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Iridium Alloys

A CRITICAL REVIEW OF THEIR CONSTITUTION AND PROPERTIES

By A. s. Darling, Ph.D., A.M.1.Mech.E.

Native crystals of iridium-platinum are Journal of Science, in 1860, remarked some- frequently found in association with platinum what incredulously: “The platinum makers of ores. These natural alloys, of rather variable are manufacturing these alloys and composition, occur as grains, and sometimes contrary to the wishes of the discoverers are as small cubes with rounded edges. Although extracting higher prices for them than for the geographical distribution is fairly wide pure platinum”. the major deposits appear to be in the Urals. Prominent physicists were, at this One such crystal, described in I940 by Masing, of the nineteenth century, devoting consider- Eckhardt and Kloiber (I) had a well-defined able effort to the problems involved in repre- two- micro-structure. senting their fundamental and derived units Synthetic alloys were produced at an early in concrete form. Iridium-platinum alloys date. Gaudin (z), who fused spheres of a came to fill a unique position in the scale 10per cent iridium-platinum on hearths of substances capable of retaining indefin- of lime and magnesia, commented in 1838 itely their mass, form, and linear dimensions. upon the lustre, malleability and extreme In addition to their resistance corrosion resistance of the resultant product. and good mechanical properties they were Twenty years later, however, iridium was readily workable and of fairly high electrical still regarded as a troublesome impurity in resistance. No internal transformations were platinum, but the researches of Sainte-Claire recognised in those days and the alloys were Deville and Debray (3) did much to dispel established as materials par exceZZence for this illusion. On behalf of the Russian instrument technology. Government they carried out extensive trials Iridium-platinum was selected, therefore, at the French Mint to determine the suit- as the logical material for the construction of ability of iridium-platinum alloys for coinage. of the International (5). A 250 The crude platinum for these experiments kilogramme ingot for this purpose was cast came from the mines of Prince Demidoff at on May 13th, 1874, at the Conservatoire des Nijne Tagilsk and contained 92.55 per cent Arts et MCtiers in Paris. The platinum, of platinum, 7 per cent of iridium and 0.45 weighing 225 kilogrammes, came from per cent of . Alloys prepared from Johnson and Matthey, the iridium being this material were greatly superior to pure supplied by the Director of Mines, St. platinum in both strength and corrosion Petersburg. Henry Tresco and George resistance. The alloy obtained by directly Matthey made the melt in a lime under fusing the crude platinum was found to be the general supervision of Sainte-Claire well suited for coinage, as were the synthetic Deville and Debray and the ingot, which alloys containing up to 20 per cent of iridium. contained 10 per cent of iridium, was prac- The 15 per cent alloy was extensively em- tically perfect and very uniform in composi- ployed for the construction of chemical plant. tion. Nickles (4), in a letter to the American Prototype and geodesique rules

Platinum Rev., 1960, 4, (l), 18-26 18 were in subsequent years manufactured by Iridium-platinum was sometimes used for George Matthey in accordance with the electrical standards. Klamenri? (9) concluded requirements of the Comitt Internationale in 1885 that of all the materials available at des Poids et Measures and the Association the time platinum- and platinum- Geodesique Internationale. Details of the iridium best fulfilled the service requirements melting, forging and rolling processes adopted of standard resistors. Two iridium-platinum during this work were published in 1879 (6). coils were included in the of British The unique combination of strength and Association resistance standards constructed high made the alioys very suitable for by Matthiesen in 1865 (10). The iridium the production of standard weights. The 10 alloy had a higher temperature coefficient per cent iridium alloy was used for the than platinum-silver but its resistance varia- National Kilogramme of France described tions, revealed by careful testing over a total by Violle in 1889 (7). Prototype kilogrammes period of 67 years, were considerably less were rigorously tested before acceptance by capricious (I I). the International Commission of the Metre. Very high pressures were applied to ascertain The Constitutional Diagram whether such treatment had any effect upon It was generally believed until compara- the apparent density. Material submitted by tively recently that platinum and iridium Johnson and Matthey successfully withstood when alloyed synthetically were completely this proving treatment, the apparent specific miscible in all proportions. Nearly all the gravity of eleven standard kilogrammes experimental evidence appeared to confirm submitted falling within the range 21.548 to this view. Feussner and Miiller’s melting 21.552 g/cm3. point determinations (12), obtained by optical

2500

2000

.* o FEUSSNER AND MULLER w 0 RAUB AND PLATE a (RESISTIVITY) 3 c A RAUB AND PLATE 2 IS00 (DILATOMETER) w a 0 RAUB AND PLATE (x RAY) 2 B MASING. ECKHARDT AND c KLOleER

1000

500

Pt ATOMIC PER CENT IRIDIUM Ir

Fig. 1 Equilibrium diagram of the iridium-platinum system

Platinum Metals Rev., 1960, 4, (1) 19 3.92 (IS), who published their ctn results in 1940, used an z3 3.90 alternating current elect- Y X rolytic etching technique which revealed with great 3.88 a facility the structure of w k- W alloys containing up to 2 3.86 40 per cent of iridium. a $ Hot forging and heat W 2 3.84 treatment for long periods c at temperatures up to t U A 1725°C were required to 3.82 20 40 60 80 100 eliminate the heavily ATOMIC PER CENT IRIDIUM cored structure previously reported by Nemilow. Fig. 2 Variation of lattice parameter with iridium concentration for All the micro-structures alloys quenched from 1700°C (Raub and Plate) shown in this paper were of typical . methods, lay on a smooth curve which Zvyagintsev, Raikhstadt and Vladimirova suggested the existence of a continuous range (16) studied the mutual diffusion of platinum of solubility below the solidus. Nemilow (13), and iridium in the solid state and concluded who studied the whole system metallo- that platinum diffused into iridium faster than graphically had previously arrived at the iridium diffused into platinum. Electrical same conclusion. He prepared alloys by resistivity measurements showed that fairly oxy-hydrogen melting and drew those con- homogeneous solid solutions could be pro- taining up to 20 per cent by weight of iridium duced by normal powder metallurgical into wire. Electrical resistivity measurements processes. were made upon these wire samples, and temperature coefficient determinations on cast The Miscibility Gap and homogenised specimens covering the The behaviour of iridium-platinum alloys entire composition range. Time and tempera- was, however, incompatible with the view ture, it was concluded, had little effect upon that no transformations occurred. As early the electrical properties, which were typical as 1886, for example, Le Chatelier (17) of those expected from simple solid observed transformations in a 20 per cent alloys. Heavy coring was observed in alloys iridium-platinum alloy which were accom- containing more than 20 per cent by weight of panied by an absorption of heat at 700°C and iridium. None of the heat treatments adopted at a second temperature midway between the completely eliminated this coarse dendritic melting points of and silver. Thermo- structure. electric studies by Barus in 1892 (29) did not, Weerts (14)~in confirmation of previous however, reveal any e.m.f. discontinuities at evidence, showed that the lattice parameter these points. Geibel (18) reported in 1911 of quenched alloys varied with concentration that the tensile strength of wires containing in a perfectly linear manner. more than 10 per cent of iridium could be Nemilow experienced great difficulty in considerably increased by heat treatment at etching his alloys, and for those containing 750°C and suggested that the effect was caused more than 40 per cent of iridium resorted to by an allotropic modification in the iridium. immersion in a fused bath of manganese Nowack (IS), commenting on these findings dioxide and sodium chloride. Raub and Buss in 19303 attributed the phenomenon to

Platinum Metals Rev., 1960, 4, (1) 20 precipitation hardening although he had no evi- dence to support his hypothesis. Ten years later Masing, Eckhardt and Kloiber (I) were still unable to dem- onstrate the existence of a second phase. These investigators carried out resistance measurements at temperatures up to I 150°C. The resistance- temperature curves had WEIGHT PER CENT IRIDIUM sharp discontinuities pro- viding evidence of trans- Fig. 3 Electrical resistivity and temperature coeficient of resistance of formations at tempera- iridium-platinum alloys tures in good agreement with those obtained from the results of tensile or in . The slow rate of establish- testing. X-ray examination provided no ment of equilibrium made the decomposition evidence of precipitation, however. It was process difficult to follow by X-ray methods, concluded that the changes in physical pro- and because of the small differences in lattice perties were caused by an order-disorder constants between platinum and iridium reaction which was not revealed because of X-ray determinations of the true solubility the similar scattering power to X-rays of limits were not particularly accurate. Elec- platinum and iridium atoms. mcal resistance measurements provided the The true nature of the platinum-iridium most accurate phase boundary determina- diagram was finally elucidated by Raub and tions.

Plate (zo), who published their results in 20 1956. These investigators provided definite metallographic evidence of a miscibility gap which had its highest point at 975°C and

50 atomic per cent of iridium. At 700°C the 15 gap extended from 7 to 99 atomic per cent of iridiunn. Microscopic evidence of a duplex InI- structure over this composition range was ? confirmed by the results of electrical resist- 5 10 -J ivity, X-ray diffraction] and dilatometric I measurements. Previous workers had not arrived at these conclusions for the simple reason that they 5 had not annealed their specimens for long enough periods. Raub and Plate showed that heat treatment periods of the order of one year were required to establish some sem- 10 20 30 blance of equilibrium in the two-phase region. WEIGHT PER CENT IRIDIUM In order to avoid loss of iridium due to Fig. 4 Thermal e.m.f. between iridium-platinum oxidation, heating was carried out in argon alloys and pure platinum

Platinum Metals Rev., 1960, 4, (1) 21 22.5 50 per cent alloy. The table below, taken from v 2 Raub and Plate, gives the I concentration limits of 22.0 + a the duplex region for m 5 various temperatures \ between 700 and 976°C. 5 21.5 I The equilibrium dia- c> shown in Fig. I is ul 2 based upon the findings W P of Feussncr and Miiller 21.0 (12), Masing, Eckhardt and Kloiber (I), and Raub and Plate (20). 20 40 60 80 100 WEIGHT PER CENT IRIDIUM Fig. 2 illustrates the Fig. 5 Density and instantaneous linear expansion coeficients of linear variation of lattice iridium-platinum alloys parameter with iridium concentration for alloys and electrical conductivity quenched from 1700'C (20). changes occurred after heat treatment periods of several hours. The development of a reason- Electrical Properties ably well-defined duplex microstructure in Fig. 3 shows that the electrical resistivity, the 50 atomic per cent iridium alloy required after a rather rapid initial rise, reaches a fairly 360 hours of heat treatment at 800°C. X-ray flat maximum of 32 to 34 microhm-cm at an diffraction examination first detected the iridium concentration of approximately 40 second phase after I440 hours at 8oo0C, but per cent. Differences in purity probably the lattice parameters of the two phases explain the slight differences between the two approached constancy only after 3600 hours sets of values quoted. Although Raub and of heat treatment. After goo0 hours at 600°C Plate illustrate the resistivity of the 50 no sign of decomposition was evident in the atomic per cent alloy as approximately 62 microhm-cm at 20"C, this value requires verification. Limiting solubility concentrations of Nemilow's temperature coefficient data iridium-platinum alloys agree closely with recent determinations made (Atomic percentage of iridium) in the Johnson Matthey Research Labora- Method of Determination tories. Temper The alloys have a positive thermal e.m.f. ature Resist- Dilato- X-Ray "C ance metric Diffraction with respect to pure platinum. Fig. 4 illustrates the effect of composition upon the 976 50 e.m.f.s generated at 700'C and 1200'C (21). 974 50 Above IOOO~Cvolatilisation and oxidation 958 40 losses render iridium-platinum alloys less 946 40 stable than rhodium-platinum alloys for high 922 30 temperature measurement. 910 20 900 40 and 74 Physical and Mechanical Properties aoo 10 26-40 and 72-86 700 7-28 and 60 -99 The specific gravity curve, shown in Fig. 5, displays a slight positive displacement from

Platinum Metals Rev., 1960, 4, (1) 22 500 linearity although the effect is not pro- 1- I nounced. The second curve in this illustra- tion shows an approximately uniform decrease 500 of linear expansion coefficient with iridium concentration (20). The mechanical properties of the alloys 400 f increase rapidly with iridium content as W 2 shown by Fig. 6 (21). With 30 per cent of a < iridium the ultimate tensile strength of I 300 u) K quenched test pieces rises to 56 tons per W Y square inch. The hardness data show that the I! > maximum mechanical properties of the series 200 are obtained with iridium contents of approxi- mately 60 per cent. The hardness and tensile

strength of all the alloys increases very rapidly 100 with the degree of cold working; the tensile strength of hard drawn 20 per cent iridium- platinum wire, for example, frequently 20 40 60 exceeds IOO tons per square inch. Fig. 7 WEIGHT PER CENT IRIDIUM illustrates the effect of cold rolIing upon the hardness of the 20 and 25 per cent iridium Fig. 6 Ultimate tensile strength, limit of pro- portionality and hardness oj quenched iridiurn- alloys. A reduction in thickness of 70 per platinum ulZoys cent increases the hardness of the 25 per cent alloy from 246 to 370 V.P.N. (21). heat treatment for 16 hours illustrates the low Very high hardness values can be developed rate of precipitation in this alloy system (21). by quenching and subsequently ageing alloys containing more than 10 per cent by weight Workability of iridium. Fig. 8 illustrates the hardness Alloys containing up to 40 per cent of changes which result when alloys quenched iridium have been hot forged for many years, from 1400OC are subsequently aged at 700°C. although the ease of working decreases The steady rate of hardness increase after rapidly when the iridium content exceeds 20 per cent. In 1879 for example George

350 o , I Matthey (6) reported that although the 25 per cent iridium alloy could be worked into sheet and wire great difficulty was experienced at higher percentages, the 50 per cent alloy being unworkable. W. C. Heraeus (22) was able, in 1891, to produce cold drawn 40 per cent iridium-platinurn wire but regarded this composition as a practical working limit. The ease of worldng increases with the purity of the materials employed, and argon arc melted material appears to be more malleable than that melted in the open. Small samples of sheet capable of being subsequently PER CENT REDUCTION IN THICKNESS rolled have been produced by carefully hot forging at temperatures just below the melting Fig. 7 Work-hurdening curues of iridium- platinum alloys point of the alloy, argon arc melted ingots

Platinum Metals Rev., 1960, 4, (1) 23 chloride in hydrochloric facilitates the solution of this material (IS). 500 Alloys containing more than about 5 per cent of iridium oxidise when heated in air. The bluish film which develops above 700°C

400 disappears at temperatures above 1200°C

u) leaving a clean matt surface upon the alloy, QW z Iridium-platinum were shown 0 4 by Burgess in 1908 (24) to be less stable than 300 those of rhodium-platinum. Crookes in 1912 au) W (25) emphasised that the volatility of iridium Y U in air was many times higher than that of >- platinum, and Wise, at a later date, quoted an 200 evaporation ratio, at 1227°C of one hundred to one. The high evaporation losses from heated iridium-platinum alloys can, in fact, be attributed to the volatility of iridium . 5 10 IS 20 The table below, taken from Raub and Plate TIME OF AGING - HOURS (26) summarises the results of some tests Fig. 8 Age-hardening curves of iridium-platinum made upon sheets of iridium-platinum alloys, alloys quenched from 1400°C and subsequently heat treated at 700°C 0.5 mm thick, heated for 23 hours at IIOO"C in a current of . The weight loss of the containing up to 60 per cent of iridium (21). alloy with 10per cent of iridium amounts to Raub and Plate in their recent work employed 2.8 per cent of the iridium content, while the drawn 50 per cent iridium-platinum wire (20). corresponding value for the 40 per cent iridium Such alloys are, however, even more difficult alloy is 46.6 per cent. to work than pure iridium, and fabrication Fig. 9 illustrates some results obtained at has so far been restricted to purposes of 900°C. SmalI sheet samples were suspended research. for various periods in a furnace through which an oxygen current of 0.4 litre per Corrosion and Oxidation Resistance minute was passed. The weight losses at Iridium additions increase considerably 900°C are less than a tenth of those at I 103°C. the corrosion resistance of platinum. At a The sudden increase in volatility when the very early date it was appreciated that iridium- iridium content exceeds 20 per cent is platinum boilers used for sulphuric acid concentration resisted the attack initiated by Evaporation losses from iridium-platinum alloys heated in a current of oxygen at nitrous compounds better than those of pure 1100°C for 23 hours platinum. Heraeus (23) stated for example (After Raub and Plate) that the rate of corrosion of the 10 per cent I iridium-platinum alloy in sulphuric acid was Evaporation loss related to iridium content only 58 per cent that of the pure . The Decrease I resistance to attack increases with Iridium in wt. mgig of iridium content in a similar fashion. The Wt.%) (mg/cm2) alloy content 40 per cent iridium alloy is virtually un- 10 I .35 2.8 attacked by boiling but can be 20 13.99 29.2 14.6 slowly etched by nascent at elevated 30 58. I0 123.6 41.2 temperatures. An alternating current process 40 89.29 186.4 46.6 utilising a saturated electrolyte of sodium II

Platinum Metals Rev., 1960, 4, (1) 24 associated with the fact that the 30 and 40 per a value of A H,,, -41 h 10Kcals was arrived cent alloys are, at 900"C, well within the at. At very high temperatures it was suggested duplex region. Raub and Plate postulated that evaporation occurred via an Ir,O that under such conditions most of the molecule, since this was the only one having evaporation occurred from the iridium-rich a positive entropy of formation from atomic phase. The evaporation mechanism suggested oxygen. Iridium-platinum alloys melt before was simple volatilisation of the oxide formed such conditions arise. when metal atoms diffused to the surface of an oxygen layer adsorbed on the solid metal. Applications The rather limited thermodynamic data Electrical contact applications account for a support this hypothesis. Holborn and Austen fair proportion of the iridium-platinum alloys (27) reported that the volatility of iridium at supplied to industry. The high hardness and 1670°C increased less rapidly than the first low contact resistance of the 10 and 20 per power of the oxygen pressure. Raub and cent iridium alloys make them particularly Plate demonstrated that the rate of volatilisa- suitable for low current, low pressure tion of iridium-platinum alloys was practic- applications where dust contamination pre- ally independent of oxygen pressure except judices the performance of softer materials. when this was very low indeed. This In medium current applications when resist- behaviour suggests the volatilisation of an ance to hammering becomes important alloys oxide rather than the dissociation of an un- containing up to 20 per cent of iridium are stable oxide. Brewer (28) assumed the oxide sometimes employed. A recent study showed to be IrO, and estimated A H,,, for the that material transfer from the cathode was expression proportional to the number of coulombs passed through the arc and decreased steadily IrO,(solid) = IrO,(gas) with iridium contents up to 35 per cent (30). to be 85 57Kcals. For the reaction Pfann (31) has shown that interesting results 20 Idsolid)+ O,(gas) =IrO,(gas) can be achievcd by pairing a negative per cent iridium-platinum contact with a positive contact of pure gold. With medium currents the material transfer caused by an asym- metrical molten bridge was found to balance the other transfer processes, thus minimising 2 contact wear. N: Precision resistors and slide wire potentio- b4 meters utilise considerable quantities of 10 E c and 20 per cent iridium-platinum wire. The I u combination of moderate resistivity, low w- s6 temperature coefficient and high corrosion t resistance exhibited by these alloys is of great value in the construction of high quality transducers. In the temperature measure- ment field we find the 10 per cent iridium- platinum alloy used in combination with a 10 40 per cent -gold wire. This 10 20 30 40 TIME IN HOURS , capable of generating 34 millvolts at 6oo0C, has wide applications at 9 Evaporation losses from iridium-platinum Fig. moderately high temperatures. alloys heated in a current of oxygen at 900°C (Raub and Plate) The bridge wires used in electric detonators

Platinum Metals Rev., 1960, 4, (1) 25 fuses, and cartridge igniters must be very freedom from corrosion is essential the age strong and resistant to corrosion, and iridium- hardening properties of the 20 per cent platinum alloys have long been preferred for iridium alloy render it very suitable for the such applications. Needles used for the initial production of high quality hairsprings. perforation of diamond dies by electro- Little is known about the creep properties sparking are sometimes made from the to per of iridium-platinum at high temperatures. cent iridium alloy. Some tests by Stauss (32) suggested that the The elasticity and high mechanical pro- 10 per cent iridium alloy was slightly inferior perties of alloys containing up to 30 per cent to the 10 per cent rhodium alloy, but the of iridium can be successfully employed in results were not conclusive. To avoid fracture

instrument suspensions. Over the range at IIOOOC, the applied stress, it was concluded, O-IOO"C the modulus of rigidity of the 10 per should not exceed 1.5 per cent of the normal cent iridium-platinum alloy decreases by room temperature tensile strength or 6 per only 0.8 per cent (21). Where complete cent of the short term strength at IIOO'C.

References

I G. Masing, K. Eckhardt and K. Kloiber Z. Metallkunde, 1940, 32, (5), 122-124 2 A. Gaudin ...... Compt. rend., 1838, (6), 862 3 M. H. Jacobi ...... Compt. rend., 1859, 49, 896 J. Pelouze ...... ,. PoZyt. J. (Dingler), 1860, 155, 118 H. Sainte-Claire Deville and H. Debray N. arch. Sci. phys. nat., 1873, 48, 45 H. Sainte-Claire Deville . . .. Compt. rend., 1875, So, 589 4 J. Nicklks ...... Amer. J. Sci., 1860, 29, (2), 270 5 H. Morin ...... Compt. rend., 1874, 78, 1502 6 G. Matthey ...... Proc. Roy. SOC.,1879, 28, 463 7 J. Violle ...... Compt. rend., 1889, 108, 894 8 J. S. Stas . . .. *. .. J. phurm. chim., 1885, 12, (5), 45 9 I. Klementit ...... Sitzbm. Akad. Ber., 1888, 97, (2), 838 (Chem. Zeit., 1888, 12, 1080) I0 British Association Report on Electrical Standards, Fourth Report, 1865 I1 British Association Report on Electrical Standards, 1932,P. 417 12 0. Feussner and L. Miiller . . . . Heraeus Festschrift, Hanau (Main), 1930, pp. 14-15; Ann. Physik, 1930, 7, 9 13 W. A. Nemilow ...... Annals. Inst. Platine, (U.S.S.R.), 1929, 7, 13-20 Z. anorg. allgem. Chem., 1932, 204, 41 14 J. Weerts ...... Z. Metallkunde, 1932, 24, 139 15 E. Raub and G. Buss ...... Z.Elektrochem., 1940, 46, (3), 195 16 0. E. Zvyagintsev, A. G. Raikhstadt Zhur. Priklad. Khim., 1944, 17, 22-30 and M. A. Vladirnirova 17 H. Le Chatelier ...... Bull. SOC.Chim., 1886, 45, (2), 482 18 W. Geibel . . .. *. .. Z. anorg. allgem. Chem., 1911, 70, 249 19 L. Nowack ...... Z. Metallkunde, 1930, 22, 140 20 E. Raub and W. Plate ...... Z. Metallkunde, 1956, 47, 688493 21 Johnson Matthey Research Laboratories Unpublished data 22 W. C. Heraeus ...... Z. Instrum. Kunde, 1891, 11, 262 23 W. C. Heraeus ...... Z. angew. Chem., 1892, 34 24 G. K. Burgess ...... Bull. Standards, 1908, 5, 199-225 25 W. Crookes ...... Proc. Roy. SOC.,I 9 12, A, 86, 46 1-476 26 E. Raub and W. Plate ...... Z. Metallkunde, 1957, 48, 529 27 L. Holborn and L. W. Austin .. Phil. Mag., 1904, 7, (6), 388 28 L. Brewer ...... Chemical Reviews, 1953, 52, 1-75 29 C. Barus...... Phil. Mag., 1892, 34, 376 30 W. B. Ittness and H. B. Ulsh.. . . Proc. Inst. Elec. Eng., Part B, 1957, 104, 63-68 31 W. G. Pfann ...... Proc. Inst. Elec. Erg., 1949, 68, (3), 197 32 H. E. Stauss ...... Trans. A.I.M.E., 1943, 152, 286-290

Platinum Metals Rev., 1960, 4, (1) 26