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A room temperature formable magnesium–silver–calcium sheet alloy with high ductility
Journal Pre-proof A room temperature formable magnesium–silver–calcium sheet alloy with high ductility Mingzhe Bian, Xinsheng Huang, Yasumasa Chino PII:
S0921-5093(20)30015-0
DOI:
https://doi.org/10.1016/j.msea.2020.138923
Reference:
MSA 138923
To appear in:
Materials Science & Engineering A
Received Date: 31 October 2019 Revised Date:
28 December 2019
Accepted Date: 3 January 2020
Please cite this article as: M. Bian, X. Huang, Y. Chino, A room temperature formable magnesium– silver–calcium sheet alloy with high ductility, Materials Science & Engineering A (2020), doi: https:// doi.org/10.1016/j.msea.2020.138923. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Credit Author Statement Mingzhe Bian: Investigation, Methodology, Writing - Original Draft Preparation. Xinsheng Huang: Investigation, Methodology, Writing - Review & Editing. Yasumasa Chino: Writing - Review & Editing, Conceptualization.
A room temperature formable magnesium–silver–calcium sheet alloy with high ductility
Mingzhe Bian*, Xinsheng Huang, Yasumasa Chino Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Aichi 463-8560, Japan
*Corresponding author: [email protected]
Abstract A newly developed magnesium sheet alloy, Mg–1.5Ag–0.1Ca in wt.%, shows a large index Erichsen value of 8.6 mm with a large average elongation to failure of 33% at room temperature (RT). The excellent RT formability and ductility are associated with a weak ring-like basal texture. Keywords: Magnesium alloys; rolling; formability; ductility; texture.
1. Introduction Lightweight magnesium (Mg) alloys are emerging as promising candidates for structural components in the automotive industry [1,2]. The processes used to convert a sheet material into complex automobile components are generally carried out at near room temperature (RT). RT formability is therefore one of the most important properties for the sheet material used in the automotive industry. Unfortunately, due to the strong basal texture developed during wrought processing and limited active deformation modes at RT, Mg alloys in the form of sheet do not show good formability at RT, restricting their wider applications. The development of new alloys such as Mg–Zn–RE (RE: Ce [3,4], Y [5], Gd [6] and Nd [7]) and Mg–Zn–Ca [8–11] has been demonstrated
to be an effective way to weaken the strong basal texture and thus enhance the formability. However, their texture distributions are still not ideal as one directional orthotropic textures, i.e. transverse direction (TD) split or rolling direction (RD) split, are developed after annealing, which would not allow the sheet material to deform homogeneously during multi-axial stretch forming [12]. A closer look at chemical compositions of these formable Mg sheet alloys developed so far, we can find that the vast majority of them contain the Zn element. Since zinc (Zn) is added as a principal or a “default” alloying element in these alloys, its importance to formability might not be recognized. However, one of the main drawbacks of the Mg–Zn based alloys, particularly with concentrated alloys, is that they contain low melting points phases, which can trigger hot shortness during hot working. In this study, an attempt was made by the present authors to explore the possibility of developing a Zn-free Mg sheet alloy with excellent RT formability. Silver (Ag) is an extremely soft, ductile and malleable transition metal and has a large solubility of 15 wt.% in Mg at the eutectic temperature of 472 °C. It is well known that microalloying additions of Ag substantially enhance the age-hardening response of Al–Zn–Mg alloys [13,14]; therefore, Ag has been added to Mg alloys to mainly investigate the role of its additions to the age-hardening response as well. For instance, Ag was systematically added to a Mg–6Zn extrusion alloy [15], and Mg–Zn [16,17], Mg–Gd–Zr [18], Mg–Y–Zn [19] and Mg–Sn–Mn [20] based casting alloys and the resultant Ag-containing alloys showed enhanced age-hardening response by the refinement of precipitate microstructures. In contrast, effects of the Ag addition on the formability of Mg alloys has been received far less attention [21], which motivated us to visit Mg–Ag alloy system for sheet applications. We found that the co-addition of small amounts of Ag and Ca could not only improve the stretch formability substantially but also form a weak ring-like basal texture that is different from those developed in Mg–Zn–RE and Mg–Zn–Ca sheet alloys. The purpose of this paper is thus to report mechanical properties of the newly developed Mg–Ag–Ca alloy and its texture developed after annealing.
2. Experimental procedure Mg–1.5Ag–0.1Ca in wt.% alloy ingot was prepared from pure Mg (>99.9%), Ag (>99.9%) and Mg–4.92 wt%Ca master alloys. Binary Mg–1.5Ag and Mg–0.1Ca in wt.% alloy ingots were also prepared for the purpose of comparison. Casting of the Mg–1.5Ag–0.1Ca and Mg–1.5Ag alloys was conducted at 820 °C under an argon protective atmosphere, while casting of the Mg–0.1Ca alloy was conducted at 740 °C under the same atmosphere. Their actual chemical compositions were measured by induction coupled plasma (ICP) and confirmed to be Mg–1.37Ag–0.10Ca, Mg– 1.41Ag and Mg–0.10Ca in wt.%, respectively. The as-cast ingots were extruded as plates of 5 mm thickness at 380 °C with an extrusion ratio of 6 and a ram speed of 5 mm/min. The extruded plates were further rolled from 5 mm to 1 mm in thickness by 7 passes, with ~ 21% thickness reduction per pass. The sheets were quenched into water immediately after each pass and were re-heated to 450 °C prior to subsequent rolling. The as-rolled sheets were finally annealed at 350 °C for 90 min. To evaluate stretch formability of annealed sheets, Erichsen cupping tests were carried out on circular blanks with a diameter of 60 mm and a thickness of 1 mm using a hemispherical punch with a diameter of 20 mm at RT. Punch speed and blank-holder force were 5 mm/min and 10 kN, respectively. Tensile specimens having a parallel length of 12 mm, a width of 4 mm and a thickness of 1 mm were machined from the annealed sheets along the RD (0°), 45° and TD (90°). Tensile tests were conducted at RT using an Instron 5565 tensile testing machine at an initial strain rate of 2.78 × 10−3 s−1. The Lankford values (r-value) were measured on the tensile specimens deformed at a plastic strain of 9%. (0002) basal pole figures were measured at the mid-layers of as-rolled and annealed sheets by X-ray texture analyses using the Schulz reflection method operated at 40 kV and 40 mA. The X-ray diffraction (XRD) patterns were measured within the range of 30° – 80° (Rigaku RINT Ultima III, Cu target). The electron backscatter diffraction (EBSD) measurements were performed at 20 kV using a JEOL scanning electron microscope (JSM-IT500) equipped with TSL
OIM 7.0 data collection and analysis software package. Scanning electron microscope (SEM) observation was performed at 15 kV using the same SEM equipment.
3. Results and discussion
Figure 1. EBSD-IPF maps and (0002) basal pole figures showing microstructures and textures of as-rolled (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag–0.1Ca sheets. Overlay of IPF and IQ maps taken from the regions marked by white dashed squares in Fig. 1a-c showing detailed microstructural features of as-rolled (d) Mg–1.5Ag, (e) Mg–0.1Ca, and (f) Mg–1.5Ag–0.1Ca sheets. White, green and yellow lines in Fig. 1d-f indicate {1012} tensile twin, {1011} compressive twin and {1011}-{1012} double twin, respectively. Fig.1 shows EBSD inverse pole figure (IPF) maps taken from the middle region of as-rolled Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets and (0002) basal pole figures measured by XRD. Typical deformation microstructures consisting of deformed matrix, deformation twins and shear bands were developed in all sheets, as can be seen from Fig.1a-c. A noticeable difference is that the binary Mg–0.1Ca and ternary Mg–1.5Ag–0.1Ca alloy sheets showed a more homogeneous distribution of shear bands in comparison with the binary Mg–1.5Ag alloy sheet. To examine the
microstructure in more detail, the regions marked by white dashed squares in Fig. 1a-c were enlarged and rescanned with a finer step size. Fig. 1d-f shows the corresponding IPF maps superimposed with image quality (IQ) maps. Various types of deformation twins were observed, but obvious dynamic recrystallized (DRXed) grains were not detected within those twin interiors and deformed matrix. The fractions of {101 2} tensile twin, {101 1} compressive twin and {101 1}{1012} double twin in the ternary alloy were measured to be 6.2 %, 5.5 % and 13.3 %, which were essentially the same as those in the binary Mg–Ag (5.2 %, 5.5 % and 16.6 %) and Mg–Ca (7.1 %, 5.3 % and 12. 6 %) alloys. Although there was little difference in the deformed microstructures, the texture intensities and distributions were apparently different. The binary Mg–Ag alloy sheet showed the highest texture intensity of 6.8 multiple random distribution (m.r.d.), while the ternary alloy sheet exhibited a weak texture intensity as low as 3.4 m.r.d. All sheets exhibited RD-split textures; however, a broadening tendency of (0002) basal poles along the TD became stronger in the order of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets.
Figure 2. EBSD-IPF maps and (0002) basal pole figures showing microstructures and textures of annealed (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag–0.1Ca sheets. EBSD-IPF maps from the middle region of the Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets after annealing and their (0002) basal pole figures are shown in Fig. 2. Fully recrystallized
and homogeneous microstructures were developed in all sheets, as shown in Fig. 2a-c. The average grain size of the Mg–0.1Ca sheet was measured to be ~24 µm, which was smaller than that of the Mg–1.5Ag sheet (~35 µm) even though the additive amount of Ca was much less than that of Ag. The co-addition of Ag and Ca further reduced the average grain size to ~20 µm. In the meantime, the texture intensity of the ternary alloy sheet was also reduced from 3.4 to 2.4 m.r.d. Regarding the texture distribution, the co-addition of Ag and Ca resulted in the most pronounced texture modification in comparison with the sole addition of Ag or Ca. A ring-like basal texture where the maximum intensity of basal planes was oriented 25 – 35° away from the ND to all directions was developed after annealing. A schematic diagram illustrating the distribution of the (0002) basal pole figure of the Mg–1.5Ag–0.1Ca alloy is shown in Fig. 2d. The maximum intensity along the RD was located at 25°, which was approximately 10o smaller than that along the TD (35°).
Figure 3. (a) SEM image and (b) XRD pattern obtained from the middle region of annealed Mg– 1.5Ag–0.1Ca sheet. The microstructure of the annealed Mg–1.5Ag–0.1Ca sheet was further investigated by SEM and XRD. As can be seen in Fig. 3a, the Mg–1.5Ag–0.1Ca sheet did not contain any second phase
particles. XRD pattern indicated that the sheet consisted of only ɑ-Mg phase (Fig. 3b), which was consistent with the SEM observation result.
Figure 4. RT stretch formability of annealed (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag– 0.1Ca sheets. (d) RT tensile curves obtained from these sheets along the RD. (e) Plot of IE value as a function of average elongation to failure. Data for Mg–Zn–RE, Mg–Zn–Ca and commercial AZ31 sheet alloys for comparison are from Ref [4,5,8,10,11,22–26]. Stretch formability of the annealed Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets was evaluated by Erichsen cupping tests at RT. The fractured specimens after Erichsen cupping tests are shown in Fig. 4a-c. The binary Mg–Ag and Mg–Ca alloy sheets showed low index Erichsen (IE) values of 3.8 mm and 3.9 mm, respectively. In contrast, the IE value of ternary alloy sheet was as high as 8.6 mm, which was more than twice those of former ones. Fig. 4d shows nominal tensile stress-strain curves of the annealed sheets stretched along the RD. Due to the limited space, the tensile properties obtained from these sheets along the 45o and the TD are summarized in Table 1. The average elongation to failure (EF) of the ternary alloy sheet was 33 %, which was substantially higher than those of binary Mg–Ag (24 %) and Mg–Ca (21 %) alloy sheets. The average n-value increased and the average r-value decreased in the order of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–
0.1Ca. Lower TYS along the RD (107 MPa) than the TD (133 MPa) was observed in the binary Mg–Ag alloy sheet, while higher TYS along the RD (124 MPa) than the TD (95 MPa) was observed in the ternary alloy sheet. Fig. 4e plots the IE value and average elongation to failure of the Mg–1.5Ag–0.1Ca alloy obtained from the this this. For the purpose of comparison, formable Mg–Zn–RE [4,5,22], Mg–Zn–Ca [8,10,11,23] and commercial AZ31 [24–26] alloys were also included in this figure. As can be seen, the newly developed Mg–1.5Ag–0.1Ca alloy showed not only excellent formability but also excellent ductility in comparison with those representative alloys reported in the literature. Composition (wt.%) UTS (MPa) TYS (MPa) EF (%) UE (%) n-value r-value 201 107 25 16 0.25 1.36 Mg–1.5Ag 205 117 22 15 0.22 1.83 211 133 24 12 0.19 2.54 199 110 21 12 0.21 1.03 Mg–0.1Ca 201 118 22 11 0.20 1.11 200 110 21 12 0.21 1.15 216 124 29 16 0.25 0.85 Mg–1.5Ag–0.1Ca 209 103 35 22 0.34 1.04 209 95 35 21 0.41 0.74
rave 1.89
1.10
0.91
Table 1. RT tensile properties of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets stretched along the RD, 45o and TD. In this study, we report a Zn-free Mg–1.5Ag–0.1Ca sheet alloy having excellent RT stretch formability and ductility. To the best of authors’ knowledge, there is no prior research on the development of dilute Mg–Ag–Ca based sheet alloys. Indeed, a recent study investigated effects of the single Ca or dual additions Ca and Ag to a concentrated Mg–6.2Zn–0.5Zr (wt.%) alloy and reported that the resultant Mg–6.2Zn–0.5Zr–0.2Ca and Mg–6.2Zn–0.5Zr–0.2Ca–0.4Ag alloys show low IE values of 5.2 and 4.9 mm, respectively, due to strong basal textures developed after annealing [21]. Unlike the concentrated Mg–6.2Zn–0.5Zr alloy, we find that the combined addition of small amount of Ag and Ca to pure Mg can substantially weaken the basal texture, while the single addition of Ag or Ca is much less effective. More importantly, a ring-like basal texture in
which the location of the maximum intensity of basal poles is 25°–35° from the ND toward all directions is formed in the Mg–1.5Ag–0.1Ca sheet. This type of texture is believed to be beneficial in obtaining excellent stretch formability as well as ductility [27,28]. This is because more grains are favorably oriented to activate basal , thereby resulting in higher n-values and lower average r-values in comparison with center or RD-split textures developed in the counterpart Mg–1.5Ag and Mg–0.1Ca sheets. The formation mechanism of the ring-like basal texture in the Mg–1.5Ag–0.1Ca alloy remains unknown. Obvious DRXed grains are not observed in twin interiors as well as deformed matrix, revealing that a transition from the typical RD split texture to the unique texture occurs during annealing. It is speculated that {1011}-{1012} double twins would be the preferential nucleation sites for recrystallized grains at the early stages of annealing [29]. Nevertheless, they are not likely to be the predominant factor that affects the formation of ring-like texture as similar fractions of {1011}-{1012} double twin are formed in the as-rolled Mg–1.5Ag and Mg–0.1Ca sheets. Zeng et al. observed that Zn and Ca atoms in a dilute Mg–0.8Zn–0.2Ca (wt.%) alloy segregate to recrystallized grain boundaries and hypothesized that the co-segregation of Zn and Ca would reduce the boundary mobility, thereby leading to a more uniform growth of recrystallized grains with randomized orientations [30]. This mechanism appears to be a viable explanation for the unique texture observed in the Mg–1.5Ag–0.1Ca alloy because recrystallized grains are more randomized compared to the counterpart Mg–1.5Ag and Mg–0.1Ca sheets. Detailed experimental work is now in progress to clarify the mechanism that is responsible for the ring-like texture formation in Mg– Ag–Ca alloys.
4. Summary Excellent stretch formability with good ductility at RT is achieved in a newly developed Mg– 1.5Ag–0.1Ca (wt.%) sheet alloy. This alloy shows a large index Erichsen value of 8.6 mm and a
large average elongation to failure of 33% at RT. A weak ring-like basal texture where the locations of the maximum intensity of basal planes are 25 – 35° away from the ND to all directions is developed after annealing, and such texture is responsible for the excellent RT formability and ductility. The finding of the ring-like texture in a new type of Mg–Ag–Ca alloy is expected to provide new pathways for the development of Mg sheet alloys with better formability and ductility.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
Acknowledgements This study was partially supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) (KAKENHI, 16H04525).
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S0921-5093(20)30015-0
DOI:
https://doi.org/10.1016/j.msea.2020.138923
Reference:
MSA 138923
To appear in:
Materials Science & Engineering A
Received Date: 31 October 2019 Revised Date:
28 December 2019
Accepted Date: 3 January 2020
Please cite this article as: M. Bian, X. Huang, Y. Chino, A room temperature formable magnesium– silver–calcium sheet alloy with high ductility, Materials Science & Engineering A (2020), doi: https:// doi.org/10.1016/j.msea.2020.138923. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Credit Author Statement Mingzhe Bian: Investigation, Methodology, Writing - Original Draft Preparation. Xinsheng Huang: Investigation, Methodology, Writing - Review & Editing. Yasumasa Chino: Writing - Review & Editing, Conceptualization.
A room temperature formable magnesium–silver–calcium sheet alloy with high ductility
Mingzhe Bian*, Xinsheng Huang, Yasumasa Chino Structural Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Aichi 463-8560, Japan
*Corresponding author: [email protected]
Abstract A newly developed magnesium sheet alloy, Mg–1.5Ag–0.1Ca in wt.%, shows a large index Erichsen value of 8.6 mm with a large average elongation to failure of 33% at room temperature (RT). The excellent RT formability and ductility are associated with a weak ring-like basal texture. Keywords: Magnesium alloys; rolling; formability; ductility; texture.
1. Introduction Lightweight magnesium (Mg) alloys are emerging as promising candidates for structural components in the automotive industry [1,2]. The processes used to convert a sheet material into complex automobile components are generally carried out at near room temperature (RT). RT formability is therefore one of the most important properties for the sheet material used in the automotive industry. Unfortunately, due to the strong basal texture developed during wrought processing and limited active deformation modes at RT, Mg alloys in the form of sheet do not show good formability at RT, restricting their wider applications. The development of new alloys such as Mg–Zn–RE (RE: Ce [3,4], Y [5], Gd [6] and Nd [7]) and Mg–Zn–Ca [8–11] has been demonstrated
to be an effective way to weaken the strong basal texture and thus enhance the formability. However, their texture distributions are still not ideal as one directional orthotropic textures, i.e. transverse direction (TD) split or rolling direction (RD) split, are developed after annealing, which would not allow the sheet material to deform homogeneously during multi-axial stretch forming [12]. A closer look at chemical compositions of these formable Mg sheet alloys developed so far, we can find that the vast majority of them contain the Zn element. Since zinc (Zn) is added as a principal or a “default” alloying element in these alloys, its importance to formability might not be recognized. However, one of the main drawbacks of the Mg–Zn based alloys, particularly with concentrated alloys, is that they contain low melting points phases, which can trigger hot shortness during hot working. In this study, an attempt was made by the present authors to explore the possibility of developing a Zn-free Mg sheet alloy with excellent RT formability. Silver (Ag) is an extremely soft, ductile and malleable transition metal and has a large solubility of 15 wt.% in Mg at the eutectic temperature of 472 °C. It is well known that microalloying additions of Ag substantially enhance the age-hardening response of Al–Zn–Mg alloys [13,14]; therefore, Ag has been added to Mg alloys to mainly investigate the role of its additions to the age-hardening response as well. For instance, Ag was systematically added to a Mg–6Zn extrusion alloy [15], and Mg–Zn [16,17], Mg–Gd–Zr [18], Mg–Y–Zn [19] and Mg–Sn–Mn [20] based casting alloys and the resultant Ag-containing alloys showed enhanced age-hardening response by the refinement of precipitate microstructures. In contrast, effects of the Ag addition on the formability of Mg alloys has been received far less attention [21], which motivated us to visit Mg–Ag alloy system for sheet applications. We found that the co-addition of small amounts of Ag and Ca could not only improve the stretch formability substantially but also form a weak ring-like basal texture that is different from those developed in Mg–Zn–RE and Mg–Zn–Ca sheet alloys. The purpose of this paper is thus to report mechanical properties of the newly developed Mg–Ag–Ca alloy and its texture developed after annealing.
2. Experimental procedure Mg–1.5Ag–0.1Ca in wt.% alloy ingot was prepared from pure Mg (>99.9%), Ag (>99.9%) and Mg–4.92 wt%Ca master alloys. Binary Mg–1.5Ag and Mg–0.1Ca in wt.% alloy ingots were also prepared for the purpose of comparison. Casting of the Mg–1.5Ag–0.1Ca and Mg–1.5Ag alloys was conducted at 820 °C under an argon protective atmosphere, while casting of the Mg–0.1Ca alloy was conducted at 740 °C under the same atmosphere. Their actual chemical compositions were measured by induction coupled plasma (ICP) and confirmed to be Mg–1.37Ag–0.10Ca, Mg– 1.41Ag and Mg–0.10Ca in wt.%, respectively. The as-cast ingots were extruded as plates of 5 mm thickness at 380 °C with an extrusion ratio of 6 and a ram speed of 5 mm/min. The extruded plates were further rolled from 5 mm to 1 mm in thickness by 7 passes, with ~ 21% thickness reduction per pass. The sheets were quenched into water immediately after each pass and were re-heated to 450 °C prior to subsequent rolling. The as-rolled sheets were finally annealed at 350 °C for 90 min. To evaluate stretch formability of annealed sheets, Erichsen cupping tests were carried out on circular blanks with a diameter of 60 mm and a thickness of 1 mm using a hemispherical punch with a diameter of 20 mm at RT. Punch speed and blank-holder force were 5 mm/min and 10 kN, respectively. Tensile specimens having a parallel length of 12 mm, a width of 4 mm and a thickness of 1 mm were machined from the annealed sheets along the RD (0°), 45° and TD (90°). Tensile tests were conducted at RT using an Instron 5565 tensile testing machine at an initial strain rate of 2.78 × 10−3 s−1. The Lankford values (r-value) were measured on the tensile specimens deformed at a plastic strain of 9%. (0002) basal pole figures were measured at the mid-layers of as-rolled and annealed sheets by X-ray texture analyses using the Schulz reflection method operated at 40 kV and 40 mA. The X-ray diffraction (XRD) patterns were measured within the range of 30° – 80° (Rigaku RINT Ultima III, Cu target). The electron backscatter diffraction (EBSD) measurements were performed at 20 kV using a JEOL scanning electron microscope (JSM-IT500) equipped with TSL
OIM 7.0 data collection and analysis software package. Scanning electron microscope (SEM) observation was performed at 15 kV using the same SEM equipment.
3. Results and discussion
Figure 1. EBSD-IPF maps and (0002) basal pole figures showing microstructures and textures of as-rolled (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag–0.1Ca sheets. Overlay of IPF and IQ maps taken from the regions marked by white dashed squares in Fig. 1a-c showing detailed microstructural features of as-rolled (d) Mg–1.5Ag, (e) Mg–0.1Ca, and (f) Mg–1.5Ag–0.1Ca sheets. White, green and yellow lines in Fig. 1d-f indicate {1012} tensile twin, {1011} compressive twin and {1011}-{1012} double twin, respectively. Fig.1 shows EBSD inverse pole figure (IPF) maps taken from the middle region of as-rolled Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets and (0002) basal pole figures measured by XRD. Typical deformation microstructures consisting of deformed matrix, deformation twins and shear bands were developed in all sheets, as can be seen from Fig.1a-c. A noticeable difference is that the binary Mg–0.1Ca and ternary Mg–1.5Ag–0.1Ca alloy sheets showed a more homogeneous distribution of shear bands in comparison with the binary Mg–1.5Ag alloy sheet. To examine the
microstructure in more detail, the regions marked by white dashed squares in Fig. 1a-c were enlarged and rescanned with a finer step size. Fig. 1d-f shows the corresponding IPF maps superimposed with image quality (IQ) maps. Various types of deformation twins were observed, but obvious dynamic recrystallized (DRXed) grains were not detected within those twin interiors and deformed matrix. The fractions of {101 2} tensile twin, {101 1} compressive twin and {101 1}{1012} double twin in the ternary alloy were measured to be 6.2 %, 5.5 % and 13.3 %, which were essentially the same as those in the binary Mg–Ag (5.2 %, 5.5 % and 16.6 %) and Mg–Ca (7.1 %, 5.3 % and 12. 6 %) alloys. Although there was little difference in the deformed microstructures, the texture intensities and distributions were apparently different. The binary Mg–Ag alloy sheet showed the highest texture intensity of 6.8 multiple random distribution (m.r.d.), while the ternary alloy sheet exhibited a weak texture intensity as low as 3.4 m.r.d. All sheets exhibited RD-split textures; however, a broadening tendency of (0002) basal poles along the TD became stronger in the order of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets.
Figure 2. EBSD-IPF maps and (0002) basal pole figures showing microstructures and textures of annealed (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag–0.1Ca sheets. EBSD-IPF maps from the middle region of the Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets after annealing and their (0002) basal pole figures are shown in Fig. 2. Fully recrystallized
and homogeneous microstructures were developed in all sheets, as shown in Fig. 2a-c. The average grain size of the Mg–0.1Ca sheet was measured to be ~24 µm, which was smaller than that of the Mg–1.5Ag sheet (~35 µm) even though the additive amount of Ca was much less than that of Ag. The co-addition of Ag and Ca further reduced the average grain size to ~20 µm. In the meantime, the texture intensity of the ternary alloy sheet was also reduced from 3.4 to 2.4 m.r.d. Regarding the texture distribution, the co-addition of Ag and Ca resulted in the most pronounced texture modification in comparison with the sole addition of Ag or Ca. A ring-like basal texture where the maximum intensity of basal planes was oriented 25 – 35° away from the ND to all directions was developed after annealing. A schematic diagram illustrating the distribution of the (0002) basal pole figure of the Mg–1.5Ag–0.1Ca alloy is shown in Fig. 2d. The maximum intensity along the RD was located at 25°, which was approximately 10o smaller than that along the TD (35°).
Figure 3. (a) SEM image and (b) XRD pattern obtained from the middle region of annealed Mg– 1.5Ag–0.1Ca sheet. The microstructure of the annealed Mg–1.5Ag–0.1Ca sheet was further investigated by SEM and XRD. As can be seen in Fig. 3a, the Mg–1.5Ag–0.1Ca sheet did not contain any second phase
particles. XRD pattern indicated that the sheet consisted of only ɑ-Mg phase (Fig. 3b), which was consistent with the SEM observation result.
Figure 4. RT stretch formability of annealed (a) Mg–1.5Ag, (b) Mg–0.1Ca, and (c) Mg–1.5Ag– 0.1Ca sheets. (d) RT tensile curves obtained from these sheets along the RD. (e) Plot of IE value as a function of average elongation to failure. Data for Mg–Zn–RE, Mg–Zn–Ca and commercial AZ31 sheet alloys for comparison are from Ref [4,5,8,10,11,22–26]. Stretch formability of the annealed Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets was evaluated by Erichsen cupping tests at RT. The fractured specimens after Erichsen cupping tests are shown in Fig. 4a-c. The binary Mg–Ag and Mg–Ca alloy sheets showed low index Erichsen (IE) values of 3.8 mm and 3.9 mm, respectively. In contrast, the IE value of ternary alloy sheet was as high as 8.6 mm, which was more than twice those of former ones. Fig. 4d shows nominal tensile stress-strain curves of the annealed sheets stretched along the RD. Due to the limited space, the tensile properties obtained from these sheets along the 45o and the TD are summarized in Table 1. The average elongation to failure (EF) of the ternary alloy sheet was 33 %, which was substantially higher than those of binary Mg–Ag (24 %) and Mg–Ca (21 %) alloy sheets. The average n-value increased and the average r-value decreased in the order of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–
0.1Ca. Lower TYS along the RD (107 MPa) than the TD (133 MPa) was observed in the binary Mg–Ag alloy sheet, while higher TYS along the RD (124 MPa) than the TD (95 MPa) was observed in the ternary alloy sheet. Fig. 4e plots the IE value and average elongation to failure of the Mg–1.5Ag–0.1Ca alloy obtained from the this this. For the purpose of comparison, formable Mg–Zn–RE [4,5,22], Mg–Zn–Ca [8,10,11,23] and commercial AZ31 [24–26] alloys were also included in this figure. As can be seen, the newly developed Mg–1.5Ag–0.1Ca alloy showed not only excellent formability but also excellent ductility in comparison with those representative alloys reported in the literature. Composition (wt.%) UTS (MPa) TYS (MPa) EF (%) UE (%) n-value r-value 201 107 25 16 0.25 1.36 Mg–1.5Ag 205 117 22 15 0.22 1.83 211 133 24 12 0.19 2.54 199 110 21 12 0.21 1.03 Mg–0.1Ca 201 118 22 11 0.20 1.11 200 110 21 12 0.21 1.15 216 124 29 16 0.25 0.85 Mg–1.5Ag–0.1Ca 209 103 35 22 0.34 1.04 209 95 35 21 0.41 0.74
rave 1.89
1.10
0.91
Table 1. RT tensile properties of Mg–1.5Ag, Mg–0.1Ca and Mg–1.5Ag–0.1Ca sheets stretched along the RD, 45o and TD. In this study, we report a Zn-free Mg–1.5Ag–0.1Ca sheet alloy having excellent RT stretch formability and ductility. To the best of authors’ knowledge, there is no prior research on the development of dilute Mg–Ag–Ca based sheet alloys. Indeed, a recent study investigated effects of the single Ca or dual additions Ca and Ag to a concentrated Mg–6.2Zn–0.5Zr (wt.%) alloy and reported that the resultant Mg–6.2Zn–0.5Zr–0.2Ca and Mg–6.2Zn–0.5Zr–0.2Ca–0.4Ag alloys show low IE values of 5.2 and 4.9 mm, respectively, due to strong basal textures developed after annealing [21]. Unlike the concentrated Mg–6.2Zn–0.5Zr alloy, we find that the combined addition of small amount of Ag and Ca to pure Mg can substantially weaken the basal texture, while the single addition of Ag or Ca is much less effective. More importantly, a ring-like basal texture in
which the location of the maximum intensity of basal poles is 25°–35° from the ND toward all directions is formed in the Mg–1.5Ag–0.1Ca sheet. This type of texture is believed to be beneficial in obtaining excellent stretch formability as well as ductility [27,28]. This is because more grains are favorably oriented to activate basal , thereby resulting in higher n-values and lower average r-values in comparison with center or RD-split textures developed in the counterpart Mg–1.5Ag and Mg–0.1Ca sheets. The formation mechanism of the ring-like basal texture in the Mg–1.5Ag–0.1Ca alloy remains unknown. Obvious DRXed grains are not observed in twin interiors as well as deformed matrix, revealing that a transition from the typical RD split texture to the unique texture occurs during annealing. It is speculated that {1011}-{1012} double twins would be the preferential nucleation sites for recrystallized grains at the early stages of annealing [29]. Nevertheless, they are not likely to be the predominant factor that affects the formation of ring-like texture as similar fractions of {1011}-{1012} double twin are formed in the as-rolled Mg–1.5Ag and Mg–0.1Ca sheets. Zeng et al. observed that Zn and Ca atoms in a dilute Mg–0.8Zn–0.2Ca (wt.%) alloy segregate to recrystallized grain boundaries and hypothesized that the co-segregation of Zn and Ca would reduce the boundary mobility, thereby leading to a more uniform growth of recrystallized grains with randomized orientations [30]. This mechanism appears to be a viable explanation for the unique texture observed in the Mg–1.5Ag–0.1Ca alloy because recrystallized grains are more randomized compared to the counterpart Mg–1.5Ag and Mg–0.1Ca sheets. Detailed experimental work is now in progress to clarify the mechanism that is responsible for the ring-like texture formation in Mg– Ag–Ca alloys.
4. Summary Excellent stretch formability with good ductility at RT is achieved in a newly developed Mg– 1.5Ag–0.1Ca (wt.%) sheet alloy. This alloy shows a large index Erichsen value of 8.6 mm and a
large average elongation to failure of 33% at RT. A weak ring-like basal texture where the locations of the maximum intensity of basal planes are 25 – 35° away from the ND to all directions is developed after annealing, and such texture is responsible for the excellent RT formability and ductility. The finding of the ring-like texture in a new type of Mg–Ag–Ca alloy is expected to provide new pathways for the development of Mg sheet alloys with better formability and ductility.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
Acknowledgements This study was partially supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) (KAKENHI, 16H04525).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: