Magnesium alloy

Abstract

A magnesium alloy containing, in % by mass, 0.95 to 2.00% of Zn, 0.05% or more and less than 0.30% of Zr, 0.05 to 0.20% of Mn, and the balance consisting of Mg and unavoidable impurities, wherein the magnesium alloy has a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm and a standard deviation of 0.7 or smaller.

Claims

1. A magnesium alloy containing, in % by mass, 0.95 to 2.00% of Zn, 0.05% or more and less than 0.30% of Zr, 0.05 to 0.20% of Mn, and the balance consisting of Mg and unavoidable impurities, wherein a total content of the unavoidable impurities is 30 ppm or less, a content of each of Fe, Ni, Co, and Cu as unavoidable impurities is less than 10 ppm, the magnesium alloy consists of a matrix phase consisting of single-phase solid solution and Zr-bearing precipitates dispersed in the matrix phase, wherein the particle size of the precipitates is smaller than 100 nm, the matrix phase of the magnesium alloy has a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm and a standard deviation of 0.7 or smaller, and the magnesium alloy has a fracture elongation of 38% or more and a tensile strength from 288 to 300 MPa in a value measured in accordance with JIS Z2241.

2. The magnesium alloy as claimed in claim 1, wherein the magnesium alloy does not contain rare-earth elements and aluminum.

3. The magnesium alloy as claimed in claim 1, wherein the fracture elongation of the magnesium alloy is 50% or less in a value measured in accordance with JIS Z2241.

4. The magnesium alloy as claimed in claim 1, wherein the magnesium alloy has a proof strength from 145 to 220 MPa in values measured in accordance with JIS Z2241.

5. A medical device comprising a metal member including the magnesium alloy as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the figures,

(2) FIG. 1 shows an SEM (scanning electron microscope) image of a microstructure of a magnesium alloy according to Example 1 of the present invention;

(3) FIG. 2 shows an SEM image of a microstructure of a magnesium alloy according to Example 2 of the present invention;

(4) FIG. 3 shows a graph of a particle size distribution of a magnesium alloy according to Example 1 of the present invention; and

(5) FIG. 4 shows a graph of a particle size distribution of a magnesium alloy according to Example 2 of the present invention.

DESCRIPTION OF THE EMBODIMENTS

(6) Hereinafter, an embodiment of the present invention will be described.

(7) Magnesium Alloy

(8) A magnesium alloy of the present invention contains, in % by mass, 0.95 to 2.00% of Zn, 0.05% or more and less than 0.30% of Zr, 0.05 to 0.20% of Mn, and the balance consisting of Mg and unavoidable impurities, wherein the magnesium alloy has a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm and a standard deviation of 0.7 or smaller.

(9) The present invention has revealed that plastic working of the magnesium alloy is improved by controlling a composition of the alloy to the above ranges and that properties of the magnesium alloy such as fracture elongation are improved by achieving fine and uniform particle size of the alloy.

(10) The magnesium alloy having the above features can avoid formation of coarse precipitates which may be triggers (starting points) of fractures and thereby reduce the possibility of breakage during and after deformation. It should be noted that although Zr, which is added in order to reduce the crystal particle size of the alloy, may form precipitates, the precipitates are typically dispersed at a nanometer scale (in a size smaller than 100 nm) in the matrix phase and thus has a negligible impact on deformation and corrosion of the alloy. For example, FIG. 1 shows an SEM image of an alloy of Example 1, and FIG. 2 shows an SEM image of an alloy of Example 2, as described later. In the figures, the areas having dark contrast show the magnesium alloy (brightness differs among crystal grains), and the white bars at the bottom show a scale of 1 μm. In both of FIG. 1 and FIG. 2, only a few precipitates having particle sizes smaller than 100 nm are observed in some crystal grains of the magnesium alloy, and there are virtually no precipitates in the crystal grain boundaries.

(11) Zinc (Zn): in % by mass, 0.95% or more and 2.00% or less

(12) Zn is added in order to enhance the strength and elongation ability of the alloy by forming a solid solution with Mg. Where the content of Zn is less than 0.95%, a desired effect cannot be obtained. An amount of Zn exceeding 2.00% is not preferred because such an amount may exceed a solid solubility limit of Zn in Mg so that Zn-rich precipitates are formed, resulting in reduced corrosion resistance. For this reason, Zn content is regulated to 0.95% or more and 2.00% or less. The content of Zn may be less than 2.00%.

(13) Zirconium (Zr): in % by mass, 0.05% or more and less than 0.30%

(14) Zr hardly forms a solid solution with Mg and forms fine precipitates, providing an effect of preventing formation of coarse crystal particles of the alloy. Addition of Zr at an amount less than 0.05% cannot provide a sufficient effect. Addition of Zr at an amount equal to or exceeding 0.30% leads to formation of a large amount of precipitates, with a reduced effect of particle size reduction. In addition, corrosion and breakage would start occurring at portions where the precipitates are segregated. For this reason, content of Zr is regulated to 0.05% or more and less than 0.30%. The content of Zr may be 0.10% or more and less than 0.30%.

(15) Manganese (Mn): in % by mass, 0.05% or more and 0.20% or less

(16) Mn allows the alloy to have extremely fine particle size and have improved corrosion resistance. Where an amount of Mn is less than 0.05%, a desired effect cannot be obtained. An amount of Mn exceeding 0.20% is not preferred because plastic workability of the alloy tends to decrease. For this reason, Mn content is regulated to 0.05% or more and 0.20% or less. A preferable content of Mn may be 0.10% or more and 0.20% or less.

(17) Unavoidable Impurities

(18) Preferably, the content of unavoidable impurities is also controlled in the magnesium alloy for medical use. Since Fe, Ni, Co, and Cu promote corrosion of the magnesium alloy, the content of each of these unavoidable impurities is preferably lower than 10 ppm, further preferably 5 ppm or lower, and preferably substantially absent. The total content of the unavoidable impurities is preferably 30 ppm or less, and further preferably 10 ppm or less. Preferably, the magnesium alloy is substantially free from rare-earth elements and aluminum. Where an amount of an impurity element in the alloy is less than 1 ppm, it is regarded that the alloy is substantially free from the impurity element. The amount of impurity may be determined, for example, by ICP optical emission spectrometry.

(19) Production of Magnesium Alloy

(20) In accordance with an ordinal production method of a magnesium alloy, the magnesium alloy may be produced by throwing ground metals or alloys of Mg, Zn, Zr, Mn into a crucible, melting the ground metals and/or alloys in the crucible at a temperature from 650 to 800° C., and casting the molten alloy. Where necessary, the cast alloy may be subjected to solution heat treatment. The ground metals do not contain rare-earth elements (and aluminum). It is possible to suppress the amounts of Fe, Ni, Co, and Cu in the impurities by the use of high purity ground metals. Fe, Ni, and Co in the impurities may be removed by de-ironing treatment to the molten alloy. In addition, or alternatively, it is possible to use ground metals produced by distillation refining.

(21) Metal Microstructure and Mechanical Properties

(22) By the above-described controls of composition and production process, the magnesium alloy can have a fine and uniform structure as seen in a particle size distribution with an average crystal particle size from 1.0 to 3.0 μm (for example, from 1.0 to 2.0 μm) and a standard deviation of 0.7 or smaller (for example, from 0.5 to 0.7). The standard deviation is preferably 0.65 or smaller. Fine precipitates containing Zr may each have a particle size smaller than 500 nm (preferably smaller than 100 nm). A matrix phase excluding the Zr precipitates may preferably be an single-phase solid solution of Mg—Zn—Mn ternary alloy.

(23) The alloy has the following mechanical properties: a tensile strength from 230 to 380 MPa (for example, from 250 to 300 MPa), a proof strength from 145 to 220 MPa, and a fracture elongation from 15 to 50% (for example, from 25 to 40%) in accordance with JIS Z2241. The alloy preferably has a tensile strength exceeding 280 MPa. The alloy preferably has a fracture elongation exceeding 30%.

(24) Medical Device

(25) The magnesium alloy of the present invention has excellent properties as a metal for medical purposes because the alloy has excellent elongation ability and the components of the alloy is controlled to be non-toxic components with non-toxic concentrations for living tissue. The magnesium alloy of the present invention may be suitably used as a metal member constituting a medical device, such as stents, staplers, screws, plates, and coils. For example, the magnesium alloy may be processed to a pipe-shaped member by hot extrusion. The thus-obtained pipe-shaped member may be processed to have a tubular tubular shape by cold-drawing and be further laser-processed to form a stent.

Example

(26) Preparation of Magnesium Alloy

(27) High purity ground metals of Mg, Zn, Mn, and Zr were prepared as initial materials. Each of the metals was weighed so as to have a component concentration as described in Table 1 and was thrown into a crucible. Then, at 730° C. the metals were molten with stirring, and a thus-obtained melt was cast to form ingots. Thus-obtained magnesium alloys of Example 1 and Example 2 contained the main components at formulation ratios which fall within the present invention. The initial materials used did not contain rare earth elements or aluminum even as unavoidable impurities. In this regard, 99.99% pure magnesium ground metal having a low concentration of impurity Cu was used. De-ironing treatment was carried out in the furnace in order to remove iron and nickel from the melt. Concentrations of impurities in the thus-obtained samples were determined using an ICP optical emission spectrometer (AGILENT 720 ICP-OES manufactured by AGILENT). Table 1 shows the compositions of Example 1 and Example 2. The concentrations of Fe, Ni, and Cu were all lower than 8 ppm (Ni and Cu were lower than 3 ppm). Al and the rare-earth elements were not detected, and Co was also below a detection limit. The total content of the unavoidable impurities was 11 ppm.

(28) TABLE-US-00001 TABLE 1 Component Impurity concentration (%) concentration (ppm) Mg Zn Mn Zr Fe Ni Cu Total Example 1 the balance 1.86 0.14 0.12 5 3 3 11 Example 2 the balance 0.95 0.11 0.24 8 3 1 11

(29) Measurement of Mechanical Properties

(30) Each alloy according to the examples was formed into a round bar material through hot extrusion. In accordance with JIS Z2241, a tensile strength, a proof strength, and a fracture elongation of the round bar material were determined. Table 2 shows the results.

(31) Observation of Metal Microstructure

(32) A cross section of an extruded material of the alloy was cleaned by Ar ion beam sputtering and was observed using a scanning electron microscope (JSM-7000F manufactured by JEOL). From the observation, an average particle size was determined using electron back scattering diffraction (EBSD) technique, and a standard deviation of a particle size distribution was calculated. Table 2 shows the results, and FIG. 3 and FIG. 4 show graphs of particle size distributions. As for each sample, observation of precipitates was carried out over an observation region of 2 mm×2 mm, and no precipitate having a particle size of 100 nm or larger was found.

(33) TABLE-US-00002 TABLE 2 Tensile Proof Average strength strength Elongation crystal particle Standard (MPa) (MPa) (%) size (μm) deviation Example 1 288 213 38 1.97 0.62 Example 2 297 217 27 1.97 0.63

(34) The present invention provides a magnesium alloy which has an excellent deformation property and can prevent corrosion due to potential difference because the magnesium alloy includes a matrix phase which forms a single-phase solid solution. Thus, it is possible to suitably control a decomposition rate of the magnesium alloy in living tissues. For this reason, the magnesium alloy is highly applicable, for example, as a metal member for a medical device, such as stents and staplers, which involves deformation during use and requires stable biodegradability.