JP2012057227A - Heat-resistant magnesium alloy excellent in high temperature fatigue strength characteristic, method for manufacturing the heat-resistant magnesium alloy, and heat-resistant parts for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat-resistant magnesium alloy excellent in high temperature fatigue strength characteristics, a method of manufacturing the heat-resistant magnesium alloy and heat-resistant parts for an engine manufactured by using the heat-resistant magnesium alloy.SOLUTION: The magnesium alloy includes, in mass%, Y of 1.8 to 8% and Sm and/or Nd of 1.4 to 8% in total amounts with the remainder consisting of Mg and inevitable impurities. The mean crystal grain size of the crystal grains of a magnesium alloy structure falls within the range of 10 to 50 μm, and 10 or more pieces of plate-like precipitates each having a major axis of 5 μm or more and an aspect ratio of 10 or more are precipitated in the crystal grains. Further, preferably, Ho of 0.5 to 2% is contained.

Description

  The present invention relates to a heat-resistant magnesium alloy excellent in high-temperature fatigue strength characteristics that can be used for engines for automobiles or motorcycles, leisure engines, small engines such as lawn mowers, and peripheral devices such as turbochargers, and high temperatures thereof. The present invention relates to a method for producing a heat-resistant magnesium alloy having excellent fatigue strength characteristics, and a heat-resistant component for an engine produced using the heat-resistant magnesium alloy having excellent high-temperature fatigue strength characteristics.

  Magnesium has a specific gravity of 1.8, and is the lightest specific gravity among metals that can be used as structural materials for machine parts (about 2/3 of aluminum and about 1/4 of iron). In addition, it has characteristics such as excellent specific strength, specific rigidity, thermal conductivity and the like.

  In recent years, from the viewpoint of global environmental protection such as prevention of global warming, magnesium has been used as a constituent material for peripheral devices such as engines, turbochargers, etc. for automobiles, motorcycles, aircraft, etc., and casings for electrical and electronic devices. The application is increasing. In particular, when applied to vehicles such as automobiles and motorcycles, fuel consumption can be improved, and this tendency is remarkable.

  However, when magnesium is used as a structural material for a vehicle used in a high temperature atmosphere, particularly when used in peripheral equipment such as an engine or a turbocharger, it is exposed to a high temperature of 200 to 300 ° C., so at least Heat resistance (high temperature strength) in the temperature range up to about 300 ° C. and stability of high temperature fatigue strength in the temperature range up to about 300 ° C. are required.

  Therefore, various heat-resistant magnesium alloys having improved heat resistance (high-temperature strength) and methods for producing the heat-resistant magnesium alloys have been proposed. For example, Patent Document 1 includes one or more of Si, Cu, Ni, Be, Fe, Ti, V, Mn, Cr, Zr, Nb, Mo, Hf, Ta, W, and Nd. Describes a method for producing a magnesium alloy extruded material excellent in elastic modulus, which is produced by melting a magnesium-based alloy containing 1 to 80% by weight. However, although the magnesium alloy obtained by the manufacturing method described in Patent Document 1 is certainly excellent in heat resistance (high temperature strength), it does not improve fatigue strength at high temperatures.

  Patent Document 2 discloses that Y: 60% by weight and the balance of Y component consisting of rare earth elements is 1.5 to 10% by weight, Nd: 60% by weight and La: 25% by weight or less, and balance of Nd consisting of Pr. A magnesium alloy containing 1 to 10% by weight of components with the balance being Mg is described. However, although the magnesium alloy described in Patent Document 2 is described as having excellent tensile properties at high temperatures, it does not improve fatigue strength at high temperatures.

  Patent Document 3 describes a particle-reinforced Mg alloy having excellent heat resistance, containing 0.2 to 12% by weight of a rare earth element in Group 3a of the periodic table, with the balance being Mg and inevitable impurities. . Certainly, the particle-reinforced Mg alloy described in Patent Document 3 is excellent in strength at high temperatures, but was developed assuming high-temperature strength at about 150 ° C. It is an alloy and does not improve fatigue strength at a high temperature of 300 ° C.

  At present, there is almost no prior art that has improved the fatigue strength of the heat-resistant magnesium alloy as described above, but as the prior art that has been aimed at improving the fatigue strength, there is a magnesium-based alloy described in Patent Document 4. be able to. This magnesium-based alloy is a magnesium-based alloy containing 1 to 8% rare earth element and 1 to 6% calcium by weight%, and the maximum crystal grain size of magnesium constituting the substrate is 30% or less.

  The magnesium-based alloy described in Patent Document 4 is a heat-resistant magnesium-based alloy to which Ca is added in order to impart heat resistance. However, from the viewpoint of fatigue strength, the addition of Ca adversely affects the improvement of fatigue strength. It is possible. Moreover, since it is manufactured including the process of solidifying the powder, it is considered that there is a problem that the manufacturing cost increases. Furthermore, although the Example of patent document 4 is shown that an intensity | strength and ductility improve, it is not shown at all that the fatigue strength improves concretely, and fatigue strength actually improves. It can be said that it is unclear whether it can be done.

In addition, the present inventors have proposed, as Patent Document 5, a magnesium alloy having excellent creep characteristics at high temperatures and a method for producing the same. The magnesium alloy described in Patent Document 5 contains, by mass%, Y: 1.8 to 8.0% and Sm: 1.4 to 8.0%, respectively, and the remaining Mg and inevitable impurities. The solid solution amount in the magnesium matrix is, by mass%, Y: 0.8 to 4.0%, Sm: 0.6 to 3.2%, and the average crystal grain size of the magnesium alloy structure is Excellent in creep characteristics at high temperature, in which the average number of precipitates having a diameter of 2 nm or more observed in a 300,000-fold TEM is present in these crystal grains is 160 / μm ² or more. Magnesium alloy.

  The magnesium alloy described in Patent Document 5 is characterized in that a part of contained Y and Sm are dissolved in a magnesium matrix, thereby improving strength and elongation at about 250 ° C. In addition, it is intended to improve the creep characteristics at about 250 ° C. by depositing the remaining portions of Y and Sm in the crystal grains as precipitates, but the high temperature fatigue strength characteristics at around 300 ° C. Has not been studied at all.

JP-A-62-218527 Japanese Patent Laid-Open No. 57-210946 JP-A-5-202443 JP 2006-2184 A JP 2009-249647 A

  The present invention has been made as a solution to the above-described conventional problems, and includes a heat-resistant magnesium alloy having excellent high-temperature fatigue strength characteristics in a temperature range up to at least about 300 ° C., a method for producing the heat-resistant magnesium alloy, and its An object of the present invention is to provide a heat-resistant component for an engine manufactured using a heat-resistant magnesium alloy.

  The invention according to claim 1 is a magnesium alloy containing, by mass%, Y in an amount of 1.8 to 8%, Sm and / or Nd in a total amount of 1.4 to 8%, with the balance being Mg and inevitable impurities The average grain size of the crystal grains of the magnesium alloy structure is in the range of 10 to 50 μm, and 10 plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more are contained in the crystal grains. This is a heat-resistant magnesium alloy having excellent high-temperature fatigue strength characteristics characterized by precipitation.

  The invention described in claim 2 is a heat-resistant magnesium alloy excellent in high temperature fatigue strength characteristics according to claim 1 further containing 0.5 to 2% by mass of Ho.

  The invention according to claim 3 is the heat-resistant magnesium alloy having excellent high temperature fatigue strength characteristics according to claim 1 or 2, wherein at least one rare earth element of Sm, Nd, and Ho is added as misch metal.

  The invention according to claim 4 is a processing temperature of 330 to 550 after casting the magnesium alloy melt having the component composition according to claim 1 or 2 and performing a solution treatment at a treatment temperature of 330 to 550 ° C. Under the condition of ° C., a forming process is performed by hot working which gives a strain having a true strain of 1.1 to 2.1 at a strain rate of 0.1 / s to 1.7 / s, and the processing is completed. Heat resistance excellent in high temperature fatigue strength characteristics characterized by having a process of cooling to 300 ° C. or less within 5 seconds and then performing an aging treatment for 40 hours or more under the condition of a treatment temperature of 300 to 330 ° C. It is a manufacturing method of a magnesium alloy.

  A fifth aspect of the present invention is a heat-resistant component for an engine produced by using the heat-resistant magnesium alloy according to any one of the first to third aspects.

  According to the heat-resistant magnesium alloy of the present invention, excellent fatigue characteristics can be ensured in a temperature range up to at least about 300 ° C. In particular, peripheral devices such as engines or turbochargers of automobiles, motorcycles, airplanes and the like can be used as suitable metal materials because they are exposed to high temperatures of 200 to 300 ° C.

  Moreover, according to the manufacturing method of the heat-resistant magnesium alloy of this invention, the heat-resistant magnesium alloy excellent in the high temperature fatigue strength characteristic can be manufactured.

  Furthermore, according to the heat-resistant component for engines of the present invention, the heat-resistant component for engines having high-temperature fatigue strength characteristics can be obtained.

It is the microscope picture (secondary electron image) which observed the cut surface of the magnesium alloy of this invention by FE-SEM.

  Since the engine or turbocharger is exposed to a high temperature of 200 to 300 ° C., the inventors have conducted diligent experiments to obtain a heat-resistant magnesium alloy having excellent fatigue strength characteristics in a temperature range up to at least 300 ° C. , Advanced research.

  First, the inventors have found that in order to ensure high temperature fatigue strength, it is important to uniformly disperse the dislocation cells introduced into the magnesium alloy while the magnesium alloy is repeatedly loaded. Since the dislocation cell accumulation site is usually a crystal grain boundary, uniform refinement of crystal grains is considered to be effective for ensuring high-temperature fatigue strength, but uniform refinement of crystal grains by rolling, forging, etc. However, it is difficult to make the average crystal grain size smaller than 5 μm in practice.

  Therefore, the inventors focused on a technique for dispersing precipitates precipitated in crystal grains as sites where dislocation cells accumulate while the magnesium alloy is subjected to repeated loads, and examined the form of the precipitates. In order to obtain a heat-resistant magnesium alloy having excellent fatigue strength characteristics, it has been concluded that it is effective that the form of precipitates satisfies the following three conditions. The first point is that the precipitate is not spherical but plate-like. The second point is that the size of the precipitate is not a fine size of nanometer size but a relatively coarse size of the order of μm. Finally, the third point is to give the precipitate an appropriate thickness so that it does not crack during repeated loading.

  Furthermore, the inventors examined the composition of precipitates effective for ensuring fatigue characteristics in the temperature region near 300 ° C., and found that the alloy elements added to the magnesium alloy were Y, Sm and / or Nd. I found it effective.

  This is the end of the study on the precipitates precipitated in the crystal grains, but it is not sufficient to simply control the morphology and composition of the precipitates. It has been found that it is also effective to define the particle size within an appropriate range.

  As a result, the alloy elements to be added to the magnesium alloy are Y, Sm and / or Nd, and the addition amount of the alloy elements is specified within an appropriate range, and the average crystal grain size of the crystal grains of the magnesium alloy structure is specified. Furthermore, the inventors have found that a heat-resistant magnesium alloy having desired high-temperature fatigue strength characteristics can be obtained by defining the form and the number of precipitates precipitated in the crystal grains. It was completed.

  Hereinafter, the present invention will be described in detail based on embodiments.

  In the present invention, the component composition of the magnesium alloy, the average crystal grain size of the crystal grains of the magnesium alloy structure, the form of the plate-like precipitates precipitated in the crystal grains, and the number of precipitates are respectively defined. Explained.

(Component composition)
The magnesium alloy of the present invention is constituted by adding Y and Sm and / or Nd as alloy elements to magnesium, but the content of Y in the magnesium alloy is 1.8 to 8%, Sm and / or The total content of Nd is 1.4 to 8%. Moreover, you may contain 0.5 to 2% of Ho. The balance is Mg and inevitable impurities. In addition,% described above shows mass%, and what is described as% in the following specification shows mass%.

Y: 1.8-8%
Y is an element necessary for ensuring excellent fatigue strength at high temperatures in addition to heat resistance (high temperature strength) and high temperature elongation in combination with Sm and / or Nd. If the Y content is less than 1.8%, it becomes impossible to ensure excellent fatigue resistance at high temperatures in addition to heat resistance (high temperature strength) and high temperature elongation. On the other hand, if the Y content exceeds 8%, the amount of precipitation of Y-based intermetallic compounds at the grain boundaries increases, and on the contrary, heat resistance (high temperature strength) high temperature elongation and heat resistance are reduced. Therefore, the Y content is set to 1.8 to 8%.

Sm and / or Nd: 1.4-8%
Sm and / or Nd coexist with Y, in addition to heat resistance (high temperature strength) and high temperature elongation, it is an element necessary for ensuring excellent fatigue resistance at high temperatures, and high temperature fatigue strength. It is a constituent element of the plate-like precipitate necessary for ensuring. If the total content of Sm and / or Nd is less than 1.4%, in addition to heat resistance (high temperature strength) and high temperature elongation, it is impossible to ensure excellent fatigue strength at high temperatures. Become. On the other hand, if the total content of Sm and / or Nd exceeds 8%, the amount of Sm-based intermetallic compound and / or Nd intermetallic compound crystallized at the grain boundary increases, and on the contrary, the heat resistance (high temperature strength). ) Reduces high temperature elongation and heat resistance. Therefore, the total content of Sm and / or Nd is set to 1.4 to 8%.

  The total content of Y and Sm and / or Nd is preferably 10% or more in order to ensure heat resistance (high temperature strength) and excellent fatigue strength at high temperatures.

Ho: 0.5-2%
Ho is an element that dissolves in a magnesium alloy and exhibits an effect of increasing the high-temperature strength, and is combined with the dispersion control of the plate-like precipitate containing Sm or Nd to raise the fatigue resistance strength at a high temperature. Demonstrate the effect. When this Ho is contained, the content is 0.5 to 2%.

  Sm, Nd, and Ho can be added as pure elements, but can also be added as a misch metal mixed with rare earth elements. When added as misch metal, there is a secondary effect that the separation cost for each rare earth element is omitted.

(Average crystal grain size)
The average grain size of the crystal grains of the magnesium alloy structure should be 50 μm or less, desirably 30 μm or less. When the average crystal grain size exceeds 50 μm, it becomes impossible to ensure ductility in a high temperature environment around 300 ° C. The smaller the average crystal grain size, the more sufficient ductility can be ensured. However, when the average crystal grain size is less than 10 μm, the precipitation conditions for the plate-like precipitates required by the present invention can be ensured. Disappear. Therefore, the average crystal grain size of the crystal grains of the magnesium alloy structure is in the range of 10 to 50 μm.

  The average crystal grain size of the crystal grains should be observed using an optical microscope after cutting the magnesium alloy material after aging treatment on an arbitrary surface and then finishing the surface smoothly by mechanical polishing, electrolytic etching, etc. Determined by Specifically, it was obtained by obtaining a microstructure with a field of view of 300 μm × 400 μm centered on a position 1 mm inside substantially perpendicular to the surface of the magnesium alloy material, and calculating the average value of the equivalent circle diameters of the existing crystal grains. .

(Long diameter of plate-like precipitate)
The major axis of the plate-like precipitate is the maximum length (diameter) of one plate-like precipitate. The major axis of the plate-like precipitate must be at least 5 μm. A plate-like precipitate having a shorter major axis than this cannot contribute to the improvement of the high temperature fatigue strength characteristics. On the other hand, the upper limit of the major axis of the plate-like precipitate is not particularly defined, but it is not longer than the major axis of the crystal grains containing the plate-like precipitate.

(Aspect ratio of plate-like precipitate)
The aspect ratio of the plate-like precipitate must be at least 10 or more. Even if the major axis of the plate-like precipitate is 5 μm or more, if the aspect ratio is less than 10, the plate-like precipitate will be broken (cracked) by repeated loading, ensuring sufficient high-temperature fatigue strength characteristics. The form for doing so cannot be maintained. On the other hand, the upper limit of the aspect ratio is not particularly defined, but the minimum thickness for securing the thickness of the plate-like precipitate is 5 to 15 nm in terms of the crystal structure, so the actual upper limit is considered to be in the range of 8000 to 10,000. It is done.

(Number of deposited plate-like precipitates)
The number of precipitates per crystal grain of the plate-like precipitates satisfying the above-mentioned major axis and aspect ratio is required to be at least 10. If the number of plate-like precipitates per crystal grain is 9 or less, sufficient high temperature fatigue strength characteristics cannot be ensured. By dispersing 10 or more plate-like precipitates in each crystal grain, it is possible to provide a barrier for dislocation cell structure integration introduced by fatigue damage effective for ensuring high temperature fatigue strength characteristics.

(Production method)
Next, a preferable production method and production conditions for obtaining the heat-resistant magnesium alloy of the present invention will be described. First, in the method for producing a heat-resistant magnesium alloy of the present invention, Y is 1.8 to 8% in mass%, Sm and / or Nd is contained in a total amount of 1.4 to 8%, and the balance is Mg and inevitable. In order to cast a magnesium alloy melt containing impurities or a magnesium alloy melt containing 0.5 to 2% by mass of Ho and 0.5% to 2%, and hot-working the ingot as necessary. The billet is machined and then subjected to a solution treatment, followed by hot working such as forging and extrusion for crystal grain refinement. In a general magnesium alloy manufacturing process, these manufacturing processes are not carried out, but are used as a product in an ingot state or only heat treatment such as solution treatment is performed on the ingot.

  The solution treatment is necessary for solid solution of Sm and / or Nd that forms a plate-like precipitate in the subsequent aging treatment. In order to sufficiently dissolve Sm and / or Nd, a solution treatment at 330 ° C. or higher is necessary, and a more preferable solution treatment temperature is higher than 450 ° C. On the other hand, when the solution treatment temperature exceeds 550 ° C., a sufficient amount of solid solution of Sm and / or Nd can be secured, but the crystal grain size of the magnesium alloy structure is controlled in the next hot working process. Therefore, the upper limit was set to 550 ° C. The time for performing the solution treatment is preferably 5 to 30 hours. If the solution treatment time is less than 5 hours, the solution of the additive elements such as Y, Sm, Nd, and Ho becomes insufficient, and high temperature strength and fatigue characteristics cannot be obtained. On the other hand, if the solution treatment time exceeds 30 hours, the crystal grains may be coarsened, and even if the conditions of the post-process are controlled, the desired crystal grain size cannot be adjusted.

  Moreover, the processing temperature of the hot working by forging, hot isostatic pressing, normal hot extrusion, or the like is set to 330 to 550 ° C. similarly to the solution treatment temperature. In this way, by aligning the hot working temperature with the solution treatment temperature, it is possible to serve both as a solution heat treatment and a hot work material heating, but after reheating the material once cooled after the solution treatment. It doesn't matter. A more preferable hot working temperature is 400 to 500 ° C. When hot working is performed at a processing temperature of less than 330 ° C., cracks may occur, and the manufacturing yield deteriorates. On the other hand, if the hot working temperature exceeds 550 ° C., the crystal grains cannot be refined, the crystal grains become coarse, and the balance between strength and ductility becomes unstable. The hot working means may be any of hot extrusion, rolling and forging.

  Further, the strain applied by hot working needs to be 1.1 or more, preferably 1.3 or more in terms of true strain. If the strain amount is less than 1.1, the recrystallization driving force for refining the crystal grains cannot be ensured, and the average crystal grain size of the crystal grains exceeds the upper limit of 50 μm. In the case of a forged member or the like, the amount of strain introduced varies depending on the site, so the strain amount of the minimum strain introduction portion needs to be 1.1 or more. In addition, the average crystal grain diameter of a crystal grain can be 30 micrometers or less by making distortion amount 1.3 or more. On the other hand, it is impossible to apply a strain with a strain amount exceeding 2.1 because forging cracks and the like occur during hot working. Therefore, the strain applied by hot working is 1.1 to 2.1, preferably 1.3 to 2.1 in terms of true strain.

  The strain rate in hot working at the time of applying this strain is 0.1 / s to 1.7 / s. If this strain rate is too low, the strain once applied recovers continuously, and it becomes impossible to accumulate driving force for crystal grain refinement, and the average crystal grain size of the crystal grain is the upper limit. It exceeds 50 μm. Although it is desirable to set a larger strain rate, forging cracks or the like may occur during hot working. Therefore, an upper limit of the strain rate is set, and in the present invention, the strain rate in hot working is 0. The range was from 1 / s to 1.7 / s.

  Next, it cools rapidly to 300 degrees C or less within 5 second after completion | finish of the shaping | molding process by a hot process. By applying this cooling, the crystal grain size of the magnesium alloy structure can be reduced, and the average crystal grain size can be 10 to 50 μm. As a result, excellent fatigue strength characteristics can be imparted to the heat-resistant magnesium alloy. The cooling may be performed by any of air cooling, gas cooling, and water cooling. However, in order to reliably cool to the center of the member, it is preferable to employ water cooling that is performed by cooling in the water.

  Although the fatigue strength characteristic excellent in the heat-resistant magnesium alloy can be imparted by the above process, the excellent fatigue strength characteristic at a high temperature around 300 ° C. cannot be imparted. Therefore, in the manufacturing method of the present invention, in order to give excellent high temperature fatigue strength characteristics, after the cooling treatment, an aging treatment for 40 hours or more is performed under the treatment temperature of 300 to 330 ° C.

  When the temperature of the aging treatment is less than 300 ° C., the growth of the precipitate is substantially stopped, so the form of the precipitate does not become the form defined in the present invention, and as a result, the high temperature fatigue strength characteristics cannot be secured. . On the other hand, when the temperature of the aging treatment exceeds 330 ° C., the temperature becomes close to the solution treatment temperature at which the precipitation elements Sm and / or Nd are dissolved, and as a result, the reprecipitation of the precipitate starts and the necessary precipitation occurs. The number and shape of the objects cannot be ensured, and high temperature fatigue strength characteristics cannot be obtained.

  The aging treatment needs to be performed for 40 hours or more. When the aging treatment time is less than 40 hours, the necessary number and form of precipitates cannot be ensured, and high temperature fatigue strength characteristics cannot be obtained. From the viewpoint of securing the number and form of precipitates more stably, it is preferable to perform an aging treatment for 100 hours or more. In the present invention, the upper limit of the aging treatment time is not particularly defined, but considering the industrial rationality and the change behavior of the precipitation form, the high temperature fatigue strength characteristics can be further improved even after aging treatment for 200 hours or more. Absent.

  In addition, after completion | finish of a shaping | molding process (after rapid cooling), you may add a re-solution treatment before performing an aging treatment. Specifically, a re-solution treatment is performed at 330 to 450 ° C. for 1 to 2 hours. By performing the re-solution treatment under such conditions, the average grain size of the crystal grains of the magnesium alloy structure is reduced. It can be maintained in the range of 10 to 50 μm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  Examples of the present invention will be described below. In this example, the component composition of the magnesium alloy and the conditions relating to the manufacturing method were changed in various ways to measure the average crystal grain size of the crystal grains and the number of plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more. In addition, high temperature fatigue strength characteristics at 300 ° C. were measured and evaluated. Tables 1 and 2 show these test conditions and test results. Table 1 shows an example of adding Y and Sm to a magnesium alloy, and Table 2 shows No. Nos. 31 to 34 are examples in which Y and Nd are added to magnesium alloys. Nos. 35 to 38 are examples in which Y, Sm and Nd are added to a magnesium alloy. Examples of addition of Ho in addition to these additional elements are shown as 39 to 42, respectively.

  Specifically, first, magnesium alloys having the component compositions shown in Table 1 and Table 2 were melted in an electric melting furnace under an argon inert atmosphere, and cast into a cast iron mold at a temperature of 750 ° C., φ950 mm × length A 100 mm magnesium alloy ingot was obtained. Subsequently, the surface of the magnesium alloy ingot was chamfered by machining to obtain magnesium alloy billets each having a diameter of 68 mm and a length of 100 mm.

  The magnesium alloy billet was subjected to a solution treatment for 20 hours under the temperature conditions shown in Tables 1 and 2, and then hot-worked by flat plate forging under the temperature conditions shown in Tables 1 and 2. In addition, the true strain and strain rate in this hot working are also shown. Then, it cooled to 300 degrees C or less within 5 second after completion | finish of hot processing by water cooling, Then, the aging treatment was performed on the temperature conditions shown in Table 1 and Table 2, and it was set as the test material.

  Using the sample cut out from the test material manufactured by the method described above, the average grain size of the crystal grains, the number of plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more, and at 300 ° C. High temperature fatigue strength characteristics were measured and evaluated. The measurement results are shown in Tables 1 and 2.

(Measurement of average grain size of crystal grains)
The average crystal grain size of the crystal grains was determined by the method described above by observing the magnesium alloy structure after the aging treatment with an optical microscope.

(Number of plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more)
The number of plate-like precipitates with a major axis of 5 μm or more and an aspect ratio of 10 or more is determined by cutting the magnesium alloy after aging treatment and fitting it into a resin plate, then mirror-polishing the surface and finishing the surface smoothly. Was obtained by observing a secondary electron image with FE-SEM (manufactured by JEOL, JSM-7001F).

  Specifically, the observation magnification is 2000 times, the acceleration voltage is 10 kV, and a microstructure of a 300 μm × 400 μm visual field is obtained centering on a position 1 mm inside substantially perpendicular to the surface of the magnesium alloy material, and exists in the visual field. Among the crystal grains to be produced, the number of plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more was determined and displayed in Tables 1 and 2. The aspect ratio was determined from the major axis / thickness of the observed plate-like precipitate. In addition, since the plate-like precipitate is precipitated with a certain orientation relationship with the magnesium matrix, the crystal grains (the crystal grains in which the [0001] plane can be substantially observed can be clearly observed within the observation field). ) Was selected and the above observations were made. FIG. 1 shows an example of a secondary electron image observed with an FE-SEM.

(Fatigue test)
The high temperature fatigue rupture life was confirmed and evaluated by carrying out a rotating bending fatigue test using an Ono type rotating bending fatigue tester. The test piece has a diameter (D0): 12.0 mm, a length (L): 90 mm, a maximum detail diameter (d): 8.0 mm, and a smooth part curvature radius (R): 48.0 mm, JIS Z2274 No. 2 A fatigue test was carried out under the condition of a rotational speed of 3000 rpm with the test piece heated with an infrared heater and kept at a temperature of 300 ° C. Repeated 10 ⁷ times fatigue test were measured 10 ⁷ times fatigue strength of the test piece. The 10 ⁷ times fatigue strength fatigue test is determined to excellent heat magnesium alloy that exceed 40MPa in high temperature fatigue strength properties.

  No. shown in Table 1. 1-16 and No. 2 shown in Table 2. Nos. 31, 32, 35, 36, and 39 to 41 satisfy the requirements of the present invention in terms of the magnesium alloy component composition and the manufacturing method. As a result, the average crystal grain size of the crystal grains, the form of the plate-like precipitate that satisfies the requirements specified in the present invention and the number of precipitates satisfying the requirements specified in the present invention also satisfied the requirements specified in the present invention.

These No. 1-16 and No.1. For 29 and 30, result of the fatigue test at 300 ° C., 10 ⁷ times fatigue strength beyond all 40 MPa, all of which can be said to be excellent thermal magnesium alloy in high-temperature fatigue strength characteristics. In particular, No. 1 which added Ho in addition to Y, Sm and Nd. In 39 to 41, the 10 ⁷ times fatigue strength was as high as 49 MPa.

  In contrast, no. No. 17 is a comparative example in which the Y content is too low, No. 17 No. 18 is a comparative example in which the Y content is too high, No. 18 No. 19 is a comparative example in which the Sm content is too low, No. 19 No. 20 is a comparative example in which the Sm content is too high, No. 20 No. 21 is a comparative example in which the solution treatment temperature is too low, No. 21. No. 22 is a comparative example in which the solution treatment temperature is too high. No. 23 is a comparative example in which the true strain is too small, No. 23. No. 24 is a comparative example in which the true strain is too large, No. 24. No. 25 is a comparative example in which the strain rate is too slow. No. 26 is a comparative example in which the strain rate is too high, No. 26. No. 27 is a comparative example in which the aging treatment temperature is too low, No. 27. No. 28 is a comparative example in which the aging treatment temperature is too high, No. 28. No. 29 is a comparative example in which the aging treatment time is too short. 30 is a comparative example in which the aging treatment temperature is too low and the aging treatment time is too short.

  No. No. 33 is a comparative example in which the Nd content is too low, No. 33. No. 34 is a comparative example in which the content of Nd is too high, No. 34. No. 37 is a comparative example in which the amount of misch metal added is small and the total content of Sm and Nd is too small. No. 38 is a comparative example in which the amount of misch metal added is large and the total content of Sm and Nd is too large. 42 is a comparative example in which the Ho content is too high.

  As a result, no. For 18, 20, 34, 38 and 42, forging cracks occurred during hot working. In addition, No. in which cracking did not occur during hot working. In 17, 19, 21 to 30, 33, and 37, the average grain size of crystal grains, the major axis is 5 μm or more, and the number of plate-like precipitates having an aspect ratio of 10 or more is at least one item. The specified requirements could not be satisfied. No. In 30, the plate-like precipitate itself could not be confirmed.

In addition, No. in which cracking did not occur during hot working. As a result of carrying out a fatigue test at 300 ° C. only for ¹⁷ , 19, 21 to 30, 33, 37, the 107 times fatigue strength is all 40 MPa or less, and these are all heat-resistant magnesium excellent in high temperature fatigue strength characteristics. Not applicable to alloys.

Claims (5)

A magnesium alloy containing 1.8 to 8%, Y and Sm and / or Nd in a total amount of 1.4 to 8% by mass, with the balance being Mg and inevitable impurities,
The average grain size of the crystal grains of the magnesium alloy structure is in the range of 10-50 μm,
A heat-resistant magnesium alloy having excellent high-temperature fatigue strength characteristics, wherein 10 or more plate-like precipitates having a major axis of 5 μm or more and an aspect ratio of 10 or more are precipitated in the crystal grains.   Furthermore, the heat-resistant magnesium alloy excellent in the high temperature fatigue strength characteristic of Claim 1 which contains 0.5 to 2% of Ho by mass%.   The heat-resistant magnesium alloy having excellent high-temperature fatigue strength characteristics according to claim 1 or 2, wherein at least one rare earth element of Sm, Nd, and Ho is added as a misch metal. After casting the magnesium alloy melt having the component composition according to claim 1 or 2,
After performing the solution treatment under conditions where the treatment temperature is 330 to 550 ° C.,
Forming is performed by hot working which gives a strain of true strain of 1.1 to 2.1 at a strain rate of 0.1 / s to 1.7 / s under a processing temperature of 330 to 550 ° C. ,
Furthermore, it is cooled to 300 ° C. or less within 5 seconds after the processing is completed.
Then, the manufacturing method of the heat-resistant magnesium alloy excellent in the high temperature fatigue strength characteristic characterized by having the process of performing the aging treatment for 40 hours or more on the conditions whose process temperature is 300-330 degreeC.   A heat-resistant component for an engine produced by using the heat-resistant magnesium alloy according to any one of claims 1 to 3.