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Intermetallic Compounds of the Platinum Group Metals SELECTED MATERIALS and THEIR PROPERTIES

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Intermetallic Compounds of the Platinum Group Metals SELECTED MATERIALS and THEIR PROPERTIES Intermetallic Compounds of the Platinum Group Metals SELECTED MATERIALS AND THEIR PROPERTIES By I. R. McGill Group Research Centre, Johnson Matthey & Co Limited towards advances in coating technology. ivany of the platinum group inter- The demand for high purity single crystals metallic compounds possess unique of mixed refractory oxides is rapidly properties which may occur in most increasing. Consequently, the choice of interesting combinations. This paper suitable non-reactive container materials be- draws attention to a number of these comes more selective. At temperatures and briejly describes some applications around zo00"C iridium and rhodium are for them. It suggests that further study obvious choices, although are more often is required before a full appreciation used under an atmosphere of nitrogen to can be made of their perhaps con- minimise precious metal loss. In terms of siderable potential for use in hostiIe thermodynamic stability and resistance to environments. environmental attack, a number of the plati- num group intermetallics offer exciting alter- natives to the present range of crucible The search for new high temperature materials. materials has gained increasing impetus over From an extensive survey by Paine et a1 (I) the last ten years. Greater demands are in 1960 covering 798 binary metal systems, being placed on existing materials as the only 95 were shown to either contain or environmental conditions under which they expected to contain intermetallic compounds must operate become increasingly severe. In with high temperature ranges of stability. particular the trend towards optimising Only two systems containing platinum group performance and efficiency in modern gas metals were studied with respect to high turbines requires the development of high temperature oxidation resistance, namely strength materials with structural stability beryllium-platinum and beryllium-palladium. and corrosion resistance at temperatures in As a result of the unexpected volume ex- excess of 10oo"C. With few exceptions the pansion of the intermetallics during isother- desire for higher creep rupture strength mal oxidation tests, experimental work on linked to better corrosion resistance cannot these systems was discontinued. It was be economically fulfilled by the use of any unfortunate that other precious metal inter- single material. Greater emphasis has there- metallics were not considered. The following fore been placed on the development of text gives some insight into the current protective coating systems which are com- and potential uses of intermetallics of the patible with new generation high strength precious metals and is based on theoretical materials and offer suitable corrosion pro- prediction supported by a selection of pub- tection at operating temperatures. Already lished experimental data. the platinum group metals, and in particular The extraodinary stability of intermetallic the platinum aluminides, have contributed compounds formed by combination of the Platinum Metals Rev., 1977, 21, (3), 85-89 85 platinum group metals with elements from therefore, be based on attaining the right the left of the 3d, 4d and 5d transition series combination of transition elements such that has been satisfactorily explained by the d orbital overlap can be maximised. This Engel-Brewer approach to metallic bonding. condition can be fulfilled by the interaction It would be appropriate, therefore, to sum- of suitable electron-deficient and electron- marise briefly the basis and implications of rich elements. The position of the platinum the Engel-Brewer correlation before con- group metals in the 4d and 5d transition sidering a selection of published data on the series is most appropriate as their electron stability and high temperature properties of configuration and in particular d orbital a number of platinum group intermetallics. structure make them primary source electron donors. The choice of electron acceptor can A Basis For Prediction therefore be made from the transition ele- In general the high melting points of ments of Groups V to 111. intermediate phases in binary alloys of the A simple illustration of the Engel-Brewer transition elements can be explained on the concept can be made by comparing the melt- basis of availability of unpaired s, p and d ing or decomposition temperatures of the bonding electrons. Although s andp electrons compounds AIr3, where A is a transition do contribute to bonding their concentration element taken from Group I11 or IV. This and ratio have more influence on long range is shown in the Table. order and crystal symmetry. The d electrons, in transition metal alloys are, however, directly Intermetallies responsible for the degree of metallic bonding and Their Application and as such their distribution affects bonding Generally high stability compounds of the capacity. Elements to the left of and includ- platinum group metals have received little ing rhenium and technetium in the 4d and 5d attention, reference only being made to them transition series make use of all their valence as prime examples of the Engel-Brewer electrons for bonding as their electron con- approach to metallic bonding. However, their figurations are not limited by the Pauli stability and high melting or decomposition exclusion principle. Optimisation of d elec- temperatures make them attractive materials tron bonding is obtained for elements with for use in demanding environments. To the d5 configuration and is clearly reflected limit the scope of transition metal combin- by the melting points of Group VI elements, ations with the platinum group metals, only molybdenum and tungsten. The effect of those elements from Groups IVB and VB resonance coupling of unpaired d electrons will be considered with occasional reference on atomic bond strength is reflected by a to other combinations worthy of mention. lowering of melting point of the elements A general lack of ductility restricts the from ruthenium to palladium and osmium use of these materials for structural com- to platinum. Similarly, the general effective- ponents, although a number of exceptions ness of d electron bonding increases from the havc been reported in the literature. In first to third transition series. The d electrons, particular a number of isostrvctural phases therefore, ultimately dictate thermodynamic with narrow ranges of compositional stability stability and in this sense are of more con- in the systems of niobium and tantalum with cern to the metallurgist as a means of pre- iridium and rhodium are known to have dicting the degree of transition metal somewhat higher degrees of ductility than interaction and the potential existence of would normally be expected of intermetallic high temperature materials in a number of compounds. selected alloy systems. The constitution of the tantalum-iridium The genesis of such a prediction must, system was first reported by Ferguson et a1 (2) Platinum Metals Rev., 1977, 21, (3) 86 An Indication of the High Temperature Stability of Same Platinum Group Intermetallics Resulting from Predictions Made by the Engel-Brewer Concept I Transition Series I Group 111 I Group IV Compound Melting Point Compound Melting Point "C "C 3d Tilr, 2115 Vlr, 21 008 4d Zrlr, - Nblr, 2435 5d Hflr, 2470 Talr, 2450 decomposition temperature in 1963 from which confirmation was then improvement in oxidation resistance of the made on a number of terminal solid solutions refractory elements niobium and tantalum at and intermediate compounds. The iridium- temperatures above IOOO~Cby alloying with rich compound u-TaIr, melts congruently a variety of suitable elements has proved to at ~450°C.Three other phases y, cc, and be somewhat ineffective. Single phase ac-Nb az were also reported all of which are stable containing 5 atomic per cent iridium shows up to at least 1860°C.The phase ccl, nominal- only a marginal improvement over pure ly TaIr, decomposes peritectically at 2120°C niobium. and shows a respectable degree of ductility Although further alloying with elements and toughness. The tantalum-rhodium such as tungsten, titanium and nickel might system (3) shows close resemblance to the well be foreseen, the interesting combination tantalum-iridium system and similar prop- of strength, ductility and high decomposition erties were recorded for a1 (TaRh), although temperatures of a number of alloys of niobium no explanation could be given. The com- and tantalum with the platinum group metals pound TaRh decomposes peritectoidally at are such as to warrant further attention. 1860°C. Ritter et a1 (4) reported the con- Of the many and varied coating methods stitution of the niobium-rhodium system in developed for high temperature corrosion 1964 and it was used to form the basis for a protection of nickel and cobalt-based alloys, comparative evaluation of the intermediate pack aluminising remains one of the most phases of the related systems tantalum- economical and convenient processes. The iridium and tantalum-rhodium. Although a mode of degradation of aluminide coatings number of these phases were reported to be is well understood and has prompted the hard but ductile, none had stability above development of superior coatings containing 1430°C. The isostructural phase ap in the precious metal diffusion barriers (5-9). There niobium-iridium system is known to have still remains some controversy as to the similar mechanical properties to the a1 mechanism by which elements such as plat- phase in the tantalum-rhodium and tantalum- inum
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