JP2009084685A - Warm working method of magnesium alloy, magnesium alloy for warm working and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optimum warm working method of a magnesium alloy utilizing superplasticity, to provide a magnesium alloy suitable for the warm working method utilizing the superplasticity, and to provide a method for manufacturing the same. <P>SOLUTION: In the warm working method of the magnesium alloy, the magnesium alloy has a quasicrystal phase, and the warm working temperature and strain rate are set such that A in the formula is >1.8×10<SP>6</SP>and <7.2×10<SP>6</SP>. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a magnesium alloy warm working method, a warm working magnesium alloy and a manufacturing method thereof, and relates to warm working using superplasticity.

Magnesium is expected to be used in various fields because of its light weight and abundant resources. However, due to its crystal structure, plastic working ability is very poor at room temperature. The superplastic behavior shows a huge deformation of several hundred% or more like water tanks, and is considered to be a very effective means for improving the formability of difficult-to-work materials such as magnesium alloys.
On the other hand, since superplasticity is manifested in a relatively high temperature range, it is difficult to maintain the initial structure (particularly, giant grain growth occurs). Therefore, composite addition and precipitation particles are used as grain boundary pinning particles to stabilize the structure, but this greatly affects the superplastic deformation response. (These particles act as pile-up sources of dislocation activity that act by an accompanying adjustment mechanism during superplastic deformation, and are the starting point for the generation of cavities that inhibit ductility.)
For these, the following prior arts are known.
Patent Document 1 shows a method for producing a high-strength magnesium alloy by quasicrystal dispersion of an Mg—Zn—Y alloy. (Application range: Mg-1 to 10 at.% Zn—0.1 to 3 at.% Y) A sample is prepared by a casting method, and then subjected to strain processing at a temperature of about 1/2 or more of the melting temperature to produce a quasicrystal. Create a dispersed phase.
Superplastic behavior is manifested when fine crystal grains of about several microns cause grain boundary sliding. However, when the processing temperature is high, recrystallization proceeds easily during the processing, it is difficult to obtain a fine crystal structure, and it is difficult to obtain a superplastic behavior such as several hundred% of elongation at break. Further, in the prior patent, the additive solute atoms of rare earth elements are limited to yttrium, and there is no description regarding other rare earth elements.
Patent Documents 2 and 3 show a method for producing a high-strength magnesium alloy by quasicrystal dispersion of an Mg—Zn—Y alloy. (Application range: Mg-1.5 to 10 at.% Zn-0.125 to 3.3 at.% Y) A sample is prepared by a casting method, and a quasicrystalline particle phase and its approximate crystal phase are obtained by warm working and heat treatment. We are developing magnesium alloys that crystallize and show high strength and high creep properties.
Superplastic behavior is due to the occurrence of grain boundary sliding. However, in these patents, particles made of Mg—Zn exhibiting a mismatched interface with the parent phase are precipitated by warm working and heat treatment.
This particle acts as a grain boundary pinning particle, suppresses grain boundary sliding, and improves creep characteristics, so that the development of superplasticity is suppressed. (Even if superplasticity appears, it becomes a starting point of cavity generation that inhibits ductility.) In addition, in the prior patent, the solute atoms of rare earth elements are limited to yttrium, and there is no description about other rare earth elements.
Patent Document 4 shows a manufacturing method in which a pre-strain of 0.4 or more is applied to a cast magnesium alloy and then warm strain processing is performed (rare earth metal is not used). Pre-strain is applied before warm working to introduce uniform precipitation nucleation sites. For this reason, it is possible to form a finely dispersed particle phase in a fine magnesium matrix and to expect the appearance of a superplastic phenomenon. However, since the interface of the formed precipitation particles is inconsistent with the parent phase, it acts as a pile-up source of dislocation activity that acts by the accompanying adjustment mechanism during superplastic deformation, and becomes the starting point of cavity generation. Therefore, the stress at the time of warm working using superplasticity is larger than that of the superplastic magnesium alloy without precipitated particles, and it is predicted that the loss of the mold during molding will be accelerated. It is also predicted that ductility will be reduced.
JP 2002-309332 A JP 2005-113234 A JP 2005-113235 A JP 2003-277899 A

  In view of such circumstances, the present invention provides an optimum warm working method for a magnesium alloy utilizing superplasticity, a magnesium alloy suitable for a warm working method utilizing superplasticity, and a method for producing the same. Objective.

The present invention provides the following inventions in order to solve the above problems.
Invention 1 is a method for warm working of a magnesium alloy, wherein the magnesium alloy has a quasicrystalline phase, and A in the following formula 1 is more than 1.8 × 10 ⁶ and less than 7.2 × 10 ^6. It is characterized by setting the processing temperature and strain rate.
(Formula 1)

Invention 2 is the warm working method of Invention 1, wherein the magnesium alloy is an Mg—Zn—RE alloy (RE: rare earth metal).
Invention 3 is a magnesium alloy for warm working, the magnesium matrix has a quasicrystalline phase with a crystal grain size of 15 μm or less and an average particle size of 2 μm or less, and the following general formula (3): It has the composition which satisfy | fills.
(Formula 3)
Mg {100- (y + x)} Zn _y RE _x (3)
0.2 ≦ x ≦ 1.5, 5x ≦ y ≦ 7x, x and y are atomic%,
RE is a rare earth element of any one of Ho, Y, Gd, Tb, Dy, and Er.

  Invention 4 is a method for producing a magnesium alloy of Invention 3, wherein the mother alloy having the composition represented by the formula (3) is homogenized at 460 ° C. or less for 4 hours or more, and then at 200 to 300 ° C. for 15 to 15 hours. After holding for 90 minutes, strong strain processing is performed at a processing ratio of 8: 1 to 400: 1.

Since the quasicrystalline phase has no translational order and exhibits a very good connection (matching interface) to the parent phase, there is an advantage that it does not affect superplastic deformation while acting as a grain boundary stabilizing particle. Therefore, when molding using superplasticity is applied, molding can be performed with a lower processing force than conventional precipitated particle-dispersed superplastic materials. A specific example is shown in Example 1 and FIG. When the strain rate is constant, it can be seen that the deformation stress of the magnesium alloy in which the general precipitated particles are dispersed instead of the quasicrystalline particles shows a larger value than the deformation stress of the Mg-Zn-RE alloy in which the quasicrystalline particles are dispersed. . Further, from FIG. 11, the superplastic behavior (particularly, elongation at break) of the Mg—Zn—RE alloy to which the rare earth element is added is superior to that of the Mg—Zn alloy, and the dispersion of the quasicrystalline phase is effective for the development of the superplastic behavior. I understand that.
In inventions 1 and 2, it is a method of realizing processing by utilizing the superplasticity of a magnesium alloy without causing alteration while taking advantage of such a quasicrystalline phase. This example is clear from Example 2 and FIG. In other words, when the room temperature mechanical properties of the sample after the forming process under the condition that the superplasticity of the present invention is developed and the sample before the superplastic forming process (only heat treatment) are compared, the deterioration of the strength and ductility characteristics is observed. I understand that it is not done.
The magnesium alloy of the invention 3 uses the superplasticity of the magnesium alloy without causing alteration while taking advantage of the quasicrystalline phase even if the conditions are slightly different from those of claim 1 or 2. Can be processed.
The invention 4 provides a method for producing the magnesium alloy of the invention 2.

The magnesium alloy warm working method of the present invention is such that the magnesium alloy has a quasicrystalline phase and A in the following (formula 1) is more than 1.8 × 10 ⁶ and less than 7.2 × 10 ^6. Sets temperature and strain rate.
(Formula 1)

As shown in Example 1 of the present invention and FIG. 11, the Mg—Zn—RE alloy in which the quasicrystalline particle phase is dispersed in the magnesium matrix shows superplastic behavior. In addition, if the rare earth element of any one of Y, Gd, Tb, Dy, Er, and Ho and zinc are within the following composition range, the quasicrystalline phase is dispersed in the magnesium matrix. 1 to 6 are clear.
(Formula 3)
Mg {100- (y + x)} ZnyREx
In the composition formula, 0.2 ≦ x ≦ 1.5, 5x ≦ y ≦ 7x.
Where x and y are atomic%
In order to show superplastic behavior, it is an essential condition that the size of the magnesium matrix is fine (the crystal grain size is 15 μm or less). Therefore, the sample produced by the casting method is homogenized at 460 ° C. or lower for 4 hours or longer, then held at 200 to 300 ° C. for 15 to 90 minutes, and then processed at a processing ratio of 8: 1 to 400 at the same temperature. : 1 to give strong strain processing. The strong strain processing method may be anything as long as it is effective for extending the magnesium alloy, such as extrusion or rolling.

3 atomic% zinc and 0.5 atomic% holmium were melt cast in commercial pure magnesium (purity 99.95%) to prepare a master alloy.
Experiment No. 1 in Table 1 In No. 2, the mother alloy was held in a furnace at 400 ° C. (in units of 50 ° C., hereinafter the same) for 24 hours, and homogenized. After removal from the furnace, the tissue was frozen by water quenching. Thereafter, an extruded billet having a diameter of 40 mm was prepared by machining. After raising the billet to 200 ° C., the billet was held for 1/2 hour and subjected to warm extrusion at an extrusion ratio of 18: 1 to obtain an extruded material.
Also, transmission electron microscopy structure observation results with an average grain size 2μm or less (see FIG. 1 (a)), the quasi-crystal phase having a composition of 100nm approximately _Mg 3 _Zn 6 Ho (FIG. 1 ( It was confirmed that the structure of (b)) was formed and showed a consistent interface with the magnesium matrix. The experimental method and the results of the tissue observation are summarized in Table 1.
Other rare earth elements: Y, Gd, Tb, Er, and Dy were prepared by the same method as in Example 1. That is, 3 atomic% zinc and 0.5 atomic% of various rare earth elements were melt cast in commercial pure magnesium (purity 99.95%) to prepare a master alloy. Thereafter, the mother alloy was held in a furnace at 400 ° C. for 24 hours, homogenized, and quenched by water quenching. Thereafter, an extruded billet was prepared by machining, and the billet was heated at 200 ° C., then held for 1/2 hour, and subjected to warm extrusion at an extrusion ratio of 18: 1 to obtain an extruded material. The result of the structure observation using the transmission electron microscope of Mg-Zn-RE alloy extruded material is shown in FIGS. Regardless of the kind of rare earth element added, the same morphological form as in FIG. 1, that is, the average crystal grain size of the mother phase: 2 μm or less, and a quasicrystalline phase consisting of a composition of Mg ₃ Zn ₆ RE of about 100 nm is dispersed in the mother phase Confirmed to do.
On the other hand, Mg—Zn alloys containing no rare earth elements were all produced by the same method except for the homogenization treatment conditions. The homogenization treatment was carried out under conditions of 300 ° C. and 48 hours because it did not contain rare earth elements. FIG. 7 shows the result of the structure observation of the Mg—Zn alloy extruded material using a transmission electron microscope. Since no rare earth element was contained, there was no quasicrystalline phase, and it was confirmed that only the b phase composed of Mg—Zn was dispersed in the magnesium matrix.
The composition analysis of each sample was investigated using ICP emission spectroscopy.

Mg-3at. % Zn-0.5 at. Table 2 shows the measured values obtained as a result of the high-temperature tensile test of the% Ho alloy. Table 3 shows measured values obtained from the results of high-temperature tensile tests of Mg—Zn—RE alloys and Mg—Zn alloys other than Ho.

A tensile test piece having a parallel part diameter of 2.5 mm and a length of 5.0 mm was collected from the extruded Mg-Zn-Ho material. Using these test pieces, a high-temperature tensile test was performed within a range of a temperature of 200 to 350 ° C. and a strain rate of 1 × 10 ⁻⁵ to 10 ⁻² s ⁻¹ .
A typical true stress-true strain curve is shown in FIG. It can be seen that the slower the strain rate during the tensile test, the greater the elongation.
The true stress when the true strain reaches 0.1 is defined as flow stress, and the relationship between flow stress-strain rate and break elongation-strain rate at each temperature is shown in FIG. When the strain rate sensitivity index: m value (m = 1 / n, n is a stress index) in FIG. 9A is 0.5, it is generally said that the plastic film has superplastic behavior. Based on this, the conditions under which the superplastic behavior is expressed are outlined.
From FIG. 9 (b), it was found that a large elongation was obtained under the condition where the superplastic behavior was exhibited, and the maximum elongation was exhibited under the conditions of temperature: 200 ° C. and strain rate: 1 × 10 ⁻⁵ s ^−1. I understand. An appearance photograph at that time is shown in FIG.
Except for RE = Ho, Mg-Zn-RE alloy extrudates and Mg-Zn alloy extrudates were also subjected to a high-temperature tensile test in the same manner as described above. However, the tensile test temperature is only 200 ° C. FIG. 11 shows the relationship between the flow stress-strain rate and the elongation at break-strain rate. Similar to the result of the Mg—Zn—Ho alloy extruded material, there is a condition that the strain rate sensitivity index: m value is 0.5, and the development of superplastic behavior can be confirmed. Moreover, strain rate: 1 In × ¹⁰ -5 ^{to 10} within the range of -3 ^{s -1,} it can be confirmed that it shows a huge extend. On the other hand, it can be seen that the Mg-Zn alloy having no quasicrystalline phase has a small m value and is smaller than the elongation of the Mg-Zn-RE alloy. In particular, the tendency is remarkable at a strain rate of 1 × 10 ⁻⁵ to 10 ⁻³ s ⁻¹ . Therefore, it is suggested that the dispersion of the quasicrystalline phase is effective for exhibiting superplastic behavior exhibiting a huge ductility.

The constitutive equation describing the high temperature deformation behavior is generally described by the following (Equation 1).
(Formula 1)

In superplastic deformation of a magnesium alloy, an effective diffusion coefficient: D _eff is used, and n = 2 (m = 0.5) and p = 2 are used as typical values of each value.
The effective diffusion coefficient is described by the following (Equation 2).
(Formula 2)

FIG. 12 shows the relationship between the crystal grain size obtained in the high-temperature tensile test of the Mg-Zn-RE alloys shown in Tables 2 and 3, the strain rate corrected with the effective diffusion coefficient, and the flow stress corrected with the rigidity. (Crystal grain size used at the time of normalization in FIG. 12: The crystal grain size of a sample held for 30 minutes at each temperature was measured, and the value was used.)
FIG. 12 also shows a general superplastic magnesium alloy having no precipitated particles.
The cases of formulas (1) and (2) (thick solid line in FIG. 12) and a superplastic magnesium alloy in which precipitated particles exist (thin solid line in FIG. 12) are also shown.
It can be seen that the deformation stress of the quasicrystalline particle-dispersed superplastic magnesium is higher than that of the superplastic material in which no precipitated particles are present, but is lower than that of general precipitated particle-dispersed magnesium.
That is, when the quasicrystal-dispersed magnesium exhibits superplastic behavior, the material constant A is found to be in the range of 1.8 × 10 ^{6 to} 7.2 × 10 ⁶ .
In addition, these results show that when superplastic deformation and combined warm working are performed, larger deformation stress is required than general superplastic magnesium alloy, but compared with general precipitated particle dispersed magnesium alloy. This means that molding is possible with low deformation stress.

The effects on room temperature mechanical properties before and after superplastic forming were investigated. The Mg—Zn—Y alloy extruded material used in Example 1 was held in a furnace at 200 ° C. for ½ hour. Then, after water cooling, a tensile test piece having a parallel part diameter of 2.5 mm and a length of 10 mm was collected (heat treated material), and a tensile test was performed at room temperature. A typical nominal stress-strain curve is shown in FIG. The yield stress was 242 MPa and the elongation at break was 23%. The yield stress is a stress value at a nominal strain of 0.2%, and the elongation at break is a nominal strain value when the nominal stress is reduced by 30% or more. On the other hand, the Mg—Zn—Y alloy extruded material used in Example 1 was subjected to 50% high temperature tensile deformation at 200 ° C. and 1 × 10 ⁻⁴ s ⁻¹ , and then water-cooled, and the parallel part diameter was 2.5 mm. A tensile test piece having a length of 10 mm was collected (superplastic deformation material). The result of the room temperature tensile test of the sample is shown in FIG. The yield stress is 233 MPa, the elongation at break is 21%, and it can be confirmed that there is almost no difference compared to the mechanical properties of the heat-treated material.

  The present invention opens the way to various material applications of magnesium alloys that have been regarded as promising as lightweight and high-strength materials, and realizes the use of magnesium alloys in machines and devices that require weight reduction such as aircraft and automobiles. Is.

Example of TEM structure observation of Mg-3at% Zn-0.5at% Ho alloy: (a) crystal grain size structure, (b) quasicrystalline phase The TEM structure observation photograph of a Mg-Zn-Y alloy extrusion material. The TEM structure observation photograph of a Mg-Zn-Gd alloy extrusion material. The TEM structure observation photograph of a Mg-Zn-Tb alloy extrusion material. The TEM structure observation photograph of a Mg-Zn-Dy alloy extrusion material. The TEM structure observation photograph of a Mg-Zn-Tb alloy extrusion material. TEM structure observation photograph of Mg-Zn alloy extruded material. True stress-true strain curve obtained at a test temperature of 200 ° C. High temperature tensile test results of Mg-3at% Zn-0.5at% Ho alloy: (a) Flow stress-strain rate, (b) Elongation at break-strain rate Appearance photograph after tensile test. Mg-3at% Zn-0.5at% RE alloy and Mg-3at% Zn alloy high temperature tensile test results: (a) flow stress-strain rate, (b) elongation at break-strain rate Relationship between crystal grain size obtained by high-temperature tensile test of Mg-Zn-RE alloy extruded material, strain rate corrected by effective diffusion coefficient, and flow stress corrected by rigidity Nominal stress-strain curve obtained by room temperature tensile test using samples before and after superplastic forming

Claims (4)

A method for warm processing of a magnesium alloy, wherein the magnesium alloy has a quasicrystalline phase, and the processing temperature is set so that A in the following formula 1 is more than 1.8 × 10 ⁶ and less than 7.2 × 10 ^6. A method of warm working a magnesium alloy, characterized by setting a strain rate.
(Formula 1)
  2. The warm working method according to claim 1, wherein the magnesium alloy is an Mg—Zn—RE alloy (RE: rare earth metal). 3. A magnesium alloy for warm working, having a quasicrystalline phase with a magnesium parent phase having a crystal grain size of 15 μm or less and an average particle diameter of 2 μm or less, and satisfying the following general formula (3) A magnesium alloy characterized by that.
(Formula 3)
Mg {100- (y + x)} Zn _y RE _x (3)
0.2 ≦ x ≦ 1.5, 5x ≦ y ≦ 7x, x and y are atomic%,
RE is a rare earth element of any one of Ho, Y, Gd, Tb, Dy, and Er. It is a manufacturing method of the magnesium alloy of Claim 3, Comprising: The mother alloy which has the composition which shows the said (Formula 3) is homogenized at 460 degrees C or less for 4 hours, and then 15-90 at 200-300 degrees C A method for producing a magnesium alloy, characterized in that after holding for a minute, a high strain processing is performed at a processing ratio of 8: 1 to 400: 1.