Processing of Iridium and Iridium Alloys
Processing of Iridium and Iridium Alloys
METHODS FROM PURIFICATION TO FABRICATION
Iridium and its alloys have been considered to be difficult to fabricate due to their high melting temperatures, limited ductility, sensitivity to impurity content and particular chemical properties. The variety of processing methods used for iridium and its alloys are reviewed, including purification, melting, forming, joining and powder metallurgy techniques. Also included are coating and forming by the methods of electroplating, chemical and physical vapour deposition and melt particle deposition.
The metal Ir has a unique combination of properties, including a high melting temperature, strength at high temperature, oxidation resistance and corrosion resistance, that are useful in a range of applications, particularly at elevated temperature. However Ir is also one of the more difficult materials to process into finished products, in many cases due to the same properties that recommend its use. This was highlighted by the review ‘A History of Iridium’ published twenty years ago in this Journal with the subtitle ‘Overcoming the Difficulties of Melting and Fabrication’ (1). The purpose of the present review is to summarise advances in the processing of Ir during the past two decades. In some cases processing methods have been refined and improved, while in others entirely new processing methods have been developed to serve new applications of Ir. The scope of this review is limited to the processing of pure Ir and Ir alloy materials in which Ir constitutes the majority of the composition. The processing methods that are reviewed include purification, melting, powder processing, forming, joining and coating.
The purity of Ir has been shown to have important effects on the mechanical properties of both the nominally pure metal and its alloys (2, 3). Ir is separated from platinum group metal (pgm) concentrates and purified either by conventional chemical refining methods or by a solvent extraction process (4). In the conventional methods Ir oxide is dissolved in aqua regia (a mixture of concentrated nitric and hydrochloric acids) and precipitated with ammonium chloride. After a series of dissolutions and precipitations the salt is heated in a hydrogen atmosphere to produce Ir sponge. In the solvent extraction method, a series of organic liquids are used to concentrate various pgms from aqueous solutions. The Ir can then be precipitated and heated in hydrogen to produce Ir sponge. Solvent extraction methods may offer cost and environmental benefits over conventional chemical precipitation methods.
The purity of the sponge is sensitive to both the details of the processing and the starting materials. A method of chemical purification of Ir compounds following dissolution is reported to achieve removal of platinum, rhodium, ruthenium and palladium to levels below 5 parts per million by weight (ppm) (5). A method of scrap purification, as well as fabrication, uses electrodeposition of Ir from molten salt solution (6). Purification is effective in removing many base metals, but less so for some others including pgms. A pyrometallurgical method for purification of Ir scrap employs induction melting in a crucible of magnesium oxide with an air atmosphere, to remove both volatile impurities through vaporisation and oxide-forming impurities as a vapour or as a slag (7). This is a relatively low-cost method, although some impurities, including iron, are not easily removed by it.
Electron beam melting has been used for purification and is effective for removal of most impurities with the exception of refractory metal elements (8, 9). Recent work treated removal of a large number of impurity elements by electron beam melting (10). It showed that titanium, vanadium and zirconium were not removed by electron beam melting, due to negative deviations from ideal solution behaviour (10). In addition, several other elements were only partially removed even under optimal conditions of melt stirring. The impurity ratio, defined as the ratio by weight of the final impurity content to the initial impurity content, was measured for electron beam melting of Ir buttons under conditions producing Ir vaporisation of 5.6% by weight. Low ratios were observed for iron, aluminium and chromium of 0.004, 0.014 and 0.06, respectively. Intermediate ratios were obtained for the impurities platinum, silicon and carbon of 0.11, 0.2 and 0.3, respectively. The results were shown to be consistent with ideal mixing and vaporisation. The observed purification behaviour for the impurity elements Fe, Al, Cr and Si was explained by their low activity coefficients.
Melting of Ir is performed by induction melting, electron beam melting and arc melting methods. Induction melting, frequently used for initial melting (11), is performed in air with zirconia or magnesia crucibles. Due to excessive volatilisation of the crucible material at the Ir melting temperature (5), the crucibles are not suitable for use in vacuum. Ceramic inclusions are avoided by the use of electron beam, plasma or arc melting with water-cooled copper crucibles. Plasma melting of Ir is performed at a moderate pressure of about 50 Pa and provides for some purification by vaporisation of impurities, although to a lesser extent than with electron beam melting. In some cases, the final melting operation prior to deformation processing (12) is electron beam melting (discussed above under purification). Button arc melting is used for relatively small quantities of Ir and Ir alloys, particularly those with alloying additions subject to vaporisation during melting. Vacuum arc remelting (VAR) is applicable to larger-size melts, and provides ingots with little or no internal porosity (13). The VAR ingots have a relatively large grain size with some degree of directional solidification. Melting of 27 mm diameter electrodes to produce 63 mm diameter ingots of an Ir alloy is performed with a direct current (DC) of 3000 A at 31 V in a vacuum of about 13 mPa with a melt rate of about 3 kg min−1. Control of porosity is important even in the case of ingots to be forged or rolled, since porosity in the melted ingot may never be completely sealed even with extensive hot deformation, such as by hot extrusion (14).
The powder metallurgy methods of pressing and sintering can be used to prepare billets for subsequent hot working to produce a fully dense product (15), but progress in producing a fully dense near-net shape by these means has been limited. In a study of powder metallurgy methods for pgms and their alloys, including pure Ir and Ir-Pt alloys, neither pure Ir nor Ir-30 wt.% Pt were amenable to pressing and sintering, nor to hot isostatic pressing (HIP) (16). The problems with Ir include its high melting temperature, which precludes standard gas atomisation methods, and contamination from can materials during HIP. An Ir-50 wt.% Pt alloy was processed to powder by a plasma rotating electrode process and brought to near theoretical density by HIP (16). Ir powder or sponge typically consists of agglomerates of Ir crystals, with crystal sizes of the order of 1 µm and agglomerates in the size range 10 µm to 150 µm. Sintering of Ir powder with subsequent hot pressing has been used for the fabrication of Ir crucibles (17). An Ir composite with 15% by weight of yttrium oxide, selected for the necessary combination of high melting temperature and electrical conductivity together with a low electron work function, was developed as an electrode material for plasma cutting (18). The material was prepared by pressing and sintering in hydrogen at 2273 K for 30 minutes to obtain a density of 94%. Ir-base alloys containing up to 15% alloy additions of niobium, titanium, zirconium or hafnium have been consolidated on a laboratory scale from pre-alloyed powders using a pulse electric current sintering method (19). Homogeneity was improved over earlier work with elemental powders, and densities close to 98% were achieved. The use of elemental powders was also found unsuitable for preparing quaternary alloys of Ir because the desired microstructures could not be obtained in melted alloys (20).
Powder metallurgy methods have been used to produce porous Ir for use as filters or in other applications. Conventional pressing and sintering methods have been used to produce a porous Ir metal filter which is bonded to Ir alloy components (21). The bonding and final sintering at 2173 K were performed in a vacuum furnace under an applied load of 22 N over an area of 0.5 cm2 using graphite tooling. A density of 47% of theoretical was achieved. Compression at room temperature subsequently increased the density to 67% in order to obtain the desired flow rates for the filter. Slurry casting has been investigated as a method for producing porous Ir components with controlled density and pore size (22). After removal of the wax binder the samples were presintered in argon for one hour at 1273 K to achieve a density of 33%. Sintering in vacuum for one hour at 1473 K and 1573 K resulted in densities of 35% and 46% respectively.
Ir and its alloys can be processed using standard metalworking methods including forging, extrusion, rolling and drawing, but only with some difficulty (23). In general deformation is performed at elevated temperature in order to avoid crack formation and propagation (15). The deformation behaviour depends on impurity content, impurity distribution, microstructure and texture. Annealing at temperatures of 2273 K and higher was shown to result in homogenisation of some potentially deleterious impurities and to decrease the tendency for grain boundary fracture (24). In contrast, small additions of elements such as thorium and cerium have been shown to segregate to grain boundaries and to improve ductility (2, 25). The very coarse grain structure of Ir and Ir alloys in cast ingot form makes the material particularly sensitive to cracking at relatively small tensile strains. Preheat temperatures for the initial deformation of cast Ir by extrusion, forging or rolling are 1500 K or higher (26). Initial working temperatures for Ir ingots can be as high as 2075 K (27). Hot extrusion minimises tensile stresses during the initial deformation processing. Canning in molybdenum for extrusion minimises cooling of the Ir during the extrusion process and permits preheat temperatures in the range of 1600 to 1700 K with an extrusion ratio of 6.4:1 (28). (The extrusion ratio is defined as the ratio of the container bore area to the total cross-sectional area of extrusion.)
Hot rolling of Ir and its alloys following initial ingot breakdown is generally performed with preheat temperatures in the range 1100 to 1500 K. In order to minimise chill during rolling, covers of Mo have been used (29). In-process recrystallisation is also used to minimise cracking during rolling. Ir alloy sheet of about 0.5 mm thickness or less can be rolled to foil at room temperature with cold work levels of more than 80%. A schematic flow diagram of the production of the DOP-26 alloy sheet material from Ir powder through multiple melting and deformation processing steps is shown in Figure 1. (DOP-26 contains (by weight) 3000 ppm tungsten, 60 ppm thorium and 50 ppm aluminium.)
Ir wire is normally produced from a melted Ir stock with deformation performed at elevated temperature. An enriched 191Ir isotope was forged square at about 1775 K and then rod rolled beginning at 1675 K preheat temperature (12). The final circular cross section of about 0.6 mm was achieved by swaging. Another method uses repeated hot extrusion of a melted ingot followed by warm drawing, and is applicable for initial ingots as small as 30 g (30). Trials of upset forming of annealed Ir wire determined that limited cold deformation of up to 25 to 30% could be achieved, but forming of complex geometries was unsuccessful (27).
Forming of sheet or plate of Ir and its alloys is generally performed at elevated temperature. Warm drawing, that is deformation below the recrystallisation temperature, has been successfully performed on Ir and its alloys. Hemispherical cups of an Ir alloy were hydroformed using tooling preheated to 775 K (31). Prior to forming, the blanks were encapsulated in evacuated stainless steel covers that were preheated to 1175 K. Similar encapsulation in stainless steel was used for deep drawing with preheated steel tooling (32). The encapsulation and subsequent removal of the capsule materials does add processing steps and potentially decreases dimensional control. Crack-free cups may be formed from similar Ir DOP-26 alloy without encapsulation, using preheated tooling, isothermal forming temperatures of 825 K to 875 K and a draw ratio of about 2 (33). Cracking occurred at a draw temperature of 775 K. An earlier work on deep drawing of Ir cups reported wrinkling of the drawn cups (34), and incorrectly concluded that drawing below the recrystallisation temperature was not possible. Increasing the hold-down pressure minimises wrinkling, and improved lubrication minimises loads on the cup wall. By way of example, blanks of 51 mm diameter and 0.65 mm thickness, in stress-relieved conditions, were drawn with a hold-down force of 65 kN using flexible graphite sheet as a lubricant (33). The advantages and disadvantages of various deep drawing methods for DOP-26 Ir alloy have been evaluated (35). No substantial differences were found between the inspection yields of formed cups with and without encapsulation. Additional processing steps associated with encapsulation and later removal of the encapsulation material increase processing costs, but the potential for surface contamination of the Ir alloy is minimised. Also, as reported elsewhere (36), the tendency for fracture near the cup radius might possibly be minimised by maintaining the punch at a lower temperature to increase the strength of the material in that region. Hot drawing of Ir has also been reported, using a preheat temperature up to 1625 K with tooling heated to about 650 K. The blanks were of 2 mm thickness and the draw ratio was 1.1 (27). In general, Ir sheet material has been formed by deep drawing, spinning or pressing at temperatures in the range 1175 to 1675 K (26). The minimum working temperature increases with increased material thickness and increased applied tensile strain levels.
Ir is weldable by a number of methods, as are some Ir alloys. The applications for welding Ir and its alloys include the fabrication of spark plug electrodes, nuclear fuel containers and crucibles for crystal growth. Arc welding 0.63 mm thick Ir metal was performed in a helium atmosphere with a 3.2 mm diameter thoriated tungsten electrode, an arc gap of 1 mm, a weld velocity of 5 mm s−1 and a current of 41 A DC with straight polarity (37). The weld metal along the centreline exhibited an unfavourable microstructure, with single grains extending through the thickness of the weld. An increase in weld velocity, with corresponding increases in welding current, reduced the size of grains along the centreline. Oscillation of the arc across the weld centreline by magnetic deflection, equivalent to a square wave at a frequency of 6.25 Hz, produced a microstructure with smaller and more equiaxed grains along the weld centreline.
Some modifications to these methods were required for welding of the DOP-26 Ir alloy containing 60 ppm Th (38). The alloy was found to be more susceptible to hot cracking during welding, a phenomenon associated with grain boundary separation during the solidification process. A weld current of 83 A was used and hot cracking was minimised by using long initial and final tapers, and a short arc length. The use of arc deflection with a four-pole oscillator tended to further reduce hot cracking and gave a smaller average grain size in the weld (39). The grain orientations and grain sizes in the weld were associated with changes in the shape of the weld pool from teardrop to elliptical (40). This improvement in grain structure with four-pole oscillation resulted in increased tensile elongation from 4% to 14%, as measured in tests at 923 K.
The effect of weld width on the DOP-26 alloy was studied under similar conditions at a weld speed of 12.5 mm s−1, with current adjusted to give weld bead widths of 3.7 mm or 2.5 mm (41). The narrow welds exhibited a finer grain size, and a tensile impact elongation more than double that for the wider bead as measured at 5000 s−1 and temperatures of 1253 and 1373 K. Scanning electron microscopy showed equiaxed particles of Ir-Th intermetallic along grain boundary fracture surfaces within the narrow weld, whereas the wider weld gave aligned intermetallic precipitates similar to a eutectic structure. The cracking at grain boundaries during solidification was attributed to segregation of Th and local melting point depression, resulting in strain concentration at weld grain boundaries.
Arc welding of the Ir alloys has been automated. Autogenous, full joint penetration gas tungsten arc (GTA) girth welds were made to join two DOP-26 Ir alloy shells over a plutonium oxide fuel pellet using computer-based control (38). This process was later updated to use a PC-based commercial controller that continuously monitors and controls welding current, rotational speed, and the composition and flow of the torch gas (42). The use of precision tooling and controls for location, rotation and joint loading allowed tack welding to be eliminated and improved yields of defect-free welded components (43).
Electron beam welding provides some advantages in the welding of Ir alloys that are difficult to join by arc welding. An alloy of Ir with 200 ppm Th was successfully welded without cracking by electron beam, but was not weldable by arc welding (44). At low speed, successful electron beam welds could only be made over a narrow range of beam focus conditions, whereas at high speeds welds could be made over a wide range of focus conditions. This behaviour at high welding speeds was associated with fusion zone grain structure and positive segregation of Th at the fusion boundary under conditions leading to cracking. High heat input led to more rapid solidification, finer grain size and lower levels of segregation (45). The micrograph in Figure 2 shows the alignment of grain boundaries in the weld metal along the centreline of a weld obtained at the relatively low travel speed of 2.5 mm s−1. At higher speeds the grain boundaries become less uniformly oriented. The shape of the weld pool during electron beam welding was also found to influence the orientation of grains in the weld pool (46), as discussed above for arc welding of Ir. Electron beam welding of Ir alloy components has been practiced, although control of heat input and tooling is essential to obtaining acceptable welds (47). Figure 3 shows an example of an electron beam weld in a DOP-26 Ir alloy cup assembly.
Laser welding has shown benefits in welding an Ir alloy that is subject to hot cracking (48). An alloy containing 200 ppm Th was welded with a continuous-wave high-power carbon dioxide laser system without hot cracking. This was explained in a manner similar to that for electron beam welding, a highly concentrated heat source and a relatively fine fusion zone microstructure. Laser welding has been used to join Ir wire segments to nickel or nickel alloy for use in spark plug applications (49). Electric resistance welding, a potentially lower-cost alternative for this joining process, was also evaluated using a programmable high-frequency power supply. Direct bonding of Ir to the Ni was not achieved but metallurgical bonding was achieved with the use of an intermediate layer. The intermediate layers were chosen to assist in alloying between the dissimilar joint materials and to provide an orderly transition in the thermal expansion coefficient of the material across the joint.
Ir alloys have been characterised with respect to weldability, defined as the capacity to produce crack-free welds. Since there are multiple mechanisms by which cracks may initiate and propagate, there are a variety of methods for measuring weldability of Ir and its alloys.
The simplest method of determining whether autogenous welds can be made without cracks in sheet material under specific conditions was employed to show that Ir containing up to 100 ppm Th produced sound welds by arc welding, whereas alloys containing 200 ppm or greater did not produce crack-free welds (44). The more sensitive modified circular patch test employs a disk of material which is clamped to a test fixture and arc welded to produce two concentric autogenous welds in sequence (50). This test introduces thermal stresses from mechanical constraint as well as increased residual stresses from the multiple welds. The modified circular patch test was used to characterise various individual lots of the DOP-26 alloy with respect to hot cracking. Another test employed the detection of underbead cracks in arc-welded DOP-26 Ir alloy capsules (51). Here, a closure weld was made by arc welding the circumference of two mating Ir alloy cups, followed by additional circumferential welds and shorter arc welds in the same locations as the previous welds. The welded cups were examined for cracks by both non-destructive and destructive methods. This permitted the selection of improved weld parameters, in particular current ramp rates, as well as characterisation of various lots of Ir alloy sheet materials.
In the Sigmajig weldability test for hot cracking, an initial tensile stress is applied to the test sample, and threshold cracking stress is determined; this is the maximum applied stress at which a crack-free weld can be made (52). This test has been used both to characterise the weldability of Ir alloy materials and to characterise the effects of varying some welding parameters (53). A study of the effect of Th concentration in Ir-0.3 wt.% W alloy showed that the threshold cracking stress decreased from 170 MPa at 37 ppm Th to 85 MPa at 94 ppm Th. Neither oxygen impurities up to 2000 ppm by volume nor water vapour up to 1000 ppm in the argon atmosphere of the glove box affected the threshold cracking stress. However, significant increases in weld width were observed with increased gas impurity levels, an effect attributed to changes in the surface tension of the liquid. The test was also used to evaluate the weldability of alloys with Ce or both Ce and Th at levels (in atom fraction) up to 100 ppm (54). An alloy with 50 ppm Ce and alloys with 40 ppm Ce and 10 ppm Th or 30 ppm Ce and 20 ppm Th all showed threshold cracking stresses of 170 MPa or greater. A number of other alloys with boron and Y additions exhibited low threshold cracking stresses.
The performance of welded Ir crucibles has also been characterised. Crucibles fabricated by welding of electron beam melted Ir have shown longer service life than those made by powder metallurgy (55). Grain growth during service at temperatures of 1800 to 2400 K resulted in large grains, up to 10 mm in size. Crucible failures were attributed to segregation of impurities at these boundaries. One unusual application is the autogenous welding of an entire wrought Ir crucible, with a goal of increasing crucible life (56). The grains in the welded structure, although large by normal standards, can impede further grain growth and delay crucible failure.
Nondestructive examination of Ir welds and base metal has been performed using both dye penetrant and ultrasonic methods. Fluorescent dye methods have been used for deep drawn cups (57). Ultrasonic inspection methods initially used in the 1980s (38, 58) have been further developed to provide better sensitivity and diagnostic capability (59).
Deposition processes for Ir include electrolytic deposition, chemical and physical vapour deposition and melt deposition. Each of these methods has advantages and limitations depending on the requirements of the application. The plating of Ir from aqueous solutions has been the subject of a recent review (60). Plating of Ir from Ir chloride solutions with sulfamic acid produces deposits up to 25 µm thick, although the deposits exhibit cracks. Plating of Ir from solution in hydrobromic acid produces crack-free deposits of up to 1 µm thick using a plating rate of about 1 µm per hour. Improved plating efficiencies and decreased cracking of the coating were reported for sodium hexabromoiridate(III) baths with additions of oxalic acid. While typical thicknesses of Ir plating of 1 µm or less can minimise corrosion and serve for many electronic applications, thicker coatings are necessary for use at elevated temperature.
Electrodeposition from molten salts has been used to produce coatings of Ir up to 0.4 mm thick, and net shape components up to 3 mm thick. Initial work on electrodeposition demonstrated that Ir plating could be performed in a bath of fused sodium cyanide or a mixture of sodium cyanide and potassium cyanide under inert atmosphere at rates up to 10 µm h−1 (61). Coatings up to 0.125 mm thick were produced in a single coating cycle (62) and coatings up to 0.4 mm were produced in multiple cycles with intermediate surface removal (63). Later work demonstrated that deposition of Ir coating from fused chlorides was also possible, with thicknesses up to 350 µm deposited at temperatures of 800 to 920 K (64). Coatings have been made using a ternary eutectic molten salt bath of NaCl-KCl-CsCl containing 2 to 7 wt.% Ir (65). This bath composition has also been used to electroform crucibles by deposition of thick deposits on to a graphite mandrel that is later removed (66). Deposition rates up to 100 µm h−1 were reported. The grain size was 10 to 15 µm for direct current deposits with a thickness of 200 µm. The grain size was about 70 µm using intermittent current reversal. Electrodeposition has been used for a variety of other components, including crucibles, tubes, nozzles and jewellery (6, 67). Some examples are shown in Figure 4.
The chemical vapour deposition (CVD) processing of Ir has progressed significantly in recent years, particularly for very high-temperature applications. Ir was first applied by CVD to rhenium rocket thruster chambers for high-temperature oxidation protection in 1986, and a 490 N chamber was flight-qualified in 1997 (68). Ir coatings are deposited by CVD to a thickness of about 50 µm onto a Mo or graphite mandrel of the appropriate shape, and Re, to a thickness of about 2 mm, is subsequently deposited over the Ir by CVD and machined (69), as shown schematically in Figure 5. Typical deposition rates are about 10 µm h−1 for Ir and 40 µm h−1 for Re at a temperature stated to be about 1475 K (70). Further performance improvements and weight savings are reported using a carbon/carbon composite outer shell and CVD Re inner liner with a CVD Ir coating (71).
The range of methods reported for CVD of Ir was recently reviewed (72). Ir hexafluoride can be used to produce non-porous coatings at rates of up to 10 µm h−1, but the compound requires high deposition temperatures and is corrosive. Organic complexes of Ir offer the potential for lower deposition temperatures. Ir acetylacetonate has been used to produce Ir coatings up to 50 µm thick at rates up to 25 µm h−1, although these deposits contain up to 20% carbon by weight. Controlled additions of oxygen can produce essentially carbon-free coatings, but deposition rates are about 0.2 µm h−1. A variety of carbonyl, allyl and cyclooctadienyl complexes of Ir have been evaluated for CVD of Ir coatings. Temperatures for decomposition are in the range of 400 to 760 K and oxygen or hydrogen is generally added to control the carbon content of the coating. A methylcyclopentadienyl complex of Ir, Ir(COD)(MeCp) (COD = 1,5-cyclooctadiene) has been used to produce coatings of 1 to 2 µm thickness, of good purity, at temperatures in the range 573 to 673 K, but with low deposition rates of 0.25 µm h−1 or less (73).
Physical vapour deposition methods for Ir include both thermal evaporation and sputtering. Electron beam vapour deposition (EBVD) has been used to produce thin coatings on mirrors for infrared telescopes (74). Ir alloys have been deposited by EBVD for use as diffusion barriers on coated Ni-base superalloys (75). Pulsed laser vaporisation of Ir has also been studied (76). Studies of coatings of Ir for high-temperature oxidation protection of carbon materials showed that continuous coatings were produced via radio-frequency magnetron sputtering, but not with direct current sputtering methods (77).
A number of melt deposition processes have been investigated for Ir component production. The production of near-net shape parts with Ir by directed light fabrication has shown some promise (78). In this method metal powder is transported in a stream of inert gas and fused to a surface in the focus of a high-power laser beam, to form fully fused near-net-shaped components. Initial work on this process indicates that porosity originating from gases during melting and solidification is an issue. Plasma spray, and, in particular, vacuum plasma spray or low-pressure plasma spray of Ir has been proposed as a method for achieving high-density coatings. While there is little published literature available on plasma spraying of Ir, it is expected to perform similarly to that of a number of refractory metals (79). The use of spherical and/or pre-alloyed powders of Ir may also offer advantages, as it does for other refractory metals (80).
During the past twenty years improvements have been made in the processing of Ir and its alloys and also in the fundamental understanding of some processing methods. These advances have supported the use of Ir and its alloys in applications such as rocket combustion chambers, fuel containers for nuclear power in space, radiation sources for medical treatments, engine ignition devices and crucibles for the growth of electronic and photonic materials. Research on new methods for Ir processing, including novel powder metallurgy and metal deposition techniques, may facilitate future applications such as the production of Ir alloys as high-temperature structural materials.
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The author acknowledges the assistance of George B. Ulrich and Stan A. David, both of the Oak Ridge National Laboratory, in providing some of the illustrations and in reviewing the manuscript. This work was sponsored by the Office of Radioisotope Power Systems (NE-34) of the United States Department of Energy and performed at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725.
Dr Evan K. Ohriner is a Distinguished Development Staff member in the Materials Science and Technology Division of the Oak Ridge National Laboratory, U.S.A. His main interest is in processing of refractory metals and alloys. In 2005 Dr Ohriner was honoured by ASM International as an ASM Fellow ‘for the development of iridium alloys and high-temperature and wear-resistant materials used in space exploration and energy generation and transmission’.