JP5419061B2 - Magnesium alloy - Google Patents

Description

  The present invention relates to a magnesium alloy exhibiting age hardening characteristics.

Magnesium alloys are expected to be applied to automobiles and aircraft as lightweight structural materials.
Crystal grain refinement, which is the main strengthening means, is very effective in improving room temperature strength, but it is difficult to improve high temperature strength. On the other hand, strengthening by dispersing the second phase composed of magnesium and additive elements can improve the strength in a wide temperature range depending on the type and distribution state of the phase to be dispersed.
In particular, aging precipitation can disperse fine precipitates in the grains and grain boundaries by combining the solution treatment for forming a supersaturated solid solution and the subsequent aging treatment. Inhibits the movement of dislocations and can improve the strength in a wide temperature range. Further, if the precipitated phase dispersed in the crystal grain boundary is thermally stable, the grain boundary slip can be suppressed and the high temperature strength can be improved.
An alloy system in which such an aging precipitation phenomenon can be expected is an alloy having a difference in solubility of alloy elements between a high temperature region and a low temperature region. Many magnesium alloys such as Mg-Al and Mg-Zn alloys are aging precipitation type, but the alloy system that can disperse the high melting point precipitation phase by aging precipitation is composed of magnesium and rare earth metal. Most of the expensive alloys are Mg-Al-based and Mg-Zn-based commercial magnesium alloys that do not contain rare earth metals. Heat resistance was imparted by forming a network and sacrificing ductility.
For example, the production cost of magnesium alloys containing rare earth metals of Patent Documents 1 to 5 is higher than that of rare earth metal-free magnesium alloys. In addition, the alloys shown in Patent Documents 1 to 5 in which an expensive rare earth metal element is contained in the constituent elements are difficult to die cast and are uneconomical for mass production of automobile parts.
On the other hand, in an alloy that does not contain a rare earth metal such as Patent Documents 6 to 11, a high melting point intermetallic compound is dispersed on the grain boundary in order to improve the high temperature strength. It causes ductility reduction and casting crack.
Further, even if the high temperature strength is improved by dispersing fine particles in the matrix as in Patent Document 12, if the process is complicated, the cost may increase.

Publication title: Phase Diagrams of Binary Magnesium alloys Publication date: 1988 Author: Nayeb-Hashemi AA and Clark JB JP-A-9-172948 JP 2005-113235 A JP-A-2005-213535 JP2008-75176 JP2008-1227639 JP-A-6-256833 JP-A-7-34172 JP 10-324941 A JP2002-266044 JP 2004-277761 A JP2007-70688A JP2008-69438

  In view of such circumstances, an object of the present invention is to provide a magnesium alloy that causes an aging precipitation phenomenon without using rare earth elements.

Magnesium alloy of the present invention, bismuth 0.85 atomic% or less, zinc contains less than 2 atomic% 0.5 atom% or more, a magnesium alloy and the balance being magnesium and unavoidable impurities, grain size 100 to 200 μm, bismuth and zinc are dissolved in the matrix by solution treatment to form a supersaturated solid solution, and a plate-like precipitate is deposited on the (1120) plane which is the columnar surface of magnesium. (Here, the correct notation for the (1120) plane is the one described in the following equation 1).
In the magnesium alloy of the present invention, preferably, the plate-like precipitate has a diameter of 100 to 250 nm and a thickness of 20 nm.
In the magnesium alloy of the present invention, preferably, second phase particles having a bismuth-rich crystal grain size of 2 to 3 μm are present. In the magnesium alloy of the present invention, preferably, the plate-like precipitate has a diameter of 40 to 50 nm and a thickness of 10 nm.
In the magnesium alloy of the present invention, the aging treatment is preferably performed until the maximum hardness is reached.

In the Mg-Bi alloy according to the present invention, the melting point of the precipitated phase predicted from the phase diagram is about 800 ° C., which is equivalent to that of the rare earth metal element. It is possible to develop a magnesium alloy having heat resistance without forming a coarse crystallized network that becomes a starting point of fracture like an alloy containing no metal.
In addition, by adding an appropriate amount of Zn, it was possible to disperse a large amount of plate-like precipitates deposited on the (1120) plane, which is the most effective for strengthening the magnesium alloy.

(1) Alloy composition: an alloy containing at least Bi and Zn as main components.
The following results became clear from the following experimental results.
Addition amount of Bi:
It is preferable that at least this is included and the content is 0.85 atomic% or less.
As described in Non-Patent Document 1, Bi does not substantially dissolve in the Mg matrix at room temperature, but the solid solubility limit increases rapidly with increasing temperature, and dissolves in 0.96 atomic% Mg at 550 ° C.
However, usually, the upper limit of the heat treatment temperature of the Mg alloy is 500 to 530 ° C., and when the upper limit of the solution treatment temperature is used, the amount of Bi that can be dissolved is about 0.85 atomic%. It becomes the upper limit of.
The amount of Bi that can be dissolved in the magnesium matrix during the solution treatment is limited to about 0.85 atomic% depending on the maximum temperature of the solution treatment, so even if more Bi is added, the hardness due to aging precipitation In addition, it is impossible to expect the effect of increasing the strength, and Bi added excessively forms coarse crystallized products at the grain boundaries, which causes the casting to break during water cooling after solution treatment, resulting in ductility. Cause damage.

Addition amount of Zn:
Addition of 0.5 atomic% or more is desirable. More preferably, it is 0.5 atomic% or more and less than 2 atomic%.
Zn is an additive element that improves the age hardening of the Mg-Bi alloy. As shown in Examples 1 and 2, in order to achieve a Vickers hardness of 60 VHN, addition of 0.5 atomic% or more is desirable.
It is clear from FIGS. 4 and 7 that this is caused by the refinement of precipitates and the change in shape and distribution state due to the addition of Zn.
In addition, as shown in FIG. 8, when 2 atomic% of Zn is added, not only is the amount of hardening caused by aging precipitation different from the case of adding 1 atomic%, but also after solution treatment as shown in FIG. The casting breaks during water cooling. Therefore, a more preferable addition amount of Zn is 0.5 atomic% or more and less than 2 atomic%.
If excessive Zn is added, the contained elements cannot be dissolved in the matrix during the solution treatment, and the crystallized product of Bi, Zn and Mg remains on the grain boundary. It can be easily inferred from Fig. 9 (f).

In order to maximize the strengthening effect due to aging precipitation and to have a high strength, the aging treatment is preferably performed until the maximum hardness is reached. In the case of an alloy to which 0.5 atomic% of Zn is added, it is preferably 25 hours, and in the case of an alloy to which 1.0 atomic% or more of Zn is added, it is preferably 100 hours.

Example 1 shows an example of an Mg-0.8Bi-0.5Zn alloy. Unless otherwise specified, alloy compositions are expressed in atomic percent.
The experiment was performed according to the flowchart shown in FIG.
First, pure magnesium, pure bismuth and pure zinc were melted in an iron crucible using a high frequency induction melting furnace and cast into an iron mold.
The obtained ingot was sealed with He in a Pyrex tube, homogenized at 525 ° C. for 48 hours using a muffle furnace, and cooled with water.
Next, the homogenized material was again sealed with He in the Pyrex tube, subjected to a solution treatment at 525 ° C. for 48 hours using a muffle furnace, and cooled with water.
Thereafter, an aging treatment was performed at 160 ° C. using an oil bath.
In order to examine the change in hardness during the aging treatment, the time-dependent change in hardness was measured using a Vickers hardness meter, and an age hardening curve was prepared. When measuring the age hardening curve, the alloy is taken out from the oil bath after a certain period of time, measured 10 times from any place with a load of 300 g and a load time of 10 seconds, and the maximum and minimum values are measured. The average value of the eight measured values was taken as the hardness of the alloy at that time.
The microstructure is observed using OM (Optical microscope) and SEM (Scanning electron microscope), and the precipitation structure in the grain is observed by TEM (Transmission electron microscope). Microscope)).
The mechanical properties of the material that reached the maximum hardness during aging treatment were evaluated by compression test.
As shown in the age hardening curve of FIG. 2 based on the measurement data of Table 1, the Vickers hardness after solution treatment is 47 VHN, and reaches peak aging 30 hours after the start of aging. The Vickers hardness at peak aging was 61 VHN, and the increase in hardness by aging treatment was 14 VHN.

Note: Measurement values filled in gray indicate measurement data excluded when calculating the average value.

The structure after solution treatment observed using OM and SEM is shown in FIG.
As shown in FIG. 3 (a), the crystal grain size is about 100 to 200 μm , and as shown in FIG. 3 (b), Bi and Zn are dissolved in the parent phase by solution treatment, and the supersaturated solid solution is obtained. Is forming.
FIG. 4 shows the intragranular structure at the time of peak aging observed using TEM. FIGS. 4A and 4B are obtained by observing the microstructure from the (1120) and (0001) crystallographic axis at a low magnification, and FIGS. 4C and 4D are (1120) and (0001). ) The microstructure is observed at high magnification from the zone axis.
As shown in FIGS. 4A and 4B, the precipitates are uniformly dispersed in the matrix phase. Further, as a result of the trace analysis from FIGS. 4C and 4D, many plate-like precipitates (diameter: 100 to 250 nm × thickness 20 nm) precipitated on the (1120) plane which is the columnar surface of magnesium are observed. It was.

Example 2 shows an example of an Mg-0.8Bi-1.0Zn alloy. Unless otherwise specified, alloy compositions are expressed in atomic percent.
As in Example 1, an experiment was performed according to the flowchart shown in FIG.
First, pure magnesium, pure bismuth and pure zinc were melted in an iron crucible using a high frequency induction melting furnace and cast into an iron mold.
The obtained ingot was sealed with He in a Pyrex tube, homogenized at 525 ° C. for 48 hours using a muffle furnace, and cooled with water.
Next, the homogenized material was again sealed with He in a Pyrex tube, and solution treatment was performed at 525 ° C. for 48 hours using a muffle furnace, followed by water cooling.
Thereafter, an aging treatment was performed at 160 ° C. using an oil bath.
In order to examine the change in hardness during the aging treatment, the time-dependent change in hardness was measured using a Vickers hardness meter, and an age hardening curve was prepared. When measuring the age hardening curve, the alloy is taken out from the oil bath after a certain period of time, and the hardness is measured 10 times from any different place with a load of 300 g and a load time of 10 seconds. The average value of the eight measured values excluding the minimum hardness was the hardness of the alloy at that time.
The microstructure is observed using OM (Optical microscope) and SEM (Scanning electron microscope), and the precipitation structure in the grains is observed by TEM (Transmission electron microscope). Microscope)).
The mechanical properties of the material that reached the maximum hardness during aging treatment were evaluated by compression test.
Like the age hardening curve shown in FIG. 5 based on the measurement data of Table 2, the Vickers hardness after solution treatment is 44 VHN, and reaches peak aging in 100 hours after aging start. The Vickers hardness at the time of peak aging was 68 VHN, and the increase in hardness by aging treatment was 22 VHN. Moreover, as a result of strength evaluation by a compression test, a yield strength of 139 MPa was shown.

Note: Measurement values filled in gray indicate measurement data excluded when calculating the average value.

FIG. 6 shows the microstructure after solution treatment observed by OM and SEM. As shown in FIG. 6 (a), the crystal grain size is about 100 to 200 μm , and as shown in FIG. 6 (b), Bi and Zn are substantially dissolved in the matrix by solution treatment. However, the presence of second-phase particles of about 2 to 3 μm , which seems to be a Bi-rich compound, was slightly confirmed.
FIGS. 7A and 7B show the microstructure in the grains at the maximum hardness of the Mg-0.8Bi-1.0Zn alloy during peak aging observed using TEM. FIGS. 7A and 7B are observed at low magnifications from the (0110) and (0001) crystal zone axes, respectively. The precipitate is refined compared to Mg-0.8Bi-0.5Zn. 7 (c) and 7 (d) are similar to the Mg-0.8Bi-0.5Zn alloy in which this structure is observed at a low magnification from the (0110) and (0001) crystal zone axes, respectively. Most of the precipitates were plate-like precipitates (diameter 40 to 50 nm × thickness 10 nm) with the (1120) plane being the columnar surface of magnesium as the crystal habit plane.

In addition to the above examples, the age hardening behavior of Mg-0.8Bi, Mg-0.8Bi-0.2Zn, and Mg-0.8Bi-2.0Zn alloys was also investigated.
The age hardening curves of these alloys are shown in FIG. 8, and Table 4 summarizes the Vickers hardness after solution treatment and during peak aging, and the increase in hardness due to aging treatment.

Note: Measurement values filled in gray indicate measurement data excluded when calculating the average value.

FIG. 9 shows the microstructure of the Mg-0.8Bi, Mg-0.8Bi-0.2Zn, and Mg-0.8Bi-2.0Zn alloys observed using OM and SEM. As seen in the optical microstructure shown in FIGS. 9 (a) and (b), the Mg-0.8Bi and Mg-0.8Bi-0.2Zn alloys are Mg-0.8Bi-0.5Zn shown in FIG. 3 and FIG. It has the same crystal grain size as Mg-0.8Bi-1.0Zn alloy. In addition, as seen in the backscattered electron images shown in FIGS. 9 (d) and 9 (e), in these alloys, Bi and Zn are the same as the Mg-0.8Bi-0.5Zn and Mg-0.8Bi-1.0Zn alloys. It is a solid solution in the phase.
However, as shown in Table 4 and the optical microscope image of FIG. 9 (c), the Mg-0.8Bi-2.0Zn alloy is composed of coarser grains than the alloys shown in the other experimental examples. As shown in the backscattered electron image of 9 (f), crystallized substances that could not be dissolved in the matrix even by solution treatment remain on the grain boundaries.

The magnesium alloy of the present invention is a magnesium alloy member having heat resistance strength. In particular, it may be applied to parts around an automobile engine.

Flow chart of experiment Age hardening curve of Mg-0.8Bi-0.5Zn alloy Microstructure of Mg-0.8Bi-0.5Zn alloy after solution treatment, (a): Optical microstructure, (b): Backscattered electron image In-grain microstructure TEM bright-field image of Mg-0.8Bi-0.5Zn alloy at maximum hardness, (a): (1120) In-grain TEM image observed at low magnification from the zone axis, (b): (0001 ) Intragranular TEM image observed at a low magnification from the zone axis, (c): (1120) Intragranular TEM image observed at a high magnification from the zone axis, (d): High from the (0001) zone axis TEM image in the observed grain Age hardening curve of Mg-0.8Bi-1.0Zn alloy Microstructure of Mg-0.8Bi-1.0Zn alloy after solution treatment, (a): Optical microstructure, (b): Backscattered electron image In-grain microstructure TEM bright field image of Mg-0.8Bi-1.0Zn alloy at maximum hardness, (a): (0110) Intra-grain TEM image observed at low magnification from the zone axis, (b): (0001 ) Intragranular TEM image observed at a low magnification from the zone axis, (c): (0110) Intragranular TEM image observed at a high magnification from the zone axis, (d): High from the (0001) zone axis TEM image in the observed grain Age hardening curves of Mg-0.8Bi, Mg-0.8Bi-0.2Zn, Mg-0.8Bi-2.0Zn alloys Mg-0.8Bi, Mg-0.8Bi-0.2Zn, microstructure of Mg-0.8Bi-2.0Zn alloy, (a): Optical microstructure of Mg-0.8Bi alloy, (b): Mg-0.8Bi-0.2Zn Optical microstructure of alloy, (c): Optical microstructure of Mg-0.8Bi-2.0Zn alloy, (d): Reflected electron image of Mg-0.8Bi alloy, (e): Mg-0.8Bi-0.2Zn alloy Backscattered electron image, (f): Backscattered electron image of Mg-0.8Bi-2.0Zn alloy Photograph of ingot after solution treatment of Mg-0.8Bi, Mg-0.8Bi-1.0Zn, Mg-0.8Bi-2.0Zn alloy Correct indication of (1120) in the description Correct indication of (0001) in the description Correct indication of (0110) in the description

Claims (5)

0.85 atomic% or less of bismuth,
Containing 0.5 atomic% or more and less than 2 atomic% of zinc,
The balance is magnesium alloy consisting of magnesium and inevitable impurities,
The crystal grain size is 100 to 200 μm. First, bismuth and zinc are dissolved in the matrix by solution treatment to form a supersaturated solid solution, and then a plate is formed on the (1120) plane which is the column surface of magnesium by aging treatment. A magnesium alloy characterized in that a precipitate is deposited (here, the correct notation for the (1120) plane is the one described in the following equation 1).
  The magnesium alloy according to claim 1, wherein the plate-like precipitate has a diameter of 100 to 250 nm and a thickness of 20 nm. 2. The magnesium alloy according to claim 1, wherein second phase particles having a bismuth-rich crystal grain size of 2 to 3 μm are present.   The magnesium alloy according to claim 3, wherein the plate-like precipitate has a diameter of 40 to 50 nm and a thickness of 10 nm. The magnesium alloy according to any one of claims 1 to 4, wherein the aging treatment is performed until the maximum hardness is reached.