KR101588076B1 - Magnesium alloy material and engine part - Google Patents

Abstract

In particular, the object of the present invention is to provide a Gd-Zn-based magnesium alloy material excellent in high-temperature fatigue strength. The structure of the Gd-Zn-based magnesium alloy material having a specific composition is defined as having a light gray long-period laminated structure image identified by the SEM image, a long-period laminated structure and a dark gray lamellar phase formed of? -Mg , Specific coarse-particle precipitates in the region of the long-period laminated structure, and specific coarse-like precipitates in the region of the lamellar phase are present so as to improve the high-temperature fatigue strength characteristics at 300 ° C.

Description

[0001] MAGNESIUM ALLOY MATERIAL AND ENGINE PART [0002]

The present invention relates to a Gd-Zn-based (or Zn-Gd-based) magnesium alloy material and an engine component made of the magnesium alloy material. Hereinafter, magnesium is also referred to as Mg, and the Gd-Zn-based magnesium alloy is also referred to as Mg-Gd-Zn-based alloy. The term " magnesium alloy material " in the present invention means a magnesium alloy product having a predetermined cross-sectional shape such as a shape or a plate, which is produced by forging, extruding, rolling, The material of the engine parts).

Magnesium alloys are the least dense, lightweight, and high in strength among the alloys in practical use. Magnesium has a specific gravity of 1.8, and among metals that can be used as structural materials for machine parts and the like, it is practically the most light in weight (about 2/3 of aluminum and about 1/4 of iron) , Thermal conductivity, and the like.

As a result, the magnesium alloy is being applied to automobile parts such as a housing for an electric product, a wheel of an automobile, and a chassis part. Particularly when applied to vehicles such as automobiles, motorcycles, etc., it is expected to greatly improve fuel economy by reducing the weight. Therefore, in recent years, application to engine parts (heat-resisting parts) including engines such as automobiles, motorcycles, and airplanes, and peripheral devices such as turbochargers has been studied.

Conventionally, in the case where high mechanical properties are required (applications), a Zn-REM magnesium alloy in which REM (rare-earth element) such as Gd or Zn is added as an alloying element among the magnesium alloys is attracting attention as an alloy excellent in heat resistance (See, for example, Patent Document 1, Patent Document 2, and Non-Patent Document 1).

However, in these documents, a magnesium alloy material of a product shape is manufactured by a special method such as a flat roll method or a rapid solidification method. Thus, in this special manufacturing method, although the magnesium alloy has high mechanical properties, there is a problem that a special manufacturing facility is newly required, the productivity is lower than that of the conventional method, and the shape of the magnesium alloy material that can be manufactured is limited have.

On the other hand, the Gd-Zn-based magnesium alloy, which is produced by a conventional production method (commercial method) comprising high yield, melt casting, plastic working (extrusion, forging, It is proposed in documents 3 to 6 and the like. This Gd is suitable for ordinary production methods with high productivity, such as easy casting compared to other REM (rare-earth elements) such as Y and the like.

These known Gd-Zn-based magnesium alloy materials commonly have a structure called a long-period laminated structure, whereby high mechanical properties can be obtained. The long period stacking structure (LPSO) refers to a structure in which the atomic layer structure of the top surface in the hexagonal structure of Mg has a long period structure such as an ABACAB type, not an ordinary AB type. It is known that the presence of this LPSO structure improves the tensile strength and proof stress, particularly tensile strength and proof stress at high temperature, of the magnesium alloy material.

Various means for enhancing mechanical properties such as strength and elongation have been proposed for magnesium alloy materials having these long-term laminated structure structures and produced by a common manufacturing method comprising melt casting and plastic working (extrusion) See, for example, Patent Documents 7 to 11).

In these Patent Documents 7 to 11, a Gd-Zn-based magnesium alloy containing a predetermined amount of Gd and Zn is subjected to hot extrusion after melt casting to produce a magnesium alloy material having a structure of this long-term laminated structure. Further, the structure is also a structure in which? -Mg that has been refined is formed in the divided portion of the long-period laminated structure. By such a structure, a magnesium alloy material having excellent tensile strength, proof stress and elongation can be obtained.

Among them, in Patent Document 7, the magnesium alloy cast material is further subjected to heat treatment at 200 to 300 占 폚 for 20 hours or more (in Fig. 4, maximum 40 hours in the embodiment), simulating use at a high temperature. The structure of the magnesium alloy casting material is made to have a long-term laminated structure. As shown in the photograph of the TEM structure in Fig. 1, the long diameter of the Mg-Gd or Mg-Gd- A large number of fine plate-shaped precipitates of about 0.4 nm (nm) are precipitated. However, in Patent Document 7 and the like, it is described that a long period laminating structure (LPSO) precipitates in the grain and grain boundaries of the magnesium alloy material, and in particular, LPSO having a high concentration exists in the lamellar form together with the Mg ₃ Gd compound However, it is not explicitly stated whether the presence of the plate-like precipitates is in the mouth or in the grain boundaries.

Further, in Patent Document 10, a magnesium alloy cast material is subjected to a solution treatment and then a heat treatment to precipitate precipitate in the form of a needle or a plate. In the TEM photograph of Fig. 10 of this embodiment, the cast material is heat-treated at 300 DEG C for 60 hours to deposit a large number of fine plate-like precipitates having a long diameter of about 1200 nm (1.2 mu m). This makes it possible to improve the 0.2% proof stress more simply than the magnesium alloy material having the long-period laminated structure. Further, in this patent document 10, it is described that the presence of the plate-like precipitate is present at the crystal grain boundary in the paragraph 0031 thereof.

Japanese Patent Application Laid-Open No. 06-041701 Japanese Patent Application Laid-Open No. 2002-256370 International Publication WO 2005/052204 Pamphlet International Publication No. 2005/052203 pamphlet International Publication No. 2006/036033 Japanese Patent Application Laid-Open No. 2006-97037 Japanese Patent Application Laid-Open No. 2008-127639 Japanese Patent Application Laid-Open No. 2008-138249 Japanese Patent Application Laid-Open No. 2008-150704 Japanese Patent Application Laid-Open No. 2007-284782 Japanese Patent Application Laid-Open No. 2008-75183

Yamasaki Tomoaki, et al., "New Mg-Gd-Zn Alloys with Long-term Laminated Structure by High-Temperature Heat Treatment", Abstract of Lecture at the 108th Spring Meeting of the Japan Institute of Light Metals (2005) p.43-44

In the above engine parts (heat-resisting parts), the magnesium alloy material is used in a high-temperature atmosphere of 200 to 300 캜. As a result, high temperature fatigue strength in a temperature region of 300 DEG C is required as heat resistance (high temperature strength) in a temperature range up to at least 300 DEG C or so.

On the other hand, the conventional Gd-Zn-based magnesium alloy material produced by the conventional method of plastic working (forging, extruding, rolling) the casting material is not particularly improved as to the high temperature fatigue strength There is room for.

An object of the present invention is to provide a magnesium alloy material having an excellent high-temperature fatigue strength in a temperature range of 300 캜 and a magnesium alloy material which is made of this magnesium alloy material (manufactured using this magnesium alloy material) To provide a component.

The essential point of the magnesium alloy material excellent in high-temperature fatigue strength characteristics of the present invention for achieving this object is that it contains 0.4 to 5.0% of Gd and 0.2 to 2.5% of Zn, respectively, in atomic%, and the balance Mg and inevitable impurities In a magnesium alloy material structure having a phase of a long-period laminated structure, a laminated phase of a long-term period and a lamellar phase formed of? -Mg, a maximum diameter of not less than 0.1 μm , the particle shape of the precipitate of less than 1.0, more 3㎛ range / ㎛ ² at the same time present in more than the average number density within the region of the lamella, a long diameter of coarse plate-like precipitate is more than 0.1 pcs / ㎛ ² or more average 3㎛ It is present in a number density.

Here, the magnesium alloy material structure of the present invention has a long-term laminated structure, a long-term laminated structure and a lamellar phase formed of? -Mg as specified, as shown in Fig. 1 to be described later, It can be clearly (easily) discriminated by the observing image by the color of the image. In addition, a large number of particulate precipitates to be defined in the area of the phase of this long-period laminated structure and a large number of coarse plate-like precipitates to be defined in the area of the lamellar phase are also observed in this 500- .

However, in accordance with the provisions of the present invention, in order to more accurately and reproducibly measure the average number density of the particulate precipitates and the plate-like precipitates having these specific sizes and shapes, Observation by SEM of 5000 times is necessary.

Means for improving the high-temperature fatigue strength required for applying the Gd-Zn-based magnesium alloy material to engine parts (heat-resisting parts) have been studied extensively by the present inventors. As a result, when the Gd-Zn-based magnesium alloy material is a magnesium alloy material having a lamellar phase of the above-mentioned long-period laminated structure, a long-term laminated structure and the lamellar phase formed of? -Mg, And the major precipitates that have been precipitated each other are greatly different in size and shape from each other. It has been found that the respective number density of the main precipitates having different sizes and shapes for each of these phases greatly affects the high-temperature fatigue strength.

More specifically, in the region of the phase of the long-period laminated structure, there are a number of relatively coarse-grained precipitates as main precipitates, and this is a factor that greatly improves the high-temperature fatigue strength of the Gd-Zn-based magnesium alloy material .

On the other hand, in the region of the lamellar phase formed by the long-period laminated structure and α-Mg, there exist a large number of relatively coarse plate-like precipitates as main precipitates, which greatly improves the high-temperature fatigue strength of the Gd-Zn- .

It is necessary to grasp the presence of each phase of these phases and the main precipitate precipitated in each of these phases and to grasp the effect (effect) on the high temperature fatigue strength of these main precipitates, The number density of each main precipitate can not be defined. In this respect, in the present invention, a plurality of relatively coarse grain-like precipitates are present in the region of the phase of the long-period laminated structure, and a large number of coarse plate-like precipitates are present in the region of the lamellar phase, Thereby greatly improving the fatigue strength. Here, the fact that the precipitates are coarse means that the coarse size of the order of micrometer is controlled (controlled), not the fine size (control) of the nanometer size.

In the present invention, the Gd-Zn-based magnesium alloy material can be obtained by a combination of a systematic combination of phases of a long-term laminated structure and a lamellar phase, and combinations of coarse-grained precipitates and different precipitates of a plate- , The high-temperature fatigue strength of the Gd-Zn-based magnesium alloy material is greatly improved. In other words, the fact that two coarse precipitates having different shapes are precipitated on two phases having different structures from each other, and the high-temperature fatigue strength is greatly improved by the synergistic effect of the combination of the two, to be.

Here, in order to secure the high-temperature fatigue strength, it is important to prevent the accumulation of the dislocation cells introduced into the magnesium alloy and uniformly disperse the magnesium alloy while the magnesium alloy material undergoes repeated loads. Since the accumulation site of the dislocation cell is usually a grain boundary system, it is generally considered that uniform finer grain size is effective for ensuring high-temperature fatigue strength. However, in the case of the Gd-Zn-based magnesium alloy material, there is a great limitation in uniformly finer crystal grains by calcining such as rolling or forging, and it is practically difficult to make the average crystal grain diameter smaller than 5 占 퐉. Therefore, the conventional grain refinement means can not sufficiently increase the high-temperature fatigue strength at 300 占 폚 of the Gd-Zn-based magnesium alloy material.

Further, as a technical knowledge of a person skilled in the art, it is naturally supposed that "when a precipitate is coarsened, there is a high possibility that the stress concentration portion becomes a stress concentration, and particularly the high temperature strength is lowered." However, according to the knowledge of the present inventors, high-temperature strength can be improved in comparison with the conventional technical sense only in view of coarsening of the precipitate. That is, when a plurality of particulate precipitates to be precipitated in the region of the phase of the long-term laminated structure are coarsened and a large number of such precipitates are precipitated in the region of the lamellar phase as in the present invention, Is assumed to serve as a barrier to block the accumulation of dislocation cells introduced into each phase (to the magnesium alloy material) under high temperature. By the barrier effect of the coarse precipitates, accumulation of the dislocation cells to be introduced can be prevented even when the magnesium alloy material undergoes repeated loads under high temperature, and these dislocation cells can be uniformly dispersed.

Further, the present invention can improve not only the coarsening of the precipitate but also the high temperature strength in comparison with the conventional technical sense in view of the relationship with the parent phase of the precipitate. For example, the magnitude of the effect of improving the high-temperature strength of the above-mentioned particulate precipitate is not explained only by the effect of the shape and size of the precipitate as described in detail in paragraph 0045 to be described later, It is presumed that the influence of the relationship with Can be said to have a relatively larger meaning than the coarse-like precipitates on the lamella.

Therefore, even when a large number of fine plate-like precipitates or particulate precipitates can be present on each phase, as in the conventional method, even when a large number of dislocation cells are uniformly distributed, for example, . This is also the reason why the prior art in which fine plate-like precipitates and particulate precipitates are present can not sufficiently improve the high-temperature fatigue strength at 300 ° C. On the other hand, according to the present invention, the high-temperature fatigue strength of the Gd-Zn-based magnesium alloy in the temperature region of 300 캜 can be greatly improved.

Brief Description of the Drawings Fig. 1 is a 500-times SEM image showing a structure of a magnesium alloy material of the present invention
2 is a SEM image of 5,000 times magnification (partially enlarged view of part A of Fig. 1).
Fig. 3 is a SEM image of 5000 times magnification (partially enlarged view of part B in Fig.

(Magnesium alloy material structure)

The characteristic structure of the magnesium alloy material (forging material) of the present invention is shown in Fig. 1: SEM image of 500 times, Fig. 2: SEM image of 5000 times magnification partially showing part A of Fig. 1, Fig. 3: And part B is shown in an SEM image of 5000 times magnified in part. Fig. 1 (Figs. 2 and 3) is an SEM image of the first invention example of Table 1 to be described later.

As shown in Fig. 1, the magnesium alloy material structure of the present invention is an image of a long-period laminated structure of a whitish-mesh-like shape, which appears to be light gray (gray) clearly identified on an SEM And a lamellar structure consisting of long-lenticular islands or scales in blackish light and dark-gray (gray) and a lamellar phase formed of alpha -Mg. Basically, the structure of the present invention is composed of a phase of these long-period laminated structure, a long-period laminated structure and a lamellar phase formed of? -Mg. However, depending on the manufacturing method and the manufacturing conditions, And may include, but is not limited to.

(Coarse particle type precipitate)

In such a magnesium alloy material structure, in the present invention, as shown in an enlarged scale in FIG. 2, in the region of the phase of the long-term laminated structure, various irregular shapes such as white, particle shape, rod shape, A large number of relatively coarse-grained precipitates are present. 2, in other words, the range of the size of the relatively rough coarse particulate precipitate of white irregularity measured in the field of view of Fig. 2 is a particulate precipitate having a maximum diameter of 0.1 mu m or more and less than 3 mu m , And these particulate precipitates are present at an average number density of 1.0 / 탆 ² or more.

Here, the maximum diameter of the particulate precipitate is the length of the side where the length becomes the largest among the irregular shapes of the particulate precipitate.

The particulate precipitate specified in the present invention can be clearly (easily) distinguished and specified, including its shape and size, with other precipitates existing in the region of the phase of the long-term laminated structure, for example, coarse plate- (Measured). These particulate precipitates are mainly composed of Mg ₅ Gd of the FCC structure, and in the case where they contain elements such as Zr and Al in addition to Zn, their elements may also exist as constituent elements of the coarse-like precipitates.

(Coarse plate type precipitate)

At the same time, in the present invention, as shown in FIG. 3 in an enlarged scale, the laminated structure of the long period and the lamellar phase region formed of? -Mg have a relatively uniform length (a long diameter described later) A large number of coarse plate-like precipitates appearing as a straight line (straight line) with regularity.

When the coarse plate-like precipitates are observed with a TEM of about 10,000 times, and the observation angle of the TEM image is inclined at, for example, -40 ° and observed in a three-dimensional manner, these plate- It can be confirmed that the precipitates commonly have a three-dimensional plate shape having a width (plate width = short diameter) in the depth direction of FIG. That is, the white straight line shown in Fig. 3 has a long diameter, has a short diameter in the depth direction of Fig. 3, and has a thickness (a planar width of a white straight line in Fig. 3) And can be confirmed as a shape having a rectangular (rectangular) plate shape. Therefore, the long diameter of the coarse-grains precipitates is the length of the straight line of each coarse-like precipitate (the length in the plane of the line in Fig. 3), which is seen as a straight line with white light in Fig. Coarse plate-shaped precipitates having a long diameter of 3 μm or more are present at an average number density of 0.1 / μm ² or more.

The coarse-grained precipitate of the present invention can be used in combination with other fine plate-shaped precipitates existing in the region of the lamellar phase and particles having various irregular shapes such as a particle shape, a rod shape, And can be clearly (easily) discriminated and specified (measured) from shape precipitates. In the coarse-grained precipitate of the present invention, Mg ₅ Gd having an FCC structure is mainly used as in the case of the particulate precipitate of the present invention. In the case of containing elements such as Zr and Al in addition to Zn, , And may exist as constituent elements of this coarse-like precipitate.

Specification of coarse-like precipitates:

In order to secure high-temperature fatigue strength as described in the column of the above effect, it is important to uniformly disperse the accumulation of the dislocation cells introduced into the magnesium alloy while the engine component made of the magnesium alloy material undergoes repeated loads under high temperature It becomes. Therefore, one of the sites where the dislocation cells accumulate while the magnesium alloy is subjected to the repeated load is focused on the main precipitates that are deposited in the region of the lamellar phase, and the presence of the main precipitates is examined. In order to obtain a magnesium alloy material having excellent high-temperature fatigue strength characteristics, it has been found that it is effective to satisfy the condition that the existence form of the precipitate peculiar to the lamellar phase is defined.

In the case of coarse plate-like precipitates existing in the area of the lamellar phase, the precipitate is not a spherical shape but a plate shape. In addition, the size of the precipitate at the second point is not a fine size as in the conventional nanometer order, but a relatively coarse size of micrometer (mu m) order even if the shape is, for example, a plate shape. Finally, the third point is to have a thickness suitable for the precipitate so that no crack is generated during the repeated load.

On the basis of this thinking method, in the present invention, the size of the coarse plate-like precipitates to be present in a large number in the area of the lamellar phase is specified, and the long diameter is defined as 3 탆 or more. At the same time, the plate-like precipitates are defined to exist in the lamellar phase region at a number average density of 0.1 / μm ² or more.

In order to secure the high-temperature fatigue strength, it is important to prevent the accumulation of the dislocation cells introduced into the magnesium alloy and uniformly disperse the magnesium alloy while the magnesium alloy material undergoes repeated loads. As in the present invention, it is possible to control precipitates that are precipitated in the region of the lamellar phase in a coarse plate shape, and the coarse plate-like precipitates are introduced into the magnesium alloy material while the magnesium alloy material undergoes repeated loads The dislocation cells can be uniformly dispersed by the barrier effect blocking the integration of the dislocation cells. As a result, the high-temperature fatigue strength of the Gd-Zn-based magnesium alloy material can be greatly improved as compared with a conventional fine plate-like precipitate having in the structure.

Long diameter of coarse-like precipitate:

On the other hand, the presence of multiple, not smaller the long diameter of the plate-like precipitates at less than 3㎛, such a fine plate-like precipitate, for example in the region of the lamella, 0.1 pieces / ㎛ ² or more of the average number in the region of the lamella Density, the barrier effect is weak, and the dislocation cells can not be uniformly dispersed. Therefore, it is necessary that the plate-shaped precipitate has a long diameter of at least 3 mu m. A short plate-like precipitate having a diameter longer than this is not effective in improving the high-temperature fatigue strength characteristics because of a weak barrier effect. On the other hand, although the upper limit of the long diameter of the plate-like precipitate is not particularly specified, the area of the lamellar phase containing these plate-like precipitates is not longer than the long diameter (maximum diameter, maximum length).

Number density of coarse-like precipitates:

Further, even if coarse plate-like precipitates of a prescribed size and shape are allowed to exist in the region of the lamellar phase, for example, when the average number density of the coarse plate-like precipitates is less than 0.1 pieces / 탆 ² , Also, this barrier effect is weak, so that dislocation cells can not be uniformly dispersed and sufficient high-temperature fatigue strength characteristics can not be ensured. Therefore, the average number density of the plate-like precipitates satisfying the above-mentioned long diameter is required to be at least 0.1 pieces / 탆 ² . By dispersing the plate-shaped precipitates at a ratio of 0.1 / μm ² or more, it is possible to provide a barriers for accumulation of dislocation cell structures introduced by fatigue damage effective for securing high-temperature fatigue strength characteristics. The upper limit of the number density is determined by the manufacturing limit, and it is practically difficult to make the number density exceeding 0.5 number / 탆 ² , so 0.5 number / 탆 ^{2 is set} to the upper limit.

3, the average width of the lines of each plate-like precipitate (width in plane of the line in Fig. 3 = average thickness) as seen by one white light line in Fig. 3, Thickness " and the " long diameter / thickness " are preferably 10 or more. Even if the plate-shaped precipitate has a long diameter of 3 탆 or more, the plate-like precipitate is broken (cracks are generated) by receiving the cyclic load at the long diameter / thickness of less than 10 to secure sufficient high-temperature fatigue strength characteristics There is a possibility that the above-described form can not be maintained. On the other hand, although the upper limit of the long diameter / thickness is not particularly specified, the minimum thickness for securing the shape of the plate-like precipitate is 5 to 15 nm on the crystal structure, and therefore, the upper limit is actually in the range of 8000 to 10000.

Specification of Particle Deposits:

The mechanism for improving the high-temperature fatigue strength characteristics in the case of the particulate precipitates existing in the region of the phase of the long-term laminated structure is the same as that of the coarse-plate precipitates. In order to secure high-temperature fatigue strength as described in the column of the above effect, it is important to uniformly disperse the accumulation of the dislocation cells introduced into the magnesium alloy while the engine component made of the magnesium alloy material undergoes repeated loads under high temperature It becomes. Therefore, while the magnesium alloy is subjected to cyclic loading, a main precipitate which is deposited in another region of the phase of the long-period laminated structure as a site where dislocation cells accumulate has been considered, and the presence of the main precipitate has been examined. Further, in order to obtain a magnesium alloy material excellent in high-temperature fatigue strength characteristics, it has been found that it is also effective to satisfy the condition that the existence form of a precipitate peculiar to the long-period laminated structure is defined.

However, in the case of this particulate precipitate, it is not the same shape as the coarse-like precipitate but is a comparatively coarse size effect of micrometer (탆) order. As described above, by increasing the size to a micrometer (mu m) order rather than a minute size such as a conventional nanometer order, there is no such a shape effect as a plate shape, but a precipitate has an appropriate thickness, So that no cracks are generated.

It is presumed that this particulate precipitate not only has an effect on the shape and size but also has a large effect on the effect of the relationship with the parent phase of the existing long-period laminated structure. That is, it is presumed that the presence of a large number of coarse particulate precipitates on the parent phase of the long-period laminated structure increases the effect of uniformly dispersing the potential cells as the parent phase. This is because the ratio of the lamellar structure to that of the lamellar phase is extremely small, about 20% of the total length of the lamellar structure. If the addition amount (content) of Gd is small, for example, the number of coarse-grained precipitates, which are the main precipitates, decreases further in addition to the ratio of the amount of Gd. However, as described above, the effect of improving the high-temperature fatigue strength characteristics is larger than that of the lamellar phase having a large ratio or the coarse plate-like precipitates, as compared with the case where the phase ratio is small. In other words, the meaning of satisfying the range of the number density is relatively larger than that of the coarse-like precipitate on the lamella in terms of improvement in high-temperature characteristics, I can tell. Also, at least about 3% of the total length of the long-period laminated structure is required.

Based on this, so as to present in the present invention, relatively coarse, the maximum diameter of 0.1㎛ above, the region on the long-period layered structure of the particulate precipitate is less than 1.0 in the range 3㎛ dog / ㎛ ² average number density of Respectively. As in the present invention, it is possible to control precipitates that are precipitated in the region of the phase of the long-period laminated structure with such relatively coarse-grained precipitates. When the magnesium alloy material undergoes repeated loads, The dislocation cells can be uniformly dispersed by the barrier effect blocking the integration of the dislocation cells introduced into the magnesium alloy material. As a result, the high-temperature fatigue strength of the Gd-Zn-based magnesium alloy material can be greatly improved as compared with a conventional fine precipitate having in the structure.

On the other hand, if the maximum diameter of the particulate precipitates to be present in a large number in the area of the phase of the long-period laminated structure is smaller than 0.1 占 퐉, such fine plate- Even if a large number of such cells can be present at an average number density of at least ² / 탆, this barrier effect is weak and the dislocation cells can not be uniformly dispersed. Furthermore, even if a coarse grain shaped precipitate of a prescribed size can exist in a crystal grain, for example, the coarse grain shaped precipitate is too small when the average number density of the coarse grain shaped precipitates is less than 1.0 / μm ² , The effect is weak and the dislocation cell can not be uniformly dispersed. Further, the upper limit of the average number density is determined by the manufacturing limit, and it is practically difficult to set the number density to exceed 10 / m ^2. Therefore, the upper limit value is 10 / m ² .

On the other hand, coarse plate-like precipitates having a long diameter of 3 mu m or more exist in the area of the phase of the long-period laminated structure. Such a coarse plate-like precipitate can not be said to be effective, but its contribution to the high-temperature fatigue strength is smaller than that of the particulate precipitate because of its small number density. For this reason, in order to accurately grasp the number density of the particulate precipitates separately from the rough plate-like precipitates, the maximum diameter of the particulate precipitates has an upper limit of less than 3 mu m.

Process for preparing coarse precipitates:

Each of the coarse precipitates specified in the present invention is distinguished from the conventional precipitates even from the generation history thereof. The structure of the present invention and the coarse precipitates related to these structures can be obtained by subjecting a Gd-Zn-based magnesium alloy cast material (ingot) to heat treatment two times (two stages) at a high temperature and a low temperature, Is produced by artificial aging treatment for a long time after plastic working in hot such as forging or hot extrusion. Therefore, there is a problem in that, as in the prior art, there is a problem in that it is difficult to obtain a sintered product to be cast during casting or a precipitate to be precipitated by artificial aging treatment after solution treatment of the cast product, And is not a precipitate precipitated by plastic working in the hot state. That is, the present invention is a novel precipitate which is not conventionally obtained and which has been subjected to the artificial aging treatment again after the heat treatment of the casting material in two steps, followed by the plastic working in the hot state. Are clearly distinguished. In the above-mentioned prior art, it is described that the cast material is subjected to artificially aging treatment after the solution treatment, or thereafter, subjected to a calcining process such as hot extrusion to produce a plate-like precipitate. However, in such a manufacturing method (manufacturing history), the plate-like or particulate precipitate can not be coarsened in the same manner as in the present invention.

This is because the shape and coarseness of the plate-like or particulate precipitate to be precipitated (generated) is deeply related to the structure and composition of the image forming the matrix. Most of the precipitates from the phase of the long-period laminated structure are particulate precipitates, and most of them from the lamellar structure of the long-period and the lamellar phase formed of -Mg are respectively produced as plate-like precipitates. In addition, the shape and coarseness of the precipitate in the form of a plate or a particle precipitated (formed) depend greatly on the production conditions. For example, if the temperature of the artificial aging treatment is appropriate and the treatment time is not long, the plate-like or particulate precipitate does not coarsen. Further, as in the case of the prior art, when the casting material is subjected to the plasticizing treatment such as the hot extrusion after the artificially aging treatment after the solution treatment, the precipitate produced in the artificial aging treatment is as if the load which the magnesium alloy material repeatedly receives , It is destroyed by the load caused by the plastic working in the hot state, becomes fine, and does not coarsen. Therefore, after the structure of the Mg-Gd-Zn-based magnesium alloy material is made up of the phase of the long-period laminated structure, the laminate structure of the long-period period and the lamellar phase formed of? -Mg as in the present invention, A large number of coarse granular precipitates are present in the region, and a large number of coarse plate-like precipitates can not be present in the region of the lamellar phase.

(Magnesium alloy component composition)

In the present invention, the composition of the magnesium alloy to be used as a base contains 0.4 to 5.0% of Gd and 0.2 to 2.5% of Zn, respectively, in terms of atomic% as a basis for obtaining excellent mechanical properties, And a Gd-Zn-based magnesium alloy composition composed of an impurity. Each component element will be described below. Note that the percentages of the content of each element are all atomic%.

Gd:

Gd (gadolinium) has a great advantage in that it is easier to cast by the conventional method than other rare earth elements (REM: Rare-Earth-Metal) such as Y, Dy, Ho, Er and Tm having the same effect. Gd contains a specific amount together with Zn, so that it is easy to form a long-period stacking (LPSO) structure in the alloy structure of Mg-Gd-Zn alloy. It is also an element constituting a coarse plate-like precipitate in the crystal mouth, which is required in the present invention, for ensuring high-temperature fatigue strength.

When the Gd content is excessively small, it is impossible to form a long-term laminate structure or a plate-like precipitate. On the other hand, if the Gd content is excessively large, the coarse Mg-Gd intermetallic compound is dispersed in the grain boundaries, and the elongation of the magnesium alloy forging material is greatly lowered (brittle). Therefore, Gd is contained in the range of 0.4 to 5.0 at%.

Zn:

Zn (zinc) contains a specific amount together with Gd, thereby forming a long-term laminated structure in the alloy structure of the Mg-Gd-Zn alloy. If the Zn content is too small, it is impossible to form a laminate structure for a long period. On the other hand, if the Zn content is too large, the coarse Mg-Zn intermetallic compound is dispersed in the grain boundaries, and the elongation of the magnesium alloy forging material is lowered (brittle). Therefore, Zn is contained in the range of 0.2 to 2.5 atomic%.

Zr, Mn:

Zr (zirconium) and Mn (manganese) are elements having an effect of refining the crystal grains. If necessary, they are contained in the range of 0.05 to 1.0 atomic percent.

Al, Ni, Cu, Ca:

Al (aluminum), Ni (nickel), Cu (copper), and Ca (calcium) are elements that enhance the high temperature strength of a magnesium alloy by the action of solid solution strengthening or dispersion strengthening, The effect of raising the strength to the endothelium at a high temperature is exhibited. When these elements are selectively contained, the total content thereof is set to 0.05 to 6.0 atomic%.

Inevitable impurities:

In addition, the Mg-Gd-Zn based alloy may inevitably include elements other than the above-mentioned components, such as Mg metal as well as Mg scrap as a dissolving raw material. In this respect, in addition to the above-mentioned additive elements, other ingredients may be contained in the range of unavoidable impurities as far as they do not adversely affect the effect of the magnesium alloy forging material of the present invention. For example, Fe (iron) and Si (silicon) may be contained in an amount of 0.2 atomic% or less, respectively.

(Manufacturing method)

A preferable manufacturing method and manufacturing conditions for obtaining the magnesium alloy material of the present invention and the engine component made of this magnesium alloy material will be described below.

The magnesium alloy of the present invention is produced by subjecting a cast material (ingot) which has been melt-cast to heat treatment in two steps, then performing artificial aging treatment again through plastic working in hot, and newly precipitating and growing.

Heat treatment:

The heat treatment is necessary to form coarse-grained precipitates on the long-period laminating structure or the long-period laminating structure. This heat treatment is carried out at a temperature of 480 to 550 占 폚, more preferably 500 to 530 占 폚, which is the first heat treatment, for 1 to 20 hours and a second heat treatment at 360 to 500 占 폚, more preferably 380 (2) heat treatment in which the substrate is held at a temperature of 480 DEG C to 480 DEG C for 1 to 20 hours.

Among them, the first heat treatment is performed so that the temperature of the first heat treatment is higher than the second temperature, and the treatment (solubilization treatment) of sufficiently solving Gd or Zn is performed by this first heat treatment. If the heat treatment temperature is too low or the treatment time is too short, there is a possibility that the high dose of alloying elements such as Gd and Zn is insufficient. On the other hand, if the heat treatment temperature is excessively high or the time is too long, the crystal grains may be coarsened. Immediately after this first heat treatment (solution treatment), it is quenched to 200 ° C or lower at an average cooling rate of 10 ° C / s or higher. When the cooling rate is less than 10 DEG C / s, the dispersed state of the particulate precipitates produced by the subsequent second heat treatment does not become the state specified in the present invention. Cooling may be performed by air cooling, gas cooling, or water cooling. However, in order to reliably cool the member to the center thereof, it is preferable to put it in cold water or warm water of several tens of degrees Celsius.

By the second heat treatment subsequent to the first heat treatment, a particulate precipitate having a maximum diameter of 0.1 mu m or more and less than 3 mu m is formed on the long-period laminating structure or the long-period laminating structure. If the second heat treatment temperature deviates from the range of 360 to 500 占 폚 in the up and down direction or if the treatment time is too short, the amount of the particulate precipitates to be formed becomes small and the number density thereof becomes small. Further, if the treatment time is too long, there is a possibility that crystal grains are coarsened.

Plastic working:

In the plastic working, well-known processing such as forging in hot, extrusion, rolling and the like are appropriately selected in accordance with the product shape, and then well-known processing such as forging, drawing and rolling in cold may be appropriately selected. Hereinafter, the hot forging is taken as an example (the following sentence can be applied to hot extrusion or hot rolling, and is applied as it is). The magnesium alloy ingot subjected to the heat treatment is once cooled and reheated after the second heat treatment, or after the second heat treatment, cooled to the start temperature for plastic working (forging, etc.) in the hot, and subjected to plastic working. In the hot forging, the lamellar phase generated by the above-described casting and heat-treating steps is made finer and a kink is formed to improve the high-temperature fatigue strength. Therefore, it is preferable to carry out plastic working at a low temperature as possible, and to give necessary and sufficient deformation.

For this reason, in hot forging the Gd-Zn-based magnesium alloy cast material having the above-mentioned composition, the hot forging is performed using a mold at a temperature range of 300 to 400 ° C. The upper limit of the heating temperature (monotonous temperature) of the ingot at the time of hot forging is 400 占 폚 or lower, preferably 380 占 폚 or lower, and the lower limit is 300 占 폚 or higher, preferably 340 占 폚 or higher. When the heating temperature (forging temperature) of the ingot is less than 300 캜, cracking occurs or the pressing ability is insufficient.

Artificial aging:

In the method for producing the Gd-Zn-based magnesium alloy material of the present invention, the artificial aging treatment is performed after the hot forging is performed in order to impart excellent high-temperature fatigue strength characteristics. Concretely, aging treatment is performed for 50 hours or more in the range of 270 to 330 占 폚 to form coarse plate-like precipitates having a long diameter of 3 占 퐉 or more in the lamellar phase region formed by? -Mgr and the long-term laminated structure. If the temperature of the artificial aging treatment is less than 270 캜, the precipitates can not grow, so that the coarse plate-like precipitates do not take the form prescribed in the present invention, and as a result, the high-temperature fatigue strength characteristics can not be secured. On the other hand, if the temperature of the artificial aging treatment is higher than 330 ° C, the solidification temperature becomes close to the solid-solution temperature of Gd which is the main element of the precipitate, and the coarse plate-like precipitates do not take the form prescribed in the present invention.

The artificial aging treatment is required to be carried out for 50 hours or more. When the artificial aging treatment time is less than 50 hours, the required number density of coarse plate-like precipitates can not be ensured and high-temperature fatigue strength characteristics can not be obtained. From the viewpoint of securing the number density of the rough plate-like precipitates more stably, it is preferable to carry out the artificial aging treatment for 100 hours or more. In the present invention, although the upper limit of the artificial aging treatment time is not particularly defined, when considering the industrial rationality and the change behavior of the precipitation form, improvement of the high-temperature fatigue strength property is not achieved even if artificial aging treatment for 200 hours or more is carried out .

The magnesium alloy material produced as described above is subjected to cold working such as cutting, grinding, punching, etc. in accordance with the application shape and structure, and then attached parts and jigs are mounted, And so on.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, but the present invention is not limited to the following Examples. Are also possible, and they are all included in the technical scope of the present invention.

<Examples>

Hereinafter, embodiments of the present invention will be described. In the compositions shown in Table 1, the hot forging temperature shown in Table 2 and the heat treatment conditions before and after the hot forging were changed to variously produce Gd-Zn-based magnesium alloy forgings having different average number densities of prescribed precipitates in the structure . Then, the obtained high-temperature fatigue strength characteristics of the magnesium alloy forging material at 300 캜 were measured and evaluated. The results are shown in Table 2.

More specifically, the magnesium alloy having the chemical composition shown in Table 1 was melted in an electric melting furnace under an inert atmosphere of argon, cast at a temperature of 750 DEG C with a cast iron base mold, and magnesium Alloy ingot. The surfaces of these ingots were then machined to form magnesium alloy billets each having a size of 90 mm? 35 mm.

Each of the billets was subjected to a first heat treatment in which each of the examples was heated at 520 캜 for 4 hours in common. After this heat treatment, the billets were cooled to 80 캜 at each cooling rate shown in Table 2. Subsequently, at the heating temperatures shown in Table 2, a second heat treatment for 4 hours was commonly performed. Thereafter, they were commonly cooled to room temperature, reheated at respective forging start temperatures (forging temperatures) shown in Table 2, and subjected to hot forging to mold disc-shaped test materials.

The test materials were subjected to artificial aging treatment at the respective temperatures and time conditions shown in Table 2.

In each of the examples, the samples cut out from the test material after the artificial aging treatment were used to measure the roughness of the surface of the long grain laminate structure (LPSO) of the magnesium alloy structure in the range of 0.1 μm or more and less than 3 μm The average number density of precipitates and the average number density of coarse plate-like precipitates having a long diameter of 3 mu m or more in the region of the lamellar phase were measured and the high temperature fatigue strength characteristics at 300 DEG C were measured and evaluated.

Here, in the Gd-Zn-based magnesium alloy shown in Table 1, the remaining part composition excluding the content of the elements described is magnesium except for the trace impurity components such as oxygen, hydrogen, and nitrogen. The symbol "-" in the content of each element in Table 1 indicates that the element content is below the detection limit.

Average number of precipitates Density:

The average number density of the precipitates was measured by cutting the test material after the artificial aging treatment and burying the test materials in a resin and mirror-polished the surface of the test material and finishing it smoothly. The surface was polished by FE-SEM (JSM-7001F, JSM- And the electron image was observed. The magnification of the FE-SEM was 5000 times and the acceleration voltage was 8 kV.

The coarse-like precipitates in the region of the lamellar phase have a certain orientation relationship with the magnesium matrix and precipitate. As a result, the crystal grain (crystal grains capable of observing the [0001] plane) clearly observed in which the precipitate is clearly observed in the observation field is selected, and the lamellar phase is observed and measured to average the number density of the lamellar phases , And the average number density was obtained.

Also, the average number density of the coarse grained precipitates in the area of the phase of the long period laminating structure (LPSO) can be obtained by selecting, observing and measuring phases of a plurality of long period laminating structures and averaging the results of the number density measurement of the phases of the respective long- , And the average number density was obtained. Figs. 2 and 3 show examples of the reflection electron image observed by this FE-SEM in the first invention example of the table.

High temperature fatigue strength test:

The high-temperature fatigue strength (high-temperature fracture life) was confirmed and evaluated by using a rotary bending fatigue tester of the Ono type and performing a rotational bending fatigue test. The specimen was cut from the test material and subjected to JIS (DIN) measurement with a diameter (D0) of 12.0 mm, a length (L) of 90 mm, a narrowest part diameter (d) of 8.0 mm and a smooth radius of curvature Z2274 No. 2 test specimen was heated with an infrared heater and the fatigue test was carried out under the condition of the number of revolutions of 3000 rpm while the temperature of the test piece was maintained at 300 캜. 10 ^Seven times fatigue test was repeated, and the fatigue strength of the test piece was measured at 10 ⁷ times. In this fatigue test, when the fatigue strength at 10 ⁷ times exceeds 45 MPa, it was judged to be a heat-resistant magnesium alloy material having excellent high-temperature fatigue strength characteristics.

Identification of organizations:

Incidentally, in all the examples of the inventive example and the comparative example of Table 1, the disc-shaped test sample has the structure of the light gray long-period laminated structure shown in Fig. 1, the long- &Lt; RTI ID = 0.0 &gt; lamellar &lt; / RTI &gt; The phase of the long-period laminated structure of the inventive example was less than 20% of the total.

Confirmation of mechanical properties:

In addition, JIS No. 4 test pieces were cut out from the disk-like test material and tensile strength, proof stress (0.2%) and elongation (%) were measured according to the tensile test prescribed in JIS. As a result, although not individually shown in the table, the examples of the inventive examples and the comparative examples of the table all satisfied the mechanical properties required for the heat resistant material having a TS of 250 to 350 MPa and a YS of 200 to 300 MPa Respectively.

As apparent from Table 1, the forgings of the first to sixth inventions, which are Gd-Zn-based magnesium alloys in the composition of the present invention, are produced under the above-described preferable conditions for forging and heat-treating. Thus, the forging material structures of the first to sixth inventions are formed by first forming, as a premise, a phase of a light gray long-period laminating structure as shown in Fig. 1, a long-period laminating structure and a- A magnesium alloy material structure having a dark gray lamellar phase.

In addition, the first to the sixth invention in honor to example forged material, and the average number density within the area, the maximum diameter of more than 0.1㎛, the particle shape of the precipitate of less than 1.0, more 3㎛ range / ㎛ ² or more on the long-period layered structure Coarse plate-like precipitates having a long diameter of 3 mu m or more exist in an area of the lamellar phase at an average number density of 0.1 / mu m ² or more. As a result, the forgings of the first through sixth inventions have the necessary mechanical properties, the fatigue strength at the 10 ⁷ times in the fatigue test is 50 MPa or higher, and the high-temperature fatigue strength characteristics are excellent.

Incidentally, all of these examples were forged without cracking in the hot forging process, and it was confirmed that the productivity in the plastic working such as hot forging was high.

On the other hand, the seventh comparative example shows that the average number density of the coarse grained precipitates in the region of the phase of the long-term laminated structure is not too high even though the content of the alloying element of Gd is too small and the manufacturing conditions are within the preferable range, The average number density of the coarse-grained precipitates in the region on the surface of the support is also excessively small. This has the necessary mechanical properties, but the fatigue strength at 10 ⁷ times is only 30 MPa, and the high-temperature fatigue strength characteristics are remarkably lower than those in the prior art.

The eighth comparative example is a magnesium alloy in the composition of the present invention, but the cooling rate after the first heat treatment is too slow. As a result, the average number density of the coarse-grained precipitates in the area of the lamellar phase is satisfied, but the average number density of the coarse-grained precipitates in the area of the phase of the long-term laminated structure is excessively small. As a result, although it has the necessary mechanical properties, the fatigue strength at the 10 ⁷ fatigue strength is only 45 MPa, and the high-temperature fatigue strength characteristics are lower than those of the conventional art.

The ninth comparative example is a magnesium alloy in the composition of the present invention, but the second heat treatment temperature is too high. In addition, the artificial aging treatment time is too short. As a result, both the average number density of the coarse grained precipitates in the area of the phase of the long-period laminated structure and the average number density of the coarse-grained precipitates in the area of the lamellar phase are all excessively small. As a result, although the required mechanical characteristics are obtained, the fatigue strength at the 10 ⁷ fatigue test is only 35 MPa, and the high-temperature fatigue strength characteristics are remarkably lower than those in the conventional art.

Comparative Example 10 is a magnesium alloy in the composition of the present invention, but the artificial aging treatment time is not performed. As a result, the average number density of the coarse-grained precipitates in the area of the phase of the long-period laminated structure is satisfied, but the coarse-like precipitates in the lamellar phase region are not precipitated. As a result, although the required mechanical characteristics are obtained, the fatigue strength at the 10 ⁷ times is only 40 MPa, and the high-temperature fatigue strength characteristics are remarkably lower than those in the prior art.

The 11th comparative example was a magnesium alloy in the composition of the present invention, but the hot forging temperature was too low to cause cracking, and thus the forging material itself could not be produced.

Comparative Examples 12 and 13 are magnesium alloys within the composition of the present invention, but the temperature of the artificial aging treatment is excessively low or too short. Therefore, the average number density of the coarse grained precipitates in the area of the phase of the long-period laminated structure is satisfied, but the average number density of the coarse-grained precipitates in the region of the lamella is excessively small. As a result, although the mechanical characteristics are required, the fatigue strength at the 10 ⁷ times is only 45 MPa in the fatigue test, and the high-temperature fatigue strength characteristics are lower than those in the conventional art.

Here, the eighth comparative example in which the average number density of the coarse grained precipitates on the long-term laminated structure is excessively small is the twelfth comparative example in which the average number density of the coarse-grained precipitates on the lamellae is excessively small, Compared with the comparative example, the high-temperature fatigue strength characteristics are poor. Therefore, from this fact, it can be concluded that the number of coarse-grained precipitates in the high-temperature characteristic improvement is larger than the coarse-like precipitate on the lamellar in the range of the number density, Is supported.

From the above results, it can be seen that the critical composition of the magnesium alloy forging material of the present invention, the structure thereof, and the preferable forging conditions, which satisfy the high temperature fatigue strength characteristics, Able to know. In order to obtain excellent high-temperature fatigue strength characteristics, the engine parts made of these magnesium alloy materials are required to have, in particular, an average number density of coarse grained precipitates in the long-term laminated structure condition and an average number density of coarse- Density, both of which need to be satisfied (technical significance).

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to provide a Gd-Zn-based magnesium alloy material which can be produced with excellent mechanical strength, high-temperature fatigue strength and high productivity. As a result, the present invention can be applied to an automobile, a motorcycle, an aircraft, and other peripheral devices such as a turbocharger, which are required to have heat resistance as well as automobile parts such as a housing of an electric product, a wheel of an automobile, It is suitable for engine parts (heat-resistant parts) made of ash.

Claims (5)

And the balance Mg and inevitable impurities, and is composed of a phase of a long-period laminating structure, a long-period laminating structure and a-Mg in the magnesium alloy material structure having a lamellar phase, in a region on the long-period layered structure, a maximum diameter of more than 0.1㎛, the particle shape of the precipitate of less than 3㎛ range of not more than 1.0 pieces / ㎛ ² 10 or more / ㎛ ² And coarse plate-like precipitates having a long diameter of 3 mu m or more exist in an average number density of 0.1 / mu m ² or more and 0.5 / mu m ² or less in the region of the lamellar phase,
Wherein the magnesium alloy material further contains 0.05 to 1.0 at% in total of one or both of Zr and Mn. delete The magnesium alloy material according to claim 1, wherein the magnesium alloy material further contains 0.05 to 6.0 at% of at least one of Al, Ni, Cu and Ca in total. An engine part comprising the magnesium alloy material according to claim 1. An engine component comprising the magnesium alloy material according to claim 3.

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