JP5637378B2 - Magnesium alloy plate - Google Patents

Description

  The present invention relates to a magnesium alloy plate suitable for materials of various members such as a casing of electric / electronic devices. In particular, it relates to a magnesium alloy plate having excellent corrosion resistance.

  Magnesium alloys containing various additive elements in magnesium are being used as constituent materials for various members such as casings for portable electric and electronic devices such as mobile phones and notebook personal computers and automobile parts.

  The main component of the magnesium alloy is a cast material (ASTM standard AZ91 alloy) produced by die casting or thixomolding. In recent years, a member obtained by pressing a plate made of a magnesium alloy for extension represented by ASTM standard AZ31 alloy is being used. For example, Patent Document 1 proposes a magnesium alloy plate made of an alloy equivalent to the AZ91 alloy in the ASTM standard and having excellent press workability.

  Since magnesium is an active metal, the surface of the above-mentioned member and the magnesium alloy used as the material is usually subjected to anticorrosion treatment such as anodizing treatment or chemical conversion treatment.

JP 2007-098470 A

  Magnesium alloys containing Al such as the AZ31 alloy and AZ91 alloy described above tend to have better corrosion resistance as the Al content increases. For example, AZ91 alloy is said to be excellent in corrosion resistance among magnesium alloys. However, even with the AZ91 alloy, the problem of corrosion resistance has not been sufficiently solved, and the above-described anticorrosion treatment is required. When anticorrosion treatment is not applied, even with AZ91 alloy, corrosion proceeds when a corrosion test such as a salt spray test or a salt water immersion test is performed. In addition to the above anti-corrosion treatment for the purpose of improving corrosion resistance, etc., if the coating is wrinkled due to impact or the coating is peeled off due to deterioration over time, the magnesium alloy will be exposed. Corrosion proceeds. Therefore, it is desired that the magnesium alloy plate itself constituting the magnesium alloy member is excellent in corrosion resistance.

  The present invention has been made in view of the above circumstances, and one of the objects of the present invention is to provide a magnesium alloy plate having excellent corrosion resistance.

  As a result of intensive studies, the present inventors have found that a magnesium alloy plate having a specific structure exhibits excellent corrosion resistance, and have completed the present invention.

  The magnesium alloy plate of the present invention is made of a magnesium alloy containing an additive element. And the particle | grains of the intermetallic compound containing an additive element and Mg exist in a board | substrate disperse | distributed. Moreover, the ratio of the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound in the XRD analysis of the plate surface divided by the c-plane (0,0,2) diffraction intensity of the Mg alloy phase is 0.040 or more. It is characterized by.

The reason why the magnesium alloy sheet of the present invention exhibits excellent corrosion resistance is not necessarily clear, but the presence state of an intermetallic compound containing an additive element (eg, Al) and Mg (typical example, Mg ₁₇ Al ₁₂ ) is closely related. The ratio between the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound and the c-plane (0,0,2) diffraction intensity of the Mg alloy phase in the XRD analysis of the plate surface (metal It is considered that the main diffraction plane (4,1,1) diffraction intensity / intermediate compound c-plane (0,0,2) diffraction intensity) is 0.040 or more. In the present invention, the magnesium alloy contains 50% by mass or more of Mg.

  Hereinafter, the magnesium alloy plate of the present invention will be described.

≪Magnesium alloy plate≫
[composition]
Magnesium alloys constituting the magnesium alloy plate include various compositions containing additive elements (remainder: Mg and impurities). In the present invention, Al is contained as an additive element in an amount of 3.0% to 11.0% by mass. It is preferable to use an Mg-Al based alloy. The higher the Al content, the better the corrosion resistance and the mechanical properties such as strength and plastic deformation resistance. In addition, when Al is contained, particles of an intermetallic compound (β phase) containing Al and Mg as precipitates can be precipitated when manufacturing a magnesium alloy sheet. On the other hand, when there is too much content of Al, there exists a possibility of causing the fall of plastic workability. A more preferable content of Al is 8.3 mass% to 9.5 mass%.

  The additive element other than Al includes one or more elements selected from Zn, Mn, Si, Ca, Sr, Y, Cu, Ag, Zr, Ce, Be, and rare earth elements (excluding Y and Ce). It is done. When these elements are contained, the total content is preferably 0.01% by mass or more and 10% by mass or less, and more preferably 0.1% by mass or more and 5% by mass or less. The rare earth element is preferably contained in an amount of 0.1% by mass or more, and among them, Y is preferably contained in an amount of 0.5% by mass or more. More specific Mg-Al alloys include, for example, AZ alloys (Mg-Al-Zn alloys, Zn: 0.2 to 1.5% by mass), AM alloys (Mg-Al-Mn alloys, Mn) according to ASTM standards. : 0.15-0.5 mass%), Mg-Al-RE (rare earth element) alloy, AX alloy (Mg-Al-Ca alloy, Ca: 0.2-6.0 mass%), AJ alloy (Mg-Al-Sr) Alloy, Sr: 0.2-7.0 mass%) and the like. In particular, an Mg-Al-Zn alloy, typically AZ91 alloy, containing 8.3 mass% to 9.5 mass% of Al and 0.5 mass% to 1.5 mass% of Zn is preferable in terms of excellent corrosion resistance. Examples of the impurity include Fe, Ni, and Cu.

[Organization]
<Intermetallic compound>
(composition)
The present invention has a structure in which intermetallic compound particles are dispersed in the plate. As an intermetallic compound, Mg ₁₇ Al ₁₂ containing Al and Mg is typical when it is made of a magnesium alloy containing Al as an additive element.

(Ratio between the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound and the c-plane (0,0,2) diffraction intensity of the Mg alloy phase in XRD analysis)
In the present invention, the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound (Mg ₁₇ Al ₁₂ etc.) in the XRD analysis of the plate surface is the c-plane (0,0,2) diffraction intensity of the Mg alloy phase. The divided ratio is 0.040 or more. This ratio is preferably larger, more preferably 0.055 or more, and even more preferably 0.060 or more. The upper limit of this ratio is not particularly limited, but 0.10 is considered appropriate from the viewpoint of practical production.

  Specific examples of the apparatus used for the XRD analysis and analysis conditions will be described later in detail.

(Area ratio)
In the present invention, the area ratio of the intermetallic compound (Mg ₁₇ Al ₁₂ or the like) in the SEM observation of the cross section of the plate is preferably 10.0% or more. Here, the area ratio expresses the ratio of the total area of the intermetallic compound to the observation visual field area in the SEM observation of the cross section of the plate as a percentage (%). The area ratio is preferably larger, more preferably 10.5% or more, and still more preferably 10.6% or more. The upper limit of the area ratio is not particularly limited, but 15% is considered appropriate from the viewpoint of practical production.

(Particle morphology, average particle size)
In the present invention, particles of an intermetallic compound (such as Mg ₁₇ Al ₁₂ ) preferably include spherical particles having an aspect ratio of less than 2. Here, the aspect ratio is represented by the ratio of the major axis to the minor axis (major axis / minor axis) of the particles. In particular, it is more preferable that spherical particles having an aspect ratio of less than 2 and rod-shaped particles having an aspect ratio of 2 or more are included. By including the rod-like particles having an aspect ratio of 2 or more, the corrosion resistance can be further improved. It is even more preferable that rod-shaped particles having an aspect ratio of 3 or more are included.

In the present invention, it is preferable that the average particle diameter of spherical particles (with an aspect ratio of less than 2) is 0.4 μm or more among particles of the above-described intermetallic compound (Mg ₁₇ Al ₁₂ or the like). Here, the average particle diameter is a value obtained by obtaining the number of spherical particles of the intermetallic compound in the observation field in the SEM observation of the plate cross section and dividing the total area of the particles existing in the observation field by the number of the particles. Is a value obtained by calculating the diameter of a circle equal to this area. The average particle size is preferably larger and more preferably 0.5 μm or more. The upper limit of the average particle diameter is not particularly limited, but it is considered that 5 μm is appropriate because cracks and the like are likely to occur during plastic working if there are too many coarse intermetallic compound particles. .

[Corrosion resistance]
In the present invention, it has excellent corrosion resistance and little corrosion weight loss by a salt spray test (test method based on JIS Z 2371: 2000). For example, the weight loss after 96 hours of the salt spray test can be 0.25 mg / cm ² or less. Corrosion weight loss is preferably as small as possible, more preferably 0.20 mg / cm ² or less. In the salt spray test, 5% strength salt water (1 liter of an aqueous solution in which 50 g of salt is dissolved) is used.

[Production method]
The magnesium alloy plate of the present invention can be manufactured, for example, by a manufacturing method including the following steps.
Casting step: A step of producing a cast material made of a magnesium alloy containing an additive element by continuous casting.
Heat treatment step: A step of producing a heat treatment material by holding the cast material at 400 ° C. or higher and then cooling it at a cooling rate of 30 ° C./min or less.
Rolling step: A step of warm rolling the heat treated material to produce a rolled plate.

  Furthermore, you may provide the correction process which warm-corrects the said rolled sheet.

It is difficult to roll the cast material as it is, and the heat treatment process is performed to soften the cast material before rolling. In addition, in the heat treatment step, by holding at a predetermined temperature for a certain period of time, the composition of the magnesium alloy can be homogenized, and an additive element such as Al can be dissolved in the magnesium alloy. Conventionally, it was thought that the corrosion resistance would be reduced if a large amount of coarse intermetallic compound (Mg ₁₇ Al ₁₂ etc.) particles were precipitated during the cooling process of the heat treatment process. It was forcibly cooled by blast and gusts. Specifically, in order to quickly pass through a temperature range (350 ° C to 250 ° C) where the precipitation rate of intermetallic compounds is high, the temperature range of 350 ° C to 250 ° C is cooled at a cooling rate of 100 ° C / min or more (rapid cooling). ) To obtain a solid solution. However, as a result of intensive research by the present inventors, in the heat treatment process, instead of rapid cooling, cooling (slow cooling) at a cooling rate of 30 ° C./min or less provides excellent corrosion resistance as described above. It was found that a rolled plate (magnesium alloy plate) having the specific structure shown was finally obtained.

  Hereinafter, each step will be described.

<Casting process>
In the casting process, a cast material having a predetermined composition is produced by a continuous casting method such as a twin roll method. For example, the continuous casting technique described in WO2006 / 003899 can be used. Since the continuous casting method can be rapidly solidified, it is possible to reduce oxides and segregation and to suppress the formation of coarse precipitates (intermetallic compounds) exceeding 10 μm. . The thickness of the cast material is not particularly limited, but is preferably 10 mm or less, more preferably 5 mm or less, because segregation is likely to occur if it is too thick.

<Heat treatment process>
In the heat treatment step, the cast material is held at 400 ° C. or higher and then cooled at a cooling rate of 30 ° C./min or less to produce a heat treated material. The heat treatment includes heating to a temperature of 400 ° C. or higher and 420 ° C. or lower, preferably 410 ° C. or lower, and maintaining that state for 60 minutes or longer and 2400 minutes or shorter (1 hour to 40 hours). Moreover, it is preferable to make this holding time long, so that there is much content of Al. On the other hand, the temperature range for cooling at a cooling rate of 30 ° C./min or less is, for example, 400 ° C. to 250 ° C. More preferably, it is divided into a temperature range of 400 ° C. to 350 ° C. and a temperature range of 350 ° C. to 250 ° C., and the cooling rate in each temperature range is adjusted as follows.

  It is preferable to cool from 400 ° C. to 350 ° C. at a cooling rate of 30 ° C./min or less and from 350 ° C. to 250 ° C. at a cooling rate of 10 ° C./min or less. In particular, in the temperature range of 400 ° C. to 350 ° C., it is more preferable to cool at a cooling rate of 2.0 ° C./min or less, and it is even more preferable to cool at a cooling rate of 0.2 ° C./min or less. On the other hand, in the temperature range of 350 ° C. to 250 ° C., it is more preferable to cool at a cooling rate of 1.0 ° C./min or less.

Thus, the rolling plate (magnesium alloy plate) which has the specific structure as mentioned above can be manufactured by gradually cooling the cooling condition in the heat treatment step. Further, by adjusting the cooling rate in each of the above temperature ranges, the precipitation state of the intermetallic compound (Mg ₁₇ Al ₁₂ etc.) (specifically, the main diffraction surface (4,1,1, 1) The ratio of the diffraction intensity to the c-plane (0,0,2) diffraction intensity, the area ratio, the particle form, and the average particle diameter) of the Mg alloy phase can be controlled.

<Rolling process>
In the rolling step, the heat treated material is warm-rolled to produce a rolled plate. In rolling the heat treated material, the plastic workability (rolling workability) can be improved by heating the material (the heat treated material or the plate material in the middle of rolling until the final rolling is performed). In particular, when the material is heated to more than 300 ° C., the plastic workability is sufficiently improved and the rolling process is easy to perform. However, if the heating temperature of the material is increased, seizure occurs during the rolling process, the crystal grains of the magnesium matrix become coarse, and a large amount of coarse intermetallic compounds are produced, resulting in the mechanical properties of the final rolled sheet. It may happen that the characteristics deteriorate. Therefore, the heating temperature of the raw material in a rolling process shall be 300 degrees C or less. In particular, the heating temperature of the material is preferably 150 ° C. or higher and 280 ° C. or lower. In addition, by rolling a plurality of times (multi-pass), it can be processed to a desired plate thickness (for example, 0.3 mm to 3.0 mm) and the average crystal grain size of the parent phase is small (for example, 10 μm or less) , Preferably 5 μm or less), and plastic workability such as rolling and pressing can be improved. For the rolling, known conditions can be used. For example, in addition to heating not only the raw material but also the rolling roll, the rolling control described in Patent Document 1 may be used in combination.

  In addition, in the processes after the heat treatment process including the rolling process, the total material holding time in the temperature range of 150 ° C. or higher and 300 ° C. or lower is set to 12 hours or less, and the above-mentioned material is used so as not to be heated above 300 ° C. It is preferable to control the heat history. By controlling the time for maintaining in the temperature range of 150 ° C. to 300 ° C., excessive growth (coarseness) of the intermetallic compound can be suppressed. Preferably, the temperature range is set to 150 ° C. or more and 280 ° C. or less, and the total time is controlled to be 6 hours or less.

  When performing multi-pass rolling, intermediate heat treatment may be performed between passes within a range in which the time for maintaining in the temperature range of 150 ° C. to 300 ° C. is included in the total time. By this intermediate heat treatment, strain, residual stress, texture, etc. introduced into the material by plastic working (mainly rolling) up to the intermediate heat treatment can be removed and reduced. It is possible to prevent cracking, distortion, and deformation and perform smoother rolling. Even when performing the intermediate heat treatment, the heating temperature of the material is set to 300 ° C. or lower. A preferable heating temperature of the material in the intermediate heat treatment is 250 ° C. or higher and 280 ° C. or lower.

<Correction process>
In the straightening step, straightening is performed in a state where the rolled plate is heated to 100 ° C. or higher and 300 ° C. or lower. Also in this case, the time for maintaining in the temperature range of 150 ° C. to 300 ° C. is included in the total time. The rolled sheet produced by the rolling process may be subjected to the final heat treatment (final annealing) described in Patent Document 1, but if the warm correction is performed without performing the final heat treatment or after the final heat treatment, Plastic workability such as processing can be improved. The correction may be performed by heating the rolled plate to 100 ° C. or higher and 300 ° C. or lower, preferably 150 ° C. or higher and 280 ° C. or lower, using a roll leveler described in WO2009 / 001516. When plastic working such as press working is performed on a rolled plate that has been subjected to such warm correction, dynamic recrystallization occurs during the plastic working, which makes it easy to perform plastic working.

<Final heat treatment>
When the final heat treatment is performed, distortion introduced into the rolled plate by rolling can be removed. As the final heat treatment, for example, the rolled plate is heated to a temperature of 100 ° C. or higher and 300 ° C. or lower and held in that state for 5 minutes or longer and 60 minutes or shorter. Also in this case, the time for maintaining in the temperature range of 150 ° C. to 300 ° C. is included in the total time. Here, Patent Document 1 describes that the heating temperature is set to 300 ° C. to 340 ° C. However, in order to suppress the crystal grain growth of the parent phase as much as possible, when the heating temperature is increased, the heating time is shortened (for example, Less than 30 minutes).

  In addition, a magnesium alloy member can be obtained by subjecting the rolled plate (magnesium alloy plate of the present invention) obtained by the above production method to plastic working such as press working. When the plastic working is performed in a temperature range of 200 ° C. or higher and 300 ° C. or lower, the plastic workability of the magnesium alloy plate can be improved and the plastic working can be easily performed. The time for holding the magnesium alloy plate at 200 ° C. to 300 ° C. during plastic working is very short, for example, it is 60 seconds or less in press working, and it is considered that defects such as coarsening of intermetallic compounds do not substantially occur. .

  In addition, a finish heat treatment can be performed after the plastic working to remove strain and residual stress introduced into the magnesium alloy member by the plastic working and to improve the mechanical characteristics. The finish heat treatment may be the same conditions as the final heat treatment (heating temperature: 100 ° C. to 300 ° C., heating time: 5 minutes to 60 minutes). However, also in this case, it is desirable that the time to be held in the temperature range of 150 ° C. to 300 ° C. is included in the total time.

  Furthermore, after the plastic working, the magnesium alloy member may be coated for the purpose of protecting the magnesium alloy member, improving aesthetics (designability), and corrosion resistance.

  The magnesium alloy plate of the present invention has a ratio obtained by dividing the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound by the c-plane (0,0,2) diffraction intensity of the Mg alloy phase in the XRD analysis of the plate surface. Has a structure of 0.040 or more, it has excellent corrosion resistance.

It is a SEM photograph of the section of the magnesium alloy plate of sample No. 1. It is a SEM photograph of a section of a magnesium alloy plate of sample No. 3. It is a SEM photograph of a section of a magnesium alloy plate of sample No. 4.

Embodiments of the present invention will be described below.
[Test Example 1]
Various magnesium alloy plates having different structures were produced, and the structure and corrosion resistance of each plate were evaluated.

  In this test, magnesium alloy plates of sample Nos. 1 to 4 manufactured as follows were prepared.

  Casting material (thickness: 4mm) made of magnesium alloy with a composition equivalent to AZ91 alloy (9.0% Al-1.0% Zn-0.15% -0.5% Mn (all mass%), balance Mg), twin roll continuous casting method A plurality of samples were prepared. Here, sample Nos. 1, 3 and 4 were coil materials obtained by producing a long cast material and winding it in a coil shape. Sample No. 2 was a sheet material obtained by cutting a cast material into a sheet having a predetermined length.

  Next, each cast material (coil material or sheet material) was placed in a heat treatment furnace and held at 400 ° C. for 24 hours, and then cooled under the conditions shown in Table 1 to produce a heat treatment material. In addition, the cooling rate in Table 1 is a value obtained by measuring the surface temperature of the coil material for the coil material and the surface temperature of the sheet material for the sheet material.

  Here, as for sample No. 1, the coil material taken out from the heat treatment furnace was put in a water tank as it was and forcedly cooled from 400 ° C. to 250 ° C. by water cooling. Sample No. 2 is a sheet material taken out from the heat treatment furnace, put in a temperature-controlled constant temperature room and cooled by air cooling from 400 ° C to 350 ° C, and then put in another temperature-controlled room set to a lower temperature at 350 ° C. To 250 ° C. by air cooling. In sample No. 3, the coil material taken out from the heat treatment furnace was left as it was and naturally cooled from 400 ° C. to 250 ° C. In sample No. 4, the coil material was left in the heat treatment furnace where heating was stopped and allowed to cool naturally from 400 ° C to 350 ° C. Then, the coil material was taken out of the heat treatment furnace and left as it was, from 350 ° C to 250 ° C. Naturally cooled.

Next, each heat-treated material was subjected to multi-pass rolling under the following conditions to produce a rolled plate (thickness: about 0.6 mm).
(Rolling conditions)
Reduction ratio: 5% / pass to 40% / pass Heating temperature of material: 250 ° C to 280 ° C
Rolling roll heating temperature: 100 ° C-250 ° C

  Furthermore, warm correction was performed with each rolled plate heated to 200 ° C. Warm correction was performed using a roll leveler apparatus including a heating furnace that heats the rolled sheet and a roll unit that has a plurality of rolls that continuously bend (strain) the rolled sheet heated in the heating furnace. . The roll unit includes a plurality of rolls arranged in a staggered manner facing the top and bottom. The roll leveler device is configured to sequentially bend the rolled sheet by these rolls each time the rolled sheet is fed to the roll part while being heated in a heating furnace and passed between the upper and lower rolls of the roll part.

  Finally, wet belt type polishing was performed on the rolled plate subjected to warm correction using a # 600 polishing belt to smooth the surface of the rolled plate, and the thickness of the rolled plate was adjusted to 0.6 mm. In the process after the heat treatment process, the heat history was controlled so that the total time kept in the temperature range of 150 ° C. to 300 ° C. was 12 hours or less and not heated to a temperature exceeding 300 ° C.

  A part of each rolled plate manufactured as described above was cut out to obtain magnesium alloy plates of Sample Nos. 1 to 4.

<XRD analysis of plate surface>
Each sample is subjected to XRD (X-Ray Diffraction) analysis of the plate surface, the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound (Mg ₁₇ Al ₁₂ ) in the XRD analysis of the plate surface, and Mg alloy The number of counts indicating the c-plane (0,0,2) diffraction intensity of the phase was measured. Then, the ratio of diffraction intensity was obtained by dividing the former by the latter. For XRD analysis, a multifunctional X-ray diffractometer X'pertPRO manufactured by Philips was used. Moreover, the conditions of XRD analysis are as follows. Table 2 shows the ratio of diffraction intensity in each sample.
(XRD analysis conditions)
X-ray used: Cu-Kα
Excitation conditions: 45kV, 40mA
Receiving optical system: Solar slit Scanning method: θ-2θ scan Measurement range: 2θ = 20 ° to 50 ° (step width: 0.03 °)
Integration time: 1 sec

<SEM observation of plate cross section>
About each sample, the cross-section processing was performed in the plate | board thickness direction along the direction orthogonal to a rolling direction with the cross section polisher using Ar ion beam, and the cross section was observed by SEM (Scanning Electron Microscope). For SEM observation, a low acceleration voltage scanning electron microscope Ultra55 manufactured by Carl Zeiss was used. The SEM observation conditions were an acceleration voltage of 5 kV and no sample coating. Observation was performed with an in-lens image. Here, FIG. 1 is an SEM photograph of sample No. 1, FIG. 2 is an SEM photograph of sample No. 3, and FIG. 3 is an SEM photograph of sample No. 4. 1-3, light gray particles are intermetallic compounds (Mg ₁₇ Al ₁₂ ). In addition, the stripe | line | column seen in the vertical direction in a figure is a trace of a cross-section out process.

For each sample, it was determined area ratio of the intermetallic compound in the SEM observation of the plate section (Mg ₁₇ Al _12). Here, cross-section processing was performed five times, and any three fields of view were observed in each of the five sections, and for each observation field, the area of all intermetallic compound particles present in the field of view was examined. The total area was calculated. Then, in each of the 15 observation visual fields, a ratio obtained by dividing the total area of the intermetallic compound by the observation visual field area was obtained, and the average value was defined as the area ratio. The observation field size is 4 μm × 6 μm (area: 24 μm ² ), and the observation field is a region where no rod-shaped (aspect ratio of 2 or more) particles exist, that is, only spherical particles (with an aspect ratio of less than 2). The area to be selected was selected. Table 2 shows the area ratio (%) in each sample.

Similarly, the average particle diameter of spherical particles (with an aspect ratio of less than 2) of the intermetallic compound (Mg ₁₇ Al ₁₂ ) was determined by SEM observation of the plate cross section. Here, in the above-mentioned observation visual field, the number of all spherical particles existing in the observation visual field was examined for each observation visual field. Then, in each of the 15 observation fields, the area obtained by dividing the total area of the intermetallic compound by the number of particles was calculated, the diameter of a circle equal to this area was obtained, and the average value was taken as the average particle diameter. . Table 2 shows the average particle diameter (μm) of each sample.

Furthermore, the particle morphology of the intermetallic compound (Mg ₁₇ Al ₁₂ ) was examined by SEM observation of the plate cross section. Here, in any one observation field (however, observation field size: 120 μm × 90 μm), the shape of the particles of the intermetallic compound existing in the observation field was determined visually. As a result, Samples No. 1 and No. 2 had only spherical particles having an aspect ratio of less than 2. On the other hand, Samples No. 3 and No. 4 had a mixture of spherical particles having an aspect ratio of less than 2 and rod-shaped particles having an aspect ratio of 2 or more. In addition, for sample No. 3 and No. 4, when the ratio of the presence of rod-shaped particles having an aspect ratio of 2 or more was compared, sample No. 4 was more rod-shaped with an aspect ratio of 2 or more than sample No. 3. Many particles were confirmed. Specifically, sample No. 3 has 3 or more rod-shaped particles per observation field, while sample No. 4 has 5 or more rod-shaped particles per observation field. It was. Further, most of the rod-like particles confirmed in Samples No. 3 and No. 4 had an aspect ratio of 3 or more.

<Corrosion resistance>
About each sample, the salt spray test was done and the corrosion weight loss was calculated | required. Here, the test was performed according to JIS Z 2371: 2000. In the salt spray test, a cas tester CY-90 manufactured by Suga Test Instruments Co., Ltd. was used. The salt spray test conditions were a test temperature of 35 ° C., a salt water concentration of 5%, and a test time of 96 hours. Table 2 shows the corrosion weight loss (mg / cm ² ) in each sample.

  Corrosion weight loss was measured as follows. Test specimens are collected from each of samples No. 1 to No. 4, and the mass of each specimen (mass before test) is measured. Each test piece is set in a test tank of a salt spray tester, and a salt spray test is performed for 96 hours. After completion of the test, each test piece is taken out from the test tank, and the corrosion products of each test piece are removed. To remove the corrosion products, first prepare 1000 ml of a solution of 100 g of chromium (VI) oxide and 10 g of silver chromate and distilled water, and immerse each test piece in this boiled solution for 1 minute. Remove the product. Further, prepare 1000 ml of a solution of 200 g of chromium (VI) oxide, 10 g of silver chromate and 20 g of barium sulfate with distilled water, and immerse each test piece in this solution at 20 ° C to 25 ° C for 1 minute for corrosion. Remove the product. Then, after removing the deposit on the surface of each test piece with a brush or the like, each test piece is washed with water and dried. After removing the corrosion products of each test piece, the mass (post-test mass) of each test piece is measured. And the value which remove | divided the mass which reduced the mass after a test from the mass before a test by the area of each test piece is made into corrosion weight loss. The mass was measured using an electronic analytical balance AEU-210 manufactured by Shimadzu Corporation.

From the results in Table 2, the ratio between the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound (Mg ₁₇ Al ₁₂ ) and the c-plane (0,0,2) diffraction intensity of the Mg alloy phase in the XRD analysis. It can be seen that Sample Nos. ^{2 to 4} having 0.040 or more have a corrosion weight loss of 0.25 mg / cm ² or less after 96 hours of the salt spray test and are excellent in corrosion resistance as compared with Sample No. 1. From the viewpoint of corrosion resistance, the area ratio of the intermetallic compound (Mg ₁₇ Al ₁₂ ) in the SEM observation of the plate cross section is 10% or more, and the average particle size of the intermetallic compound (Mg ₁₇ Al ₁₂ ) is 0.4 μm or more. It turns out that it is preferable. In particular, samples No. 3 and No. 4 containing rod-shaped intermetallic compound (Mg ₁₇ Al ₁₂ ) particles have a corrosion loss of 0.20 mg / cm ² or less after 96 hours of salt spray test, and are superior in corrosion resistance. I understand that.

  Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, the composition of the magnesium alloy and the manufacturing conditions of the magnesium alloy plate can be changed as appropriate.

  The magnesium alloy plate of the present invention can be suitably used for various members of electric / electronic devices, in particular, various members that require corrosion resistance in addition to casings of portable devices such as mobile phones and notebook personal computers. .

Claims (8)

A magnesium alloy plate made of a magnesium alloy containing an additive element,
The magnesium alloy is AZ91 alloy,
Mg ₁₇ Al ₁₂ intermetallic particles are present dispersed in the plate,
The particles of the intermetallic compound include spherical particles having an aspect ratio of less than 2, and rod-shaped particles having an aspect ratio of 2 or more.
In the XRD analysis of the plate surface, the ratio obtained by dividing the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound by the c-plane (0,0,2) diffraction intensity of the Mg alloy phase is 0.040 or more,
The area ratio of the intermetallic compound in SEM observation of the plate cross section is 10.0% or more,
A magnesium alloy plate in which spherical particles of the intermetallic compound have an average particle size of 0.4 μm or more.   2. The magnesium alloy according to claim 1, wherein a ratio obtained by dividing the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound by the c-plane (0,0,2) diffraction intensity of the Mg alloy phase is 0.055 or more. Board.   The ratio obtained by dividing the main diffraction surface (4,1,1) diffraction intensity of the intermetallic compound by the c-plane (0,0,2) diffraction intensity of the Mg alloy phase is 0.060 or more. The magnesium alloy plate described. The magnesium alloy sheet according to any one of claims 1 to 3, wherein the corrosion weight loss after 96 hours of the salt spray test is 0.25 mg / cm ² or less. The magnesium alloy plate according to any one of claims 1 to 4, wherein the corrosion weight loss after 96 hours of the salt spray test is 0.20 mg / cm ² or less.   The magnesium alloy plate according to any one of claims 1 to 5, wherein an area ratio of the intermetallic compound in SEM observation of the plate cross section is 10.5% or more.   The magnesium alloy plate according to any one of claims 1 to 6, wherein an area ratio of the intermetallic compound in SEM observation of the cross section of the plate is 10.6% or more.   The magnesium alloy sheet according to any one of claims 1 to 7, wherein an average particle diameter of the spherical particles of the intermetallic compound is 0.5 µm or more.