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Interactions between long-period stacking ordered phase and β′ precipitate in Mg–Gd–Y–Zn–Zr alloy: Atomic-scale insights from HAADF-STEM
Author’s Accepted Manuscript Interactions between Long-period Stacking Ordered Phase and β’ Precipitate in Mg-Gd-Y-Zn-Zr Alloy: Atomic-scale Insights from HAADF-STEM Jingxu Zheng, Bin Chen www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30601-2 http://dx.doi.org/10.1016/j.matlet.2016.04.114 MLBLUE20711
To appear in: Materials Letters Received date: 19 February 2016 Revised date: 8 April 2016 Accepted date: 14 April 2016 Cite this article as: Jingxu Zheng and Bin Chen, Interactions between Longperiod Stacking Ordered Phase and β’ Precipitate in Mg-Gd-Y-Zn-Zr Alloy: Atomic-scale Insights from HAADF-STEM, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.04.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interactions between Long-period Stacking Ordered Phase and β’ Precipitate in Mg-Gd-Y-Zn-Zr Alloy: Atomic-scale Insights from HAADF-STEM
Jingxu Zhenga,b, Bin Chena,b* a: Frontier Research Center for Materials Structure, Shanghai Jiao Tong University, Shanghai, China b: School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China *Corresponding author: Bin Chen (Dr.), [email protected]
Abstract This paper reports on an atomic-scale investigation into the interactions between β′ precipitates
and
long-period
stacking
ordered
phase
(LPSO)
in
Mg-10Gd-5Y-2Zn-0.5Zr (wt%) alloy, using Cs-corrected high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM). The general morphology within the alloy is LPSO phase intercalated with β′ precipitates and the two structures interconnect into a 3-D network. The β′/LPSO interfaces are fully characterized and categorized into three types: RE-absent gaps parallel to (112̅0)Mg, redistributions of heavy atoms within LPSO structures and interceptions of β′ precipitates by LPSO structures from [101̅0]Mg direction. Keywords: Metals and alloys; Phase transformation; Microstructure; Interfaces; Electron microscopy
1. Introduction The strengthening of Mg-based materials has long been a popular research topic because of the urgent demand for weight-reduced materials for structural applications. The strengthening effect of RE addition in alloys such as Mg-Gd-Y and Mg-Nd-Y has been investigated by many papers [1-7]. Besides, the addition of zinc into Mg-RE alloys has been proved to have significant effect in improving both the strength and ductility. For example, Mg-Gd-Y-Zn-Zr was reported to be an ultra-high strength, high-ductility alloy, which has attracted numerous research interests [8-12]. In contrast to the deep investigation into the properties of Mg-Gd-Y-Zn-Zr, the 1
microstructure of the Mg-RE-Zn is poorly understood. The strengthening structures in Mg-Gd-Y-Zn-Zr are generally reported to be coexisting β′ and long-period stacking ordered phase[8]. However, the spatial relationship and interaction between the two structures, to the authors’ best knowledge, are not clearly revealed yet. This paper, based on atomic-scale direct imaging, aims to fully unravel the microstructure in T6-treated Mg-Gd-Y-Zn-Zr, including spatial relationship and interaction between β′ and LPSO, providing insights into the relationship between structure and property. In addition, the paper also deepens our understanding of LPSO from a different perspective by characterizing β′/LPSO interfaces.
2. Experimental Procedures The Mg-Gd-Y-Zn-Zr alloy in this research was prepared from pure Mg, Mg-20Gd, Mg-30Y, Mg-30Zr master alloys (wt.%) and pure zinc by melting the components in an electrical resistance furnace under the protection of argon gas. The ingot was solution-treated at 500℃ in a sulfur atmosphere for 5 hours followed by quenching in water. The nominal composition was Mg-10Gd-5Y-2Zn-0.5Zr (wt%). The alloy was cut into small bulks, the sizes of which were about 5mm × 5mm × 5mm. The bulks were aged in silicon oil at a temperature of 225℃ for 48 hours. Disks with a diameter of 3 mm and a thickness of ~80𝜇m were punched from manually ground slices cut from the aged bulks. The disks were polished and thinned by Gatan Precision Ion Polishing System 695 to avoid the possible sample damage induced in a twin jet electro-polishing method. Atomic-resolution HAADF-STEM characterization was carried out on JEM-ARM200F, equipped with a probe-Cs corrector and a cold field emission gun. The accelerating voltage was 200kV. The camera length was set to 8 cm which yields a collection semi-angle of 48-327 mrad.
3. Results and Discussions
2
Figure 1. (a) General morphology of 𝐋𝐏𝐒𝐎 structures intercalated with 𝛃′ ̅0]Mg; (b) Schematic diagram illustrating the precipitates. Incident beam // [11𝟐 spatial relationship between 𝛃′ precipitates and LPSO structures. The LPSO phase in Mg-Gd-Y-Zn-Zr alloy forms during the casting process and does not dissolve during the solution-treatment at 500℃ whereas the β′ precipitates forms during the artificial ageing carried out at 225℃, as shown in supplementary material S1. The general morphology viewed from [112̅0]Mg of the alloy aged for 48 hours is presented in figure 1a and the morphology from [0001]Mg is presented in supplementary material S2. Because LPSO phase is not observable from [0001]Mg, the alloy is beamed from [112̅0]Mg direction, from which both LPSO and β′ precipitates can be characterized. As shown in figure 1a, the β′ precipitates and LPSO form an intercalated structure. The red arrows denote the LPSO structures. The β′ precipitates, one of which is enclosed by a dashed yellow curve, lie between the LPSO structures. Because the LPSO structures do not dissolve during the ageing, the spatial interaction between β′ and LPSO hinders the coarsening of both β′ and LPSO. In figure 1b, the schematic diagram illustrating the spatial relationship between β′ precipitates and LPSO structures is presented. The feature of LPSO is closed-packing planes aligning in certain sequence with RE enrichment and Zn enrichment in certain planes as denoted by the stacked light blue planes in figure 1b. The plate-like β′ precipitates grow between the separated LPSO structures during the isothermal ageing at 225℃. When the density and sizes of β′ precipitates are large enough, they begin to interact with the neighboring LPSO structures and the growth of β′ precipitates along [0001]Mg direction is, thus, restrained by the LPSO structures as a framework. In consequence, the β′ precipitates form on the prismatic planes and the LPSO structures form on the basal planes, interconnecting into a 3-D network within the alloy and thus leading to the unique mechanical properties of the alloy.
3
̅0]Mg Figure 2. The interface between 𝛃′ and LPSO. Incident beam // [11𝟐 In figure 2, type-I interfaces between β′ and LPSO are presented. As shown in figure 2a, straight gaps with a width about ~1.3 nm exist between β′ and LPSO, denoted by the parallel dashed yellow lines. The low contrast of the gaps reveals that heavy atoms, such as RE and Zn, are absent within the area. In figure 2b and 2c, a subset of the atoms in β′ precipitates are denoted by red circles; a subset of the RE-enriched layers in LPSO are denoted by red arrows and a subset of the layers without RE-enrichment are denotes by yellow arrows. The gaps consist of four RE/Zn-free atomic layers and as a result the theoretic distance is 1.315 nm in pure Mg model, which is consistent with the observed value. The stacking sequence of the LPSO in fig. 3b and 3c is 14H — “ABABCACACACBABA…” in the inner region of LPSO. The stacking sequence in the interfacial gap is consistent with the normal HCP stacking model in the α-Mg matrix — “AB…”.
According to Nie’s papers[13, 14] about the building block
theory in LPSO, each building block in a 14H LPSO consists of a complete stacking sequence of “ABABCACACACBABA” and the number of the atomic layers in the interfacial gap is, therefore, three instead of the observed value of four. From another 4
perspective, when considering the LPSO as heterogeneous nucleation sites for β′, the gap of four atomic layers should not exist. In heterogeneous nucleation, the energy required for nucleation is reduced because the precipitates nucleate right on the heterogeneous nuclei and thus the interfacial area is smaller. In the LPSO/β′ interface, the interfacial area for β′ nuclei is not reduced due to the gaps composed of pure Mg atomic layers between LPSO and β′. Hence, the building blocks that interface with α-Mg matrix may have different compositions and properties than the internal ones in LPSO phase.
Figure 3. Redistribution of the heavy atoms in LPSO close to 𝛃′ precipitates. ̅0]Mg. Incident beam // [11𝟐 In figure 3, type-II interactions between LPSO and β′ precipitates are presented, during which the heavy atoms, such as RE and Zn, undergo a redistribution within the RE-enriched layers. In normal LPSO structures, the contrast within one RE-enriched 5
layer is homogeneous, as shown in figure 2. However, in type-II interaction, the contrast within a RE-enriched layer redistributes with a periodic fluctuation in [0002]Mg plane along [101̅0]Mg direction, as shown in figure 3a, 3b and 3c. The periodicity is 1.1±0.1 nm, which is approximately equivalent to the distance of 5 atomic sites along [101̅0]Mg direction. Actually, as shown in figure 3d, the distance between two adjacent RE zigzag lines in β′ structure is 1.1 nm, which indicates that one zigzag line in β′ may correspond to one period in the redistributed LPSO structure.
̅0]Mg Figure 4. A 𝛃′ precipitate intercepted by LPSO. Incident beam // [11𝟐 In figure 4, the dashed yellow lines are parallel to (0002)Mg, denoting a type-I interaction as discussed. In addition to the type-I interaction, the β′ precipitates are intercepted by LPSO structures from [101̅0]Mg direction, which is categorized as a type-III interaction. As a consequence, the zigzag lines in β′ are broken by gaps with low contrast that indicates, also, the absence of heavy atoms, as shown in the red rectangles. In addition, the β′ structure are atomic-resolutioned while the LPSO are not since the two are at different focus values, indicating that type-III interaction may occur along not only [101̅0]Mg, as denoted by the red arrow, but also [112̅0]Mg that is parallel to the incident beam direction. On the other hand, the growth of LPSO is impeded by the existence of β′ since the contrast of LPSO intercepting β′ precipitates are lower than normal LPSO.
6
4. Conclusions Based on the HAADF-STEM characterization, (1) The general morphology within the alloy is LPSO phase intercalated with β′ precipitates and the two structures interconnect into a 3-D network which is the microstructural origin of the unique mechanical properties of Mg-RE-Zn materials. (2) The β′/LPSO interfaces are fully characterized and categorized into three types: RE-absent gaps parallel to (112̅0)Mg with a width about 1.3nm; redistributions of heavy atoms within LPSO structures; interceptions of β′ precipitates by LPSO structures from [101̅0]Mg direction.
Acknowledgements This paper is financially supported by the National Natural Science Foundation of China (Grant no.51171107). The authors extend their gratitude to Yunwen Chen (Zhejiang University), Laijin Luo (Shenzhen Foreign Languages School), Haiyan Wang, Zhihua Zheng, Xiangzhong Ren (Shenzhen University) and Chengli Wang.
References [1] Antion C, Donnadieu P, Perrard F, Deschamps A, Tassin C, Pisch A. Hardening precipitation in a Mg– 4Y–3RE alloy. Acta Materialia. 2003;51:5335-48. [2] Kawabata T, Matsuda K, Kamado S, Kojima Y, Ikeno S. HRTEM observation of the precipitates in Mg-Gd-Y-Zr alloy.
Materials Science Forum: Trans Tech Publ; 2003. p. 303-6.
[3] Honma T, Ohkubo T, Hono K, Kamado S. Chemistry of nanoscale precipitates in Mg–2.1 Gd–0.6 Y– 0.2 Zr (at.%) alloy investigated by the atom probe technique. Materials Science and Engineering: A. 2005;395:301-6. [4] 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. Journal of Alloys and Compounds. 2006;421:309-13. [5] Rzychoń T, Michalska J, Kiełbus A. Effect of heat treatment on corrosion resistance of WE54 alloy. Journal of Achievements in Materials and Manufacturing Engineering. 2007;20:191-4. [6] Xin R, Li L, Zeng K, Song B, Liu Q. Structural examination of aging precipitation in a Mg–Y–Nd alloy
7
at different temperatures. Materials Characterization. 2011;62:535-9. [7] Zheng J, Xu X, Zhang K, Chen B. Novel structures observed in Mg–Gd–Y–Zr during isothermal ageing by atomic-scale HAADF-STEM. Materials Letters. 2015;152:287-9. [8] Li Y, Zhu G-z, Qiu D, Yin D, Rong Y, Zhang M-X. The intrinsic effect of long period stacking ordered phases on mechanical properties in Mg-RE based alloys. Journal of Alloys and Compounds. 2016;660:252-7. [9] Yamada K, Okubo Y, Shiono M, Watanabe H, Kamado S, Kojima Y. Alloy development of high toughness Mg-Gd-Y-Zn-Zr alloys. Materials transactions. 2006;47:1066-70. [10] Zhang S, Yuan G, Lu C, Ding W. The relationship between (Mg, Zn) 3 RE phase and 14H-LPSO phase in Mg–Gd–Y–Zn–Zr alloys solidified at different cooling rates. Journal of Alloys and Compounds. 2011;509:3515-21. [11] Honma T, Ohkubo T, Kamado S, Hono K. Effect of Zn additions on the age-hardening of Mg–2.0 Gd–1.2 Y–0.2 Zr alloys. Acta Materialia. 2007;55:4137-50. [12] Xu C, Zheng M, Xu S, Wu K, Wang E, Kamado S, et al. Ultra high-strength Mg–Gd–Y–Zn–Zr alloy sheets processed by large-strain hot rolling and ageing. Materials Science and Engineering: A. 2012;547:93-8. [13] Zhu Y, Weyland M, Morton A, Oh-Ishi K, Hono K, Nie J. The building block of long-period structures in Mg–RE–Zn alloys. Scripta materialia. 2009;60:980-3. [14] Zhu Y, Morton A, Nie J. The 18R and 14H long-period stacking ordered structures in Mg–Y–Zn alloys. Acta Materialia. 2010;58:2936-47.
1. The general morphology of this alloy is 3-D network composed of β′ and LPSO.
2. Three types of interactions between β′ and LPSO are fully revealed. 3. The LPSO that interfaces with α-Mg matrix may have unique characteristics.
8
9
PII: DOI: Reference:
S0167-577X(16)30601-2 http://dx.doi.org/10.1016/j.matlet.2016.04.114 MLBLUE20711
To appear in: Materials Letters Received date: 19 February 2016 Revised date: 8 April 2016 Accepted date: 14 April 2016 Cite this article as: Jingxu Zheng and Bin Chen, Interactions between Longperiod Stacking Ordered Phase and β’ Precipitate in Mg-Gd-Y-Zn-Zr Alloy: Atomic-scale Insights from HAADF-STEM, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.04.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interactions between Long-period Stacking Ordered Phase and β’ Precipitate in Mg-Gd-Y-Zn-Zr Alloy: Atomic-scale Insights from HAADF-STEM
Jingxu Zhenga,b, Bin Chena,b* a: Frontier Research Center for Materials Structure, Shanghai Jiao Tong University, Shanghai, China b: School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China *Corresponding author: Bin Chen (Dr.), [email protected]
Abstract This paper reports on an atomic-scale investigation into the interactions between β′ precipitates
and
long-period
stacking
ordered
phase
(LPSO)
in
Mg-10Gd-5Y-2Zn-0.5Zr (wt%) alloy, using Cs-corrected high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM). The general morphology within the alloy is LPSO phase intercalated with β′ precipitates and the two structures interconnect into a 3-D network. The β′/LPSO interfaces are fully characterized and categorized into three types: RE-absent gaps parallel to (112̅0)Mg, redistributions of heavy atoms within LPSO structures and interceptions of β′ precipitates by LPSO structures from [101̅0]Mg direction. Keywords: Metals and alloys; Phase transformation; Microstructure; Interfaces; Electron microscopy
1. Introduction The strengthening of Mg-based materials has long been a popular research topic because of the urgent demand for weight-reduced materials for structural applications. The strengthening effect of RE addition in alloys such as Mg-Gd-Y and Mg-Nd-Y has been investigated by many papers [1-7]. Besides, the addition of zinc into Mg-RE alloys has been proved to have significant effect in improving both the strength and ductility. For example, Mg-Gd-Y-Zn-Zr was reported to be an ultra-high strength, high-ductility alloy, which has attracted numerous research interests [8-12]. In contrast to the deep investigation into the properties of Mg-Gd-Y-Zn-Zr, the 1
microstructure of the Mg-RE-Zn is poorly understood. The strengthening structures in Mg-Gd-Y-Zn-Zr are generally reported to be coexisting β′ and long-period stacking ordered phase[8]. However, the spatial relationship and interaction between the two structures, to the authors’ best knowledge, are not clearly revealed yet. This paper, based on atomic-scale direct imaging, aims to fully unravel the microstructure in T6-treated Mg-Gd-Y-Zn-Zr, including spatial relationship and interaction between β′ and LPSO, providing insights into the relationship between structure and property. In addition, the paper also deepens our understanding of LPSO from a different perspective by characterizing β′/LPSO interfaces.
2. Experimental Procedures The Mg-Gd-Y-Zn-Zr alloy in this research was prepared from pure Mg, Mg-20Gd, Mg-30Y, Mg-30Zr master alloys (wt.%) and pure zinc by melting the components in an electrical resistance furnace under the protection of argon gas. The ingot was solution-treated at 500℃ in a sulfur atmosphere for 5 hours followed by quenching in water. The nominal composition was Mg-10Gd-5Y-2Zn-0.5Zr (wt%). The alloy was cut into small bulks, the sizes of which were about 5mm × 5mm × 5mm. The bulks were aged in silicon oil at a temperature of 225℃ for 48 hours. Disks with a diameter of 3 mm and a thickness of ~80𝜇m were punched from manually ground slices cut from the aged bulks. The disks were polished and thinned by Gatan Precision Ion Polishing System 695 to avoid the possible sample damage induced in a twin jet electro-polishing method. Atomic-resolution HAADF-STEM characterization was carried out on JEM-ARM200F, equipped with a probe-Cs corrector and a cold field emission gun. The accelerating voltage was 200kV. The camera length was set to 8 cm which yields a collection semi-angle of 48-327 mrad.
3. Results and Discussions
2
Figure 1. (a) General morphology of 𝐋𝐏𝐒𝐎 structures intercalated with 𝛃′ ̅0]Mg; (b) Schematic diagram illustrating the precipitates. Incident beam // [11𝟐 spatial relationship between 𝛃′ precipitates and LPSO structures. The LPSO phase in Mg-Gd-Y-Zn-Zr alloy forms during the casting process and does not dissolve during the solution-treatment at 500℃ whereas the β′ precipitates forms during the artificial ageing carried out at 225℃, as shown in supplementary material S1. The general morphology viewed from [112̅0]Mg of the alloy aged for 48 hours is presented in figure 1a and the morphology from [0001]Mg is presented in supplementary material S2. Because LPSO phase is not observable from [0001]Mg, the alloy is beamed from [112̅0]Mg direction, from which both LPSO and β′ precipitates can be characterized. As shown in figure 1a, the β′ precipitates and LPSO form an intercalated structure. The red arrows denote the LPSO structures. The β′ precipitates, one of which is enclosed by a dashed yellow curve, lie between the LPSO structures. Because the LPSO structures do not dissolve during the ageing, the spatial interaction between β′ and LPSO hinders the coarsening of both β′ and LPSO. In figure 1b, the schematic diagram illustrating the spatial relationship between β′ precipitates and LPSO structures is presented. The feature of LPSO is closed-packing planes aligning in certain sequence with RE enrichment and Zn enrichment in certain planes as denoted by the stacked light blue planes in figure 1b. The plate-like β′ precipitates grow between the separated LPSO structures during the isothermal ageing at 225℃. When the density and sizes of β′ precipitates are large enough, they begin to interact with the neighboring LPSO structures and the growth of β′ precipitates along [0001]Mg direction is, thus, restrained by the LPSO structures as a framework. In consequence, the β′ precipitates form on the prismatic planes and the LPSO structures form on the basal planes, interconnecting into a 3-D network within the alloy and thus leading to the unique mechanical properties of the alloy.
3
̅0]Mg Figure 2. The interface between 𝛃′ and LPSO. Incident beam // [11𝟐 In figure 2, type-I interfaces between β′ and LPSO are presented. As shown in figure 2a, straight gaps with a width about ~1.3 nm exist between β′ and LPSO, denoted by the parallel dashed yellow lines. The low contrast of the gaps reveals that heavy atoms, such as RE and Zn, are absent within the area. In figure 2b and 2c, a subset of the atoms in β′ precipitates are denoted by red circles; a subset of the RE-enriched layers in LPSO are denoted by red arrows and a subset of the layers without RE-enrichment are denotes by yellow arrows. The gaps consist of four RE/Zn-free atomic layers and as a result the theoretic distance is 1.315 nm in pure Mg model, which is consistent with the observed value. The stacking sequence of the LPSO in fig. 3b and 3c is 14H — “ABABCACACACBABA…” in the inner region of LPSO. The stacking sequence in the interfacial gap is consistent with the normal HCP stacking model in the α-Mg matrix — “AB…”.
According to Nie’s papers[13, 14] about the building block
theory in LPSO, each building block in a 14H LPSO consists of a complete stacking sequence of “ABABCACACACBABA” and the number of the atomic layers in the interfacial gap is, therefore, three instead of the observed value of four. From another 4
perspective, when considering the LPSO as heterogeneous nucleation sites for β′, the gap of four atomic layers should not exist. In heterogeneous nucleation, the energy required for nucleation is reduced because the precipitates nucleate right on the heterogeneous nuclei and thus the interfacial area is smaller. In the LPSO/β′ interface, the interfacial area for β′ nuclei is not reduced due to the gaps composed of pure Mg atomic layers between LPSO and β′. Hence, the building blocks that interface with α-Mg matrix may have different compositions and properties than the internal ones in LPSO phase.
Figure 3. Redistribution of the heavy atoms in LPSO close to 𝛃′ precipitates. ̅0]Mg. Incident beam // [11𝟐 In figure 3, type-II interactions between LPSO and β′ precipitates are presented, during which the heavy atoms, such as RE and Zn, undergo a redistribution within the RE-enriched layers. In normal LPSO structures, the contrast within one RE-enriched 5
layer is homogeneous, as shown in figure 2. However, in type-II interaction, the contrast within a RE-enriched layer redistributes with a periodic fluctuation in [0002]Mg plane along [101̅0]Mg direction, as shown in figure 3a, 3b and 3c. The periodicity is 1.1±0.1 nm, which is approximately equivalent to the distance of 5 atomic sites along [101̅0]Mg direction. Actually, as shown in figure 3d, the distance between two adjacent RE zigzag lines in β′ structure is 1.1 nm, which indicates that one zigzag line in β′ may correspond to one period in the redistributed LPSO structure.
̅0]Mg Figure 4. A 𝛃′ precipitate intercepted by LPSO. Incident beam // [11𝟐 In figure 4, the dashed yellow lines are parallel to (0002)Mg, denoting a type-I interaction as discussed. In addition to the type-I interaction, the β′ precipitates are intercepted by LPSO structures from [101̅0]Mg direction, which is categorized as a type-III interaction. As a consequence, the zigzag lines in β′ are broken by gaps with low contrast that indicates, also, the absence of heavy atoms, as shown in the red rectangles. In addition, the β′ structure are atomic-resolutioned while the LPSO are not since the two are at different focus values, indicating that type-III interaction may occur along not only [101̅0]Mg, as denoted by the red arrow, but also [112̅0]Mg that is parallel to the incident beam direction. On the other hand, the growth of LPSO is impeded by the existence of β′ since the contrast of LPSO intercepting β′ precipitates are lower than normal LPSO.
6
4. Conclusions Based on the HAADF-STEM characterization, (1) The general morphology within the alloy is LPSO phase intercalated with β′ precipitates and the two structures interconnect into a 3-D network which is the microstructural origin of the unique mechanical properties of Mg-RE-Zn materials. (2) The β′/LPSO interfaces are fully characterized and categorized into three types: RE-absent gaps parallel to (112̅0)Mg with a width about 1.3nm; redistributions of heavy atoms within LPSO structures; interceptions of β′ precipitates by LPSO structures from [101̅0]Mg direction.
Acknowledgements This paper is financially supported by the National Natural Science Foundation of China (Grant no.51171107). The authors extend their gratitude to Yunwen Chen (Zhejiang University), Laijin Luo (Shenzhen Foreign Languages School), Haiyan Wang, Zhihua Zheng, Xiangzhong Ren (Shenzhen University) and Chengli Wang.
References [1] Antion C, Donnadieu P, Perrard F, Deschamps A, Tassin C, Pisch A. Hardening precipitation in a Mg– 4Y–3RE alloy. Acta Materialia. 2003;51:5335-48. [2] Kawabata T, Matsuda K, Kamado S, Kojima Y, Ikeno S. HRTEM observation of the precipitates in Mg-Gd-Y-Zr alloy.
Materials Science Forum: Trans Tech Publ; 2003. p. 303-6.
[3] Honma T, Ohkubo T, Hono K, Kamado S. Chemistry of nanoscale precipitates in Mg–2.1 Gd–0.6 Y– 0.2 Zr (at.%) alloy investigated by the atom probe technique. Materials Science and Engineering: A. 2005;395:301-6. [4] 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. Journal of Alloys and Compounds. 2006;421:309-13. [5] Rzychoń T, Michalska J, Kiełbus A. Effect of heat treatment on corrosion resistance of WE54 alloy. Journal of Achievements in Materials and Manufacturing Engineering. 2007;20:191-4. [6] Xin R, Li L, Zeng K, Song B, Liu Q. Structural examination of aging precipitation in a Mg–Y–Nd alloy
7
at different temperatures. Materials Characterization. 2011;62:535-9. [7] Zheng J, Xu X, Zhang K, Chen B. Novel structures observed in Mg–Gd–Y–Zr during isothermal ageing by atomic-scale HAADF-STEM. Materials Letters. 2015;152:287-9. [8] Li Y, Zhu G-z, Qiu D, Yin D, Rong Y, Zhang M-X. The intrinsic effect of long period stacking ordered phases on mechanical properties in Mg-RE based alloys. Journal of Alloys and Compounds. 2016;660:252-7. [9] Yamada K, Okubo Y, Shiono M, Watanabe H, Kamado S, Kojima Y. Alloy development of high toughness Mg-Gd-Y-Zn-Zr alloys. Materials transactions. 2006;47:1066-70. [10] Zhang S, Yuan G, Lu C, Ding W. The relationship between (Mg, Zn) 3 RE phase and 14H-LPSO phase in Mg–Gd–Y–Zn–Zr alloys solidified at different cooling rates. Journal of Alloys and Compounds. 2011;509:3515-21. [11] Honma T, Ohkubo T, Kamado S, Hono K. Effect of Zn additions on the age-hardening of Mg–2.0 Gd–1.2 Y–0.2 Zr alloys. Acta Materialia. 2007;55:4137-50. [12] Xu C, Zheng M, Xu S, Wu K, Wang E, Kamado S, et al. Ultra high-strength Mg–Gd–Y–Zn–Zr alloy sheets processed by large-strain hot rolling and ageing. Materials Science and Engineering: A. 2012;547:93-8. [13] Zhu Y, Weyland M, Morton A, Oh-Ishi K, Hono K, Nie J. The building block of long-period structures in Mg–RE–Zn alloys. Scripta materialia. 2009;60:980-3. [14] Zhu Y, Morton A, Nie J. The 18R and 14H long-period stacking ordered structures in Mg–Y–Zn alloys. Acta Materialia. 2010;58:2936-47.
1. The general morphology of this alloy is 3-D network composed of β′ and LPSO.
2. Three types of interactions between β′ and LPSO are fully revealed. 3. The LPSO that interfaces with α-Mg matrix may have unique characteristics.
8
9