Patent Application
20200157662
The present invention relates to a magnesium alloy sheet.
The present application claims the priority of Japanese Patent Application No. 2017-122544, filed Jun. 22, 2017, which is incorporated herein by reference in its entirety.
Patent Literature 1 discloses a magnesium alloy sheet with small elongation anisotropy in warm working and with good warm plastic formability, which is formed of a magnesium alloy containing 9% by mass Al.
PTL 1: WO2009/001516
A magnesium alloy sheet according to the present disclosure is formed of a magnesium-based alloy containing,
on a mass percent basis,
Al: 5.0% or more and 6.5% or less,
Sr: 0.2% or more and 1.0% or less,
Zn: 0.1% or more and 0.75% or less, and
Mn: 0.1% or more and 0.5% or less, the remainder being magnesium and incidental impurities.
A magnesium alloy sheet with high ductility and corrosion resistance is desired.
Magnesium alloys rich in Al, typically AZ91 alloys and magnesium alloys with an Al content comparable to the Al content of AZ91 alloys according to American Society for Testing and Materials standard, have high corrosion resistance but low ductility due to the high Al content. A magnesium alloy sheet described in Patent Literature 1 has a high elongation in warm working at 200° C. or more and high ductility in warm working but a low elongation of approximately 15% or less at room temperature, for example, at 20° C. Thus, there is a demand for a magnesium alloy sheet with a high elongation after fracture and high ductility even at room temperature and with high corrosion resistance.
Accordingly, it is an object of the present disclosure to provide a magnesium alloy sheet with high ductility and corrosion resistance.
A magnesium alloy sheet according to the present disclosure has high ductility and corrosion resistance.
First, the embodiments of the present invention are described below.
(1) A magnesium alloy sheet according to an embodiment of the present invention is formed of a magnesium-based alloy containing,
on a mass percent basis,
Al: 5.0% or more and 6.5% or less,
Sr: 0.2% or more and 1.0% or less,
Zn: 0.1% or more and 0.75% or less, and
Mn: 0.1% or more and 0.5% or less, the remainder being magnesium and incidental impurities.
The magnesium alloy sheet formed of the magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in these particular ranges has high ductility even at room temperature and high corrosion resistance, as described below. The term “room temperature”, as used herein, refers to 20° C.±15° C.
Magnesium based alloys with a high Al content, such as AZ91 alloys, are likely to form Mg17Al12, which is effective for corrosion resistance, and have high corrosion resistance. However, a high Al content tends to result in coarsening of Mg17Al12, and casting defects tend to occur around the coarse grains. These coarse grains or casting defects tend to decrease elongation after fracture and ductility. Although a low Al content results in a decrease in coarse Mg17Al12 or casting defects and high ductility, it results in low corrosion resistance. In contrast, the magnesium alloy sheet, which has a particular Al content lower than the Al content of AZ91 alloys and has a particular Sr content, can preferentially form an intermetallic compound containing Sr (hereinafter also referred to as a Sr-based compound), typically Al4Sr, over Mg17Al12 particularly in the production process. The preferential formation tends to result in fine Mg17Al12. Furthermore, a relatively low Al content as described above rarely results in coarsening of a Sr-based compound, such as Al4Sr. Thus, coarse Mg17Al12 or Al4Sr grains and casting defects around these coarse grains can be decreased in the magnesium alloy sheet. Hence, the magnesium alloy sheet is less prone to fracture due to these coarse grains or casting defects and has a high elongation after fracture and high ductility even at room temperature.
In general, rolled sheets formed of known magnesium alloys often have different mechanical properties (elongation after fracture, etc.) between the rolling direction and a direction perpendicular to the rolling direction (hereinafter also referred to as a transverse direction), that is, anisotropy of mechanical properties. Typically, the mechanical properties in the transverse direction are inferior to the mechanical properties in the rolling direction. For example, a large difference between elongation after fracture in the rolling direction and elongation after fracture in the transverse direction results in the difficulty of uniform bending in any direction with high accuracy. Due to such anisotropy of mechanical properties, the rolled sheet is of limited use. In contrast, the cause of fracture can be alleviated in the magnesium alloy sheet, as described above. Thus, the magnesium alloy sheet has a small difference between elongation after fracture in the rolling direction and elongation after fracture in the transverse direction, that is, small anisotropy of elongation after fracture, and therefore has high ductility also in this regard. A method for measuring elongation after fracture in the rolling direction and elongation after fracture in the transverse direction is described in detail below in Test Example 1.
Furthermore, the magnesium alloy sheet contains Zn in a particular range as well as Al and Sr. Zn probably contributes to partly promoted formation of Mg17Al12 after the preferential formation of the Sr-based compound. Thus, the magnesium alloy sheet can appropriately contain Mg17Al12 and has high corrosion resistance. Furthermore, the magnesium alloy sheet contains Mn in a particular range as well as Al, Sr, and Zn, and Fe, which has adverse effects on corrosion resistance, can exist as an Al—Mn—Fe compound. This also provides the magnesium alloy sheet with high corrosion resistance.
The magnesium alloy sheet has high ductility even at room temperature and, unlike known sheets, has high corrosion resistance irrespective of a relatively low Al content of 6.5% or less by mass. Thus, the magnesium alloy sheet can also be utilized as a material for an application that requires high ductility at room temperature.
(2) In one embodiment of the magnesium alloy sheet,
the difference between elongation after fracture in the rolling direction and elongation after fracture in a transverse direction in the magnesium alloy sheet is 2% or less.
This embodiment has small anisotropy of elongation after fracture and high ductility even at room temperature.
(3) In one embodiment of the magnesium alloy sheet,
the elongation after fracture is 18% or more.
This embodiment has a high elongation after fracture and high ductility even at room temperature. As in (2), the magnesium alloy sheet with small anisotropy of elongation after fracture can be suitably utilized as a material for an application that requires high ductility at room temperature.
(4) In one embodiment of the magnesium alloy sheet,
Mg17Al12 is dispersed on a grain boundary of the magnesium-based alloy, and
the Mg17Al12 has an average grain size in the range of 10 nm or more and 30 μm or less.
In this embodiment, very fine Mg17Al12 can more easily prevent fracture caused by coarse grains and imparts higher ductility. In the embodiment, dispersed fine Mg17Al12 can improve corrosion resistance.
(5) In one embodiment of the magnesium alloy sheet,
an intermetallic compound containing Sr is dispersed on a grain boundary of the magnesium-based alloy.
In this embodiment, a Sr-based compound, such as Al4Sr, dispersed on grain boundaries can more easily prevent fracture caused by coarse grains and imparts higher ductility than a localized coarse Sr-based compound. In the embodiment, a Sr-based compound, such as Al4Sr, can prevent coarsening of Mg17Al12 and imparts high ductility also in this regard.
(6) In one embodiment of the magnesium alloy sheet,
the magnesium alloy sheet has a thickness in the range of 0.5 mm or more and 5 mm or less.
In this embodiment, the magnesium alloy sheet with a small thickness in this range can be suitably utilized as a material for an application that requires both high ductility and high corrosion resistance as well as a decrease in size and a decrease in thickness. In the embodiment, the magnesium alloy sheet with a large thickness in this range can be suitably utilized as a material for an application that requires high ductility and corrosion resistance as well as high strength and rigidity. Although bending requires higher elongation on the outside than on the inside, due to its high ductility, as described above, the embodiment even with a large thickness can be easily subjected to deformation, such as bending.
A magnesium alloy sheet according to an embodiment of the present invention is more specifically described below. Unless otherwise specified, the element content of an alloy composition is based on the mass percentage (% by mass).
A magnesium alloy sheet according to an embodiment is formed of a magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in particular ranges. The magnesium-based alloy contains Al: 5.0% or more and 6.5% or less, Sr: 0.2% or more and 1.0% or less, Zn: 0.1% or more and 0.75% or less, and Mn: 0.1% or more and 0.5% or less, the remainder being magnesium and incidental impurities. The following is a detailed description.
Al contributes mainly to corrosion resistance.
At an Al content of 5.0% or more, an intermetallic compound containing Mg and Al, typically Mg17Al12, is formed on grain boundaries and ensures corrosion resistance. A higher Al content tends to result in the formation of Mg17Al12 and higher corrosion resistance. An Al content of 5.05% or more or 5.1% or more tends to result in higher corrosion resistance.
At an Al content of 6.5% or less, coarse grains composed of a compound containing Al, such as Mg17Al12, are difficult to form, and fracture caused by coarse grains composed of the compound is suppressed. This results in high ductility. An Al content of 6.45% or less, 6.4% or less, particularly less than 6.2%, tends to result in higher ductility.
Sr contributes mainly to finer grains of a magnesium-based alloy and finer Mg17Al12. Consequently, Sr contributes to improved ductility.
At an Al content of 5.0% or more as described above, coarse grains, for example, composed of Mg17Al12 are easily formed on grain boundaries of the magnesium-based alloy, and the coarse grains tend to cause casting defects around the coarse grains. The coarse grains and casting defects reduce ductility. At an Al content in the above range and at a Sr content of 0.2% or more, a Sr-based compound containing Sr, particularly a compound containing Al and Sr, such as Al4Sr, can be preferentially formed over Mg17Al12 in the production process. This is because Al4Sr has a higher melting point than Mg17Al12. Typically, a Sr-based compound, such as Al4Sr, has fine grains, which act as crystal nuclei. Thus, the magnesium-based alloy tends to have fine grains. A Sr-based compound containing Al, such as Al4Sr, suppresses the subsequent growth (coarsening) of Mg17Al12. Consequently, Mg17Al12 tends to have fine grains. Thus, the magnesium-based alloy can have a microstructure containing fine grains and containing Mg17Al12 and a Sr-based compound, such as Al4Sr, finely dispersed on grain boundaries. In other words, the magnesium-based alloy can have a microstructure in which Mg17Al12 and a Sr-based compound are finely and almost uniformly dispersed throughout the microstructure. Such a fine microstructure improves ductility. A higher Sr content tends to result in the formation of a Sr-based compound, such as Al4Sr. The Sr content can be 0.25% or more or 0.3% or more.
A Sr content of 1.0% or less results in the prevention of coarsening of a Sr-based compound, such as Al4Sr, and results in high ductility. At a Sr content of 0.95% or less or 0.9% or less, the coarsening can be more easily prevented.
Zn contributes mainly to improved corrosion resistance.
At a Zn content of 0.1% or more, the formation of Mg17Al12, which is effective for corrosion resistance, is promoted. Thus, even after the preferential formation, Mg17Al12 is appropriately formed and improves corrosion resistance. At a Zn content of 0.15% or more or 0.2% or more, the promoting effect is more easily achieved.
A Zn content of 0.75% or less results in promoted formation of Mg17Al12, prevention of coarsening, and high ductility. A Zn content of 0.74% or less or 0.73% or less results in more reliable prevention of the coarsening of Mg17Al12 and high ductility.
Mn contributes to improved corrosion resistance.
At a Mn content of 0.1% or more, a compound such as Al—Mn—Fe can be formed in the production process from Fe in the melt and Al and Mn. Thus, Mn makes phase separation of Fe, which adversely affects corrosion resistance, as the above compound and thereby decreases or removes Fe. At a Mn content of 0.15% or more or 0.2% or more, the compound can be more reliably formed, and deterioration of corrosion resistance caused by Fe can be more easily suppressed.
At a Mn content of 0.5% or less, a compound such as Al—Mn can be prevented from being excessively formed, which results in high corrosion resistance. The compound such as Al—Mn acts as a starting point of filiform corrosion, and the formation of a large amount of the compound lowers corrosion resistance. At a Mn content of 0.45% or less or 0.4% or less, the compound such as Al—Mn can be more easily prevented from being excessively formed, which results in high corrosion resistance.
A magnesium alloy sheet according to an embodiment typically has a fine microstructure. For example, when a magnesium alloy sheet according to an embodiment is a cast material produced by a continuous casting process, such as a twin-roll process, a magnesium-based alloy constituting the magnesium alloy sheet has an average grain size of 30 μm or less or 20 μm or less. Alternatively, for example, when a magnesium alloy sheet according to an embodiment is a rolled material produced by rolling, including warm rolling, of the cast material, or is a formed material produced by plastic working, such as press forming, of the rolled material, a magnesium-based alloy constituting the magnesium alloy sheet has an average grain size of 20 μm or less or 15 μm or less. The rolled material has a smaller grain size and therefore higher ductility than the cast material.
The average grain size can be measured, for example, by a line method. In the line method, a cross section of a magnesium alloy sheet is observed with an optical microscope, a measuring straight line with a predetermined length is drawn on the observed image, and the number of grains cut by the measuring straight line is counted. The length of the measuring straight line divided by the number of grains is the average grain size. For example, the length of the measuring straight line is such that the number of grains cut by the straight line is 50 or more. Alternatively, the average grain size can be measured by a SEM-EBSD crystal analysis using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) in combination.
A magnesium alloy sheet according to an embodiment has a microstructure in which fine compound grains are dispersed on grain boundaries. More specifically, a magnesium alloy sheet according to an embodiment contains Mg17Al12 dispersed on grain boundaries of a magnesium-based alloy, and the Mg17Al12 has an average grain size in the range of 10 nm or more and 30 μm or less. A magnesium alloy sheet according to an embodiment containing such fine Mg17Al12 is the above rolled material.
Mg17Al12 improves corrosion resistance, as described above. In particular, very finely dispersed Mg17Al12 with an average grain size of 10 nm or more further improves corrosion resistance.
Mg17Al12 with an average grain size of 30 μm or less rarely causes casting defects possibly formed around coarse Mg17Al12 or around coarse grains and can easily suppress deterioration of ductility caused by the casting defects, thus resulting in high ductility. Mg17Al12 with a somewhat large average grain size of 15 nm or more, 20 nm or more, 30 nm or more, 50 nm or more, 100 nm or more tends to further improve corrosion resistance. Mg17Al12 with a smaller average grain size of 25 μm or less, 20 μm or less, 15 μm or less, 10 μm or less tends to result in higher ductility.
A magnesium alloy sheet according to an embodiment contains an intermetallic compound containing Sr (Sr-based compound) dispersed on grain boundaries of the magnesium-based alloy. For example, the Sr-based compound is a compound containing Al and Sr, such as Al4Sr. As described above, Sr in the Sr-based compound probably contributes to fine Mg17Al12. Consequently, fine Mg17Al12 improves corrosion resistance and ductility.
A fine Sr-based compound further improves ductility. For example, the Sr-based compound has an average grain size in the range of 50 nm or more and 5 μm or less or 500 nm (0.5 μm) or more and 2 μm or less. A magnesium alloy sheet according to an embodiment containing such a fine Sr-based compound is the above rolled material or a cast material produced by the above twin-roll process. The rolled material contains Mg17Al12 and a Sr-based compound finely dispersed on grain boundaries of the fine microstructure as described above.
The average grain size of Mg17Al12 and the average grain size of the Sr-based compound can be measured as described below. A cross section of a magnesium alloy sheet is observed with SEM, and Mg17Al12 grains and Sr-based compound grains are extracted from the observed image. The area of each grain in the cross section is determined, and the diameter of a circle with this area is considered to be the grain size. The grain sizes of 50 or more grains are averaged to determine the average grain size.
The grain size and state of Mg17Al12 and the grain size and state of the Sr-based compound can typically depend on the composition or the production conditions of the magnesium-based alloy.
A magnesium alloy sheet according to an embodiment can have various sizes (thickness, width, length, etc.) and various shapes depending on its use. A magnesium alloy sheet according to an embodiment is a rectangular flat sheet with a predetermined uniform thickness, width, and length. Alternatively, the flat sheet is long and is wound to form a coiled material. The flat sheet or coiled material is the rolled material or cast material. When a coiled material of a rolled material is utilized as a material for a plastic worked component, such as a press formed component, the material can be continuously supplied to a processing apparatus, such as a press machine, thus contributing to the mass production of the plastic worked component. When a magnesium alloy sheet according to an embodiment is the above formed material (plastic worked component), at least part of the magnesium alloy sheet is bent to form a three-dimensional shape. Furthermore, the magnesium alloy sheet may have a through-hole, a groove, or a protrusion and may locally have a different thickness.
The thickness of a magnesium alloy sheet according to an embodiment can be appropriately selected. For example, the thickness may range from 0.5 mm or more and 5 mm or less. In this range, when the thickness is as small as 2 mm or less, 1.5 mm or less, or 1.0 mm or less, the magnesium alloy sheet is suitable as a material for small thin magnesium alloy structural components, such as housings of mobile devices. In the above range, when the thickness is as large as more than 2 mm or 2.5 mm or more, the magnesium alloy sheet is suitable as a material for magnesium alloy structural components with higher strength and rigidity, such as housings of large devices. Even when the magnesium alloy sheet has a thickness of more than 2 mm and 5 mm or less, the sheet has high ductility as described above and can stretch sufficiently on the outside when bent in the thickness direction. For a further decrease in weight or thickness, the thickness may be 4.5 mm or less, 4 mm or less, 3.8 mm or less, 3.5 mm or less.
A magnesium alloy sheet according to an embodiment typically has a high elongation after fracture even at room temperature. In particular, when a magnesium alloy sheet according to an embodiment is the above rolled material, the magnesium alloy sheet has a higher elongation after fracture at room temperature than a magnesium alloy sheet of the above cast material. For example, the elongation after fracture at room temperature is 18% or more. The term “elongation after fracture”, as used herein, refers to the value in a transverse direction of a magnesium alloy sheet. When a magnesium alloy sheet according to an embodiment is a rolled material, the elongation after fracture in the transverse direction may be lower than the elongation after fracture in the rolling direction. Thus, when the elongation after fracture in the transverse direction is as high as 18% or more, the elongation after fracture in the rolling direction is expected to be higher than or equal to the elongation after fracture in the transverse direction, and the whole magnesium alloy sheet has a high elongation after fracture.
A higher elongation after fracture results in higher ductility and easier deformation, such as bending, and the elongation after fracture is 18.5% or more, 19% or more, or 19.5% or more.
The rolling direction and the transverse direction of a magnesium alloy sheet may be determined in a simplified manner from the shape and size of the magnesium alloy sheet. For example, for the above coiled material, the longitudinal direction of the sheet can be considered to be the rolling direction, and the width direction perpendicular to the longitudinal direction of the sheet can be considered to be the transverse direction. For the above flat sheet or three-dimensional sheet with a roughly rectangular planar shape, the long side direction can be considered to be the rolling direction, and the short side direction can be considered to be the transverse direction.
A magnesium alloy sheet according to an embodiment typically has small anisotropy of elongation after fracture even at room temperature. In particular, when a magnesium alloy sheet according to an embodiment is the above rolled material, the magnesium alloy sheet has smaller anisotropy of elongation after fracture at room temperature than a magnesium alloy sheet of the above cast material. For example, the difference between elongation after fracture in the rolling direction of a magnesium alloy sheet and elongation after fracture in a transverse direction (hereinafter referred to as an elongation difference) is 2% or less. A smaller elongation difference results in smaller anisotropy of elongation after fracture, higher ductility, and easier uniform deformation, such as bending, in any direction. Thus, the elongation difference is preferably 1.8% or less, more preferably 1.5% or less, 1.3% or less, still more preferably 1.2% or less.
The elongation after fracture can typically depend on the composition or the production conditions of the magnesium-based alloy. For example, a low Al or Zn content in the above range tends to result in a high elongation after fracture.
A magnesium alloy sheet according to an embodiment also has high surface treatability when the magnesium alloy sheet has the fine microstructure as described above and contains Mg17Al12 or a compound such as a Sr-based compound finely dispersed on grain boundaries. For example, a magnesium alloy sheet subjected to corrosion protection treatment, such as chemical conversion treatment or anodic oxidation treatment, tends to have a treatment layer with a uniform thickness and has higher corrosion resistance.
When a magnesium alloy sheet according to an embodiment is the rolled material, the magnesium alloy sheet can be produced by a method including a casting step of producing a cast material by a continuous casting process, a heat-treatment step of performing solid solution treatment of the cast material to produce a solid solution treated material, and a rolling step of performing rolling, including warm rolling, of the solid solution treated material to produce a rolled material. When a magnesium alloy sheet according to an embodiment is the cast material, the magnesium alloy sheet can be produced through the casting step. When a magnesium alloy sheet according to an embodiment is the formed material, the formed material can be produced by the casting step, the heat-treatment step, the rolling step, and a processing step of performing plastic working of the rolled material to produce a plastic worked component. The outline of each step is described below.
A cast material is preferably produced by a continuous casting process. In particular, a cast material produced by a twin-roll process that can perform rapid solidification at a solidification rate of 50 K/s or more is more preferred because the cast material has substantially no or very few internal defects, such as shrinkage cavities, pores, and segregation, or substantially no or very few coarse oxides, has a small average grain size, or contains smaller crystallized precipitates (see
Solid solution treatment of the cast material to perform solid solution of additive elements is preferred because it facilitates precipitation of precipitates of a uniform size containing the additive elements during warm rolling in the next rolling step. For example, the heat-treatment conditions include a heat-treatment temperature in the range of 380° C. or more and 420° C. or less and a holding time in the range of 60 minutes or more and 600 minutes or less. For the other heat-treatment conditions, known conditions may be referred to.
A heat-treated material produced by the heat-treatment step can be subjected to rolling, including warm rolling, to have a dense microstructure with substantially no or fewer casting defects, such as pores, to have a finer microstructure, or to form precipitates, such as Mg17Al12. In particular, fine precipitates, such as Mg17Al12, can be formed from a heat-treated material composed of a magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in the particular ranges, as described above. The warm rolling conditions include a material temperature in the range of 150° C. or more and 350° C. or less and a draft in the range of 10% or more and 50% or less per pass, for example. For the other rolling conditions, known conditions may be referred to. High density with substantially no or very few casting defects, the presence of texture, or the presence of a fine microstructure (for example, an average grain size of 15 μm or less or 10 μm or less) may be an indicator of the rolled material.
The rolled material may be subjected to surface treatment, such as a leveling process, polishing processing, or chemical conversion treatment. For the conditions for the leveling process, polishing processing, and surface treatment, known conditions may be referred to.
A magnesium alloy sheet according to an embodiment can be utilized as a material for various applications. In particular, when a magnesium alloy sheet according to an embodiment is the rolled material, the magnesium alloy sheet can be utilized as a material for plastic worked components subjected to various plastic workings, such as press forming. Examples of the plastic worked components, such as housings, of various electronic and electrical devices, more specifically, components of relatively small, portable devices, such as cellular phones and laptop computers, and relatively large devices, such as TVs. Other examples of the plastic worked components of various transports, more specifically, exteriors, such as body panels, interiors, such as sheet panels, engine components, and chassis components of automobiles, aircrafts, and railway vehicles. Still other examples of the plastic worked component include various frames and structural components.
Being formed of a magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in particular ranges, a magnesium alloy sheet according to an embodiment has high ductility and corrosion resistance.
These advantageous effects are more specifically described below in Test Example 1.
Magnesium alloy sheets were formed of magnesium-based alloys with various compositions and were tested for ductility and corrosion resistance.
Each sample of the magnesium alloy sheets is a rolled material produced by the following casting step, heat-treatment step, and rolling step.
In the casting step, a cast material (5 mm in thickness) is produced from a magnesium-based alloy with the composition listed in Table 1 (each element content is expressed in % by mass) by a twin-roll process. Al, Zn, Sr, and an Al—Mn alloy are added to a melt of pure magnesium serving as the base to produce a melt of a magnesium-based alloy with the composition listed in Table 1. The solidification rate is 50 K/s or more. An ICP spectroscopic analysis of the composition of the resulting cast material confirmed the composition listed in Table 1 (the remainder is composed of Mg and incidental impurities).
In the heat-treatment step, the cast material is subjected to solid solution treatment. The heat-treatment temperature is 400° C., and the holding time is 300 minutes (5 hours).
In the rolling step, the heat-treated material is subjected to warm rolling to a thickness of 1 mm to produce a rolled material. The material temperature in the warm rolling is 350° C.
A tensile test specimen was cut from each sample of the rolled material and was subjected to a tensile test according to JIS Z 2241 (Metallic materials-Tensile testing-Method of test at room temperature, 2011) to measure the elongation after fracture (%) at room temperature (approximately 20° C.). Two tensile test specimens α and β are cut from each sample. The tensile test specimens α and β are plate-like test specimens No. 13 according to JIS Z 2241.
The tensile test specimen α was cut such that the longitudinal direction of the specimen was parallel to the rolling direction, and the tensile direction in the test was parallel to the rolling direction. The elongation after fracture of the tensile test specimen α is listed in Table 1 as elongation after fracture (%) in the rolling direction.
The other tensile test specimen β was cut such that the longitudinal direction of the specimen was perpendicular to the rolling direction, and the tensile direction in the test was perpendicular to the rolling direction. The elongation after fracture of the tensile test specimen β is listed in Table 1 as elongation after fracture (%) in the transverse direction.
The elongation after fracture in the rolling direction minus the elongation after fracture in the transverse direction, that is, the difference of the elongations after fracture is listed in Table 1 as the elongation difference (%).
A test specimen was cut from each sample of the rolled material and was subjected to a salt spray test according to JIS Z 2371 (Methods of salt spray testing, 2015). The appearance of the test specimen was observed.
After a neutral sodium chloride solution was sprayed for 192 hours, the appearance of each sample was visually inspected. Substantially no white rust over the entire surface of the sample indicates high corrosion resistance and is rated “good”. White rust on part of the sample is rated “white rust”. White rust over the entire surface of the sample is rated “full white rust”. Table 1 shows the evaluation results of each sample.
Table 1 shows that samples No. 1 to No. 13 have a high elongation after fracture at room temperature, a small elongation difference, high ductility, and high corrosion resistance. Quantitatively, for the samples No. 1 to No. 13, the elongation after fracture at room temperature (elongation after fracture in the transverse direction) is 18% or more, 18.5% or more in most samples, 19% or more, or 20% or more in many samples. The samples No. 1 to No. 13 have an elongation difference of 2% or less, 1.5% or less in most samples, or 1.0% or less in many samples. Thus, the samples No. 1 to No. 13 have small anisotropy of elongation after fracture at room temperature.
The samples No. 1 to No. 13 have a fine microstructure and have a finely dispersed grains consisting of Mg17Al12 or a compound such as a Sr-based compound on grain boundaries.
The upper in
In contrast, among samples No. 101 to No. 109, the samples No. 101, No. 103, No. 104, and No. 107 to No. 109 with high corrosion resistance have a very low elongation after fracture at room temperature (in the transverse direction), only approximately 13% or less, and have low ductility. The samples No. 105 and No. 106 with a high elongation after fracture at room temperature (in the transverse direction) have white rust and full white rust and have low corrosion resistance. The sample No. 102 has a low elongation after fracture at room temperature (in the transverse direction) and low corrosion resistance. The lower in
The reason for these results is probably that the samples No. 1 to No. 13 were composed of the magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in the particular ranges and therefore had high ductility and corrosion resistance. The samples No. 101 to No. 109 composed of the magnesium-based alloy with a composition outside the particular ranges are discussed below for comparison.
The sample No. 101 contains Al and Mn corresponding to an AM60 alloy, which is an American Society for Testing and Materials standard alloy with high ductility, and has a microstructure containing finely dispersed grains composed of Al—Mn, as shown in the lower in
The sample No. 102 has a lower Al content than the sample No. 101. The samples No. 1 to No. 13 have much higher ductility and higher corrosion resistance than the sample No. 102.
The sample No. 103 contains Al, Zn, and Mn but no Sr. The samples No. 1 to No. 13 have higher ductility than the sample No. 103.
The sample No. 105 contains Al, Sr, and Mn but no Zn. The samples No. 1 to No. 13 have higher corrosion resistance than the sample No. 105.
Thus, the comparison with the samples No. 101 to No. 103 and No. 105 shows that Al, Sr, Zn, and Mn are preferably contained to achieve both high ductility and high corrosion resistance.
The samples No. 1 to No. 13 have higher ductility than the samples No. 104 and No. 107 to No. 109, which contain Al, Sr, Zn, and Mn. A high Zn content of 0.8% or more by mass in the samples No. 104 and No. 108, a high Al content of 6.7% by mass in the sample No. 107, and a high Sr content of 1.2% by mass in the sample No. 109 tend to result in the formation of coarse compound grains and low ductility.
The samples No. 1 to No. 13 have higher corrosion resistance than the sample No. 106, which contains Al, Sr, Zn, and Mn. The sample No. 106 has low corrosion resistance probably due to a low Al content of 4.5% by mass.
Thus, the comparison with the samples No. 104 and No. 106 to No. 109 shows that Al: more than 4.5% by mass and less than 6.7% by mass, Sr: more than 0 and less than 1.2% by mass, Zn: more than 0 and less than 0.8% by mass, and Mn: 0.2% or more by mass and 0.5% or less by mass are preferred to achieve both high ductility and high corrosion resistance.
One reason for the fine microstructure of the rolled material of the samples No. 1 to No. 13, for fine Mg17Al12, and for a fine Sr-based compound is probably that the cast material produced by the twin-roll process was used as a material.
These tests show that a magnesium alloy sheet formed of a magnesium-based alloy with a particular composition containing Al, Sr, Zn, and Mn in particular ranges can have both high ductility and high corrosion resistance.
The present invention is defined by the appended claims rather than by these embodiments. All modifications that fall within the scope of the claims and the equivalents thereof are intended to be embraced by the claims.
For example, the composition and thickness of the magnesium-based alloy in Test Example 1 can be appropriately modified.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-122544 | Jun 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/021524 | 6/5/2018 | WO | 00 |
