JP6373557B2 - Magnesium wrought alloy and method for producing the same - Google Patents

Description

The present invention relates to an Mg alloy in which a quasicrystalline phase is dispersed, and more particularly, an ultrahigh strength magnesium alloy having a sufficient ductility and containing a small amount of additive elements and obtained by a simple manufacturing process, and a method for manufacturing the same. About.

  Magnesium is a lightweight metal and is abundant on the earth. Therefore, it is gaining importance as a light weight material for structural parts, electronic devices and the like. Among these application fields, when a mobile structural component such as an automobile or a railway vehicle is applied, the material is required to have high strength and high ductility for safe and reliable operation. Of the material strengthening methods, it is most common to reduce the microstructure dimension of the matrix, for example, to refine the crystal grain size. The dispersion of fine particles in the matrix phase is one of the important methods for improving the material properties. In the latter case, the strength of the alloy is increased at the expense of ductility.

  Recently, the use of quasicrystalline particles as dispersed particles has attracted attention. The quasicrystal does not have a constitution of a repeating unit having a predetermined atomic arrangement, that is, it does not have a translational symmetry like a normal crystal phase. As a result, the quasicrystal lattice matches and bonds with various crystal lattices, and unlike other types of dispersed particles, the quasicrystal dispersed particles rarely serve as a starting point of material destruction. Regarding magnesium alloys, it is known that the dispersion of quasicrystalline particles has mechanical properties with an excellent balance between strength and ductility and exhibits very high fracture toughness. This is shown, for example, in JP-A-2002-309332, JP-A-2005-113234, and JP-A-2005-113235.

  However, the tensile yield strength of these drawn alloys is limited to a range of 250 to 300 MPa and breaks at an elongation of about 13%. A high yield strength of about 400 MPa while having sufficient ductility is required for many structural applications. Due to energy consumption and expensive processes such as severe plastic deformation and powder metallurgy, the mechanical strength is further improved.

  In the Mg-6 atomic% Zn-1 atomic% Y alloy (Zn 14.3 mass%, Y 3.24 mass%), ductility and excellent mechanical properties have been reported. When a rapidly solidified cast material is extruded, the yield strength in tension and compression is about 400 MPa, and the elongation at break is 12 to 16% (A. Singh, Y. Osawa, H. Somekawa and T. Mukai, Scripta Mater. 64 (2011) 661). However, this alloy has a high density (a large amount of alloying elements), requires production by rapid solidification casting, and is heavy and expensive.

  The following describes the invention of a low density alloy with a less restrictive casting process that exhibits high mechanical properties with a tensile yield strength of 400 MPa level and an elongation of 12 to 16%.

An object of the present invention is to prepare the following Mg alloy.
(1) With a simple, energy and cost efficient approach,
(2) The amount of alloy elements is small,
(3) the characteristics of the grain structure of about 1 micron size;
(4) To be accompanied by moderate ductility (total elongation exceeding 12%) and exhibiting high tensile yield strength exceeding 350 MPa.

According to one embodiment of the present invention, the composition range is Mg-y atom% Zn-x atom% Y, where y is in the range of 5x ≦ y ≦ 7x and x is 0.2 ≦ x ≦ 0. The remaining portion is a magnesium extended alloy composed of commercial grade metal magnesium and unavoidable impurities, and a dendrite structure having a supersaturated region of zinc in the parent phase of the magnesium extended alloy. And having a Y- rich region of a quasicrystalline phase and a cubic or hexagonal phase, prepared by a drawing process of extruding or rolling a dendritic cast structure in a temperature range of 230 ° C. to 280 ° C., and 370 MPa A magnesium wrought alloy having the above tensile yield strength is shown .
The quasicrystalline phase has a size of 1 μm or less , typically 200 nm, and a 20 nm nanoscale quasicrystalline phase is dispersed in the alloy. The average grain size of the alloy is about 1 μm. In the alloy, nano-sized particles typically having a size of 15 nm are precipitated and dispersed.

According to one aspect of the present invention, supersaturation of zinc in the Mg—Zn—RE alloy leads to fine precipitation during further processing by extension processes such as extrusion and rolling. These precipitates prevent crystal growth during the drawing process and strengthen the final alloy product.
According to one embodiment of the present invention, the ternary phase of the alloy is a quasicrystalline phase, and the crystal grain size of the alloy is about 1 micron.
According to one aspect of the present invention, the alloy has a breaking elongation value of 12% or more, preferably in the range of 12-16%.
The method for producing a magnesium wrought alloy of the present invention has a composition range of (100-xy) atomic% Mg-y atomic% Zn-x atomic% Y, where y is 5x ≦ y ≦ 7x. Within the range, x is in the range of 0.2 ≦ x ≦ 0.5, and the remaining part is a method for producing a magnesium extended alloy composed of commercial grade metal Mg and inevitable impurities, Prepared by a drawing process in which a cast material having a structure is extruded in a temperature range of 230 ° C. to 280 ° C., the extrusion ratio is between 2: 1 to 50: 1, and zinc is contained in the structure of the magnesium drawn alloy. And having a tensile yield strength of 370 MPa or more, consisting of a quasicrystalline Y-rich region and a cubic or hexagonal phase .

The mechanical properties of the alloy (tensile yield strength> 370 MPa, elongation at break> 12%) are maintained over the entire composition range shown in FIG. Therefore, an ultra-high strength ultra-light alloy can be manufactured at low cost.
The alloy manufacturing process is very simple and cost effective, with only solidification and extrusion or rolling. Manufacturing is a two-step process of solidification and extrusion, so manufacturing costs are low.
Therefore, the production cost is also low for the following reasons: (1) that there is no homogeneous treatment generally performed at a high temperature (above 400 ° C.), (2) the extrusion temperature is low, (3) No heat treatment after extrusion is required.
As a result, a low density magnesium alloy with very high strength combined with ductility is obtained at a very low production cost.

FIG. 1 is a plot of density, tensile yield strength (YS) and elongation at break for extruded alloys versus zinc and yttrium alloy components. Tensile yield strength and elongation remain constant for the alloy components.

FIG. 2 is a polarized photograph of the cast structure of an Mg-3 atomic% Zn-0.5 atomic% Y (ZW82) alloy cast by rapid solidification casting of Example 1 taken with an optical microscope. Various crystal grains and their dendrite structures are clearly distinguished.

3 shows (a) a rapidly solidified casting mold, (b) a normal or conventional mold, (c) an Mg-3 atomic% Zn-0. It is a photograph of the cast structure of 5 atomic% Y (ZW82) alloy.

FIG. 4 is a photograph of the cast structure of Example 1 after casting with a rapid solidification casting mold, taken with a scanning electron microscope (SEM). (A) shows the non-uniformity of the composition in the matrix due to the dendrite solidified structure, and is taken in the backscattering mode. (B) is the chemical composition by energy dispersive spectroscopy (EDS) along the line, and shows the concentration profiles of Mg, Zn and Y along this line.

FIG. 5 shows a standard binary phase diagram of (a) Mg—Zn alloy system and (b) Mg—Y alloy system. Lines corresponding to Mg-3 atomic% Zn in (a) and Mg-0.5 atomic% Y in (b) are marked.

FIG. 6 is a photograph of the cast structure in the water-cooled mold of Example 1 taken by SEM. The compositions determined by EDS at various points marked 1 to 11 are listed in Table 2.

FIG. 7 is a photograph of the alloy after extrusion of Example 1, in which Mg-3 atomic% Zn-0.5 atomic% Y (ZW82) was photographed by SEM. Bright particles have a ternary phase distribution, mainly a quasicrystalline phase. (A) has shown the structure | tissue extruded at 235 degreeC after casting with the rapid solidification casting mold. (B) shows the structure extruded at 266 ° C. after casting with a water-cooled mold.

FIG. 8 shows Mg-3 atomic% Zn-0.5 atomic% after extrusion of Example 1 (a) rapidly solidified casting mold, (b) conventional mold, and (c) water cooled mold ingot. It is the photograph which shows the microstructure of Y (ZW82) alloy, and was image | photographed with the transmission electron microscope (TEM) in bright field mode.

[FIG. 9] FIG. 9 shows the extruded Mg-3 atomic% Zn-0.5 atomic% Y (ZW8) of Example 1.
2) A photograph showing the distribution of fine precipitates (nanometer scale) in the parent phase of the alloy, (a) Rapid solidification casting mold and (b) Size distribution of precipitates in it, (c) A solidified mold with a conventional mold and (d) a water-cooled mold is shown, and was taken with a TEM in bright field mode.

[FIG. 10] FIG. 10 shows the Mg-3 atomic% Zn-0.5 atomic% Y (ZW82) alloy solidified by the conventional mold, the rapid solidification mold and the water-cooled mold of Example 1 and obtained by tensile after extrusion. Is a nominal stress-strain curve.

[FIG. 11] FIG. 11 shows Mg-2.6 extruded in Example 2 at 257 ° C. after rapid solidification casting.
A photograph showing the microstructure of an atomic% Zn-0.34 atomic% Y (ZW61) alloy, taken with a TEM. (A) Distribution of general grain structure and ternary phase (mainly quasicrystals, dark particles, one of which is indicated by an arrow), (b) Fine precipitates in the parent phase Is shown. The size distribution of the precipitate is inserted in (b).

[FIG. 12] FIG. 12 is a diagram showing Example 2 in which M was extruded at 257 ° C. after solidification in a rapidly solidified casting mold
2 shows a representative nominal stress-strain curve obtained by tensioning a g-2.26 atom% Zn-0.34 atom% Y (ZW61) alloy.

[FIG. 13] FIG. 13 shows Mg-1.2 extruded in Example 3 at 250 ° C. after rapid solidification casting.
A photograph showing the microstructure of an atomic% Zn-0.2 atomic% Y (ZW31) alloy, taken with a TEM. (a) Coarse ternary phase (mainly quasicrystal) distributed as elongated clusters along a general crystal grain structure and diagonal line, (b) shows fine precipitates in the matrix.

[FIG. 14] FIG. 14 shows Mg-1.2 extruded in Example 3 at 250 ° C. after rapid solidification casting.
2 shows a representative nominal stress-strain curve obtained by tensioning an atomic% Zn-0.2 atomic% Y (ZW61) alloy.

  The Mg alloy is prepared to include a quasicrystalline phase. In order to achieve this, the following composition preferably has the general formula (100-xy) atomic% Mg-y atomic% Zn-x atomic% RE. Here, RE is any one of rare earth elements Y, Gd, Tb, Dy, Ho or Er, y is in the range 5x ≦ y ≦ 7x, and x is in the range 0.2 ≦ x ≦ 1.5. .

  The molten alloy is poured into a mold, usually a steel mold, and cast. The mold provides a cooling rate sufficient to form a dendrite solidified structure. A supersaturated region of zinc is also formed, which is beneficial for the final properties. As long as the dendritic solidification form is achieved, the solidification cooling rate may be varied from 4 K / s to 25 K / s and beyond. The solidified structure is important as the initial structure for effective microstructural refinement during extrusion or another stretching process.

  A finely solidified structure (small crystal grains and dendrite protrusion size) generally leads to refinement of the microstructure after extrusion. As is well known, the formation of a dendrite structure depends on the solidification rate and the solute concentration as compared with the cellular grain structure. For dendritic structures, fast solidification rates and dense solute concentrations are preferred. Each dendrite protrusion is a single crystal grain, but due to the thin dendrite arm, the structure is much finer than the crystal size.

  The dendrite structure also ensures a fine dispersion of the ternary phase (mainly quasicrystalline phase) in the alloy. Since the ternary phase is formed between the dendrite arms, it is more uniformly distributed. This also promotes fine dispersion in the matrix after extrusion. Fine dispersion of the quasicrystalline phase in the alloy is essential for strengthening.

  Further, during solidification, supersaturation of solute atoms occurs in front of the progressing solidification interface. This supersaturation is beneficial for two reasons: (1) Fine precipitates can be generated in situ during or after extrusion. (2) The solute can change the texture formed during extrusion, resulting in a desirable weak texture.

In situ precipitation during extrusion produces very fine nano-sized precipitates. These precipitates pin the grain boundaries and prevent grain growth after recrystallization during extrusion . Thus, a very fine particle size is obtained after extrusion of the alloy. Furthermore, these fine precipitates have a large number density and the fine precipitates pin the movement of dislocations, thereby imparting high strength to the alloy. Precipitates formed during extrusion are rounded in shape rather than needle-like or disk-shaped elongated, which is advantageous for excellent ductility.

  The solidified alloy is then machined into billets and extruded in a 25: 1 ratio. The low extrusion temperature results in a fine microstructure and excellent mechanical properties. A low temperature range of 230 to 280 ° C. is recommended, and the average particle size is refined to 1 micron or less.

The ternary phase is mainly quasicrystalline Mg ₃ Zn ₆ RE, but the cubic phase Mg ₃ Zn ₃ RE ₂ and the hexagonal phase are dispersed in the alloy. In the α-Mg matrix crystal grains, quasicrystalline nanosized precipitates (size is generally about 20 nm) and monoclinic Mg ₄ Zn ₇ phase are dispersed.

Table 1 shows the alloy compositions studied as examples. Alloy ZW82 was cast by casting into three different types of molds. The cast alloy was extruded into a round bar with a diameter of 8 mm. A tensile specimen having a parallel part length of 25 mm and a diameter of 3 mm was machined from a round bar. A plurality of compression tests were performed using a cylindrical test piece having a height of 8 mm and a diameter of 4 mm. The mechanical test was performed on an Instron testing machine at an initial strain rate of 10 ⁻³ / sec at room temperature.

  Extruded alloys exhibit a tensile yield strength exceeding 370 MPa with a break elongation (ductility) of 12% or more. The mechanical properties are plotted against the alloy composition in FIG. The tensile yield strength remains higher than 370 MPa for all compositions. The same applies to the elongation at break, both exceeding 12%. In this way, a magnesium alloy having a very high strength can be obtained with a composition of a very low concentration of alloy elements and an appropriate ductility. Individual examples are described below.

As shown in Table 1, 3 atomic percent zinc, 0.5 atomic percent yttrium, and commercial grade pure magnesium (purity 99.5%) (8 mass percent zinc specified by ZW82, 2 mass percent) Of yttrium with a density of 1.861 g / cm ³ ) was melt cast to produce an alloy. The alloy was cast at three different solidification rates using three different molds:
(1) Thick steel plate mold (rapidly solidified casting mold),
(2) Conventional steel mold,
(3) Water-cooled (WC) mold.
The melt at a temperature of 750 ° C. was poured into the mold. The cooling rate was measured by thermocouples at three locations on the mold. The overall cooling rate ranged from 4.1 K / sec to 25 K / sec. The cooling rate of the water-cooled mold was the slowest.

  The cast structure of the rapidly solidified cast alloy observed with a polarizing microscope is shown in FIG. Different contrasts distinguish different grains. The bright contrast crystal grains stand out especially in the upper left part. In each crystal grain, a dendrite arm is visible. The crystal grain size (or dendrite size) is about 500 μm, while the dendrite arm is about 50 μm thick.

The cast structure observed by a scanning electron microscope (SEM) is shown in FIG. These photographs show the dendrite solidification structure. Dendrites are outlined by light hues between dendrites, which are ternary phases, mainly quasicrystalline phases. Among the three types of molds, the microstructure of the water-cooled mold is rough.

These solidification structures were examined in detail by SEM. FIG. 4A shows a detailed microstructure of the rapid solidification casting under the backscattering composition contrast in the SEM. Apart from the bright white pattern which is the second phase (ternary phase) (mainly quasicrystalline phase), a bright band is observed in the dark part of the matrix, but these are on the extension of the dendritic arm. Looks like there is. Two bright bands are indicated by arrows.
The dark region shows a composition of approximately Mg-1.8 atomic% Zn, which is close to the solubility of zinc in magnesium at room temperature. The light gray area shows a composition close to Mg-2.4 atomic% Zn, which is the maximum solubility of zinc in magnesium at higher (eutectic) temperatures (state of FIG. 5 (a)). (See figure).
The composition along the line segment is shown in FIG. Along the horizontal line in this photograph, the concentration profiles of Mg, Zn and Y are individually plotted in the figure below. Where the line intersects the ternary phase (indicated by the three arrows on the right), both zinc and yttrium levels increase rapidly. In addition, there are solute-rich regions in the matrix, three of which are arrowheaded. In these regions, the zinc concentration is high, but the yttrium content is not (it remains constant). Thus, a zinc supersaturated region is formed in the parent phase.

  Such a structure is caused by the solidification process. In FIG. 5A, the Mg-rich portion of the Mg—Zn phase diagram is shown in a simplified manner. At the zinc concentration of this alloy (3 atomic% Zn), vertical lines are drawn parallel to the temperature axis. As the solidification progresses, α-Mg based crystals are first formed in the melt. With decreasing temperature, these crystals are in equilibrium with increasing melt concentration. For example, at 600 ° C. (line segment A1), the melt concentration is about 6.2 atomic%, and at 400 ° C. (line segment A2), the α-Mg based crystal grains are melted at 6.3 atomic% with zinc. It is in equilibrium with the liquid. The eutectic phase is formed at 300 ° C. (line segment A3) and is in equilibrium with α-Mg at a zinc concentration of about 2.4 atomic%. Therefore, the eutectic phase coexists with a high concentration α-Mg matrix (between dendrite protrusions). A partial Mg—Y binary phase diagram is shown in FIG. In the case of a binary Mg—Y alloy containing 0.5 atomic% yttrium, yttrium is in solid solution at 100 ° C. or higher even though the solubility at room temperature is almost zero.

  FIG. 6 shows the detailed microstructure of the water-cooled mold casting as observed by backscattering composition contrast in SEM. The composition was measured by the number of the marked point and is listed in Table 2. The points marked 1, 2 and 3 are ternary phases and show very bright contrast. A slight contrast is observed inside the matrix, which is attributed to the binary phase, as is the region of zinc enrichment. The average concentration in the dark region of the parent phase was measured to be 1.86 ± 0.03 atomic percent Zn, while it was 2.53 ± 0.17 atomic percent Zn in the bright region of the parent phase. On average, about 7% by volume is eutectic and 17% is the supersaturated matrix.

  The castings were machined into 40 mm diameter ingots and then extruded into 8 mm diameter round bars at a temperature of about 250 ° C. with an extrusion ratio of 25: 1.

  The microstructure of the extruded alloy is shown in FIGS. FIG. 7 shows the distribution of the ternary phase in the extruded alloy of the rapidly solidified cast alloy extruded at 235 ° C. and the water-cooled mold cast alloy extruded at 266 ° C. by a scanning electron microscope. The area ratio of the eutectic phase is estimated to be about 14%. The crystal grain structure of the extruded alloy observed with a transmission electron microscope is shown in FIG. The black pieces in each photo are ternary phases, mainly quasicrystalline phases. These dispersed phases are very fine, from 1 μm to 50 nm for fine ones. The crystal grain size is about 1 micron or less (Table 3).

  The photograph taken with the transmission electron microscope in FIG. 9 shows fine precipitates formed in the alloy. These precipitates have a size range of 5-20 nm. These precipitates are formed during the extrusion process when the extrusion temperature is below 300 ° C. These precipitates are formed at a high density because the parent phase before extrusion is a supersaturated solid solution. These precipitates are effective in suppressing dislocations during deformation. Due to their small size and form, these precipitates do not significantly affect the ductility of the alloy. FIG. 9 (a) shows the alloy solidified by rapid solidification casting and then extruded at 235 ° C. at high magnification. The particle size distribution of the precipitate in this micrograph is shown in FIG. The average size is 12.66 ± 3.21 nm. FIG. 9 (c) shows fine precipitates in an alloy cast with a conventional mold and then extruded at 240 ° C. FIG. 9 (d) shows fine precipitates in the alloy cast in a water-cooled mold and then extruded at 266 ° C.

From the extruded round bar, a tensile test piece having a parallel part diameter of 3 mm and a length of 15 mm and a compression test piece having a diameter of 4 mm and a height of 8 mm were prepared. The tensile and compressive properties of the test pieces were evaluated at room temperature. The direction in which the test piece was collected was parallel to the extrusion direction, and the initial tensile and compressive strain rate was 1 × 10 ⁻³ s ⁻¹ .

  FIG. 10 shows the nominal stress-strain curve obtained by tensile and compression tests of alloy Mg-3 atomic% Zn-0.5 atomic% Y extruded after rapid solidification casting, conventional mold casting and water-cooled mold casting. The tensile yield strength ranges from 370 MPa to over 400 MPa, and the elongation ranges from 12 to 16%. These values are listed in Table 3.

The alloy has a composition of Mg-2.26 atomic% Zn-0.34 atomic% Y (5.8 mass% zinc, 1.2 mass% yttrium, called ZW61, density 1.81 g / cm ³ ). (Table 1). This was rapidly solidified and cast using the method described in Example 1 and then extruded at 257 ° C. This alloy had an average grain size of about 1.1 μm. The microstructure showing the crystal grain structure was taken with a transmission electron microscope and is shown in FIG. Another detailed TEM photograph shown in FIG. 11B shows fine precipitates in the matrix phase. The size distribution of the precipitate is inserted in this figure. The average precipitate size is 8.8 ± 2.0 nm. A typical nominal stress-strain curve obtained in the tensile test is shown in FIG. It had an elongation value of about 13% and a tensile yield strength of 377 MPa (Table 3).

The alloy has a composition of Mg-1.2 atomic% Zn-0.2 atomic% Y (3.2 mass% zinc, 0.7 mass% yttrium, called ZW31, density 1.785 g / cm 3) ( Table 1). This was rapidly solidified and cast by the method described in Example 1. In addition to extrusion , tensile and compression tests were performed under the same conditions as described in Example 1. A microstructure taken by TEM is shown in FIG. The average crystal grain size is about 1 micron. As shown in the high-magnification photograph of FIG. 13 (b), high-density fine nano-sized precipitates are clearly observed. A nominal stress-strain diagram by the tensile test is shown in FIG. This alloy had an elongation value of 12.3% and a tensile yield strength of 389 MPa (Table 3). Therefore, the density is 1.785 g / cm ³ , and it can be said that the alloy is lightweight and very high strength.

The above is provided primarily for illustrative purposes. Further changes and variations in various parameters of the alloy composition and process and conditions can continue to embody the basic and novel concepts of the present invention. These will readily occur to those skilled in the art and are within the scope of the present invention.

  The present invention is applied to an Mg alloy having a quasicrystalline phase dispersion having a combination of ultrahigh strength and ductility. The Mg alloy of the present invention is suitable as a structural member for highly reliable applications. The Mg alloy of the present invention is suitable for structural uses such as various types of frames such as automobile door frames and seat frames.

JP 2002-309332 A JP 2005-113234 A JP 2005-113235 A

A. Singh, Y. Osawa, H. Somekawa and T. Mukai, Scripta Materialia 64 (2011) 661.

Claims (4)

The composition range is (100-xy) atomic% Mg-y atomic% Zn-x atomic% Y, where y is in the range of 5x ≦ y ≦ 7x, and x is 0.2 ≦ x ≦ 0. .5, the remainder being a magnesium extended alloy consisting of the commercial grade metal Mg and inevitable impurities,
It has a dendrite structure having a supersaturated region of zinc in the structure of the magnesium extended alloy, and consists of a Y-rich region of a quasicrystalline phase and a cubic or hexagonal phase,
A magnesium expanded alloy having a tensile yield strength of 370 MPa or more, prepared by a stretching process in which a cast material having a dendrite cast structure is extruded or rolled in a temperature range of 230 ° C to 280 ° C.   The magnesium wrought alloy according to claim 1, wherein the ternary phase of the magnesium wrought alloy is a quasicrystalline phase, and the crystal grain size of the alloy is 1 micron or less.   The magnesium extended alloy according to claim 1 or 2, wherein the magnesium extended alloy has a breaking elongation value of 12% or more.   The composition range is (100-xy) atomic% Mg-y atomic% Zn-x atomic% Y, where y is in the range of 5x ≦ y ≦ 7x, and x is 0.2 ≦ x ≦ 0. The remaining part is a method for producing a magnesium extended alloy comprising the commercial grade metal Mg and inevitable impurities,
  It is prepared by a drawing process in which a cast material having a dendrite cast structure is extruded in a temperature range of 230 ° C. to 280 ° C.,
  The extrusion ratio is between 2: 1 and 50: 1,
  It has a dendrite structure with a zinc supersaturated region in the structure of the magnesium wrought alloy, consisting of a Y-rich region of a quasicrystalline phase and a cubic or hexagonal phase, and has a tensile yield strength of 370 MPa or more. Manufacturing method of magnesium wrought alloy.