JP2005054233A - Heat resistant magnesium alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a magnesium alloy which has excellent creep resistance without using any rare earth element. <P>SOLUTION: The heat resistant magnesium alloy comprises, by mass, 4 to 9% aluminum, 1 to 5% copper, ≤4% zinc and 0.001 to 0.01% beryllium, and the balance magnesium, and has a metallic structure where a magnesium-aluminum-copper based compound is scattered into base phase magnesium. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnesium alloy. More specifically, the present invention relates to a heat-resistant magnesium alloy that has heat resistance typified by creep resistance and can be used in a high-temperature environment (for example, around 150 ° C.).

The weight reduction of machines and equipment, including vehicles and airplanes, is directly related to the improvement of fuel efficiency, and is therefore especially emphasized in recent years when global environmental protection is important. In such a background, the density of the magnesium alloy is about 1.8 Mg / m ³ , which is the lightest in practical metal materials. Therefore, the environment-friendly structure can be substituted for the conventional iron-based alloy and aluminum alloy. It is attracting attention as a material for use. Furthermore, since it is easy to recycle, it is often replaced with a resin when there is no problem in strength.

  As an inexpensive magnesium alloy having standard mechanical properties, an Mg—Al—Zn—Mn based alloy (for example, ASTM standard AZ91D) is well known. Also, ASTM standard AS21 is known as a magnesium alloy whose creep resistance is improved by adding silicon (Si, silicon). Furthermore, a magnesium alloy (see, for example, Patent Document 1) in which creep resistance and castability are improved by adding a rare earth element has been developed.

JP 2002-121737 A

  However, Mg—Al—Zn—Mn alloys typified by AZ91D have difficulty in creep resistance, and materials for parts used at high temperatures (for example, around the engine of an automobile exceeding 100 ° C. and transmission cases). It cannot be used for.

  Furthermore, although AS21 is excellent in creep resistance, it has poor castability (for example, fluidity so-called hot water), and in practice, it is difficult to use the die casting method. Further, since the silicon contained in AS21 increases the melting point of the alloy, the temperature of the magnesium alloy molten metal that is easy to burn must inevitably be raised, casting workability is impaired, and advanced casting techniques are required. As a result, the manufacturing process takes time and labor, resulting in an increase in product cost.

  Furthermore, since a magnesium alloy containing a rare earth element is expensive, it is mainly suitable for special applications (for example, aerospace and military applications) and lacks versatility.

  Therefore, an object of the present invention is to provide a heat-resistant magnesium alloy that is excellent in heat resistance and castability as typified by creep resistance and is inexpensive. Specifically, the present invention is an inexpensive heat-resistant magnesium that does not contain a rare earth element, has excellent creep resistance as well as a heat-resistant magnesium alloy containing a rare-earth element, and has better castability than a heat-resistant magnesium alloy containing a rare-earth element. The object is to provide an alloy.

  In order to achieve this object, the inventors of the present application have conducted various experiments and studies. As a result, by adding aluminum and copper in a certain range, they can be used as a magnesium alloy for casting without adding rare earth elements. As a result, it has been found that a magnesium alloy having extremely high creep resistance greatly exceeding that of the Mg-Al-Zn-Mn alloy (AZ91D).

  The heat-resistant magnesium alloy according to claim 1 is based on such knowledge, and an additive alloy element group (excluding rare earth elements) containing at least 3% by mass of aluminum and 0.1% by mass or more of copper, and the balance It consists of magnesium. By including aluminum and copper under the above-described conditions, high creep resistance comparable to or exceeding AS21 magnesium alloy is exhibited by the interaction between these aluminum and copper. Moreover, since it contains copper, the molten metal around the molten metal becomes good and the castability is excellent. Furthermore, since it is not necessary to add an expensive rare earth element, the alloy cost can be kept low.

  According to experiments by the inventors of the present application, the aluminum content is preferably in the range of 3 to 9% by mass, more preferably in the range of 4 to 9% by mass, and the most preferable physical properties are shown when the content is about 8% by mass. Moreover, the range of 0.1-5 mass% is preferable, as for copper content, the range of 1-5 mass% is more preferable, and when it is about 3 mass%, the most preferable physical property is shown. Based on the said knowledge, invention of Claim 2 makes content of aluminum 4-9 mass%, and invention of Claim 3 makes content of copper 1-5 mass%.

  Furthermore, the invention according to claim 4 is the heat-resistant magnesium alloy according to any one of claims 1 to 3, wherein the additive alloy element group contains 4 mass% or less of zinc. The addition of zinc is not particularly effective for creep resistance, and is equivalent or slightly improved in creep resistance. However, the inclusion of zinc provides the effect of improving corrosion resistance and improving castability. Here, from the viewpoint of prevention of porosity generation during casting, the amount of zinc is preferably 4% by mass or less, and from the viewpoint of strength, an increase in the amount of zinc is concerned about embrittlement due to the intermetallic compound phase. The amount of zinc is more preferably 2% by mass or less because it does not have a positive effect.

  Furthermore, the invention according to claim 5 is the heat-resistant magnesium alloy according to any one of claims 1 to 4, wherein 0.001 to 0.01% by mass of beryllium is included in the additive alloy element group. Beryllium is effective to prevent magnesium combustion and oxidation by forming a thin film on the surface of the molten metal. By containing 0.001% by mass or more of beryllium, the molten metal temperature can be raised or the molten metal can be handled roughly. The magnesium in the molten metal can be prevented from burning and igniting, and the casting operation becomes easy. On the other hand, from the viewpoint of magnesium combustion / ignition prevention, it is sufficient that the beryllium content is about 0.01% by mass, and even if the content exceeds 0.01% by mass, further improvement of the effect cannot be expected. . Furthermore, since beryllium is an expensive metal, even if the content is increased unnecessarily, it results in only increasing the alloy cost. For this reason, content of beryllium is 0.001-0.01 mass%.

  Here, the inventors of the present application have a magnesium-aluminum-copper-based compound in the matrix magnesium in the structure of the magnesium alloy that has obtained high creep resistance by containing aluminum and copper in a certain range. It was found that crystals were dispersed and dispersed in a piece or pseudo piece.

  The heat-resistant magnesium alloy according to claim 6 is based on such knowledge, and is a crystal in which a magnesium-aluminum-copper compound is dispersed in a matrix form or pseudo-piece form in a matrix magnesium. According to this heat-resistant magnesium alloy, in addition to solid solution strengthening of aluminum element and copper element, high creep resistance is exhibited by the formation of a magnesium-aluminum-copper intermetallic compound and dispersion strengthening by its dispersion.

  Thus, according to the heat-resistant magnesium alloy according to claims 1 to 3, high creep resistance comparable to or exceeding that of the AS21 magnesium alloy can be obtained by the interaction between aluminum and copper. Moreover, since it has copper in a composition, the molten metal of a molten metal becomes favorable and it is excellent also in castability. Furthermore, since it is not necessary to add an expensive rare earth element, the alloy cost can be kept low. This heat-resistant magnesium alloy can be used, for example, in automobile engines and transmission system parts that are used in a high-temperature environment around 150 ° C. and require heat-resistant creep characteristics.

  Furthermore, according to the heat-resistant magnesium alloy of claim 4, by containing zinc, the effect of further improving the creep resistance, and further improving the corrosion resistance and the castability can be obtained.

  Furthermore, according to the heat-resistant magnesium alloy of claim 5, by containing beryllium, even if the molten metal temperature is raised or the molten metal is handled roughly, the magnesium in the molten metal will not burn and ignite. Moreover, it is not necessary to use an expensive flameproof gas such as sulfur hexafluoride gas. Accordingly, it is possible to obtain the effects of facilitating casting work, improving safety, and reducing work costs.

  Furthermore, according to the heat-resistant magnesium alloy according to claim 6, high creep resistance comparable to or exceeding the AS21 magnesium alloy is obtained by dispersion strengthening with a magnesium-aluminum-copper compound dispersed in the matrix magnesium. be able to.

  Hereinafter, an embodiment of the present invention will be described in detail. This heat-resistant magnesium alloy is composed of an additive alloy element group including aluminum and copper and the balance magnesium. Here, the “additional alloy element group” means an element intentionally added to improve the properties of the magnesium alloy, except for magnesium (Mg) contained in the magnesium alloy and an inevitable impurity element contained therein. Refers to the group of elements.

  The additive alloy element group includes at least 3% by mass of aluminum (Al) and 0.1% by mass or more of copper (Cu). When aluminum and copper are contained under the above conditions, the creep resistance of the magnesium alloy can be improved by the interaction between these aluminum and copper. According to experiments by the inventors of the present application, the aluminum content is preferably in the range of 3 to 9% by mass, more preferably in the range of 4 to 9% by mass, and the most preferable physical properties are shown when the content is about 8% by mass. Moreover, the range of 0.1-5 mass% is preferable, as for copper content, the range of 1-5 mass% is more preferable, The most preferable physical property was shown when it was set as about 3 mass%.

  Furthermore, it is preferable that zinc (Zn) is contained in the additive alloy element group. Zinc is an element effective for improving corrosion resistance and castability. However, the addition of excess zinc exceeding 4% by mass may cause the generation of porosity during casting. Therefore, the zinc content is preferably 4% by mass or less, and more preferably 2% by mass or less. Although the addition of zinc exceeding 2% by mass has the possibility of improving the corrosion resistance, the inventors of the present application experimented did not show a good improvement in creep resistance, and in terms of strength, This is because there is a concern about embrittlement due to an increase in the compound phase.

Furthermore, it is preferable that beryllium (Be) is contained in the additive alloy element group. Beryllium is effective in preventing the combustion of magnesium. In general, it is said that beryllium of about 0.0005% by mass has an effect of preventing magnesium from burning, and the conventional AZ91D magnesium alloy actually contains this amount of beryllium. However, if the molten AZ91D alloy is vigorously stirred, or if the molten metal temperature rises to 700 ° C. or higher, magnesium will burn unless the molten metal surface is cut off from the atmosphere by sulfur hexafluoride (SF ₆ ) gas or the like. It was empirically found that it would ignite. Therefore, the beryllium content is preferably 0.001% by mass or more. In this case, even if the molten metal temperature is raised to 700 ° C. or higher, or the molten metal is handled roughly, magnesium in the molten metal is not burned and ignited, and expensive flameproofing such as sulfur hexafluoride gas. There is no need to use gas. Accordingly, it is possible to obtain the effects of facilitating casting work, improving safety, and reducing work costs. On the other hand, from the viewpoint of magnesium combustion / ignition prevention, it is sufficient that the beryllium content is about 0.01% by mass, and even if the content exceeds 0.01% by mass, further improvement of the effect cannot be expected. . Furthermore, since beryllium is an expensive metal, even if the content is increased unnecessarily, it results in only increasing the alloy cost. Therefore, in this embodiment, the beryllium content is set to 0.001 to 0.01% by mass. Moreover, it is estimated that beryllium is effective also in suppressing the high temperature oxidation of a magnesium alloy. Therefore, the addition of beryllium is expected to appear as a positive effect on the alloy strength when the creep test time is extended to tens of thousands of hours, for example, and is considered to contribute to improving the durability of the magnesium alloy.

  In addition, the additive alloy element group may contain an element (for example, an inevitable impurity element) other than Al, Cu, Zn, and Be mentioned above depending on circumstances.

  For example, in the heat-resistant magnesium alloy of this embodiment, aluminum is 3% by mass or more (preferably 3 to 9% by mass, more preferably 4 to 9% by mass, most preferably 8% by mass), and copper is 0.1% by mass or more. (Preferably 0.1 to 5 mass%, more preferably 1 to 5 mass%, most preferably 3 mass%), zinc is 4 mass% or less (more preferably 2 mass% or less), and beryllium is 0.001 to 0.001. It consists of 0.01% by mass and the remaining magnesium. The heat-resistant magnesium alloy having this component composition can be produced by appropriately adopting a casting method (for example, a die casting method or a die casting method) generally used in the production of, for example, automobile parts. For example, when a mold casting method is given as an example, first, pure magnesium is melted in a crucible to obtain a molten metal of about 680 ° C. to 800 ° C., and pure aluminum weighed to have a target composition with respect to the molten magnesium, Add pure copper and beryllium. In the refining before melting and casting, it is preferable to use a melting flux and a refining flux, respectively. After confirming that the added metals are completely dissolved, a molten metal at about 680 ° C. is cast into a room temperature mold to produce a heat-resistant magnesium alloy.

  Here, when the inventors of the present application observed the metal structure of the above heat-resistant magnesium alloy with an electronic scanning microscope, it was confirmed that magnesium-aluminum-copper compounds were scattered in the matrix magnesium. . For example, in the case of the heat-resistant magnesium alloy of the present embodiment obtained by the above-described mold casting method, aluminum, copper, zinc, and beryllium are solid-solved in the matrix magnesium in the cast structure. A magnesium-aluminum-copper compound is contained between the dendrite cell or the dendrite arm. Here, the dendrite cell refers to a petal-like dendrite petal part in which the dendrite arm is not well developed. A magnesium alloy having this metal structure has very good creep resistance. Adopting the component composition described above can be regarded as one method for obtaining this metal structure.

<Manufacture of heat-resistant magnesium alloy>
A predetermined amount of pure magnesium (for example, 99.93% purity manufactured by Norsk) was dissolved in a crucible made of SUS430 stainless steel that had been subjected to hot-dip aluminum plating. Then, pure aluminum (for example, having a purity of 4N), pure copper, and beryllium (for silicon of Comparative Example 4) weighed to obtain a target composition were added to molten magnesium at 680 ° C. to 800 ° C. The addition of beryllium was performed by first adding a master alloy of 97 mass% aluminum and 3 mass% beryllium. During refining before melting and casting, a melting flux and a refining flux were used, respectively. After confirming that each added metal was completely dissolved, a molten metal of about 680 ° C. was cast into a room temperature mold, and alloys (Examples 1 to 6 and Comparative Examples 1 to 4) having the compositions shown in Table 1 were made of gold. It was produced by a mold casting method. In the conventional example 1, an AZ91D magnesium alloy was used, and in the conventional example 2, an AS21 magnesium alloy was used.

<Creep test>
The alloys of Examples 1 to 6, Comparative Examples 1 to 4, and Conventional Examples 1 and 2 were processed into test pieces defined by the creep rupture test method, and a creep test was performed in accordance with JISZ2271. The creep test was performed under the conditions of a test temperature of 150 ° C., a test load of 50 MPa, and a test time of 100 hours.

  1 to 10 show the creep curves of the respective alloys obtained as a result of the creep test. In these drawings, reference numerals S1 to S6 indicate Examples 1 to 6, respectively, reference numerals C1 to C4 indicate comparative examples 1 to 4, and reference numerals P1 and P2 indicate conventional examples 1 and 2, respectively.

  It can be confirmed that all of Examples 1 to 6 have excellent creep resistance significantly exceeding AZ91D (Conventional Example 1). In particular, Examples 2, 3, and 6 can be confirmed to have excellent creep resistance comparable to or exceeding AS21 (Conventional Example 2).

  Here, when the creep curves of Example 2 and AZ91D (Conventional Example 1) are compared (see FIG. 2), the creep strain after the 100-hour test is about 0.18% in Example 2, whereas It can be confirmed that AZ91D (conventional example 1) reaches about 0.8%. In Example 2 and AZ91D (Conventional Example 1), it is considered that the amounts of aluminum and zinc are almost equal, and therefore, the excellent creep resistance of Example 2 is considered to be affected by copper. On the other hand, looking at the creep curve of Comparative Example 3 (see FIG. 9), the creep strain after the 100-hour test has already reached 1%, and Comparative Example 3 is inferior in creep resistance to AZ91D (Conventional Example 1). . In Comparative Example 3 and Example 2, the contents of elements other than aluminum are substantially equal, and only the aluminum content is greatly different. Therefore, it was considered that the excellent creep resistance obtained in Example 2 was due to the interaction between copper and aluminum, not just the effect of copper.

  Further, looking at the creep curve of Example 6 (see FIG. 6), the creep strain after the 100 hour test is about 0.23%, and it can be confirmed that the creep curve of Example 2 almost overlaps. The main difference between Example 6 and Comparative Example 3 is only the aluminum content. Therefore, it can be said that the difference in creep resistance between Example 6 and Comparative Example 3 is due to the difference in the aluminum content.

  In Example 1 and Example 2 where the main difference is only the zinc content, Example 2 containing zinc showed better creep resistance than Example 1 not containing zinc. From this, it can be confirmed that the creep resistance is further improved by the addition of zinc.

  As described above, it was found from the results of the creep test that the creep resistance in the magnesium alloy is greatly improved by adding copper, aluminum, and zinc and controlling the amount of these added. These metallic elements of copper, aluminum and zinc are presumed to increase the creep resistance of the magnesium alloy by solid solution strengthening and further by the formation and dispersion of intermetallic compounds.

  Next, the compound phases contained in Example 3, Example 6 and Comparative Example 3 and their distribution forms were clarified by conducting structure observation, EPMA (Electron Probe Microanalyzer) analysis and X-ray diffraction.

  FIG. 11 is a metal SEM (scanning electron microscope) photograph showing the alloy composition. In the figure, (A) shows the microstructure of Example 3, (B) shows the microstructure of Example 6, and (C) shows the microstructure of Comparative Example 3. In each alloy, a white phase that seems to be a compound was observed. Furthermore, for Example 3 and Example 6, in addition to the white phase, a gray phase was also observed adjacent to it. Further, in Comparative Example 3, the white phase forms an incomplete network (network-like) that is cut off at some points between the parent phase dendrite arms, and crystallizes in a lamellar manner, whereas Example 3 and Example In No. 6, it was confirmed that the white phase was dispersed in the form of islands, in other words, in the form of a piece or a pseudo piece and crystallized.

  12 shows an X-ray image of Example 3, FIG. 13 shows an X-ray image of Example 6, and FIG. 14 shows an X-ray image of Comparative Example 3. 12 to 14, (A) corresponds to an enlarged image of FIG. 11, (B) shows the distribution of aluminum elements corresponding to (A), and (C) shows the same figure ( The distribution of the copper element corresponding to A) is shown, and (D) shows the distribution of the zinc element corresponding to FIG. The bright white portions in the diagrams (B) to (D) indicate the locations where many target elements exist.

According to the X-ray image, the white phase observed in the composition image is a compound containing a large amount of Cu and Al, and a magnesium-aluminum-copper compound (hereinafter, this compound is also abbreviated as Mg-Al-Cu compound). Is considered.) The phase indicated by the arrow A in FIGS. 12A, 13A, and 14A is an Mg—Al—Cu compound. This Mg—Al—Cu-based compound is considered to be mainly compound λ. Here, the compound λ is basically an AlCuMg (Al: Cu: Mg = 1: 1: 1) which is a Laves phase having a Cu ₂ Mg structure and formed by replacing Cu of Cu ₂ Mg with Al. 1). However, since the compound λ has a solid solution range, the ratio of elements constituting the compound λ may not always be Al: Cu: Mg = 1: 1: 1 when strictly considered. Therefore, in the present specification, the term “compound λ” includes not only AlCuMg but also compounds whose ratio of Al: Cu: Mg is not 1: 1: 1. This compound λ corresponds to “λ ₁ phase + λ ₂ phase + λ ₃ phase” in the equilibrium state diagram shown in FIG. 16, for example (there is a theory that the λ phase has _three phases of λ ₁ , λ ₂ , λ ₃ ). According to the observation of the present inventors, it was one phase). Further, it is considered that the Mg—Al—Cu-based compound includes, for example, the Q phase in the equilibrium diagram shown in FIG. 16, that is, Al ₇ Cu ₃ Mg ₆ . That is, it is considered that the Mg—Al—Cu-based compound is mainly composed of the compound λ and the Q phase. FIG. 16 is an equilibrium diagram at atmospheric pressure (about 1 atm), showing a reaction line and a reaction point, showing that solidification proceeds in the direction of the arrow in the figure as the temperature decreases. Yes.

  In addition, the phase observed in gray in the composition images of Example 3 and Example 6 is a magnesium-aluminum-zinc compound (hereinafter, this compound is also abbreviated as Mg-Al-Zn compound). It is thought that. The phase indicated by the arrow B in FIGS. 12A and 13A is the compound. The alloy structure of Example 6 has a slightly smaller crystallization phase than the alloy structure of Example 3, but is considered to be basically composed of the same compound as Example 3. Further, most of the phases forming the network observed in the alloy structure of Comparative Example 3 contain a large amount of Cu and Al, and the aluminum-zinc binary compound (that is, the arrow in FIG. 14A). The phase indicated by C) was slightly observed, but the Mg—Al—Zn compound was not observed.

  15A, 15B, and 15C show X-ray diffraction patterns of Example 3, Example 6, and Comparative Example 3, respectively. The circles in the figure indicate the diffraction peaks of the magnesium solid solution (corresponding to the α phase in the equilibrium diagram of FIG. 16, hereinafter also simply referred to as α). As a result, in addition to the α diffraction peak indicated by ◯ in each alloy, it is clear that an unknown diffraction peak indicated by △ in the same figure exists in the vicinity where 2θ is 20 ° and 40 °. It was. As a result of paying attention to the unknown peak and examining it, it was found that the peak was due to the compound λ. From this result, it was confirmed that all of Example 3, Example 6, and Comparative Example 3 crystallize compound λ. The reason why no Q-phase peak is observed in the diffraction patterns of Example 3 and Example 6 is considered to be because the abundance of the Q-phase in the metal structure is smaller than that of the compound λ.

  The large difference in creep resistance generated between Example 3 and Example 6 and Comparative Example 3 is considered to be due to the difference in the distribution form of the crystallization phase. In the metal structure of Comparative Example 3 with inferior creep characteristics, Mg-Al-Cu compounds crystallized in a lamellar form are connected in a network form, whereas Examples 3 and In the metal structure of Example 6, the Mg—Al—Cu-based compound was dispersed in a lump shape (in other words, in an island shape). Therefore, it is considered that the metal structure in which the magnesium-aluminum-copper compound λ is dispersed in the matrix magnesium contributes to the improvement of creep resistance.

Therefore, the difference in the crystallization form of the compound λ and the Q phase mainly constituting the Mg—Al—Cu-based compound was examined from the equilibrium diagram shown in FIG. As a result, in each composition range of Examples 3 and 6 and Comparative Example 3, it was found that the compound λ was crystallized through a binary eutectic reaction on the equilibrium diagram. As described above, compound λ was actually confirmed in each alloy by structural observation and X-ray diffraction. However, it has also been found that there is a maximum point e ₄ (see FIG. 16) on the binary eutectic line, and the solidification process is greatly different at this point. Therefore, the solidification process is divided into the following three cases. That is, the first coagulation process, after crystallization of primary crystal alpha, a case that through the U ₁₃ points or E ₆ points of the equilibrium phase diagram shown in Figure 16. Second coagulation process, after crystallization of primary crystal alpha, is a case where through the e ₄ points of the equilibrium phase diagram. A third solidification process after crystallization of primary crystal alpha, is a case where through the E ₃ points of the equilibrium phase diagram.

First, the first solidification process will be described. Examples 3 and 6 are considered to correspond to this case. In this case, it is considered that the primary crystal α first crystallizes from the liquid L (liquid phase, that is, the combined financial liquid), and reaches the binary eutectic line (e ₄ -U ₁₃ line) as the temperature decreases. The resulting eutectic reaction shown in Equation 1 (560 ℃ ~435 ℃) in binary eutectic _line, is believed to result Tsutsumitomo crystals reaction shown in Equation 2 by _{U 13} points (435 ° C.).

<Formula 1>
L → α + λ
<Formula 2>
L + λ → α + Q

Here, Q in Equation 2 represents the Q phase in the equilibrium diagram shown in FIG. 16 and is Al ₇ Cu ₃ Mg ₆ . In the case of equilibrium solidification, this completes solidification. However, in rapid solidification such as die casting, the envelop eutectic reaction of Formula 2 accompanied by diffusion in the solid does not proceed, and as a result, E ₆ points (three terpolymer eutectic point) L remains until, resulting ternary eutectic reaction (423 ° C.) as shown in equation 3 in this E ₆ points, is believed to complete the coagulation.

<Formula 3>
_{L → Q + α + Al 12} Mg 17

As a result of the above solidification process, the compound λ is not lost by the reaction of Formula 2, and as a result, it is considered that the compound λ is dispersed in the form of islands in the metal structure at room temperature. In addition, when Zn is included, it is considered that Al ₁₂ Mg ₁₇ crystallizes as an Mg—Al—Zn compound.

On the other hand, in the case of the second solidification process, the primary crystal α is first crystallized from L and reaches the maximum point e ₄ as the temperature decreases, where it is considered that the reaction of Formula 1 (560 ° C.) occurs to complete the solidification. . On the other hand, in the case of the third solidification process, the primary crystal α is first crystallized from L and reaches the binary eutectic line (e ₄ -U ₁₃ line) as the temperature decreases. 560 ° C. to 483 ° C.), the ternary eutectic reaction (483 ° C.) shown in Formula 4 is generated at the E ₃ point, and solidification is considered to be completed.

<Formula 4>
L → α + λ + CuMg ₂

In Comparative Example 3, the amount of Al was small compared to Examples 3 and 6, and Al ₁₂ Mg ₁₇ or Mg—Al—Zn-based compounds that were thought to crystallize last were not observed. It is presumed that the third solidification process has been performed. As a result, it is considered that the compound λ was crystallized into a lamellar shape and connected in a network shape.

  In summary, the good creep resistance in each of the alloys of Examples 1 to 6 is considered to be due to the solid solution strengthening by Cu, Al and Zn and the dispersion strengthening by the Mg—Al—Cu based compound. . Therefore, an alloy design that can obtain a metal structure in which Mg—Al—Cu-based compounds are dispersed in the matrix magnesium (for example, setting of the amount of Cu and Al, and casting for undergoing the first solidification process described above) By setting the conditions and casting size, etc., a magnesium alloy having excellent creep resistance can be developed.

  Moreover, from the results of Comparative Examples 1 to 3, it can be confirmed that sufficient creep resistance is not exhibited when the amount of aluminum is less than 2% by mass. Therefore, the amount of aluminum is desirably 3% by mass or more. Moreover, from the results of Examples 1 to 6 showing good creep resistance, the amount of aluminum is more preferably about 4 to 9% by mass, and most preferably about 8% by mass.

  Here, since the compound λ and the Q phase are generated at the end of the solidification process, the locations where the Mg—Al—Cu compound exists in the metal structure are near the crystal grain boundary, between the dendrite cells, and the dendrite. It can be considered between arms. That is, a compound λ or Q phase is generated from the liquid L trapped between the branches of the primary crystal α (dendritic arm) or the petal portion (dendritic cell) grown in a dendritic form, or the primary crystals α are It is conceivable that the compound λ or Q phase is crystallized at the colliding solidification terminal part (that is, near the grain boundary). As a result, it is considered that the compound λ and the Q phase, that is, the Mg—Al—Cu compound are scattered in the metal structure.

By the way, Comparative Example 4 also showed excellent creep resistance comparable to AS21 (Conventional Example 2) (see FIG. 10). This is because, in Comparative Example 4, the compound λ is not formed in terms of composition, but in order to crystallize a compound (Mg ₂ Si or the like) derived from Si, excellent creep is caused by the dispersion strengthening of this compound. It is thought that the characteristics were obtained.

<Salt spray test>
Samples (cuboids) having a surface area of about 1000 mm ² were prepared by cutting and polishing the alloys of Examples 1 to 6, Comparative Examples 1 to 4, and Conventional Example 2, and a salt spray test was conducted in accordance with JISZ2371. I did it. Each sample was set in a salt spray tester so that the sample surface was not horizontal, and the test was performed for 24 hours using an aqueous solution containing 5 mass% NaCl under an atmosphere of 35 ° C.

  FIG. 17 shows the result of the salt spray test. It was confirmed that Examples 1, 3, 6 and Comparative Examples 3, 4 had corrosion resistance comparable to AS21 (Conventional Example 2). In addition, about AZ91D (conventional example 1), since the corrosion product adhered to the sample surface after the test and sufficient measurement could not be performed, it is not shown in FIG. 18 and 19 show the relationship between the Zn content and the Al content on the corrosion resistance, respectively.

FIG. 18 shows that the corrosion resistance of the magnesium alloy is improved by containing about 2% by mass of Zn. In particular, in the case of an alloy containing about 2% by mass of Al (Comparative Examples 1 to 3), the effect is remarkable, and in the case where the Zn content is about 0.5% by mass (that is, Comparative Example 1), the corrosion weight loss is reduced. Whereas it is about 80 (mg / cm ² day), the corrosion weight loss is reduced to about 30 (mg / cm ² day) when the Zn amount is about 2% by mass (ie, Comparative Example 3). . Moreover, as shown in FIG. 17, since good corrosion resistance was obtained in Comparative Example 4 containing about 5% by mass of Zn, setting the content of Zn to 2% by mass or more would bring about further improvement in corrosion resistance. is expected. Thus, Zn is an element that greatly contributes to improving corrosion resistance. On the other hand, FIG. 19 shows that an increase in the amount of Al improves the corrosion resistance. However, the effect of increasing the amount of Al is not recognized in an alloy having a Zn content of 2% by mass or more. That is, it is considered that the amount of Zn dominates the corrosion resistance of the magnesium alloy. It has been reported that the corrosion resistance of magnesium alloys is generally better as the crystal grains are finer. In the case of die casting, since the crystal grains are orders of magnitude smaller than that of the die casting material used in this example, it is considered that the corrosion resistance of any alloy shown in Table 1 is further improved by die casting.

<Creep test with different temperature conditions>
For Example 2 and Example 3 shown in Table 1 in which both the creep characteristics and the corrosion resistance are relatively balanced, the creep characteristics when the test temperature was increased to 175 ° C. and 200 ° C. were examined. The conditions other than the test temperature were the same as in Example 1 described above.

  20 and 21 show the creep curves of Example 2 and Example 3, respectively. Comparing these creep curves, it can be seen that the tendency of the creep strain to increase with the passage of time and the test temperature is substantially the same in both Example 2 and Example 3. In addition, the creep strain after the 100-hour test is 0.180% (150 ° C.), 0.623% (175 ° C.), and 2.25% (200 ° C.) in Example 2, and in Example 3, They are 0.227% (150 ° C.), 0.703% (175 ° C.), and 2.35% (200 ° C.), and there is no significant difference between Example 2 and Example 3. That is, in both Example 2 and Example 3, the creep resistance decreases as the temperature increases, but the degree of decrease is almost the same. Therefore, the creep curves almost overlap, and the creep resistance under the test conditions is the same as in Example 2. This was the same as in Example 3.

  22 and 23 show the relationship between time and creep strain rate in Example 2 and Example 3, respectively. In FIGS. 20 and 21, the vertical axis (creep strain rate [% / h]) and the horizontal axis (time [h]) are both logarithmic scales in order to clarify minute changes in the creep strain rate. In the creep test at 150 ° C. and 200 ° C., the creep strain rate is almost constant after 10 hours in both Example 2 and Example 3, and it is considered that steady creep has been reached. On the other hand, in the creep test at 175 ° C., in both Example 2 and Example 3, the section where the creep strain rate is constant is not clearly recognized on the graph so that the primary creep is not completely completed. appear. However, in the data at 175 ° C., the creep strain rate changed between 0.0035% / h to 0.00275% / h after 68 hours in Example 2, and 60 hours passed in Example 3. Later, the creep strain rate changes between 0.00550% / h and 0.00358% / h. Therefore, in the creep test at 175 ° C., it was considered that steady creep was reached after 68 hours in Example 2 and after 60 hours in Example 3. The average creep strain rate after the lapse of time considered to have reached steady creep was defined as the minimum creep rate, and the results were summarized in Table 2.

  Assuming that the elementary process of creep is due to a simple thermal activation process, the Arrhenius equation shown in Equation 5 holds between the creep rate ε ′ and the temperature T. Further, if both sides of Formula 5 are natural logarithms, Formula 6 is obtained, and if Formula 6 is a common logarithm, Formula 7 is obtained.

<Formula 5>
ε ′ = Aexp {−Q / (RT)}
<Formula 6>
lnε ′ = lnA−Q / (RT)
<Formula 7>
log ε ′ = log A−Q / (2.3RT)
However,
Q: Apparent activation energy R: Gas constant (8.314 [J / mol · K])

Next, Arrhenius plots for Example 2 and Example 3 were performed using the values in Table 2. FIG. 24 shows the Arrhenius plot results for Example 2, and FIG. 25 shows the Arrhenius plot results for Example 3. 24 and 25 show the temperature dependence of the minimum creep rate in Example 2 and Example 3. FIG. In FIGS. 24 and 25, the horizontal axis represents the reciprocal temperature (K ⁻¹ ), the vertical axis represents the minimum creep rate (s ⁻¹ ), and the vertical axis represents a logarithmic scale. As a result of the analysis, each point in FIG. 24 and FIG. 25 was able to return to the straight line shown in the figure. Here, the value on the horizontal axis in FIGS. 24 and 25 corresponds to (1 / T) in Equation 3, and the slope of the regression line corresponds to (Q / 2.3R) in Equation 3. Therefore, the apparent activation energy Q in the steady creep of Example 2 and Example 3 can be derived from the slope of each straight line and was 89.8 kJ / mol and 86.5 kJ / mol, respectively. Incidentally, the activation energy of self-diffusion of magnesium is 140 kJ / mol.

  As described above, it was found that the apparent activation energy Q in the steady creep of Example 2 and Example 3 was almost equal. Therefore, in Example 2 and Example 3, the governing factors of creep deformation are considered to be the same.

<Examples 7 to 9>
Creep test pieces of alloys having compositions shown in Table 3 (Examples 7 to 9) were produced by the same method as in Examples 1 to 6, and a creep test was performed under the same conditions as in Examples 1 to 6. Examples 7 to 9 shown in Table 3 have compositions in which the zinc content is increased as compared with Examples 1 to 6 described above.

  26 to 28 show the creep curves of the respective alloys obtained from the results of the creep test. In these drawings, reference numerals S7 to S9 denote Examples 7 to 9, respectively. As shown in these figures, the creep characteristics of Examples 7 to 9 are almost the same as those of Examples 2 and 3, and the effect of increasing the zinc content is not particularly observed. That is, the addition of zinc exceeding 2% by mass does not lead to an improvement in creep resistance, and from the viewpoint of strength, it is considered that only the concern of embrittlement due to an increase in the intermetallic compound phase causes no positive effect. .

<Examples 10 to 12>
Creep test pieces of alloys having compositions shown in Table 4 (Examples 10 to 12) were produced by the same method as in Examples 1 to 6, and a creep test was performed under the same conditions as in Examples 1 to 6. Example 10 shown in Table 4 has a composition in which the copper content is reduced as compared with Example 3 described above, and Examples 11 and 12 have compositions with an increased copper content as compared with Example 3 described above. ing.

  29 to 31 show the creep curves of the respective alloys obtained from the results of the creep test. In these drawings, reference numerals S10 to S12 denote Examples 10 to 12, respectively. As shown in these figures, the creep characteristics of Examples 10 to 12 are almost the same as those of Example 3. In addition, it was confirmed that any of Examples 10 to 12 had a metal structure close to that of Example 3. That is, it has a metal structure in which Mg—Al—Cu-based compound is dispersed in a matrix or pseudo-flaky in the matrix magnesium and crystallized.

In Example 10, since the amount of copper contained is small, the volume ratio of the Mg—Al—Cu-based compound in the metal structure is smaller than in Examples 11 and 12, but these are fine and uniform instead. A tendency to disperse was observed. In addition, it has been clarified that Mg—Al—Zn-based compounds are granulated and finely dispersed due to the presence of copper. Therefore, in Example 10, it is considered that the dispersion strengthening of the Mg—Al—Zn compound in addition to the dispersion strengthening of the Mg—Al—Cu compound contributes to the improvement of creep resistance. Incidentally, it believed to undergo E ₆ points shown in FIG. 16 in the solidification process of this case. Furthermore, even when the amount of copper was 0.1 mass% in the composition of Example 10, the same metal structure as Example 10 was obtained. Therefore, from the viewpoint of improving creep resistance, it is desirable to contain 0.1% by mass or more of copper.

  On the other hand, from the results of Examples 11 and 12, it was confirmed that no further improvement in creep resistance was observed even when copper was added in excess of 3% by mass. While the addition of a large amount of copper is concerned about embrittlement due to an increase in intermetallic compound phase, since the copper content is low, there is a concern that the yield strength and tensile strength at room temperature or high temperature are lowered. That is, an increase in the copper content has the advantage of improving the temperature of the hot water whilst there is a risk of embrittlement of the alloy. On the other hand, a decrease in the copper content reduces the risk of alloy embrittlement while maintaining the yield strength. And there is a risk of a decrease in tensile strength. In view of the above, the copper content is more preferably in the range of 1 to 5 mass%, and most preferably about 3 mass%.

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the gist of the present invention. For example, the heat-resistant magnesium alloy preferably contains zinc and beryllium as described above, but in some cases, one or both of these may not be contained. In addition, the heat-resistant magnesium alloy according to the present invention may contain elements other than Mg, Al, Cu, Zn, and Be in some cases. For example, an impurity element that does not affect the “metal structure in which the Mg—Al—Cu compound is interspersed in the matrix magnesium” may be included.

  As described above, the heat-resistant magnesium alloy according to the present invention is remarkably superior in high-temperature creep characteristics as compared with Mg-Al-Zn-Mn alloys that are generally widely used in practice. Moreover, it can be manufactured at a lower cost than conventional heat-resistant magnesium alloys to which rare earth elements are added. That is, even if it does not contain a rare earth element, the creep resistance at around 150 ° C. is superior to that of a heat resistant magnesium alloy containing a rare earth element due to the relatively inexpensive interaction between Al and Cu, and the heat resistant magnesium alloy contains a rare earth element. Since it is more fluid and suitable for casting, it is useful for constructing parts that reach a high temperature around 150 ° C., such as engine blocks and transmission cases of automobile engines. Therefore, it is possible to further reduce the weight of automobiles and the like, and not only to reduce transportation costs by improving fuel efficiency, but also to reduce energy consumption and adverse effects on the environment. It can also be used as a product.

It is a figure which shows the creep characteristic of one Example (Example 1) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 2) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 3) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 4) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 5) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 6) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of a 1st comparative example. It is a figure which shows the creep characteristic of the 2nd comparative example. It is a figure which shows the creep characteristic of the 3rd comparative example. It is a figure which shows the creep characteristic of the 4th comparative example. (A) shows the composition image of the alloy structure of one example (Example 3) of the heat-resistant magnesium alloy of the present invention, and (B) shows the alloy of another example (Example 6) of the heat-resistant magnesium alloy of the present invention. The composition image of a structure is shown, (C) shows the composition image of the alloy structure of a comparative example. (A) shows an X-ray image of the alloy structure of one embodiment (Example 3) of the heat-resistant magnesium alloy of the present invention, (B) shows the distribution of aluminum elements corresponding to FIG. ) Shows the distribution of the copper element corresponding to (A) in the figure, and (D) shows the distribution of the zinc element corresponding to (A) in the figure. (A) shows the X-ray image of the alloy structure of another example (Example 6) of the heat-resistant magnesium alloy of the present invention, (B) shows the distribution of aluminum elements corresponding to FIG. (C) shows the distribution of the copper element corresponding to (A) in the figure, and (D) shows the distribution of the zinc element corresponding to (A) in the figure. An X-ray image of an alloy structure of a comparative example is shown, (B) shows the distribution of aluminum element corresponding to FIG. (A), (C) shows the distribution of copper element corresponding to FIG. (D) shows the distribution of zinc element corresponding to FIG. (A) shows the X-ray diffraction pattern of one example (Example 3) of the heat-resistant magnesium alloy of the present invention, and (B) shows the X-ray of another example (Example 6) of the heat-resistant magnesium alloy of the present invention. A diffraction pattern is shown, and (C) shows an X-ray diffraction pattern of a comparative example. An example of an equilibrium diagram of magnesium, aluminum, and copper is shown. It is a figure which shows an example of the salt spray test result of a magnesium alloy. It is a figure which shows the relationship between the zinc content in a magnesium alloy, and corrosion resistance. It is a figure which shows the relationship between the aluminum content in a magnesium alloy, and corrosion resistance. It is a figure which shows the creep characteristic (each case of test temperature 150 degreeC, 175 degreeC, and 200 degreeC) of one Example (Example 2) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic (each case of test temperature 150 degreeC, 175 degreeC, and 200 degreeC) of the other Example (Example 3) of the heat-resistant magnesium alloy of this invention. FIG. 21 is a diagram corresponding to FIG. 20 and showing the relationship between time [h] and creep strain rate [% / h]. FIG. 22 is a diagram corresponding to FIG. 21 and showing the relationship between time [h] and creep strain rate [% / h]. FIG. 23 corresponds to FIG. 22 and represents the temperature dependence of the minimum creep rate. FIG. 24 corresponds to FIG. 23 and represents the temperature dependence of the minimum creep rate. It is a figure which shows the creep characteristic of the other Example (Example 7) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 8) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 9) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 10) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 11) of the heat-resistant magnesium alloy of this invention. It is a figure which shows the creep characteristic of the other Example (Example 12) of the heat-resistant magnesium alloy of this invention.

Claims (6)

  A heat-resistant magnesium alloy comprising an additive alloy element group (excluding rare earth elements) containing at least 3% by mass of aluminum and 0.1% by mass of copper, and the balance magnesium.   The heat-resistant magnesium alloy according to claim 1, wherein the aluminum content is 4 to 9 mass%.   The heat-resistant magnesium alloy according to claim 1 or 2, wherein the copper content is 1 to 5 mass%.   The heat-resistant magnesium alloy according to any one of claims 1 to 3, wherein the additive alloy element group contains 4 mass% or less of zinc.   The heat-resistant magnesium alloy according to any one of claims 1 to 4, wherein the additive alloy element group contains 0.001 to 0.01% by mass of beryllium.   A heat-resistant magnesium alloy having a metal structure in which a magnesium-aluminum-copper-based compound is dispersed in the form of magnesium or aluminum in the matrix magnesium and crystallized.