JP4700488B2 - Heat-resistant magnesium alloy - Google Patents

Description

  The present invention relates to a heat-resistant magnesium alloy, and more particularly, to a technique for improving the die casting property while improving the high-temperature strength by maximizing solid solution strengthening.

  In recent years, for example, magnesium is being applied to strength members that constitute engines, frames, and the like in order to reduce the weight of automobiles. Magnesium is the lightest metal that can be used as a structural material, and has practically excellent characteristics such as higher specific strength and specific rigidity than iron and aluminum. On the other hand, since magnesium has insufficient proof stress and creep strength at high temperatures, especially when used as a component of an engine, it is exposed to high temperatures around 200 ° C at the maximum. The reduction of the bolt axial force of the part is a problem.

  Therefore, for example, in Patent Document 1, solid solution strengthening is achieved by adding gadolinium, which is a kind of rare earth element, to magnesium, and the initial creep strength is improved. Further, in Patent Document 1, at least one element selected from the group consisting of lanthanides from lanthanum to europium in the periodic table of elements is further contained beyond the solid solubility limit. These saturated elements that are not dissolved in solid form a stable eutectic compound with magnesium atoms at a high temperature, and the eutectic compound is dispersed at grain boundaries, thereby improving the steady creep strength of the magnesium alloy.

JP 2003-129161 A

By the way, the use environment of a structural material using a magnesium alloy becomes severe year by year, and further improvement in yield strength and creep strength at high temperatures is required. Further, in the technique described in Patent Document 1, when the magnesium alloy is die-cast, the eutectic compound at the grain boundary is solidified at the end, so that the shrinkage nest is likely to occur, and the eutectic compound is brittle and is accompanied by the formation of the shrinkage nest. There is a problem that casting cracks are likely to occur.
Therefore, the present invention maximizes the solid solution strengthening and improves the heat resistance strength, and prevents the occurrence of casting cracks due to casting defects such as shrinkage cavities during die casting and brittle eutectic compounds. It aims at providing the heat-resistant magnesium alloy which can be performed.

  As a result of intensive studies in order to maximize the solid solution strengthening of the magnesium alloy, the present inventors have come up with the idea of dissolving an element having an atomic diameter smaller than that of magnesium. There are interstitial and substitutional solid solutions. When an element having an atomic diameter much smaller than that of the base metal is dissolved, the solid solution enters the gaps between the base metal elements to form the interstitial solid solution. Form. On the other hand, when an element having an atomic diameter equal to or larger than that of the base metal is dissolved, the solid solution element forms a substitutional solid solution that replaces the base element in the crystal lattice. The heat-resistant magnesium alloy of the present invention is a substitutional solid solution.

  FIG. 1A shows a crystal structure of a magnesium alloy in which gadolinium is dissolved in magnesium. Dissolving gadolinium having an atomic diameter larger than that of magnesium causes distortion in the crystal lattice, which is considered to improve the strength of the magnesium alloy, particularly the high temperature creep strength. FIG. 1B is a schematic diagram showing a crystal structure when zinc (or silver) is further dissolved therein.

  The atomic diameter of zinc is about 17% smaller than the atomic diameter of magnesium, and the atomic diameter of silver is about 10% smaller than the atomic diameter of magnesium. For this reason, when zinc (silver) is dissolved, the surrounding crystal lattice is distorted toward the zinc (silver) side, and as a result, a space for dissolving gadolinium is expanded. Thereby, more gadolinium atoms can be dissolved, and solid solution strengthening is exhibited to the maximum. The above-mentioned action is the same for lanthanoids from lanthanum to europium in the periodic table of elements, and the solid solution strengthening is further exhibited by increasing the amount of those solid solutions, and excessive formation of compounds. Is suppressed, and the problem of casting defects such as a decrease in elongation and shrinkage cavity during die casting is solved.

The heat-resistant magnesium alloy of the present invention has been made based on the above knowledge, and at least one kind selected from the group consisting of 0.5 to 3.8% by mass of gadolinium and lanthanoids from lanthanum to europium in the periodic table of elements. It contains a total of 1 to 15% by mass of elements and 0.1 to 0.8% by mass of silver or 0.1 to 1.8% by mass of zinc, with the balance being magnesium and inevitable impurities. Yes.

  According to the present invention, since silver and / or zinc is contained, the rare earth elements as described above are dissolved in a large amount in magnesium, and thereby the solid solution strengthening can be exhibited to the maximum. In addition, since the amount of solid solution of rare earth elements is large, it is possible to prevent the amount of compounds formed by saturation from being excessive, and therefore to prevent the formation of shrinkage cavities during die casting and the accompanying occurrence of casting cracks. Can do.

The best mode for carrying out the present invention will be described below together with the operation of the present invention.
1. In order to achieve solid solution strengthening of gadolinium gadolinium, it is necessary to add 0.5% by mass or more, and more preferably 1.0% by mass or more. Moreover, when added exceeding the maximum solid solution amount with respect to magnesium, saturated gadolinium combines with a lanthanoid element and magnesium at a grain boundary to form a brittle compound, thereby causing a problem that the elongation is significantly reduced. Since the maximum solid solution amount of gadolinium is 3.8% by mass, this is the upper limit.

2. Lanthanoid In the present invention, at least one element selected from the group consisting of lanthanides from lanthanum to europium in the periodic table of elements is contained in a total of 1 to 15% by mass. These elements are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), and europium (Eu).

  The above element is dissolved in magnesium to promote solid solution strengthening, but the saturated element forms a compound with an impurity element such as magnesium element or Fe, and this compound is dispersed at the grain boundary. Since the formed compound is stable at a high temperature, the diffusion of atoms within the grain boundary is effectively inhibited even in a high temperature environment, and the high temperature strength of the magnesium alloy can be increased.

Here, if the amount of the lanthanoid added is less than 1 % by mass, the amount of compound formed at the grain boundary decreases, so that a cast hole is likely to occur at the grain boundary, which becomes the starting point of the casting crack. On the other hand, if the addition amount exceeds 15% by mass, the effect of increasing the solid solution amount by silver or zinc is diminished and the amount of compound formation becomes too large, and the segregated portion of the compound causes the formation of shrinkage cavities during die casting. As a result, casting cracks are likely to occur. Moreover, when there is too much formation amount of a compound, the elongation of a heat-resistant magnesium alloy will fall remarkably. In particular, when applied to an automobile engine or the like, since an elongation of about 2.0% or more is required, it is necessary to ensure both high-temperature strength and elongation.

3. The reason why silver and zinc are selected as silver and zinc- added elements is that these elements have a large amount of solid solution in magnesium and it is difficult to form harmful compounds with gadolinium. Silver content is 0.1 to 0.8 wt%, the content of zinc is 0.1 to 1.8 mass%. When the addition amount of these elements is less than each lower limit value, the effect of increasing the solid solution amount of the rare earth element is poor, and it becomes difficult to achieve the object of the present invention. More preferably, the zinc content is 1% by mass or more. The upper limit of the silver and zinc contents is the solid solubility limit of these elements in magnesium. Therefore, if the content exceeds the upper limit, a compound is formed with magnesium, gadolinium and lanthanoid, and problems such as shrinkage at the time of die casting occur at the portion where the compound is segregated.

4). Area ratio of compound phase diagram 2 (A) is a diagram showing the structure of the heat resistant magnesium alloy of the present invention, it shows a compound phase surrounding the α phase of magnesium, the periphery of the α phase. Here, the structure when the amount of the rare earth element added is increased is shown in FIG. According to the study by the present inventors, it has been found that when the area ratio of the compound phase is 24% or less, the elongation is good and the die casting property is also good. Therefore, the area ratio of the compound phase is desirably 24% or less.

  As described above, since the heat-resistant magnesium alloy of the present invention is excellent in high-temperature strength and die-casting properties, structural materials used at high temperatures, such as cylinder blocks, cylinder heads, intake honey holds, head covers, chains It is suitable for use in structural members around automobile engines such as cases, oil pans, transmission cases, and ECU frames.

A. Casting Hereinafter, examples of the present invention will be described.
Pure magnesium is dissolved by an electric melting furnace in a mixed gas atmosphere of argon gas and sulfur hexafluoride gas, and the first element (gadolinium), the second element (lanthanoid) and the second element are formed so as to have the composition shown in Table 1. Each of the three elements (silver or zinc) was added in a predetermined amount. Comparative Examples 1 to 5 shown in Table 1 are examples in which the third element is not added, and Comparative Examples 6 and 7 are examples in which the addition amounts of zinc and silver exceed the preferable range of the present invention. After the molten metal was agitated and settled, it was cast into a specially prepared mold for evaluating the die-casting cracking property to obtain a material for evaluation.

  FIG. 3 is a plan view (A) and a cross-sectional view (B) showing the cavity of the mold used, and when casting into this mold, a material having the same shape and size as the cavity can be obtained. For melting, a steel crucible whose inner surface was alumer-treated was used, and misch metal (MM) was used as a lanthanoid from lanthanum to europium. Casting was performed by placing the mold in a 200-ton cold chamber die casting machine. The amount of addition of the second element shown in Table 1 was set to an amount in which die-cast casting cracks had conventionally occurred in order to confirm both the strength improvement by adding the third element and the effect of preventing casting cracks.

B. Observation of the material The appearance of the cast material was observed to investigate the presence of casting cracks. The presence or absence of the casting crack was observed at the step S at a position away from the casting hole P. The results are shown in Table 1. FIG. 4A is a photograph showing the appearance of the cast material, and an enlarged view of the step S in Example 2 is an enlarged view of the step S in Comparative Example 2. (C) of FIG. As shown in Table 1 and FIG. 4 (B), in Examples 1 to 8 of the present invention, no casting cracks were generated while adding the second element in an amount that conventionally caused die casting cracks. As a result, the effect of adding the three elements was confirmed.

On the other hand, the casting crack was confirmed by the raw material of all the comparative examples 1-7. The reason is that in Comparative Examples 1 to 5, since the third element is not added, the amount of the solid solution of the first element and the second element is small, and as a result, the formation of compounds by these saturated elements becomes excessive. This is because segregation occurred and a shrinkage nest occurred in the segregation part. 5A shows a photomicrograph of the cross section of the step S portion of Comparative Example 2. FIG. As shown in FIG. 5A, in Comparative Example 2, the metal structure composed of the α phase and the compound phase is exhibited, but the amount of the eutectic compound precipitated at the grain boundary because it does not contain the third element. As a result, the ductility is reduced and segregation of the eutectic compound occurs everywhere in the material. And the crack which started from this segregation part is shown by FIG. 5 (A). However, in Comparative Example 3, casting cracks occurred for a reason different from the above. In Comparative Example 3, since the second element is not added, the amount of the compound dispersed at the grain boundary is small. FIG. 5B is a photomicrograph of the cross section of the step S portion of Comparative Example 3, and as shown in this figure, a cast hole is formed at the grain boundary where the compound is to be generated, and this cast hole has a cast crack. Responsible.

In Comparative Examples 6 and 7, since the addition amounts of zinc and silver each exceed the preferable range of the present invention, the solid solution limit of these elements is exceeded. As a result, zinc or silver formed a compound with magnesium, gadolinium, and lanthanoid, and shrinkage cavities formed in the parts where these elements segregated, resulting in casting cracks.

C. Strength Evaluation After heating the material at 200 ° C. for 100 hours to give a thermal history, a tensile test piece and a creep test piece having a shape according to JIS No. 4 test piece were cut, and a tensile test and a creep test were performed. . The tensile test was carried out using a 5-ton autograph in an atmosphere of 200 ° C. at a tensile rate of 0.5 mm per minute, and 0.2% proof stress and elongation at break were measured. In the creep test, a load of 50 MPa was applied to the test piece at a temperature of 200 ° C., and the total amount of elongation was measured when the test piece was held for 100 hours. The results of these tests are also shown in Table 1. Moreover, it measured using the area ratio of the compound in the cross section of each test piece. The measurement results are also shown in Table 1. In addition, although the area ratio of the compound was calculated | required as two layers of black-and-white using general-purpose image analysis software, Windows (trademark) Phot Shop was used in this test.

  In Examples 1 to 8 of the present invention, the area ratio of the compound is 24% or less, and the elongation at break is 0.2% or more. On the other hand, in Comparative Examples 2, 4, 5-7 in which the area ratio of the compound exceeds 24%, the elongation at break is 1.6 or less, and the elongation is reduced as compared with the examples of the present invention. I understood that. Further, the 0.2% proof stress is generally higher in the example than the comparative example, and the creep elongation is generally shorter in the example. Therefore, it was confirmed that the present invention is excellent in yield strength and creep strength.

(A) is a figure which shows the crystal structure of the conventional heat-resistant magnesium alloy, (B) is a figure which shows the crystal structure of the heat-resistant magnesium alloy of this invention. (A) And (B) is a figure which shows the structure | tissue of a heat-resistant magnesium alloy. It is the top view (A) and side view (B) which show the cavity of the metal mold | die used in the Example of this invention. (A) is the photograph which shows the raw material cast in the Example of this invention, (B) is the photograph of Example 2 which expanded the step part of (A), (C) is the step part of (A). It is the photograph of the comparative example 2 expanded. It is a microscope picture which shows the cross section of the raw material cast in the Example, (A) is a comparative example 2, (B) is a comparative example 3.

Claims (2)

Gadolinium 0.5 to 3.8% by mass, and at least one element selected from the group consisting of lanthanides from lanthanum to europium in the periodic table of elements 1 to 15% by mass in total , 0.1 to 0.8 % by mass % Heat-resistant magnesium alloy containing 0.1% by weight of silver or 0.1 to 1.8% by weight of zinc, with the balance being magnesium and inevitable impurities. And α phase of the metal structure is magnesium, consists of a compound phase surrounding the α phase, heat the magnesium alloy according to claim 1, the area ratio of the compound phase is equal to or less than 24%.

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