Artifactless superelastic alloy

Abstract

The present invention provides an artifactless superelastic alloy including a Au—Cu—Al alloy, the superelastic alloy containing Cu in an amount of 20 atom % or more and 40 atom % or less, Al in an amount of 15 atom % or more and 25 atom % or less, and Au as a balance, the superelastic alloy having a bulk magnetic susceptibility of −24 ppm or more and 6 ppm or less. The Ni-free superelastic alloy of the present invention is capable of exhibiting superelasticity in a normal temperature range, and hardly generated artifacts in a magnetic field environment. The alloy can be produced by setting a casting time in a melting and casting step to a fixed time, and hot-pressing an alloy after casting to make material structures homogeneous.

Claims

1. An artifactless superelastic alloy comprising a Au—Cu—Al alloy, the superelastic alloy containing Cu in an amount of 20 atom % or more and 40 atom % or less, Al in an amount of 15 atom % or more and 25 atom % or less, and Au as a balance, the superelastic alloy having a bulk magnetic susceptibility of −24 ppm or more and 6 ppm or less.

2. The superelastic alloy according to claim 1, comprising Cu in an amount of 25 atom % or more and 35 atom % or less, Al in an amount of 18 atom % or more and 25 atom % or less, and Au as a balance.

3. The superelastic alloy according to claim 2, having the bulk magnetic susceptibility of −8.1 ppm or more and −0.6 ppm or less.

4. A method for producing the superelastic alloy defined in claim 1, comprising the steps of: melting and casting a Au—Cu—Al alloy containing Cu in an amount of 20 atom % or more and 40 atom % or less, Al in an amount of 15 atom % or more and 25 atom % or less, and Au as a balance, the melting and casting step including producing the Au—Cu—Al alloy with a molten metal solidification time set to 6.0 sec or less; and hot-pressing the Au—Cu—Al alloy subjected to the melting and casting step at a temperature of 550° C. or higher and 650° C. or lower.

5. The method for producing the superelastic alloy according to claim 4, comprising a homogeneity heat treatment step of heating the alloy after the hot-pressing step at a temperature of 450° C. or higher and 550° C. or lower.

6. A method for producing the superelastic alloy defined in claim 2, comprising the steps of: melting and casting a Au—Cu—Al alloy containing Cu in an amount of 20 atom % or more and 40 atom % or less, Al in an amount of 15 atom % or more and 25 atom % or less, and Au as a balance, the melting and casting step including producing the Au—Cu—Al alloy with a molten metal solidification time set to 6.0 sec or less; and hot-pressing the Au—Cu—Al alloy subjected to the melting and casting step at a temperature of 550° C. or higher and 650° C. or lower.

Description

DESCRIPTION OF EMBODIMENTS

(1) Hereinafter, an embodiment of the present invention will be described. In this embodiment, Au—Cu—Al alloys while concentrations of Cu and Al were changed, and various properties such as a superelastic property in a normal temperature range, a bulk magnetic susceptibility, mechanical properties and processability were then evaluated.

(2) For preparation of the superelastic alloy, Cu having a purity of 99.99%, Al having a purity of 99.99% and Au having a purity of 99.99% were used as melt raw materials. By use of a non-consumable W electrode-type argon arc melting furnace, these raw materials were melted in an Ar-1% H.sub.2 atmosphere, and a molten metal thus obtained was solidified to produce a Au—Cu—Al alloy ingot. In this embodiment, the molten metal was cast with a water-cooled copper mold, and a solidification time here was 1.6 sec.

(3) Next, the produced alloy ingot was treated by hot-pressing. The hot-pressing was performed by pressing the ingot in vacuum at 600° C. and 100 MPa for 3600 sec. After the hot-pressing treatment, the alloy ingot was heated at 500° C. for 3.6 ksec to make the ingot homogeneous. The ingot (thickness: 1 to 2 mm) subjected to the homogeneity treatment was gradually cooled, and used for preparation of test pieces.

(4) The alloy ingot was subjected to discharge processing to prepare tension test pieces (thickness: 0.2 mm, width: 2 mm and length: 20 mm). Alloys processed into the test pieces were subjected to final heat treatment. The heat treatment was performed by heating the alloy at 500° C. for 1 hour, followed by rapid cooling.

(5) For each test piece, the superelastic property, the bulk magnetic susceptibility, mechanical properties and processability were evaluated. Methods for evaluation of the properties are as follows.

(6) Superelastic Property

(7) A transformation temperature (Ms) was measured by a DSC (differential scanning calorimetry) method. As a heating condition, a temperature elevation or fall rate over a range of −150° C./min to 150° C. was 10° C./min. Test pieces having a measured transformation temperature of 310 K (37° C.) or lower were rated excellent (⊚) because it would be possible to exhibit superelasticity. Test pieces having a transformation temperature of higher than 310 K (37° C.) were rated poor (×).

(8) Bulk Magnetic Susceptibility (Artifact)

(9) A bulk magnetic susceptibility (Xvol) of each test piece was measured by a magnetic balance. The measurement was performed at room temperature. A deviation of the measured bulk magnetic susceptibility from a magnetic susceptibility of water (−9 ppm) was calculated. Test pieces with a deviation within ±5 ppm were rated excellent (⊚), test pieces with a deviation within ±15 ppm or less were rated good (∘), and test pieces with a deviation beyond ±15 ppm were rated poor (×).

(10) Mechanical Properties

(11) For each test piece, a Vickers hardness (HV) was measured with a Vickers tester (load: 300 gf). For evaluation, test pieces having a Vickers hardness of 300 Hv or more were rated excellent (⊚), test pieces having a Vickers hardness of 200 Hv or more were rated good (∘), test pieces having a Vickers hardness of 150 Hv or more were rated fair (Δ), and test pieces having a Vickers hardness of less than 150 Hv were rated poor (×).

(12) Processability

(13) For each test piece, a compression strain (ε) was measured with a compression tester. The measurement was performed at room temperature and a strain rate of 3.3×10.sup.−4 s.sup.−1. For evaluation, test pieces having a compression strain (ε) of 10% or more were rated excellent (⊚), test pieces having a compression strain (ε) of 5% or more were rated good (∘), and test pieces having a compression strain (ε) of less than 5% were rated poor (×).

(14) The measurement results and evaluation results of the above properties are shown in Table 1.

(15) TABLE-US-00001 TABLE 1 Measurement result Bulk magnetic Superelastic susceptibility Mechanical Composition (at %) property (artifact) property Processability No. Au Cu Al Ms/K Evaluation Xvol/ppm Evaluation HV/Hv Evaluation ε/% Evaluation 1 Balance 27.0 18.0 266 ⊚ −3.5 ◯ 232 ◯ 9.6 ◯ 2 28.0 22.0 291 ⊚ −0.6 ◯ 152 Δ 18.2 ⊚ 3 25.0 25.0 294 ⊚ −2.7 ◯ 166 Δ 14.0 ⊚ 4 30.0 25.0 291 ⊚ −1.6 ◯ 249 ◯ 12.5 ⊚ 5 35.0 25.0 297 ⊚ −1.8 ◯ 381 ⊚ 7.6 ◯ 6 31.3 28.0 274 ⊚ −8.1 ◯ 398 ⊚ 2.7 X 7 45.0 25.0 — — 7.0 X 142 X 9.5 ◯

(16) Table 1 reveals that Au—Cu—Al alloys containing Cu in an amount of 20 atom % or more and 40 atom % or less and Al in an amount of 15 atom % or more and 25 atom % or less (No. 1 to No. 5) had a favorable transformation temperature (Ms) and bulk magnetic susceptibility (Xvol), and were capable of attaining exhibition of superelasticity and an artifactless property. These Au—Cu—Al alloys had acceptable mechanical properties (hardness) and processability (compression strain).

(17) On the other hand, an alloy containing Al in an amount of more than 25% (No. 6) had an extremely favorable magnetic susceptibility value owing to the magnetic susceptibility adjustment effect of Al, but significantly poor processability (compression strain). The present invention is based on an assumption that alloys are used as medical materials and processed into medical devices having various shapes, and therefore processability is also an important property. Accordingly, the alloy has unfavorable properties overall. An alloy containing Cu in an amount of more than 40% (No. 7) was confirmed to be an unfavorable alloy because the alloy did not exhibit superelasticity, and had a magnetic susceptibility of more than 6 ppm.

INDUSTRIAL APPLICABILITY

(18) The superelastic alloy of the present invention is an alloy which exhibits superelasticity in a normal temperature range, has an appropriate bulk magnetic susceptibility, and can be artifactless in a magnetic field environment such as MRI. The alloy is free from Ni, and therefore has biocompatibility which is an essential condition as a medical material. The alloy also has a favorable X-ray-imaging property. The present invention can be expected to be applied to various medical devices.