Iridium Platinum Alloys

Transcription

1 ridium Pltinum Alloys A CRTCAL REVE OF THER CONSTTUTON AND PROPERTES By A. s. Drling, Ph.D., A.M.1.Meh.E. Ntive rystls of iridium-pltinum re frequently found in ssoition with pltinum ores. These nturl lloys, of rther vrible omposition, our s grins, nd sometimes s smll ubes with rounded edges. Although the geogrphil distribution is firly wide the mjor deposits pper to be in the Urls. One suh rystl, desribed in 940 by Msing, Ekhrdt nd Kloiber () hd well-defined two-phse miro-struture. Syntheti lloys were produed t n erly dte. Gudin (z), who fused spheres of 10 per ent iridium-pltinum lloy on herths of lime nd mgnesi, ommented in 1838 upon the lustre, mllebility nd extreme orrosion resistne of the resultnt produt. Twenty yers lter, however, iridium ws still regrded s troublesome impurity in pltinum, but the reserhes of Sinte-Clire Deville nd Debry (3) did muh to dispel this illusion. On behlf of the Russin Government they rried out extensive trils t the Frenh Mint to determine the suitbility of iridium-pltinum lloys for oinge. The rude pltinum for these experiments me from the mines of Prine Demidoff t Nijne Tgilsk nd ontined per ent of pltinum, 7 per ent of iridium nd 0.45 per ent of rhodium. Alloys prepred from this mteril were gretly superior to pure pltinum in both strength nd orrosion resistne. The lloy obtined by diretly fusing the rude pltinum ws found to be well suited for oinge, s were the syntheti lloys ontining up to 20 per ent of iridium. The 15 per ent lloy ws extensively employed for the onstrution of hemil plnt. Nikles (4), in letter to the Amerin Journl of Siene, in 1860, remrked somewht inredulously: The pltinum mkers of Pris re mnufturing these lloys nd ontrry to the wishes of the disoverers re extrting higher pries for them thn for pure pltinum. Prominent physiists were, t this period of the nineteenth entury, devoting onsiderble effort to the problems involved in representing their fundmentl nd derived units in onrete form. ridium-pltinum lloys me to fill unique position in the sle of substnes pble of retining indefinitely their mss, form, nd liner dimensions. n ddition to their orrosion resistne nd good mehnil properties they were redily workble nd of firly high eletril resistne. No internl trnsformtions were reognised in those dys nd the lloys were estblished s mterils pr exezzene for instrument tehnology. ridium-pltinum ws seleted, therefore, s the logil mteril for the onstrution of of the nterntionl Metre (5). A 250 kilogrmme ingot for this purpose ws st on My 13th, 1874, t the Conservtoire des Arts et MCtiers in Pris. The pltinum, weighing 225 kilogrmmes, me from Johnson nd Mtthey, the iridium being supplied by the Diretor of Mines, St. Petersburg. Henry Treso nd George Mtthey mde the melt in lime blok under the generl supervision of Sinte-Clire Deville nd Debry nd the ingot, whih ontined 10 per ent of iridium, ws prtilly perfet nd very uniform in omposition. Prototype metres nd geodesique rules Pltinum Metls Rev., 1960, 4, (l),

2 were in subsequent yers mnuftured by George Mtthey in ordne with the requirements of the Comitt nterntionle des Poids et Mesures nd the Assoition Geodesique nterntionle. Detils of the melting, forging nd rolling proesses dopted during this work were published in 1879 (6). The unique ombintion of strength nd high density mde the lioys very suitble for the prodution of stndrd weights. The 10 per ent iridium lloy ws used for the Ntionl Kilogrmme of Frne desribed by Violle in 1889 (7). Prototype kilogrmmes were rigorously tested before eptne by the nterntionl Commission of the Metre. Very high pressures were pplied to sertin whether suh tretment hd ny effet upon the pprent density. Mteril submitted by Johnson nd Mtthey suessfully withstood this proving tretment, the pprent speifi grvity of eleven stndrd kilogrmmes submitted flling within the rnge to g/m3. ridium-pltinum ws sometimes used for eletril stndrds. Klmenri? (9) onluded in 1885 tht of ll the mterils vilble t the time pltinum-silver nd pltinumiridium best fulfilled the servie requirements of stndrd resistors. Two iridium-pltinum oils were inluded in the group of British Assoition resistne stndrds onstruted by Mtthiesen in 1865 (10). The iridium lloy hd higher temperture oeffiient thn pltinum-silver but its resistne vritions, reveled by reful testing over totl period of 67 yers, were onsiderbly less priious ( ). The Constitutionl Digrm t ws generlly believed until omprtively reently tht pltinum nd iridium when lloyed synthetilly were ompletely misible in ll proportions. Nerly ll the experimentl evidene ppered to onfirm this view. Feussner nd Miiller s melting point determintions (12), obtined by optil 2500.* 2000 o FEUSSNER AND MULLER w 0 RAUB AND PLATE (RESSTVTY) 3 A RAUB AND PLATE 2 S00 (DLATOMETER) w 0 RAUB AND PLATE (x RAY) 2 B MASNG. ECKHARDT AND KLOleER Pt ATOMC PER CENT RDUM r Fig. 1 Equilibrium digrm of the iridium-pltinum system Pltinum Metls Rev., 1960, 4, (1) 19

3 tn z Y X 3.88 w k $ t U A ATOMC PER CENT RDUM Fig. 2 Vrition of lttie prmeter with iridium onentrtion for lloys quenhed from 1700 C (Rub nd Plte) methods, ly on smooth urve whih suggested the existene of ontinuous rnge of solubility below the solidus. Nemilow (13), who studied the whole system metllogrphilly hd previously rrived t the sme onlusion. He prepred lloys by oxy-hydrogen melting nd drew those ontining up to 20 per ent by weight of iridium into wire. Eletril resistivity mesurements were mde upon these wire smples, nd temperture oeffiient determintions on st nd homogenised speimens overing the entire omposition rnge. Time nd temperture, it ws onluded, hd little effet upon the eletril properties, whih were typil of those expeted from simple solid solution lloys. Hevy oring ws observed in lloys ontining more thn 20 per ent by weight of iridium. None of the het tretments dopted ompletely eliminted this orse dendriti struture. eerts (14)~ in onfirmtion of previous evidene, showed tht the lttie prmeter of quenhed lloys vried with onentrtion in perfetly liner mnner. Nemilow experiened gret diffiulty in ething his lloys, nd for those ontining more thn 40 per ent of iridium resorted to immersion in fused bth of mngnese dioxide nd sodium hloride. Rub nd Buss (S), who published their results in 1940, used n lternting urrent eletrolyti ething tehnique whih reveled with gret fility the struture of lloys ontining up to 40 per ent of iridium. Hot forging nd het tretment for long periods t tempertures up to 1725 C were required to eliminte the hevily ored struture previously reported by Nemilow. All the miro-strutures shown in this pper were of typil solid solutions. Zvygintsev, Rikhstdt nd Vldimirov (16) studied the mutul diffusion of pltinum nd iridium in the solid stte nd onluded tht pltinum diffused into iridium fster thn iridium diffused into pltinum. Eletril resistivity mesurements showed tht firly homogeneous solid solutions ould be produed by norml powder metllurgil proesses. The Misibility Gp The behviour of iridium-pltinum lloys ws, however, inomptible with the view tht no trnsformtions ourred. As erly s 1886, for exmple, Le Chtelier (17) observed trnsformtions in 20 per ent iridium-pltinum lloy whih were ompnied by n bsorption of het t 700 C nd t seond temperture midwy between the melting points of gold nd silver. Thermoeletri studies by Brus in 1892 (29) did not, however, revel ny e.m.f. disontinuities t these points. Geibel (18) reported in 1911 tht the tensile strength of wires ontining more thn 10 per ent of iridium ould be onsiderbly inresed by het tretment t 750 C nd suggested tht the effet ws used by n llotropi modifition in the iridium. Nowk (S), ommenting on these findings in ttributed the phenomenon to Pltinum Metls Rev., 1960, 4, (1) 20

4 preipittion hrdening lthough he hd no evidene to support his hypothesis. Ten yers lter Msing, Ekhrdt nd Kloiber () were still unble to demonstrte the existene of seond phse. These investigtors rried out resistne mesurements t tempertures up to 150 C. The resistnetemperture urves hd shrp disontinuities providing evidene of trnsformtions t tempertures in good greement with those obtined from the results of tensile testing. X-ry exmintion provided no evidene of preipittion, however. t ws onluded tht the hnges in physil properties were used by n order-disorder retion whih ws not reveled beuse of the similr sttering power to X-rys of pltinum nd iridium toms. The true nture of the pltinum-iridium digrm ws finlly eluidted by Rub nd Plte (zo), who published their results in These investigtors provided definite metllogrphi evidene of misibility gp whih hd its highest point t 975 C nd 50 tomi per ent of iridium. At 700 C the gp extended from 7 to 99 tomi per ent of iridiunn. Mirosopi evidene of duplex struture over this omposition rnge ws onfirmed by the results of eletril resistivity, X-ry diffrtion] nd diltometri mesurements. Previous workers hd not rrived t these onlusions for the simple reson tht they hd not nneled their speimens for long enough periods. Rub nd Plte showed tht het tretment periods of the order of one yer were required to estblish some semblne of equilibrium in the two-phse region. n order to void loss of iridium due to oxidtion, heting ws rried out in rgon EGHT PER CENT RDUM Fig. 3 Eletril resistivity nd temperture oefiient of resistne of iridium-pltinum lloys or in vuum. The slow rte of estblishment of equilibrium mde the deomposition proess diffiult to follow by X-ry methods, nd beuse of the smll differenes in lttie onstnts between pltinum nd iridium X-ry determintions of the true solubility limits were not prtiulrly urte. Eleml resistne mesurements provided the most urte phse boundry determintions. n -? J Fig EGHT PER CENT RDUM Therml e.m. f. between iridium-pltinum lloys nd pure pltinum Pltinum Metls Rev., 1960, 4, (1) 21

5 EGHT PER CENT RDUM 22.5 v m 5 5 \ 21.5 > 21.0 Fig. 5 Density nd instntneous liner expnsion oefiients of iridium-pltinum lloys Hrdness nd eletril ondutivity hnges ourred fter het tretment periods of severl hours. The development of resonbly well-defined duplex mirostruture in the 50 tomi per ent iridium lloy required 360 hours of het tretment t 800 C. X-ry diffrtion exmintion first deteted the seond phse fter 440 hours t 8oo0C, but the lttie prmeters of the two phses pprohed onstny only fter 3600 hours of het tretment. After goo0 hours t 600 C no sign of deomposition ws evident in the Limiting solubility onentrtions of iridium-pltinum lloys (Atomi perentge of iridium) Temper ture "C oo 700 Resistne Method of Determintion Diltometri X-Ry Diffrtion 40 nd nd nd ul 2 P quenhed from 1700'C (20). Eletril Properties 50 per ent lloy. The tble below, tken from Rub nd Plte, gives the onentrtion limits of the duplex region for vrious tempertures between 700 nd 976 C. The equilibrium digrm shown in Fig. is bsed upon the findings of Feussnr nd Miiller (12), Msing, Ekhrdt nd Kloiber (), nd Rub nd Plte (20). Fig. 2 illustrtes the liner vrition of lttie prmeter with iridium onentrtion for lloys Fig. 3 shows tht the eletril resistivity, fter rther rpid initil rise, rehes firly flt mximum of 32 to 34 mirohm-m t n iridium onentrtion of pproximtely 40 per ent. Differenes in purity probbly explin the slight differenes between the two sets of vlues quoted. Although Rub nd Plte illustrte the resistivity of the 50 tomi per ent lloy s pproximtely 62 mirohm-m t 20"C, this vlue requires verifition. Nemilow's temperture oeffiient dt gree losely with reent determintions mde in the Johnson Mtthey Reserh Lbortories. The lloys hve positive therml e.m.f. with respet to pure pltinum. Fig. 4 illustrtes the effet of omposition upon the e.m.f.s generted t 700'C nd 1200'C (21). Above OOO~C voltilistion nd oxidtion losses render iridium-pltinum lloys less stble thn rhodium-pltinum lloys for high temperture mesurement. Physil nd Mehnil Properties The speifi grvity urve, shown in Fig. 5, displys slight positive displement from Pltinum Metls Rev., 1960, 4, (1) 22

6 linerity lthough the effet is not pronouned. The seond urve in this illustrtion shows n pproximtely uniform derese of liner expnsion oeffiient with iridium onentrtion (20). The mehnil properties of the lloys inrese rpidly with iridium ontent s shown by Fig. 6 (21). ith 30 per ent of iridium the ultimte tensile strength of quenhed test piees rises to 56 tons per squre inh. The hrdness dt show tht the mximum mehnil properties of the series re obtined with iridium ontents of pproximtely 60 per ent. The hrdness nd tensile strength of ll the lloys inreses very rpidly with the degree of old working; the tensile strength of hrd drwn 20 per ent iridiumpltinum wire, for exmple, frequently exeeds OO tons per squre inh. Fig. 7 illustrtes the effet of old roling upon the hrdness of the 20 nd 25 per ent iridium lloys. A redution in thikness of 70 per ent inreses the hrdness of the 25 per ent lloy from 246 to 370 V.P.N. (21). Very high hrdness vlues n be developed by quenhing nd subsequently geing lloys ontining more thn 10 per ent by weight of iridium. Fig. 8 illustrtes the hrdness hnges whih result when lloys quenhed from 1400OC re subsequently ged t 700 C. The stedy rte of hrdness inrese fter 350 o, PER CENT REDUCTON N THCKNESS Fig. 7 ork-hurdening urues of iridiumpltinum lloys EGHT PER CENT RDUM f 2 < 300 u) K Y! > Fig. 6 Ultimte tensile strength, limit of proportionlity nd hrdness oj quenhed iridiurnpltinum ulzoys het tretment for 16 hours illustrtes the low rte of preipittion in this lloy system (21). orkbility Alloys ontining up to 40 per ent of iridium hve been hot forged for mny yers, lthough the ese of working dereses rpidly when the iridium ontent exeeds 20 per ent. n 1879 for exmple George Mtthey (6) reported tht lthough the 25 per ent iridium lloy ould be worked into sheet nd wire gret diffiulty ws experiened t higher perentges, the 50 per ent lloy being unworkble.. C. Hereus (22) ws ble, in 1891, to produe old drwn 40 per ent iridium-pltinurn wire but regrded this omposition s prtil working limit. The ese of worldng inreses with the purity of the mterils employed, nd rgon r melted mteril ppers to be more mlleble thn tht melted in the open. Smll smples of sheet pble of being subsequently rolled hve been produed by refully hot forging t tempertures just below the melting point of the lloy, rgon r melted ingots Pltinum Metls Rev., 1960, 4, (1) 23

7 u) Q z 0 4 u) - Y U > S 20 TME OF AGNG - HOURS Fig. 8 Age-hrdening urves of iridium-pltinum lloys quenhed from 1400 C nd subsequently het treted t 700 C ontining up to 60 per ent of iridium (21). Rub nd Plte in their reent work employed drwn 50 per ent iridium-pltinum wire (20). Suh lloys re, however, even more diffiult to work thn pure iridium, nd fbrition hs so fr been restrited to purposes of reserh. Corrosion nd Oxidtion Resistne ridium dditions inrese onsiderbly the orrosion resistne of pltinum. At very erly dte it ws ppreited tht iridiumpltinum boilers used for sulphuri id onentrtion resisted the ttk initited by nitrous ompounds better thn those of pure pltinum. Hereus (23) stted for exmple tht the rte of orrosion of the 10 per ent iridium-pltinum lloy in sulphuri id ws only 58 per ent tht of the pure metl. The resistne to hlogen ttk inreses with iridium ontent in similr fshion. The 40 per ent iridium lloy is virtully unttked by boiling qu regi but n be slowly ethed by nsent hlorine t elevted tempertures. An lternting urrent proess utilising sturted eletrolyte of sodium hloride in hydrohlori id filittes the solution of this mteril (S). Alloys ontining more thn bout 5 per ent of iridium oxidise when heted in ir. The bluish film whih develops bove 700 C disppers t tempertures bove 1200 C leving len mtt surfe upon the lloy, ridium-pltinum thermoouples were shown by Burgess in 1908 (24) to be less stble thn those of rhodium-pltinum. Crookes in 1912 (25) emphsised tht the voltility of iridium in ir ws mny times higher thn tht of pltinum, nd ise, t lter dte, quoted n evportion rtio, t 1227 C of one hundred to one. The high evportion losses from heted iridium-pltinum lloys n, in ft, be ttributed to the voltility of iridium oxide. The tble below, tken from Rub nd Plte (26) summrises the results of some tests mde upon sheets of iridium-pltinum lloys, 0.5 mm thik, heted for 23 hours t OO"C in urrent of oxygen. The weight loss of the lloy with 10 per ent of iridium mounts to 2.8 per ent of the iridium ontent, while the orresponding vlue for the 40 per ent iridium lloy is 46.6 per ent. Fig. 9 illustrtes some results obtined t 900 C. Sml sheet smples were suspended for vrious periods in furne through whih n oxygen urrent of 0.4 litre per minute ws pssed. The weight losses t 900 C re less thn tenth of those t 103 C. The sudden inrese in voltility when the iridium ontent exeeds 20 per ent is Evportion losses from iridium-pltinum lloys heted in urrent of oxygen t 1100 C for 23 hours (After Rub nd Plte) Evportion loss relted to iridium ontent Derese ridium in wt. mgig of t.%) (mg/m2) lloy ontent Pltinum Metls Rev., 1960, 4, (1) 24

8 ssoited with the ft tht the 30 nd 40 per ent lloys re, t 900"C, well within the duplex region. Rub nd Plte postulted tht under suh onditions most of the evportion ourred from the iridium-rih phse. The evportion mehnism suggested ws simple voltilistion of the oxide formed when metl toms diffused to the surfe of n oxygen lyer dsorbed on the solid metl. The rther limited thermodynmi dt support this hypothesis. Holborn nd Austen (27) reported tht the voltility of iridium t 1670 C inresed less rpidly thn the first power of the oxygen pressure. Rub nd Plte demonstrted tht the rte of voltilistion of iridium-pltinum lloys ws prtilly independent of oxygen pressure exept when this ws very low indeed. This behviour suggests the voltilistion of n oxide rther thn the dissoition of n unstble oxide. Brewer (28) ssumed the oxide to be ro, nd estimted A H,,, for the expression ro,(solid) = ro,(gs) to be 85 57Kls. For the retion 2 N : b4 E - u w s 6 t 10 dsolid)+ O,(gs) = ro,(gs) TME N HOURS Fig. 9 Evportion losses from iridium-pltinum lloys heted in urrent of oxygen t 900 C (Rub nd Plte) vlue of A H,,, -41 h 10Kls ws rrived t. At very high tempertures it ws suggested tht evportion ourred vi n r,o moleule, sine this ws the only one hving positive entropy of formtion from tomi oxygen. ridium-pltinum lloys melt before suh onditions rise. Applitions Eletril ontt pplitions ount for fir proportion of the iridium-pltinum lloys supplied to industry. The high hrdness nd low ontt resistne of the 10 nd 20 per ent iridium lloys mke them prtiulrly suitble for low urrent, low pressure pplitions where dust ontmintion prejudies the performne of softer mterils. n medium urrent pplitions when resistne to hmmering beomes importnt lloys ontining up to 20 per ent of iridium re sometimes employed. A reent study showed tht mteril trnsfer from the thode ws proportionl to the number of oulombs pssed through the r nd deresed stedily with iridium ontents up to 35 per ent (30). Pfnn (31) hs shown tht interesting results n be hievd by piring negtive 20 per ent iridium-pltinum ontt with positive ontt of pure gold. ith medium urrents the mteril trnsfer used by n symmetril molten bridge ws found to blne the other trnsfer proesses, thus minimising ontt wer. Preision resistors nd slide wire potentiometers utilise onsiderble quntities of 10 nd 20 per ent iridium-pltinum wire. The ombintion of moderte resistivity, low temperture oeffiient nd high orrosion resistne exhibited by these lloys is of gret vlue in the onstrution of high qulity trnsduers. n the temperture mesurement field we find the 10 per ent iridiumpltinum lloy used in ombintion with 40 per ent plldium-gold wire. This thermoouple, pble of generting 34 millvolts t 6oo0C, hs wide pplitions t modertely high tempertures. The bridge wires used in eletri detontors Pltinum Metls Rev., 1960, 4, (1) 25

9 fuses, nd rtridge igniters must be very strong nd resistnt to orrosion, nd iridiumpltinum lloys hve long been preferred for suh pplitions. Needles used for the initil perfortion of dimond dies by eletrosprking re sometimes mde from the to per ent iridium lloy. The elstiity nd high mehnil properties of lloys ontining up to 30 per ent of iridium n be suessfully employed in instrument suspensions. Over the rnge O-OO"C the modulus of rigidity of the 10 per ent iridium-pltinum lloy dereses by only 0.8 per ent (21). here omplete freedom from orrosion is essentil the ge hrdening properties of the 20 per ent iridium lloy render it very suitble for the prodution of high qulity hirsprings. Little is known bout the reep properties of iridium-pltinum t high tempertures. Some tests by Stuss (32) suggested tht the 10 per ent iridium lloy ws slightly inferior to the 10 per ent rhodium lloy, but the results were not onlusive. To void frture t OOOC, the pplied stress, it ws onluded, should not exeed 1.5 per ent of the norml room temperture tensile strength or 6 per ent of the short term strength t OO'C. Referenes G. Msing, K. Ekhrdt nd K. Kloiber 2 A. Gudin M. H. Jobi J. Pelouze......,. H. Sinte-Clire Deville nd H. Debry H. Sinte-Clire Deville J. Niklks H. Morin G. Mtthey J. Violle J. S. Sts.... * Klementit Feussner nd L. Miiller A. Nemilow J. eerts E. Rub nd G. Buss E. Zvygintsev, A. G. Rikhstdt nd M. A. Vldirnirov 17 H. Le Chtelier Geibel.... * L. Nowk E. Rub nd. Plte Johnson Mtthey Reserh Lbortories 22. C. Hereus C. Hereus G. K. Burgess Crookes E. Rub nd. Plte L. Holborn nd L.. Austin.. 28 L. Brewer C. Brus B. ttness nd H. B. Ulsh G. Pfnn H. E. Stuss Z. Metllkunde, 1940, 32, (5), Compt. rend., 1838, (6), 862 Compt. rend., 1859, 49, 896 PoZyt. J. (Dingler), 1860, 155, 118 N. rh. Si. phys. nt., 1873, 48, 45 Compt. rend., 1875, So, 589 Amer. J. Si., 1860, 29, (2), 270 Compt. rend., 1874, 78, 1502 Pro. Roy. SOC., 1879, 28, 463 Compt. rend., 1889, 108, 894 J. phurm. him., 1885, 12, (5), 45 Sitzbm. Akd. Ber., 1888, 97, (2), 838 (Chem. Zeit., 1888, 12, 1080) British Assoition Report on Eletril Stndrds, Fourth Report, 1865 British Assoition Report on Eletril Stndrds, 1932,P. 417 Hereus Festshrift, Hnu (Min), 1930, pp ; Ann. Physik, 1930, 7, 9 Annls. nst. Pltine, (U.S.S.R.), 1929, 7, Z. norg. llgem. Chem., 1932, 204, 41 Z. Metllkunde, 1932, 24, 139 Z. Elektrohem., 1940, 46, (3), 195 Zhur. Prikld. Khim., 1944, 17, Bull. SOC. Chim., 1886, 45, (2), 482 Z. norg. llgem. Chem., 1911, 70, 249 Z. Metllkunde, 1930, 22, 140 Z. Metllkunde, 1956, 47, Unpublished dt Z. nstrum. Kunde, 1891, 11, 262 Z. ngew. Chem., 1892, 34 Bull. Stndrds, 1908, 5, Pro. Roy. SOC., 912, A, 86, Z. Metllkunde, 1957, 48, 529 Phil. Mg., 1904, 7, (6), 388 Chemil Reviews, 1953, 52, 1-75 Phil. Mg., 1892, 34, 376 Pro. nst. Ele. Eng., Prt B, 1957, 104, Pro. nst. Ele. Erg., 1949, 68, (3), 197 Trns. A..M.E., 1943, 152, Pltinum Metls Rev., 1960, 4, (1) 26