Metal wire rod composed of iridium or iridium alloy

Abstract

The present invention provides a metal wire rod composed of iridium or an iridium alloy, wherein the number of crystal grains on any cross-section in a longitudinal direction is 2 to 20 per 0.25 mm.sup.2, and the Vickers hardness at any part is 200 Hv or more and less than 400 Hv. The iridium wire rod is a material which is produced by a -PD method, and has low residual stress and which has a small change in the number of crystal grains and hardness even when heated to a temperature equal to or higher than a recrystallization temperature (1200 C. to 1500 C.). The metal wire rod of the present invention is excellent in oxidative consumption resistance under a high-temperature atmosphere, and mechanical properties.

Claims

1. A metal wire rod composed of iridium or an iridium alloy, wherein the number of crystal grains on all cross-sections parallel to the longitudinal direction is 2 to 20 per 0.25 mm.sup.2, wherein the wire rod has a Vickers hardness of 200 Hv or more and less than 400 Hv.

2. The metal wire rod according to claim 1, wherein the number of crystal grains in which the aspect ratio (x/y) based on a longitudinal direction (x) and a direction (y) vertical to the longitudinal direction is 1.5 or more is 20 or less per 0.25 mm.sup.2 on all cross-sections parallel to the longitudinal direction.

3. The metal wire rod according to claim 1, wherein the iridium alloy is at least any of an iridium alloy containing platinum, ruthenium, rhodium, and nickel in a total amount of 1 to 50% by mass.

4. A method for producing the metal wire rod, the wire rod being defined in claim 1, comprising the steps of: (a) providing a raw material made of a molten metal of iridium or an iridium-containing alloy in a molten state in a crucible having a nozzle serving as a die at a bottom of the crucible; (b) bringing a growing crystal into contact with the molten metal contained in the crucible from the bottom; (c) pulling the growing crystal downwardly away from the crucible at a constant speed through the nozzle at the bottom of the crucible to cool and to solidify the molten metal and thereby forming the wire rod; (d) adjusting a pull-down speed of the growing crystal so that a solid-liquid interface between the molten metal and the solidified metal is around the center in a vertical direction of the nozzle; and (e) adjusting a cooling rate of the wire rod to 120 C./sec to 1 C./sec until the temperature of the wire rod discharged from the nozzle becomes 1200 C. or lower.

5. The metal wire rod according to claim 2, wherein the iridium alloy is at least any of an iridium alloy containing platinum, ruthenium, rhodium, and nickel in a total amount of 1 to 50% by mass.

6. A method for producing the metal wire rod, the wire rod being defined in claim 2, comprising the steps of: (a) providing a raw material made of a molten metal of iridium or an iridium-containing alloy in a molten state in a crucible having a nozzle serving as a die at a bottom of the crucible; (b) bringing a growing crystal into contact with the molten metal contained in the crucible from the bottom ; (c) pulling the growing crystal downwardly away from the crucible at a constant speed through the nozzle at the bottom of the crucible to cool and to solidify the molten metal and thereby forming the wire rod; (d) adjusting a pull-down speed of the growing crystal so that a solid-liquid interface between the molten metal and the solidified metal is around the center in a vertical direction of the nozzle; and (e) adjusting a cooling rate of the wire rod to 120 C./sec to 1 C./sec until the temperature of the wire rod discharged from the nozzle becomes 1200 C. or lower.

7. A method for producing the metal wire rod, the wire rod being defined in claim 3, comprising the steps of: (a) providing a raw material made of a molten metal of an iridium-containing alloy in a molten state in a crucible having a nozzle serving as a die at a bottom of the crucible; (b) bringing a growing crystal into contact with the molten metal contained in the crucible from the bottom; (c) pulling the growing crystal downwardly away from the crucible at a constant speed through the nozzle at the bottom of the crucible to cool and to solidify the molten metal and thereby forming the wire rod; (d) adjusting a pull-down speed of the growing crystal so that a solid-liquid interface between the molten metal and the solidified metal is around the center in a vertical direction of the nozzle; and (e) adjusting a cooling rate of the wire rod to 120 C./sec to 1 C./sec until the temperature of the wire rod discharged from the nozzle becomes 1200 C. or lower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view explaining a configuration of an apparatus for producing an iridium wire rod based on a -PD method.

(2) FIG. 2 is a view explaining a process for production of an iridium wire rod of comparative examples.

(3) FIG. 3 shows photographs depicting the cross-section structures of wire rods of Example 8 and Comparative Example 8.

(4) FIG. 4 shows photographs depicting material structures after the wire rods of Example 8 and Comparative Example 8 are heated at a high temperature.

(5) FIG. 5 shows photographs of wire rods of Example 2 and Comparative Example 2 after heating at a high temperature and a bending test.

DESCRIPTION OF EMBODIMENT

(6) Hereinafter, preferred examples of the present invention will be described. In this embodiment, wire rods formed of iridium and various kinds of iridium alloys were produced by a -PD method (Examples 1 to 10). As conventional iridium wire rods, iridium wire rods having the same compositions as in examples were produced in a production process with processing in combination with a heat treatment as described in Patent Document 1 (Comparative Examples 1 to 10). Further, as reference examples, an ingot produced by a CZ method was processed and heat-treated to produce a wire rod. Hereinafter, processes for production of the iridium wire rods of examples, comparative examples and reference examples will be described.

Examples 1 to 10

(7) FIG. 1 illustrates an apparatus for producing an iridium wire rod based on the -PD method as applied in this embodiment. In the apparatus for producing an iridium wire rod, a molten iridium raw material is placed in a crucible as shown in FIG. 1. A die having a though-hole is embedded at the bottom of the crucible. In production of a wire rod by the -PD method, first a growing crystal is brought into contact with the raw material in the crucible from the bottom, and the growing crystal is then pulled down (moved downward) at a constant speed.

(8) In this embodiment, iridium or an iridium alloy (each having a purity of 99% or more) provided beforehand was placed in a zirconia crucible (container dimension: 403050). On the other hand, the growing crystal (seed crystal of 0.8 mm) was introduced from below a nozzle (dimension: 1 mm (inner diameter)5 mm (length)) provided on the bottom of the crucible. The raw material was melted by high frequency induction heating. Thereafter, the raw material was pulled down at a pull-down speed of 5 mm/min. At this time, a nitrogen gas (1 L/min) was fed downward from the upper part of the crucible. In this embodiment, cooling was performed slowly at a cooling rate of 50 C./sec until the wire rod temperature decreased to 1200 C. in a section of 30 mm from the nozzle outlet. In this way, a wire rod having a wire diameter of 1 mm and a length of 150 mm was produced.

Comparative Examples 1 to 10

(9) An ingot (diameter: 12 mm) composed of iridium or an iridium alloy was produced by a nitrogen arc melting method, and the ingot was processed into a wire rod by passing through the process shown in FIG. 2. In this processing process, processing is repeatedly performed until an intended dimension is obtained in each of the steps of hot forging and hot grooved roll rolling by biaxial pressurization. The repetition of biaxial pressurization in comparative examples is intended to impart a high orientation to the wire rod. In the processing process in comparative examples, the hot processing temperature and the heat treatment temperature are each set to a crystallization temperature or lower. Accordingly, rupture at grain boundaries generated by recrystallization in the middle of processing is prevented.

Reference Examples 1 and 2

(10) Iridium and iridium alloy ingots having a diameter of 5 mm were each produced by a CZ method (pull-up speed: 10 mm/min) from a high-frequency-melted iridium hot metal by use of a water-cooled copper mold. The wire rod was subjected to hot wire drawing processing to obtain a fine wire. For processing conditions at this time, the processing temperature was 1000 C. to 1200 C., and the processing ratio per one pass was 10%. The wire diameter of the wire rod was 1 mm. In the reference examples, wire rods were produced with two materials: pure iridium (corresponding to Example 1) and an iridium-rhodium alloy (corresponding to Example 5).

(11) For the iridium wire rods produced as described above, measurement of the number of crystal grains by observation of the material structure and measurement of the hardness were first performed. For these measurements, the produced wire rod was cut to a length of 1 mm, and further cut into halves in the longitudinal direction. Microscopic observation was performed, where an observation field of view with an area of 0.25 mm.sup.2 was arbitrarily set, and the number of crystal grains was measured. The presence/absence and the number of equiaxial crystals having an aspect ratio of 1.5 or more were determined. The Vickers hardness was then measured by a Vickers hardness meter. The results are shown in Table 1.

(12) TABLE-US-00001 TABLE 1 Observation results Number Number of Composition Production of crystal equiaxial Hardness Ir Pt Rh Ru Ni method grains crystals (Hv) Example 1 100 -PD 5 0 323 Example 2 Balance 10 2 0 311 Example 3 Balance 20 4 0 331 Example 4 Balance 50 2 0 389 Example 5 Balance 20 4 0 299 Example 6 Balance 20 3 0 326 Example 7 Balance 20 6 0 332 Example 8 Balance 20 20 4 0 293 Example 9 Balance 3 3 0 315 Example 10 Balance 1 2 0 298 Comparative 100 Processing + 26 3 489 Example 1 heat Comparative Balance 10 treatment 23 5 561 Example 2 Comparative Balance 20 25 4 593 Example 3 Comparative Balance 50 34 4 490 Example 4 Comparative Balance 20 25 3 467 Example 5 Comparative Balance 20 28 3 666 Example 6 Comparative Balance 20 32 6 618 Example 7 Comparative Balance 20 20 38 5 462 Example 8 Comparative Balance 3 22 8 456 Example 9 Comparative Balance 1 29 6 527 Example 10 Reference 100 CZ + 16 2 510 Example 1 processing Reference Balance 20 14 2 400 Example 2

(13) The results for samples produced in this embodiment show that in Examples 1 to 10, the number of crystal grains on a cross-section in the longitudinal direction is in the specified range, and the hardness is relatively low. In comparative examples, the number of crystal grains is not excessively large, but is larger as compared to examples, and the hardness is high. In reference examples, the number of crystal grains is small because a CZ method is applied in ingot production, while the hardness is relatively high. This may be ascribable to conditions for subsequent processing (processing temperature is lower than 1400 C.).

(14) FIG. 3 depicts the cross-section structures of the wire rods of Example 8 and Comparative Example 8. The wire rod of Example 8 includes a small number of columnar crystals. On the other hand, the wire rod of Comparative Example 8 has a material structure in which a large number of crystals extending in the longitudinal direction are densely arranged in the form of fibers.

(15) Next, each sample in Table 1 was subjected to oxidation heating at a high temperature, and a change in structure and a change in hardness after heating were examined. Further, the oxidative consumption amount after heating was measured, and high-temperature oxidation properties were evaluated. Wire rods (length: 10 mm) having the same compositions as in Table 1 were provided and heated at high temperature, and whether breakage occurred or not was determined in a bending test for the wire rods after heating. In this bending test, it was determined that breakage occurred when wire rod rupture or surface cracking occurred at the time of bending the wire rod to 90. The results of the above evaluations are shown in Tables 2 and 3.

(16) TABLE-US-00002 TABLE 2 Number of crystal grains Hardness Composition Production Before Before Ir Pt Rh Ru Ni method heating 1200 C. 1500 C. heating 1200 C. 1500 C. Example 1 100 -PD 5 5 6 323 280 288 Example 2 Balance 10 2 3 3 311 288 297 Example 3 Balance 20 4 4 3 331 318 288 Example 4 Balance 50 2 4 2 389 355 342 Example 5 Balance 20 4 3 8 299 255 221 Example 6 Balance 20 3 6 2 326 289 288 Example 7 Balance 20 6 8 9 332 284 275 Example 8 Balance 20 20 4 8 2 293 255 242 Example 9 Balance 3 3 4 2 315 283 267 Example 10 Balance 1 2 3 4 298 274 276 Comparative 100 Processing + 26 80 46 489 243 259 Example 1 heat Comparative Balance 10 treatment 23 175 31 561 285 271 Example 2 Comparative Balance 20 25 100 35 593 342 333 Example 3 Comparative Balance 50 34 175 42 490 447 493 Example 4 Comparative Balance 20 25 45 31 467 261 256 Example 5 Comparative Balance 20 28 46 38 666 331 282 Example 6 Comparative Balance 20 32 115 48 618 314 346 Example 7 Comparative Balance 20 20 38 47 41 462 271 232 Example 8 Comparative Balance 3 22 52 29 456 245 256 Example 9 Comparative Balance 1 29 65 52 527 352 335 Example 10 Reference 100 CZ + 16 35 29 510 340 430 Example 1 processing Reference Balance 20 14 25 23 400 300 310 Example 2

(17) TABLE-US-00003 TABLE 3 Oxidative Presence/absence of Composition Production consumption (%) breakage Ir Pt Rh Ru Ni method 1200 C. 1500 C. 1200 C. 1500 C. Example 1 100 -PD 38.55 60.58 Example 2 Balance 10 36.45 56.23 Example 3 Balance 20 32.34 47.15 Example 4 Balance 50 26.31 24.32 Example 5 Balance 20 0.93 7.67 Example 6 Balance 20 67.20 83.45 Example 7 Balance 20 0.48 4.46 Example 8 Balance 20 20 3.64 11.25 Example 9 Balance 3 26.30 42.56 Example 10 Balance 1 23.45 36.35 Comparative 100 Processing + 42.69 64.19 x x Example 1 heat Comparative Balance 10 treatment 47.78 58.65 x x Example 2 Comparative Balance 20 37.54 49.37 x x Example 3 Comparative Balance 50 51.78 27.56 x x Example 4 Comparative Balance 20 5.32 11.56 x x Example 5 Comparative Balance 20 72.56 87.04 x x Example 6 Comparative Balance 20 6.20 8.30 x x Example 7 Comparative Balance 20 20 8.12 13.53 x x Example 8 Comparative Balance 3 28.90 46.20 x x Example 9 Comparative Balance 1 28.52 41.76 x x Example 10 Reference 100 CZ + 43.20 63.50 x Example 1 processing Reference Balance 20 3.66 7.54 x Example 2 : Material rupture and surface cracking were not observed. x: Material rupture and surface cracking were observed. : Rupture and cracking were not observed, but wrinkles occured on the surface.

(18) From Table 3, it can be confirmed that the iridium wire rods of examples are superior in oxidative consumption and high-temperature strength to the wire rods of comparative examples which have the same compositions. Breakage occurring in comparative examples was found to be grain boundary cracking in observation of the ruptured part. For oxidative consumption, intensive corrosion occurred around grain boundaries. In this respect, Table 2 shows that in the iridium wire rods of examples, a change in the number of crystal grains is small, and a change in hardness is suppressed. In comparative examples, recrystallization is promoted by heating at a high temperature, so that the material is considerably softened while the number of crystal grains increases.

(19) FIG. 4 depicts the material structures in Example 8 and Comparative Example 8 after heating at a high temperature (1200 C. and 1500 C.). In the comparative example, the number of crystal grains increases due to recrystallization after heating at a high temperature. Particularly, the number of crystal grains on the outer peripheral section markedly increases. On the other hand, in Example 8, a change in material structure is extremely small.

(20) FIG. 5 shows photographs of external appearances in a bending test conducted after heating at a 1500 C. for 20 hours for the wire rods of Example 2 and Comparative Example 2. In the comparative example, evident rupture was observed. Further, in the comparative example, roughnesses were observed in the surface form. On the other hand, the wire rod of the example was bent without being ruptured, and had a glossy surface.

(21) The high-temperature properties in Reference Examples 1 and 2 are superior to those in Comparative Examples 1 and 5 which have the same compositions, but are inferior to those in Examples 1 and 5. Reference examples which apply a CZ method are superior to comparative examples as to material structure control. However, it is considered that in reference examples, the processing temperature was low and the plastic processing ratio was high, so that residual strain existed, and caused slight recrystallization.

INDUSTRIAL APPLICABILITY

(22) The present invention provides a material which has a satisfactory high-temperature oxidation resistance property, and is thus capable of being used for a long period of time under a high-temperature oxidizing atmosphere. The material of the present invention is suitable as a material for ignition plug electrodes, various kinds of sensor electrodes, lead wires and the like, which is used under a high-temperature oxidizing atmosphere.