, King J, Lorimer G. Precipitation reactions in magnesium-rare earth alloys containing yttrium, gadolinium or dysprosium. Scr Mater 2003;48:1023–8. [11] Nishijima M, Hiraga K. Structural changes of precipitates in an Mg–5 at% Gd alloy studied by transmission electron microscopy. Mater Trans 2007;48:10–5.