MEDICAL Pt-W ALLOY

Abstract

The present invention relates to a medical Pt—W alloy, containing 10 mass % or more and 15 mass % or less of W, with the balance being Pt and inevitable impurities, in which a Zr content is 1000 ppm or less. Limiting the Zr content can improve workability, particularly workability at the stage of hot working. Regarding impurity control, further limiting a Ca content to 250 ppm or less can provide more suitable workability. The present invention is good in workability in processing into a wire included in an embolic coil, a guide wire or the like.

Claims

1. A medical Pt—W alloy, comprising 10 mass % or more and 15 mass % or less of W, with the balance being Pt and inevitable impurities, wherein a Zr content is 1000 ppm or less.

2. The medical Pt—W alloy according to claim 1, wherein a Ca content is 250 ppm or less.

3. A medical tool, comprising the medical Pt—W alloy defined in claim 1.

4. The medical tool according to claim 3, wherein the medical tool is any one of a stent, a catheter, a coil, a guide wire, a delivery wire, dental braces, a clasp, an artificial tooth root, a clip, a staple, a bone plate, a nerve stimulation electrode, a lead for a pacemaker, and a radiation marker.

5. A medical tool, comprising the medical Pt—W alloy defined in claim 2.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0035] The FIGURE illustrates an SEM image of a cross section of a Pt-12 mass % W alloy (melt retention time: 3 min) produced in First Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] First Embodiment: A preferred embodiment of the present invention will now be described. In the present embodiment, a commercially available zirconia crucible was used to produce Pt—W alloy ingots having various compositions for evaluating workability in hot working. In the present embodiment, a Pt—W alloy (W concentration: 10 mass %, or 12 mass %) was produced precedently with a water-cooled copper mold, and this alloy was used as a mother alloy to be melted again in a zirconia crucible, and thus, Pt—W alloy ingots having the same composition were produced. The mother alloy was thus melted again to obtain alloy ingots for making melting conditions uniform among samples by using the alloys in the uniform state, and for concentrating a factor of mixture of impurities (such as Zr) to a melt retention time.

[0037] For producing each Pt—W alloy ingot, 240 g of the mother alloy was put in a commercially available zirconia crucible (product name: ZIR, capacity: 70 cc), and the resultant was heated in a high frequency melting apparatus under a reduced-pressure argon atmosphere. It was confirmed that the metal had been melted to form a melt, and a melt retention time from this time point until the output of the melting apparatus was stopped was adjusted. After stopping the output, the resultant was casted in a Cu mold over about 2 seconds using an automatic casting function of the melting apparatus. An ingot (dimension: diameter of 11 mm×length of 70 mm) produced by cooling the resultant to normal temperature was taken out of the mold. In the present embodiment, Pt-10 mass % W alloys (Example 1 and Comparative Example 1) respectively having the melt retention time set to 30 seconds and 1 minute, and Pt-12 mass % W alloys (Example 2 and Comparative Example 2) respectively having the melt retention time set to 30 seconds and 3 minutes were produced.

[0038] Thereafter, each of the ingots of the Pt—W alloys of the respective compositions thus produced was heated at 700° C., and then subjected to hot swaging at a processing rate of 15%, and workability was evaluated based on whether or not a processed sample (dimension: diameter of 9.5 mm×length of 80 mm) had a crack on the surface. A crack was checked visually and with a microscope, and when no crack was found in the entire sample, the sample was evaluated acceptable (good), and when even one crack was found, the sample was evaluated as unacceptable (poor). Besides, a sample in which no crack had been caused through hot working was subjected to cold swaging at room temperature at a processing rate of 15%, and then, similarly evaluated for whether or not a crack was caused.

[0039] It is noted that each Pt—W alloy was measured for a Zr content and a Ca content with a glow discharge mass analyzer (GD-MS, name of apparatus: Astrum) after producing the ingot. The measurement results and evaluation results are shown in Table 1.

TABLE-US-00001 TABLE 1 Melt Workability Workability Alloy retention Zr Ca in hot in cold composition time content content working working Example 1 Pt-10% W 30 sec 100 ppm 1 ppm good good Comparative 1 min 200 ppm 3 ppm good good Example 1 Example 2 Pt-12% W 30 sec  90 ppm 1 ppm good good Comparative 3 min 1050 ppm  30 ppm  poor — Example 2

[0040] It is understood from Table 1 that a Zr content corresponding to a component of the crucible is changed by adjusting the melt retention time in the crucible. It was confirmed that when the Zr content exceeds 1000 ppm, the workability was poor and a crack was caused on the surface even through hot working. On the other hand, a Pt—W alloy having a Zr content of 1000 ppm or less did not have a crack caused through hot working, and in addition, did not have a crack caused through cold working.

[0041] The FIGURE illustrates an SEM image (COMPO image) of the cross section of the Pt-12 mass % W alloy (melt retention time: 3 min) in which a crack was found. It is understood from the FIGURE that particles presumed as precipitates were randomly found on a grain boundary and in a grain in the Pt—W alloy having a high Zr content. This observation region was subjected to EDX analysis, and as a result, Pt, Zr and 0 were detected in positions of the precipitates. In EDX, peak positions of Pt and Zr are close to each other, and hence attention was paid to a detection position of 0, and the detection position of 0 overlapped the position of the precipitate, and thus, it was presumed that the precipitate was an oxide (Zr oxide).

[0042] Second Embodiment: In the present embodiment, Pt—W alloys were produced with a water-cooled copper mold used in the melting casting step and with Zr added to adjust the content. A Pt-10% W mother alloy prepared in the same manner as in First Embodiment was used, and was placed in a water-cooled copper mold together with Zr metal (purity of 99%) (total amount charged: 100 g). Thereafter, a melt was prepared by arc melting to produce an alloy ingot. In the present embodiment, the amount of Zr charged was set to 50 ppm, 100 ppm, 250 ppm, 500 ppm, 1000 ppm, or 1500 ppm in the Pt-10 mass % W alloy. It is noted that the amount of Zr charged and the Zr content (analysis value) in the produced alloy ingot do not accord with each other as described above. This is probably due to volatilization of the raw materials during melting, and in the present embodiment, evaluation was made based on the Zr content in the alloy ingot.

[0043] The Pt-10 mass % W alloy having different Zr contents thus produced were subjected to hot working and cold working in the same manner as in First Embodiment, and were evaluated for workability. Besides, in each of the alloy ingots, the Zr content was analyzed by GD-MS. The workability evaluation results thus obtained are shown in Table 2.

TABLE-US-00002 TABLE 2 Zr Content Workability Workability Alloy Amount Analysis in hot in cold composition charged value working working Example 4 Pt-10% W  50 ppm 45 ppm good good Example 5 100 ppm 80 ppm good good Example 6 250 ppm 240 ppm good good Example 7 500 ppm 365 ppm good good Example 8 1000 ppm  880 ppm good good Comparative 1500 ppm  1150 pp poor — Example 4 Ca content: substantially 0 ppm indicates data missing or illegible when filed

[0044] In the present embodiment, the Pt—W alloys with the Zr content intentionally adjusted were examined, and as shown in Table 2, a crack caused through hot working was observed in the Pt-10 mass % W alloy having a Zr content exceeding 1000 ppm. This result matches the result obtained in First Embodiment.

[0045] Third Embodiment: In the present embodiment, Pt-10 mass % W alloys were produced with a water-cooled copper mold used in the same manner as in Second Embodiment and with a Ca content adjusted. A mother alloy similar to that used in Second Embodiment was used, and was placed in a water-cooled copper mold together with a Ca powder (purity of 99%) (total amount charged: 100 g). Thereafter, a melt was prepared by arc melting to produce an alloy ingot. In the present embodiment, the Pt-10 mass % W alloys were produced with the amount of Ca charged set to 25 ppm, 50 ppm, 100 ppm, 250 ppm, 500 ppm, or 750 ppm. It is noted that the amount charged and an analysis value in the alloy ingot do not accord with each other also with respect to Ca due to volatilization during melting.

[0046] The Pt-10 mass % W alloys having different Ca contents produced in the present embodiment were subjected to hot working and cold working in the same manner as in First Embodiment, and was evaluated for workability in these processing. It is noted that the Ca content in each alloy ingot was analyzed by GD-MS. The workability evaluation results thus obtained are shown in Table 3.

TABLE-US-00003 TABLE 3 Ca content Workability Workability Alloy Amount Analysis in hot in cold composition charged value working working Example 9 Pt-10% W  25 ppm  10 pp good ∘ good ∘ Example 10  50 ppm  13 pp good ∘ good ∘ Example 11 100 ppm  62 pp good ∘ good ∘ Example 12 250 ppm 145 pp good ∘ good ∘ Example 13 500 ppm 188 pp good ∘ good ∘ Comparative 750 ppm 285 pp good ∘ poor x Example 5 Zr content: substantially 0 ppm indicates data missing or illegible when filed

[0047] As shown in Table 3, it was confirmed that even when Ca was contained in an amount beyond 250 ppm, a crack was not caused in hot working but was caused in cold working. It is deemed, based on this result, that Ca has a high possibility of affecting workability in cold working of a Pt—W alloy.

[0048] In First Embodiment using a zirconia crucible, the amount of Ca mixed was small even when the melt retention time was rather long (3 min). It is thus deemed that there is a lower possibility of excessive mixture of Ca than mixture of Zr. However, Ca is added to a constituent material of a crucible as a stabilizer, and hence mixture of a large amount of Ca may be of concern depending on the amount added (and through repeated melting operations). Besides, in some production sites, Pt alloy pieces removed in cutting processing or the like may be melted again to be used in some cases. In consideration of the variations of the composition of a crucible, and viewpoint of ensuring product cost/efficiency, when a zirconia crucible is used, the melt retention time should be noted for suppressing mixture of Zr and Ca.

[0049] Fourth Embodiment: In the present embodiment, a plurality of Pt—W alloys having different W concentrations were produced through procedures similar to those employed in First Embodiment, and wires were produced therefrom through hot working and cold working. Then, the wires thus produced were subjected to coiling processing (secondary processing) to evaluate workability in the processing.

[0050] For producing the Pt—W alloys, in the same manner as in First Embodiment, mother alloys (W concentration: 8 mass %, 10 mass %, 12 mass %, 15 mass %, and 16 mass %) of the Pt—W alloys were produced with a water-cooled copper mold used, and each of the mother alloys was melted again in a zirconia crucible to produce a Pt—W alloy ingot. Conditions for melting casting with the zirconia crucible were the same as those employed in Example 1 (melt retention time: 30 seconds). Then, the thus produced alloy ingot was heated at 700° C. for 10 minutes, and molded into a crude wire having a wire diameter of 3.5 to 7.4 mm through hot swaging. Next, the crude wire was subjected to cold wire drawing at room temperature to be processed into a wire diameter of 0.5 mm, and at this point, a process annealing treatment was performed by heating at 800° C. for 60 minutes under a nitrogen atmosphere. After the annealing, the resultant was further subjected to cold wire drawing to be processed into a Pt alloy wire having a wire diameter of 28 It is noted that a Zr content obtained after the casting with the zirconia crucible was 100 ppm or less in all the alloy ingots.

[0051] Through the hot/cold working described above, no crack/disconnection was caused in the materials to be processed. This is probably because the mixture of Zr was suppressed by applying the melting casting step similar to that of First Embodiment, and appropriately setting the retention time of the alloy melt.

[0052] Each of the Pt—W alloy fine wires (wire diameter of 28 μm) thus produced was measured for hardness on a cross section with a Vickers hardness measuring device (product name: HM-200, manufactured by Mitutoyo Corporation, load: 50 gf). Besides, a tensile testing machine for extra fine wires (Strograph E3-S: Toyo Seiki Seisaku-sho, Ltd.) was used to perform a tensile test for measuring tensile strength (UTS).

[0053] Then, each of the Pt—W alloy wires thus produced was subjected to coiling processing for evaluating workability in secondary processing. This evaluation was made depending on whether or not disconnection was caused during the coiling processing. For the coiling processing, a spring index (coil average diameter (D)/filament diameter (d)) was set to 4.5, and the wire was wound around a core material (diameter of 0.1 mm) for the processing. When an alloy wire with a length of 10 m could be processed to the last through the coiling processing, this sample was determined to have good workability (good). When a wire was disconnected during the processing, the processing was terminated at that point, and this sample was determined to have poor workability (poor). Measurement results and evaluation results obtained in the present embodiment are shown in Table 4.

TABLE-US-00004 TABLE 4 Evaluation of Mechanical properties secondary Alloy Tensile processing No. composition Hardness strength workability Example 14 Pt-8% W 368 Hv 2450 Mpa good Example 15 Pt-10% W 455 Hv 2783 Mpa good Example 16 Pt-12% W 498 Hv 2996 Mpa good Example 17 Pt-15% W 550 Hv 3159 Mpa good Comparative Pt-16% W 605 Hv 3359 Mpa poor Example 6

[0054] As described above, even when a Zr concentration in a Pt—W alloy exceeds 15 mass %, the alloy can be processed into a wire by suppressing Zr mixture. A Pt—W alloy wire having a W concentration of 16 mass % was, however, disconnected during secondary processing (coil processing). W contained in a Pt—W alloy contributes to strength increase of the alloy. This can be confirmed also from the measurement results of the mechanical properties shown in Table 4. Excessively high strength/hardness affects, however, workability in secondary processing. In consideration of the usage of the Pt—W alloy of the present invention, it was confirmed that an upper limit of the W concentration should be 15 mass %. Regarding a lower limit of the W concentration, it is not that an alloy wire having a W concentration of 8 mass % has low strength/hardness, but with a reference standard set to a hardness value of 400 Hv (tensile strength of 2500 MPa), it was confirmed that the W concentration is adequately 10 mass % or more.

INDUSTRIAL APPLICABILITY

[0055] As described above, a medical Pt—W alloy of the present invention is excellent in workability, and an alloy ingot of this alloy can be easily processed into alloy materials in various shapes. In particular, workability in processing into a wire is also excellent, and hence, the Pt—W alloy can be applied to various tools in the shape of a wire. Besides, the Pt—W alloy of the present invention is good also in mechanical properties and X-ray visibility. Owing to these advantages, the present invention can be expected to be used in a medical tool such as an embolic coil or a guide wire.