Patent Publication Number: US-2010116378-A1

Title: Heat-resistant magnesium alloy

Description:
TECHNICAL FIELD 
     The present invention is one which relates to a heat-resistant magnesium alloy that are capable of withstanding services under high loads and at high temperatures. 
     BACKGROUND ART 
     Magnesiumalloy, which is much more lightweight than aluminum alloy is, is about to come to be used widely for aircraft material, vehicle material, and the like, from the viewpoint of weight saving. However, in magnesium alloy, since the strength and heat resistance are not sufficient depending on applications, further improvement of the characteristics has been sought. 
     Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-162,090, and in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-232,060, there are disclosed magnesium alloys in which calcium (Ca) and aluminum (Al) are contained in adequate amounts. In these literatures, since Ca—Al compounds and Mg—Ca compounds crystallize or precipitate at the grain boundaries between the Mg crystalline grains in the magnesium alloys, the movements of dislocations are held back. As a result, the magnesium alloys undergo creep deformations less even in high-temperature regions, and therefore exhibit good heat resistance. Further, in the aforementioned magnesium alloys, Mn is solidified into the Mg crystalline grains, and thereby the magnesium alloys are subjected to solid-solution strengthening. 
     DISCLOSURE OF THE INVENTION 
     The metallic structure of alloy affects its characteristics greatly. Accordingly, in order to obtain a magnesium alloy that possesses strength and creep resistance being sufficient for services at high temperatures, it is necessary to adapt the types and amounts of additive elements into adequate ones in order to control the metallic structure. 
     It is an object of the present invention to provide a magnesium alloy, both of whose crystalline grains&#39; interior and crystalline grain boundaries are strengthened and which therefore exhibits good heat resistance, by means of controlling the metallic structure of the magnesium alloy using adequate alloying elements. 
     Specifically, a heat-resistant magnesium alloy according to the present invention is characterized in that it includes: 
     magnesium (Mg), a major component; 
     a first alloying element “M 1 ” being any one or more members that are selected from the group consisting of aluminum (Al) and nickel (Ni); 
     a second alloying element “M 2 ” being any one or more members that are selected from the group consisting of manganese (Mn), barium (Ba), chromium (Cr) and iron (Fe); and 
     calcium (Ca); and 
     it has a metallic structure including: 
     Mg crystalline grains; 
     plate-shaped precipitated substances being precipitated within grains of the Mg crystalline grains; and 
     grain-boundary crystallized substances being crystallized at grain boundaries between the Mg crystalline grains to form networks that are continuous microscopically. 
     Note that, in the present description, the “networks that are continuous microscopically” take on network structures (three-dimensionally mesh structures) macroscopically, and are states in which crystals exist continuously even inside the networks (see  FIG. 2 ). Therefore, the following are not involved: discontinuous states whose interior is constituted of small crystals, even though they take on network structures (see  FIG. 3 ). 
     Since the heat-resistant magnesium alloy according to the present invention includes the second alloying element “M 2 ,” it has the plate-shaped precipitated substances within the grains of the Mg crystalline grains, and the grain-boundary crystallized substances, which form the networks that are continuous microscopically, at the grain boundaries, as will be detailed later. Since the plate-shaped precipitated substances exist within the Mg crystalline grains, the movements of dislocation within the Mg crystalline grains are prevented, and accordingly it becomes less likely to deform. Moreover, since the grain-boundary crystallized substances, which form the networks, are present continuously microscopically at the grain boundaries between the Mg crystalline grains, the strength at the grain boundaries improves. As a result, the heat-resistant magnesium alloy according to the present invention exhibits high mechanical characteristics even in high-temperature regions. That is, in the magnesium alloy according to the present invention, the mechanical characteristics in high-temperature regions are improved by strengthening it not only within the Mg crystalline grains&#39; granular interior but also at the grain boundaries between the Mg crystalline grains. 
     Said precipitated substances can desirably comprise a Laves-phase compound with type-“C15” crystalline structure. Moreover, said precipitated substances can desirably be precipitated parallel to the {001} plane of Mg crystal. 
     Said grain-boundary crystallized substances, which form the networks that are continuous microscopically, can desirably comprise an Mg-“M 1 ”-Ca-system compound. Moreover, said grain-boundary crystallized substances can desirably comprise a mixed-crystal phase of a Laves-phase compound with type-“C14” crystalline structure and a Laves-phase compound with type-“C36” crystalline structure; on this occasion, it is allowable that said mixed-crystal structure can include the type-“C14” crystalline structure more than the type-“C36” crystalline structure. 
     When the precipitated substances are precipitated parallel to the {001} plane of Mg crystal, the movements of dislocation on the sliding plane of hexagonal Mg crystal are suppressed. When the grain-boundary crystallized substances comprise a mixed-crystal phase of a Laves-phase compound with type-“C14” crystalline structure and a Laves-phase compound with type-“C36” crystalline structure, compounds, which constitute the networks, do not undergo any phase separation, and consequently turn into single crystals virtually in appearance (see  FIG. 4 ), the area of the crystalline-grain boundaries between crystalline grains that constitute the networks, and the number of the crystalline grains that constitute the networks become minimum. 
     Note that the aforementioned “type-‘C14’,” “type-‘C15’,” and “type-‘C36’” are codes in accordance with a magazine, “STRUKTURBERICHTE,” and express three similar basic crystalline structures that are represented by MgZn 2 , MgCu 2  and MgNi 2  of the Laves phases. 
     Further, it is desirable that it can have fine particles that include said second alloying element “M 2 ” within said Mg crystalline grains. 
     The heat-resistant magnesium alloy according to the present invention can preferably include: Ca in an amount of from 2% by mass or more to 4% by mass or less; said first alloying element “M 1 ” in an amount of from 0.9 or more to 1.1 or less by mass ratio with respect to Ca (“M 1 ”/Ca); said second alloying element “M 2 ” in an amount of from 0.3% by mass or more to 0.6% by mass or less; and the balance comprising Mg and inevitable impurities; when the entirety is taken as 100% by mass. 
     Alternatively, the heat-resistant magnesium alloy according to the present invention can preferably include: Ca in an amount of from 1.235 atomic % or more to 2.470 atomic % or less; said first alloying element “M 1 ” in an amount of from 1.34 or more to 1.63 or less by atomic ratio with respect to Ca (“M 1 ”/Ca); said second alloying element “M 2 ” in an amount of from 0.13 atomic % or more to 0.27 atomic % or less; and the balance comprising Mg and inevitable impurities; when the entirety is taken as 100 atomic %. 
     Heat-resistance magnesium alloys, which possess metallic structures that are desirable from the viewpoints of mechanical characteristics at high temperatures, are obtainable by setting the content proportions of the first alloying element, second alloying element and Ca that the heat-resistance magnesium alloy according to the present invention contains to appropriate ranges. 
     Note that the “heat resistance” being referred to in the present specification is one that is evaluated by mechanical properties of magnesium alloy in high-temperature atmospheres (creep characteristics or high-temperature strengths that are determined by means of stress relaxation tests or axial-force retention tests, for instance). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a metallic-structure photograph in which a cross section of a test specimen being labeled # 01  was observed with a metallographic microscope. 
         FIG. 2  is a metallic-structure photograph in which an observational sample being labeled # 01  was observed with a transmission electron microscope (or TEM). 
         FIG. 3  is a metallic-structure photograph in which an observational sample being labeled #C 1  was observed with a TEM. 
         FIG. 4  is a dark-field scanning-transmission-electron-microscope (or DF-STEM) image on the observational sample being labeled # 01 . 
         FIG. 5  is a DF-STEM image on the observational sample being labeled #C 1 . 
         FIG. 6  is a TEM image on the observational sample being labeled # 01 , and an electron diffraction pattern thereof (the incident direction being &lt;110&gt;). 
         FIG. 7  is another TEM image on the observational sample being labeled # 01 , and another electron diffraction pattern thereof (the incident direction being &lt;111&gt;). 
         FIG. 8  is a TEM image on the observational sample being labeled #C 1 , and an electron diffraction pattern thereof (the incident direction being &lt;111&gt;). 
         FIG. 9  is a DF-STEM image in which the interior of Mg crystalline grains in the observational sample being labeled # 01  was observed. 
     
    
    
     Note that “# 01 ” and “#C 1 ” are codes for distinguishing magnesium alloys whose compositions differed in later-described examples. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the best mode for carrying out the heat-resistant magnesium alloy according to the present invention (hereinafter being abbreviated to as “magnesium alloy”) will be explained. 
     The magnesium alloy according to the present invention includes: magnesium (Mg), a major component; a first alloying element “M 1 ”; a second alloying element “M 2 ”; and calcium (Ca); and it has a metallic structure that includes: Mg crystalline grains; plate-shaped precipitated substances being precipitated within grains of Mg crystalline grains; and grain-boundary crystallized substances being crystallized at grain boundaries between the Mg crystalline grains to form networks that are continuous microscopically. 
     In the magnesium-alloy according to the present invention, the plate-shaped precipitated substances are present within the Mg crystalline grains. The plate-shaped precipitated substances prevent the movements of dislocation within the Mg crystalline grains. The deformations of crystal occur because the dislocation moves on sliding plane. Therefore, it is allowable that they can be plate-shaped precipitated substances that are parallel to the “c” plane of hexagonal Mg crystal, that is, to the {001} plane of Mg crystal. Note that the plate-shaped precipitated substances come to exhibit a plate thickness of 2-20 nm, and that the thicker the plate thickness is the more mechanical characteristics improve. 
     Moreover, it is allowable that the plate-shaped precipitated substances can comprise a Laves-phase compound with type-“C 15 ” crystalline structure. The “c” plane of Mg crystal, and the {111} plane of “C 15 ” structure are likely to form interfaces that are stable to each other crystallographically, and therefore it is possible to predict that the formation of the plate-shaped precipitated substances is facilitated. It is allowable that compounds constituting the precipitated substances that have such crystalline structures can be “M 1 ”-Ca-system compounds and/or Mg-“M 1 ”-Ca-system compounds. 
     It is allowable as well that the magnesium alloy according to the present invention can have fine particles within the granular interior of the Mg crystalline grains. The fine particles are present within the Mg crystalline grains, and most of them exist around the plate-shaped precipitated substances. It is believed that, although these fine particles are present within the Mg crystalline grains, they are not those which contribute to the improvement of strength inside the Mg crystalline grains. However, the presence of the fine particles is related to the generation of the precipitated substances (will be described later), and the fine particles are fine particles, which include “M 2 ,” like “M 1 ”-“M 2 ”-system compounds, for instance. Note that the fine particles are sphere-shaped ones substantially and exhibit particle diameters of 10-15 nm approximately. 
     In the magnesium alloy according to the present invention, the grain-boundary crystallized substances, which form networks that are continuous microscopically, are crystallized at the grain boundaries between the Mg crystalline grains to be present therein. For example, even in compositions being made by excluding the second alloying element “M 2 ” from that of the magnesium alloy according the present invention, grain-boundary crystallized substances might be crystallized at the grain boundaries between the Mg crystalline grains, and additionally might form networks. However, in magnesium alloys that do not include any “M 2 ,” it has been understood that no microscopic continuity can be seen in the grain-boundary crystallized substances that form the networks. On the other hand, in the magnesium alloy according to the present invention, because of including “M 2 ,” the grain-boundary crystallized substances form the networks that are continuous microscopically. Because of the fact that the networks are continuous microscopically, the crystalline grain-boundary area of compounds that constitute the networks, and the number of crystalline grains are reduced greatly. As a result, the grain-boundary strength is improved, and is then strengthened. On this occasion, it is desirable that the networks of the grain-boundary crystallized substances can cover 70% or more of the grain boundaries between the Mg crystalline grains that are observed linearly in a regional cross section with 400 μm×600 μm approximately in the magnesium alloy (this value will be abbreviated to as a “covering ratio of networks”). 
     Moreover, it is allowable that the grain-boundary crystallized substances can comprise a mixed-crystal phase of a Laves-phase compound with type-“C14” crystalline structure and a Laves-phase compound with type-“C36” crystalline structure. The type-“C14” crystalline structure, and the type-“C36” crystalline structure are desirable, because they are hexagonal ones to each other and are likely to form mixed-phases. Since the Laves-phase compounds in the mixed-crystal phase come to be approximated to the single crystals extremely, the grain-boundary crystallized substances are continuous microscopically; and accordingly the crystalline-grain boundary area of crystalline grains, that is, compounds that constitute the networks, and the number of the crystalline grains that constitute the networks become minimum. 
     Moreover, it is desirable that the grain-boundary crystallized substances can comprise an Mg-“M 1 ”-Ca-system compound. Since Mg 2 Ca has a type-“C14” crystalline structure, it is assumed that a mixed-crystal phase of a type-“C14” crystalline structure and a type-“C36” crystalline structure is formed by solidifying “M 1 ” into Mg 2 Ca. In this instance, it is allowable that the mixed-crystal phase can include the type-“C14” crystalline structure more than the type-“C36” crystalline structure. 
     The magnesium alloy according to the present invention that has the metallic structure as described above includes: magnesium (Mg), a major component; a first alloying element “M 1 ”; a second alloying element “M 2 ”; and calcium (Ca). 
     For the first alloying element “M 1 ,” it is possible to use at least one member that is selected from the group consisting of aluminum (Al) and nickel (Ni). Although not only Al but also Ni are elements that react with Ca to form compounds and take on a type-“C15” Laves structure, a mixed-crystal phase of a type-“C14” Laves structure and a type-“C36” Laves structure is formed under such a condition that Mg 2 Ca, which takes on a type-“C14” Laves structure, is dominant, because Al and/or Ni are dissolved into Mg 2 Ca For the second alloying element “M 2 ,” it is possible to use at least one member that is selected from the group consisting of manganese (Mn), barium (Ba), chromium (Cr) andiron (Fe). The reason why it is possible to use these elements as “M 2 ” can be explained by means of structural changes of the magnesium alloy according to the present invention in the cooling process. 
     It was understood from the cooling curve when casting a cast product comprising the magnesium alloy according to the present invention by a general solidifying process (air cooling) that three temperature-halting points (the respective temperatures are labeled “T 1 ,” “T 2 ” and “T 3 ”; and “T 1 ”&gt;“T 3 ,” and “T 2 ”&gt;“T 3 ”) appear. When the molten-metal temperature reaches a primary-crystal temperature (i.e., a temperature at which the solidification begins: “T 1 ”=from 600° C. or more to 620° C. or less), primary-crystal Mg crystallized. Moreover, when it reaches “T 2 ,” it is predicted that “M 1 ” and “M 2 ” react to generate fine particles of “M 1 ”-“M 2 ”-system compounds, high-temperature-generated compounds. Next, when it reaches the eutectic temperature “T 3 ,” the grain-boundary crystallized substances, which form the networks, crystallize along with eutectic Mg. However, as a result of Carrying out an elementary analysis on the fine particles of the resulting cast product, it was found that “M 2 ” was included therein more than the theoretical value. Specifically, in regions of low temperatures that are much lower than “T 3 ,” it is possible to predict that “M 1 ” is spewed out from the fine particles (or “M 1 ”-“M 2 ”-system compounds), and that the spewed-out “M 1 ” forms compounds with Ca and then precipitates being accompanied by the agglomeration of Ca that dissolves into the Mg crystalline grains. 
     Therefore, it is necessary that not only the second alloying element “M 2 ” can react with the first alloying element “M 1 ” at high temperatures that are higher than “T 3 ” but also it can be less likely to dissolve into Mg. Because of such reasoning, it is possible to use at least one member that is selected from the group consisting of manganese (Mn), barium (Ba), chromium (Cr) and iron (Fe), especially from among the transition elements. These elements exhibit atomic radii being comparable with each other, and take on similar crystalline structures; further they react with “M 1 ” to generate the compounds in comparatively high-temperature regions, to be concrete, between “T 1 ” and “T 3 ” alone. 
     Note that the magnesium alloy according to the present invention includes at least one species of the aforementioned first alloying elements and second alloying elements, respectively. It is also allowable that it can include one species of them as for the first element and second element, respectively; and it is even allowable that it can include plural species of them as for either one of them or both of them. 
     It is preferable that the magnesium alloy according to the present invention can include: Ca in an amount of from 2% by mass or more to 4% by mass or less; said first alloying element “M 1 ” in an amount of from 0.9 or more to 1.1 or less by mass ratio with respect to Ca (“M 1 ”/Ca); said second alloying element “M 2 ” in an amount of from 0.3% by mass or more to 0.6% by mass or less; and the balance comprising Mg and inevitable impurities; when the entirety is taken as 100% by mass. Alternatively, it is preferable that the magnesium alloy according to the present invention can include: Ca in an amount of from 1.235 atomic % or more to 2.470 atomic % or less; said first alloying element “M 1 ” in an amount of from 1.34 or more to 1.63 or less by atomic ratio with respect to Ca (“M 1 ”/Ca); said second alloying element “M 2 ” in an amount of from 0.13 atomic % or more to 0.27 atomic % or less; and the balance comprising Mg and inevitable impurities; when the entirety is taken as 100 atomic % 
     When “M 1 ”/Ca is less than 0.9 by mass ratio (namely, being less than 1.34 by atomic ratio), it is not preferable because the content of Ca is so great that the castability deteriorates. On the other hand, when “M 1 ”/Ca surpasses 1.1 by mass ratio (namely, surpassing 1.63 by atomic ratio), it is not preferable because the grain-boundary crystallized substances are less likely to turn into a mixed-crystal phase, and because crystalline grains, which are constituted of type-“C36” Laves structure alone, are likely to be formed so that they undergo phase separation. Further, when type-“C36” crystalline structure is exposed to high temperatures, it is likely to undergo phase transition to type-“C15” crystalline structure (Scripta Materialia 51 (2004) 1005-1010). Since the type-“C15” crystalline structure is likely to undergo massive agglomeration in high-temperature regions, and since it does not form the networks of the crystallized substances, networks which are continuous microscopically, the mechanical characteristics at high temperatures lower remarkably. A more preferable “M 1 ”/Ca value can be from 0.95 or more to 1.05 or less (namely, being 1.42-1.56 by atomic ratio). 
     When the content proportion of the second alloying element “M 2 ” is less than 0.3% by mass (namely, 0.13 atomic %), it is not preferable because it is impossible to retain the “M 1 ,” which constitutes the precipitated substances in the cooling step (or solidifying step), as compounds so that the precipitated substances are not precipitated sufficiently. Moreover, it is not preferable because many “M 1 ” reside without ever combining with “M 2 ” so that crystalline grains, which possess type-“C36” Laves structure alone that does not take on any mixed-crystal structure as the grain-boundary crystallized substances, are likely to be formed, and so that they undergo phase separation. On the other hand, when the content proportion of “M 2 ” surpasses 0.6% by mass (namely, 0.27 atomic %), it is not preferable because compounds that contain “M 2 ” are precipitated within the grain-boundary crystallized substances so that they might possibly cut off the networks. The lower limit of a more preferable content proportion of “M 2 ” can be 0.34% by mass (namely, 0.15 atomic %) or more. The upper limit of a more preferable content proportion of “M 2 ” can be 0.55% by mass (namely, 0.25 atomic %) or less, and can much more preferably be 0.5% by mass (namely, 0.23 atomic %) or less. 
     Ca is an element that forms type-“C14” and type-“C36” Laves structures together with Mg. When a content proportion of Ca is less than 2% by mass (namely, 1.235 atomic %), it is not preferable because the precipitated substances and grain-boundary crystallized substances are not generated sufficiently so that the effect of improving the heat-resistant characteristic is not sufficient. On the other hand, when the content proportion of Ca surpasses 4% by mass (namely, 2.470 atomic %), it is not preferable because the generation amounts of the precipitated substances and grain-boundary crystallized substances become too great so that problems might arise in post-processes. A more preferable content proportion of Ca can be from 2.5% by mass or more to 3.5% by mass or less (namely, from 1.54 atomic % or more to 2.16 atomic % or less). 
     The magnesium alloy according to the present invention is not limited to those made by ordinary gravity casting and pressure casting, but can even be those made by die-cast casting. Moreover, even the casting mold being utilized for the casting does not matter if it is sand molds, metallic molds, and the like. Although even the solidification rate in the solidifying step is not limited in particular, it is allowable to let it stand to cool in air atmosphere. 
     Beginning with the fields of space, military and aviation, applications of the magnesium alloy according to the present invention can be extended to various fields, such as automobiles and home electric instruments. In reality, however, it is all the more suitable that, taking advantage of its heat resistance, the magnesium alloy according to the present invention can be utilized in products being utilized in high-temperature environments, such as engines, transmissions, compressors for air conditioner or their related products that are put in place within the engine room of automobile, for instance. To be concrete, the following can be given: cylinder heads, cylinder blocks and oil pans of internal combustion engine; impellers for turbocharger of internal combustion engine, transmission cases being used for automobile and the like, and so forth. 
     So far, the embodiment modes of the heat-resistant magnesium alloy according to the present invention have been explained, however, the present invention is not one which is limited to the aforementioned embodiment modes. It can be conducted in various modes to which modifications, improvements, and the like, which one of ordinary skill in the art can carry out, are performed, within a range not departing from the scope of the present invention. 
     Hereinafter, while giving specific examples, the present invention will be explained in detail. 
     Two kinds of test specimens whose contents (or addition amounts) of Al, Ca and Mn in magnesium alloys were varied were made, and then not only their metallic structures were observed but also a stress relaxation test was carried out. 
     (Making of Test Specimens) 
     A chloride-system flux was coated onto the inner surface of a crucible being made of iron that had been preheated within an electric furnace, and then a weighed pure magnesium base metal, pure Al, and an Mg—Mn alloy, if needed, were charged into it and were then melted. Further, weighed Ca was added into this molten metal that was held at 750° C. (i.e., a molten-metal preparing step). 
     After fully stirring this molten metal to melt the raw materials completely, it was held calmly at the same temperature for a while. During this melting operation, a mixture gas of carbon dioxide gas and SF 6  gas was blown onto the molten metal&#39;s surface in order to prevent the burning of Mg, and the flux was sprayed whenever being deemed appropriate. 
     The thus obtained various alloy molten metals were poured into a metallic mold with a predetermined configuration (i.e., a molten-metal pouring step), and were then solidified in air atmosphere (i.e., a solidifying step). Thus, test specimens with 30 mm×300 mm×40 mm were made by means of gravity casting. The obtained test specimens were labeled # 01  (an example including Mn), and #C 1  (a comparative example not including Mn). The chemical compositions of the respective test specimens were specified in Table 1. Note that, in the magnesium-alloy compositions being given in Table 1, the balances are Mg, respectively. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Magnesium-alloy 
                 Magnesium-alloy 
               
               
                   
                 Composition 
                 Composition 
               
               
                   
                 (% by mass) 
                 (atomic %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Test Specimen 
                 Al 
                 Ca 
                 Mn 
                 Al 
                 Ca 
                 Mn 
                 Al/Ca 
               
               
                   
               
               
                 #01 
                 3 
                 3 
                 0.5 
                 2.75 
                 1.85 
                 0.23 
                 1.49 
               
               
                 #C1 
                 3 
                 3 
                 — 
                 2.74 
                 1.85 
                 — 
                 1.48 
               
               
                   
               
            
           
         
       
     
     Note that, in Table 1, “% by mass” and “atomic %” are used as the units for the alloy compositions being labeled # 01  and #C1. Here, the values that used the unit, “% by mass,” were the charged quantities in the molten-metal preparing step, and those values were converted into the “atomic %.” 
     (Observation on Metallic Structure) 
     Test specimens # 01  and #C 1  were observed with a metallographic microscope or transmission electron microscope (TEM). 
       FIG. 1  is a metallic-structure photograph in which a cross section of the test specimen being labeled # 01  was observed with a metallographic microscope. The Mg crystalline grains (bright parts), and the grain-boundary crystallized substances (black parts) that existed like networks at the grain boundaries between the Mg crystalline grains were observed. Note that, although not being shown diagrammatically, a metallic-structure photograph being similar to  FIG. 1  was obtained even when a cross section of the test specimen being labeled #C 1  was observed. That is, in either one of the test specimens, network-shaped grain-boundary crystallized substances were observed macroscopically. 
     Next, in order to observe micro-fine constructions of the metallic structures, the respective test specimens were adapted into a flake-shaped observational sample, respectively, and were then observed using a TEM. 
       FIG. 2  and  FIG. 3  are metallic-structure photographs in which the observational samples being labeled # 01  and #C 1  were observed with the TEM. In both of them, crystalline grain boundaries in which two or more crystalline grains of primary-crystal Mg neighbor to each other were observed. In  FIG. 2  (# 01 ), the grain-boundary crystallized substances (black parts) were grown as a lamellar shape, and were continuous. In  FIG. 3  (#C 1 ), the grain-boundary crystallized substances were interrupted partially, and were discontinuous. Note that the covering ratio of networks in # 01  was about 90%. 
     Moreover,  FIG. 4  and  FIG. 5  are a dark-field scanning-transmission-electron-microscope (DF-STEM) images in which the grain-boundary crystallized substances in the observational samples according to # 01  and #C 1  were observed, respectively. In the test specimen being labeled # 01 , no phase separation was seen as shown in  FIG. 4 ; whereas, in the test specimen being labeled #C 1 , phase separation was seen as shown in  FIG. 5 . When an elementary mapping was carried out with respect to the DF-STEM images of  FIG. 4  and  FIG. 5  by means of energy-dispersion-type X-ray spectroscopy (EDX), Mg, Al and Ca were distributed uniformly in  FIG. 4  (# 01 ); whereas the concentration of Al was high in the crystalline grains, which were agglomerated granularly to undergo phase separation, in  FIG. 5  (#C 1 ). And, the electron diffraction of type-“C36” crystalline structure was obtained from the crystalline grains with high Al concentrations. On the other hand, the electron-diffraction pattern of type-“C14” crystalline structure was obtained mainly from the crystals in which each of Mg, Al and Ca was distributed uniformly in  FIG. 4  and  FIG. 5 ; however, the diffraction spot of type-“C36” crystalline structure, which coincided with the twofold cycle to type-“C14” crystalline structure, appeared partially, even though they did not undergo any phase separation. Specifically, it was understood that the crystals in which Mg, Al and Ca were distributed uniformly were a mixed-crystal phase of type-“C14” crystalline structure and type-“C36” crystalline structure, and were virtually single crystals visually. Therefore, in the test specimens being labeled # 01 , the grain-boundary crystallized substances forming the networks were continuous microscopically, and they virtually turned into single crystals visually. On the contrary, in the test specimen being labeled #C 1 , although the grain-boundary crystallized substances formed networks macroscopically, the networks were discontinuous microscopically, and the Laves-phase compounds, which comprised type-“C36” crystalline structure alone and had undergone phase separation, were present. 
     Note that, on a magnesium alloy in which the Mn content in # 01  was changed to 0.2% by mass (namely, 0.09 atomic %), the grain-boundary crystallized substances were observed with the TEM, though not being shown diagrammatically. According to the obtained DF-STEM image, the massive agglomerations that were seen in #C 1  ( FIG. 5 ) decreased so that compounds extending as strip shapes came to account for it greatly when the Mn amount increased; however, it was understood that no continuity that was observed in # 01  ( FIG. 4 ) was seen when the Mn content was 0.2% by mass. 
       FIG. 6  and  FIG. 7  are TEM images on Test Specimen # 01 , and  FIG. 8  is a TEM image on Test Specimen #C 1 . In  FIG. 6 , the interior of the Mg crystalline grains was observed while setting the incident direction to &lt;110&gt;, whereas it was observed while setting the incident angle to &lt;111&gt; in  FIG. 7  and  FIG. 8 . In  FIG. 6  (# 01 ), streak-shaped precipitated substances that were parallel to the {001} plane were seen. And, from  FIG. 7  in which the observation was carried out at the same position as that in  FIG. 6  but while inclining the incident direction, the precipitated substances were found to have plate shapes that were parallel to the {001} plane. When the STEM-EDX analysis was carried out onto these plate-shaped precipitated substances, Al and Ca were detected mainly. Moreover, from the plate-shaped precipitated substances, the electron-diffraction pattern of type-“C15” crystalline structure that coincided with Al 2 Ca was obtained. 
     On the contrary, in  FIG. 8  (#C 1 ), no clear streak-shaped contrast was seen. Note that, even when the same STEM-EDX analysis as that was done for # 01  was carried out, Al and Ca were hardly detected. Therefore, the precipitated substances hardly existed in the test specimen being labeled #C 1 . 
       FIG. 9  is a DF-STEM image in which the interior of the Mg crystalline grains in the observational sample being labeled # 01  was observed. A plurality of fine particles were seen around the plate-shaped precipitated substances. When an elementary analysis was carried out onto the fine particles (e.g., “B” in  FIG. 9 ), Mn was detected. Note that no Mn was detected even when the plate-shaped precipitated substances (e.g., “A” in  FIG. 9 ) were analyzed. 
     (Stress Relaxation Test) 
     A stress relaxation test was carried out not only onto Test Specimens # 01  and #C 1  given in Table 1 but also onto AXE662, AE42 and AZ91D (all as per ASTM standards), thereby examining the magnesium alloys&#39; heat resistance (e.g., creep resistance). In the stress relaxation test, a process was measured, process in which the stress, which was needed when a load was applied to a test specimen until it exhibited a predetermined deformation magnitude, decreased with time in the course of testing time. To be concrete, in 150° C. air atmosphere, a compression stress of 100 MPa was loaded to the test specimens, and then the compression stress was lowered in agreement with the elapse of time so as to keep the displacements of the test specimens at that time constant. 
     In Table 2 and Table 3, the respective test specimens&#39; alloy compositions, and their stresses after 40 hours since the stress relaxation test started are given. Note that, in the magnesium-alloy compositions being given in Table 2 and Table 3, the balances are Mg, respectively. Moreover, “RE” is a mish metal. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                 Magnesium-alloy 
                   
               
               
                   
                 Test 
                 Composition (% by mass) 
                 Stress 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Specimen 
                 Al 
                 Zn 
                 Ca 
                 RE 
                 Mn 
                 (MPa) 
               
               
                   
                   
               
               
                   
                 #01 
                 3 
                 — 
                 3 
                 — 
                 0.5 
                 92 
               
               
                   
                 #C1 
                 3 
                 — 
                 3 
                 — 
                 — 
                 86 
               
               
                   
                 AXE662 
                 6 
                 — 
                 6 
                 2 
                 — 
                 83 
               
               
                   
                 AE42 
                 4 
                 — 
                 — 
                 2 
                 — 
                 74 
               
               
                   
                 AZ91D 
                 9 
                 1 
                 — 
                 — 
                 — 
                 68 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Magnesium-alloy 
                   
               
               
                 Test 
                 Composition (atomic %) 
                 Stress 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Specimen 
                 Al 
                 Zn 
                 Ca 
                 RE 
                 Mn 
                 (MPa) 
               
               
                   
               
               
                 #01 
                 2.75 
                 — 
                 1.85 
                 — 
                 0.23 
                 92 
               
               
                 #C1 
                 2.74 
                 — 
                 1.85 
                 — 
                 — 
                 86 
               
               
                 AXE662 
                 5.67 
                 — 
                 3.81 
                 0.36 
                 — 
                 83 
               
               
                 AE42 
                 3.68 
                 — 
                 — 
                 0.36 
                 — 
                 74 
               
               
                 AZ91D 
                 8.23 
                 0.38 
                 — 
                 — 
                 — 
                 68 
               
               
                   
               
            
           
         
       
     
     Test Specimen # 01  exhibited a decrease proportion of the loaded stress especially less, compared with those of the other test specimens, and therefore showed high creep resistance even under high temperatures. This is due to the following: the firm networks, which were continuous microscopically, were formed at the grain boundaries between the Mg crystalline grains because of the presence of Mn; and the deformation resistance of Test Specimen # 01  enlarged so that the strength thereof improved because the movements of dislocation were suppressed by the plate-shaped precipitated substances within the Mg crystalline grains.