JP6080067B2 - Magnesium alloy - Google Patents

Description

  The present invention relates to a magnesium alloy having magnesium as a main component, good workability, and high strength (or high tensile strength / high compressive strength).

Magnesium alloys having high strength have been developed in recent years, and are attracting attention as constituent materials for automobiles, aircraft, and the like as new materials that replace aluminum alloys.
However, in order to use these as industrial materials, the processability is poor, and various developments have been made to improve them, but sufficient products have not yet been obtained.
For example, as a measure to improve ductility, it is also considered to use extruded materials, but in this case it is difficult to improve compressive strength, and deformation anisotropy is the ratio of compressive yield stress to tensile yield stress. There is a problem that it is difficult to use as a lightweight structural material.
On the other hand, the inventors have clarified that a new magnesium alloy having high workability and sufficient workability can be provided by controlling the crystal structure, and has filed a patent application. (Patent Document 1)
And it was desired to use this organization structure more effectively.

Japanese Patent Application No. 2010-060144

Inoue, Tsuji, Kimura, Nagai; Journal of the Japan Institute of Metals, 69 (2005) 943.T. INOUE, F. Yin, Y. Kimura; Mater. Sci. Eng. , A466 (2007) 114. Y. Kimura et. al. , Scripter Mater. , 57 (2007) 465.

  In view of such circumstances, the present invention has an object to improve the possibility that such an organizational structure can be easily maintained and other functions can be added.

The magnesium alloy of the invention 1 has magnesium as a main component, the crystal structure thereof has a large-angle grain boundary, and the inside of the crystal grain surrounded by the large-angle grain boundary is composed of sub-crystal grains. In addition, only 14.5% by mass or less of tin that can be dissolved in magnesium as an additive element is contained, and the average crystal grain size of the crystal grains is 10 μm or less, and the average grain size of the sub-crystal grains is characterized 2μm Ri der hereinafter elongation of 7% or more, a tensile yield stress (a) 250 MPa or more, the compressive yield stress (B) 200 MPa or more, that is the yield stress anisotropy ratio (B / a) 0.7 or more And
Invention 2 is characterized in that the crystal grains having a crystal grain size of 10 μm or less occupy 70% or more of the whole.

With the above configuration, the presence of sub-crystal grains enables deformation of the crystal grains themselves, but it is presumed that slipping between the crystal grains is prevented, and both good ductility and high strength are compatible. While having a crystal structure, it is possible to have the following advantages. Since tin is the only element that dissolves in magnesium, industrial applications are expected to expand.

FIG. 3 is an example of microstructural observation by SEM / EBSD in Example 1. FIG. The nominal stress-nominal strain curve obtained by the room temperature tensile / compression test of Example 1. 5 is a microstructural observation example using a transmission electron microscope of Comparative Example 1. FIG.

The magnesium alloy of the present invention is characterized by its crystal structure.
1) has a large tilt grain boundary,
2) The inside of the crystal grain surrounded by this large tilt grain boundary has a configuration in which it is a sub-crystal grain.
Here, the “large tilt grain boundary” is defined as a grain boundary having a misorientation angle of 15 degrees or more. For such large tilt grain boundaries, crystal orientation mapping by SEM / EBSD (Scanning Electron Microscope: Electron Back-Scattered Diffraction: electron beam backscatter diffraction) and means for measuring misorientation by a transmission electron microscope Is specifically confirmed.
In addition, “subcrystalline grain” is defined as having a grain boundary with an orientation difference angle of 5 degrees or less.
In the magnesium alloy of the present invention, the level of its characteristics is superior to that of the prior art, and not only has the material structure of 1) and 2), but also an element that dissolves in magnesium is selected. Thus, the following characteristics were obtained. Elongation value 7% or more Tensile strength 275 MPa or more is realized. And
Tensile yield stress (A) 250 MPa or more Compressive yield stress (B) 200 MPa or more Yield stress anisotropy ratio (B / A) 0.7 or more is realized.
The elements that can be selected as the additive element of the present invention and the amount of addition are as listed in Table 1 below. If the amount added exceeds these values, it is presumed that a coarse intermetallic compound is formed, resulting in a decrease in ductility and toughness.

The permanent deformation by applying a load at a predetermined temperature is defined as processing strain, and the introduction of such processing strain is, for example, groove roll rolling exemplified in the examples or extrusion processing at a high extrusion ratio. It is considered as an application of means such as rolling under high pressure, high strain shearing such as ECAE (Equal-channel-angular-extrusion).
The groove roll rolling is also shown in Non-Patent Documents 1 and 2, for example, but is provided with grooves having a cross-sectional shape such as a triangle on the surface of the rolling roll. When these are brought into contact with each other, a diamond-shaped hole is formed. In the production of the magnesium alloy of the present invention, such groove roll rolling is a preferable means, and the groove shape in this case is one in which hexagonal or elliptical holes are formed in addition to the diamond shape described above. Is preferably considered, and the roll peripheral speed is preferably considered in the range of 1 to 50 m / min. Moreover, in the case of groove roll rolling, it is preferable to heat-process in the range of 100-500 degreeC beforehand for the time for the range of 5-120 minutes.
In the “introduction of processing strain” by various means such as groove roll rolling as described above, for example, heating is preferably performed so that the entire material is uniform at a temperature at which the material can pass without cracking. Hold and then introduce strain repeatedly. The cross-sectional reduction rate at that time can be appropriately set in relation to various conditions for introducing processing strain. In other words, the cross-section reduction rate may be set as a condition for forming the characteristic crystal structure as 1) and 2) in the alloy of the present invention. For example, the cross-section reduction rate is set as 92%, 95%, etc. as exemplified in the embodiment.

In the magnesium alloy of the present invention, for example, as shown in the examples, it is possible to significantly increase the strength without reducing the good ductility by introducing a working strain with a cross-sectional reduction rate of 90% or more. Unless a processing strain of 90% or more is introduced, crystal grains of 10 μm or less cannot be obtained, and an increase in strength cannot be expected.
In introducing the strain, it is preferable to continuously perform a plurality of strain introducing steps. In this case, the strain introduced by a single pass may be, for example, a cross-sectional reduction rate of 10 to 20%.

  The proportion of crystal grains having an average grain size of 10 μm or less surrounded by large-angle grain boundaries increases as the processing strain introduction (area reduction ratio) increases. For example, when the area reduction ratio is 90% or more. In this case, the ratio can be 90% or more, and the crystal structure in which the average grain size of the sub-crystal grains in the crystal grains is 1.5 μm or less can be 70% or more of the whole.

For example, the characteristics of the magnesium alloy of the present invention, which has the above-mentioned unique crystal structure by the introduction of the above processing strain, are tensile yield stress (A) 250 MPa or more, compressive yield stress (B) 200 MPa or more, The yield stress anisotropy ratio (A / B) is 0.7 or higher.

  Furthermore, it can be applied to a material having a large cross-sectional area or a long material having a complicated shape, and can be applied to an increase in size of the material.

Commercial pure magnesium (purity 99.95%) and yttrium were melted and cast to prepare a 1.1 mass% yttrium-magnesium alloy ingot. After ingot solution treatment, a billet for rolling with a diameter of 40 mm was prepared by machining. After holding the rolling billet in a furnace heated to 400 ° C. for 1 hour, groove rolling was performed. Here, the roll surface temperature was room temperature, and the roll peripheral speed was 30 m / min. Moreover, rolling was repeated 15 times so that the cross-sectional area reduction by groove roll rolling was 18% per pass, and the total area reduction rate was 92%. FIG. 1 shows an example of microstructure observation by SEM / EBSD. Through the crystal orientation analysis by EBSD, a large tilt grain boundary having a misorientation angle of 15 degrees or more is indicated as a grain boundary by a black curve group in the figure. In the figure: The average size of the crystal grains surrounded by the large-angle grain boundary denoted as G was 2.7 μm.

A tensile test piece showing a parallel part diameter of 3 mm and a length of 15 mm, and a compression test piece showing a diameter of 4 mm and a height of 8 mm were taken from the rolled material. The specimen collection direction is parallel to the rolling direction, and the initial tensile / compressive strain rate is 1 × 10 ⁻³ s ⁻¹ . FIG. 2 shows a nominal stress-nominal strain curve obtained by room temperature tension / compression. A summary of mechanical properties is shown in Table 2. The yield stress was an offset value of 0.2% strain.

Comparative Example 1

  An extruded billet having a diameter of 40 mm was prepared by the same manufacturing method as in Example 1. The extruded billet is put into an extrusion container heated to about 300 ° C., held for about 30 minutes, and then subjected to warm extrusion at an extrusion ratio of 25: 1 with a surface reduction rate of 94%, and an extruded material having a diameter of 8 mm is obtained. Obtained. The extrusion speed was 0.2 mm / second. FIG. 3 shows an example of microstructure observation using a transmission electron microscope. Notation: As indicated by G, adjacent crystal grains are composed of large tilt grain boundaries, and the average grain size of the crystal grains composed of the large tilt grain boundaries is about 2 μm. A tensile test was performed under the same conditions as in Example 1, and the results obtained are shown in Table 2. The crystal grains of 2 μm or less of the groove roll material are composed of sub-crystal grains, whereas the crystal grains of 2 μm or less of the extruded material are composed of large-angle grain boundaries. That is, it is difficult to form subcrystalline grains by extrusion.

Table 2 shows the tensile and compression test results of the magnesium alloy materials of Example 1 and Comparative Example 1. In a general high strain processing method, most crystal grains form a fine grain structure consisting of large-angle grain boundaries, and the bottom texture is very developed. Therefore, the yield anisotropy peculiar to magnesium, that is, the compressive yield stress becomes extremely low. However, since the magnesium alloy created by this process (groove roll method) has many subcrystalline grains, the development of the bottom texture is suppressed and the yield anisotropy is improved while maintaining a high strength level.

  Commercially pure magnesium (purity 99.95%) and calcium were dissolved and cast to prepare a 0.5 mass% calcium-magnesium alloy ingot. After ingot solution treatment, a billet for rolling with a diameter of 40 mm was prepared by machining. After holding the rolling billet in a furnace heated to 400 ° C. for 1 hour, groove rolling was performed. The rolling conditions are the same as in Example 1. A tensile test and a compression test were performed under the same conditions as in Example 1, and the obtained results are shown in Tables 3 and 4.

  Commercially pure magnesium (purity 99.95%) and tin were melt cast to produce a 1.5 mass% tin-magnesium alloy ingot. After ingot solution treatment, a billet for rolling with a diameter of 40 mm was prepared by machining. After the billet for rolling was kept in a furnace heated to 300 ° C. for 1 hour, groove roll rolling was performed. The rolling conditions are the same as in Example 1. A tensile test and a compression test were performed under the same conditions as in Example 1, and the obtained results are shown in Tables 3 and 4.

  Commercial pure magnesium (purity 99.95%) and tin were melted and cast to prepare a 4.7 mass% tin-magnesium alloy ingot. After solution treatment of the ingot mother alloy, a billet for rolling having a diameter of 40 mm was prepared by machining. After the billet for rolling was kept in a furnace heated to 300 ° C. for 1 hour, groove roll rolling was performed. The rolling conditions are the same as in Example 1. A tensile test and a compression test were performed under the same conditions as in Example 1, and the obtained results are shown in Tables 3 and 4.

G: Crystal grains (grain boundaries surrounded by large-angle grain boundaries (orientation angle of 15 ° or more))
RD: groove rolling parallel direction TD: groove rolling vertical direction

Claims (2)

A magnesium alloy mainly composed of magnesium, the crystal structure of which has a large tilt grain boundary, and the inside of the crystal grain surrounded by the large tilt grain boundary is composed of sub-crystal grains,
Only 14.5% by mass or less of tin that can be dissolved in magnesium as an additive element is contained,
The average crystal grain size of the crystal grains at 10μm or less state, and are 2μm or less as an average particle size of the subgrains,
A magnesium alloy having an elongation value of 7% or more, a tensile yield stress (A) of 250 MPa or more, a compressive yield stress (B) of 200 MPa or more, and a yield stress anisotropy ratio (B / A) of 0.7 or more . 2. The magnesium alloy according to claim 1 , wherein crystal grains having a crystal grain size of 10 μm or less occupy 70% or more of the whole.