Platinum Metals Rev., 1990, 34, (4), 192
Ternary and Complex Rhodium Alloys
An Investigation of Mechanical Properties
A previous study on binary alloys of rhodium has shown that the addition of small quantities of suitable alloying elements produce useful commercial alloys (i). It was found that certain binary alloys did not colour liquid glass, had an acceptable creep life and could be fabricated into products used in the production of glass fibre. At high loading and high temperatures, some binary rhodium alloys had a superior creep life to zirconia grain stabilised (ZGS) 10 per cent rhodium-platinum. Binary alloys which contain hafnium, zirconium and niobium form a protective surface oxide layer which prevents metal loss by the evaporation that is normally associated with the platinum group metals.
This investigation of rhodium alloys was carried out to see if the useful properties of binary alloys could be enhanced by the addition of further alloying elements, and if the creep life could be improved by the addition of grain boundary strengthening elements, such as carbon, or by the formation of a gamma prime precipitate or an oxidation-resistant oxide spinel.
The concentration of the alloying elements used in this study was generally i weight per cent, which for most alloys is below the solid solubility limit, thus preventing precipitation of a second intermetallic phase. The alloys were argon arc melted and fabricated into sheets, and their stress rupture properties were determined at loadings of 110 to 345 bar at temperatures of 1200,1300, 1400 and 1500°C.
The addition of a further alloying element to the binary alloys only produced a small increase of up to 50 Hv in the as-cast hardness of the ternary alloys, as shown in Table I. In the previous paper it was shown that the binary alloys containing scandium, holmium, zirconium and lutetium had the longest stress rupture lives of 127, 99, 94 and 73 hours, respectively, at a loading of 345 bar and at a temperature of 1200°C; but the rare earth alloys had poor workability (1). The addition of tantalum or rhenium to these alloys improved their workability. The addition of tantalum and rhenium to binary alloys of the other refractory group metals, and certain other elements, improved their cold formability, as shown in Table II.
|Ta||43||59||100 +||75 +||38||54||43||56||46|
The principal mode of oxidation in the binary alloys was by grain boundary diffusion and internal oxidation. The addition of a spinel oxide former, such as chromium, was made to improve the oxidation resistance of these alloys. At the lower temperature of 1200°C chromium did improve the oxidation resistance of binary alloys. This improvement was similar to that found in stainless steels and nickel superalloys. At the higher temperature of 1400°C, chromium no longer formed a protective oxide spinel. The oxidation resistance of the more complex alloys was similar to that found in the binary alloys where the addition of a strong oxide former prevented rhodium evaporation. Most of the alloys revealed a weight gain after oxidation, as a result of the alloying element being converted to an oxide, Table III. The most resistant oxide layers formed upon the ternary alloys containing tantalum or niobium.
Glass compatibility tests carried out on binary alloys revealed that the addition of a strong oxide former, such as chromium or hafnium, prevented rhodium from colouring liquid glass. The effect of small additions of these elements to those binary alloys which mildly coloured glass was to prevent them from colouring liquid glass (Table III). The addition of a third or fourth element to most of the binary alloys did not reduce the contact angle of liquid glass. Only two of the more complex alloys, lutetium-tantalum-rhodium and hafnium-rhenium-scandium-rhodium, reduced the contact angle of liquid glass to below the 33° angle that occurs with rhodium to 20° and 27°, respectively.
In the previous study it was shown that binary alloys formed by the addition of refractory group metals, rare earth metals and indium produced the longest stress rupture lives. Therefore a series of ternary alloys was made to determine the effect of additions of 0.75 weight per cent niobium and 1 weight per cent tantalum to binary alloys, and the results are given in Tables IV and V. These results show that the most significant improvement in creep life occurred in those alloys which contained either another refractory group metal, a rare earth metal, indium or another platinum group metal. The addition of 1 weight per cent of osmium or iridium to a 0.75 per cent niobium-rhodium alloy increased the stress rupture life from 56 to 105 hours. The effect of osmium concentration on the 0.75 per cent niobium-rhodium alloy is seen in Table VI, which shows the optimum osmium content to be 1 per cent. The ternary alloys of niobium-rhodium and tantalum-rhodium which had the longest creep life were zirconium-niobium-rhodium, molybdenum-niobium-rhodium and tungsten-tantalum-rhodium of 156,151 and 106 hours, respectively, at 345 bar and 1200°C.
Therefore a further series of ternary alloys including refractory group metals was made to determine which combination would produce the longest creep life, and the results are seen in Table VII. This shows that some of the strongest alloys are produced from combinations of elements on the same line of the Periodic Table (zirconium/niobium/molybdenum and hafnium/ tantalum/tungsten). Two of these alloys, 0.75 per cent niobium-1 per cent zirconium and 0.75 per cent niobium-1 per cent molybdenum, had a longer stress rupture life than a cobaltbased superalloy at a loading of 345 bar and at a temperature of 1200°C
The effect of the addition of a refractory group metal on the stress rupture properties of the binary alloys of scandium, holmium, erbium and lutetium is seen in Table VIII. This shows that most refractory group metals reduced the stress rupture life of these rare earth-containing alloys. However, the addition of hafnium to lutetium-rhodium, chromium to scandium-rhodium and tantalum to erbium-rhodium, increased the stress rupture life compared to that of the binary alloys, while only chromium improved the formability. The effect of the addition of osmium, iridium and rhenium upon the stress rupture properties of ternary alloys is given in Table IX. Of these elements, the addition of osmium and rhenium produced the most significant improvements in the stress rupture life of these ternary alloys.
|Stress rupture lives, hours|
|Ternary Alloy||Complex Alloy|
|Alloying elements||Temperature, °C||Alloying elements||Temperature, °C|
The effects of the addition of non-metallic elements carbon, boron, phosphorus, sulphur and silicon on the creep lives of 0.75 per cent niobium-rhodium and 1 per cent zirconium-rhodium alloys are seen in Table X. This shows that even at low concentrations these elements can substantially reduce the stress rupture life achieved by the binary rhodium alloys. A similar result produced by small additions of carbon to other ternary alloys is given in Table VI. Very low concentrations of carbon in alloys which contain a strong carbide forming element, such as niobium, improved the stress rupture life; but at higher concentrations the stress rupture life was decreased.
The effect of loading upon the creep life of complex alloys is seen in Table XI. The addition of 0.75 per cent niobium to 0.1 per cent silicon-rhodium alloy at a loading of 110 bar at 1200°C reduced the stress rupture life from 2147 to 2045 hours. Most of the alloy additions to the binary niobium-rhodium alloys substantially reduced the stress rupture life, at the loading of no bar at 1200°C, from 1919 hours. Only one alloy, the 0.3 per cent hafnium-1 per cent zirconium-rhodium, produced a significant increase in the stress rupture life compared with that of the binary rhodium alloys containing hafnium or zirconium, at a loading of no bar and a temperature of 1200°C. Lives were increased from 869 and 677 hours to 1565 hours, respectively. The stress rupture results show that at high loadings the addition of a third element improves the creep life, but at lower loadings a further alloy addition makes no significant improvement. This lack of improvement in creep properties is due to the fact that the alloy addition does not prevent the oxygen diffusion which produces large oxide particles, which then act as crack initiators.
It was found that the ternary alloy 0.75 per cent niobium-1 per cent of tantalum-rhodium could be used as a welding rod for rhodium alloys. Stress rupture results obtained from welded rhodium alloys of niobium-tantalum, hafnium-vanadium and hafnium-titanium at 110 bar and a temperature of 1200°C gave lives of 792,1919 and 869 hours, respectively. This ternary alloy was used to weld fabricated rhodium spinner baskets.
Some alloys were made which contained a gamma prime precipitate, of either Ni3 Ti or Nb 3 Ti, in order to determine their effect upon stress rupture properties. The results revealed that at 345 bar and a temperature of 1200°C, the gamma prime precipitates produced only a small increase in the stress rupture life of 51 hours for 3 per cent nickel-titanium-rhodium and 48 hours for 3 per cent niobium-titanium-rhodium, when compared to the values of 21, 34, and 41 hours for the binary alloys of nickel-rhodium, niobium-rhodium and titanium-rhodium, respectively.
Other possible hardening ternary alloys were investigated. The addition of cobalt to a tungsten-rhodium alloy increased the stress rupture life from 11 to 85 hours, at a loading of 345 bar and at 1200°C. The addition of gallium and indium, which are used to harden platinum, reduced the stress rupture life of indium-rhodium from 40 to 16 hours, at 345 bar and 1200°C.
The formation of an oxide spinel used to protect the low alloy steels of chromium, and molybdenum, together with the strong oxide 3 formers, hafnium and tantalum, produced one of the longest lives of 156 hours, when tested at a loading of 345 bars and at 1200°C.
Studies of nickel and cobalt superalloys have shown that the presence of low levels of non-metallic elements can significandy reduce the stress rupture lives of these alloys (2). The present investigation has shown that small additions of boron, carbon, phosphorus, sulphur and silicon to a 0.75 weight per cent niobium-rhodium alloy all reduce the stress rupture life of the binary alloy, see Figure 1. This effect was also found in a 1 weight per cent zirconium-rhodium alloy, and in other ternary alloys. At very low levels of carbon, alloys which form very stable carbides, such as niobium, revealed an increased stress rupture life at 345 bar and at 1200°C. This improvement in stress rupture life may be due to the formation of fine particles of NbC which increased the grain boundary cohesion. A similar improvement is found in the stress rupture properties of nickel superalloys with low carbon content. Studies of nickel superalloys have shown that the carbon increases the grain boundary adhesion, but at higher concentrations of carbon it forms large carbides at the grain boundaries which act as crack initiators promoting premature failure. In addition, at high temperature these elements are readily oxidised to volatile gases which form voids, which then cause rapid failure.
The results of this investigation show that additions of non-metallic elements have a deleterious effect upon the high temperature properties of rhodium, and this effect may occur in other platinum group metals or alloys. The use of electron beam melting makes it possible to produce alloys with a lower non-metallic content than can be achieved by conventional melting techniques. The use of such equipment for melting platinum group metals and their alloys for high temperature applications will reduce the concentration level of non-metallic elements, and hence may improve their high temperature properties.
The superior stress rupture lives of binary and ternary rhodium alloys which contain additions of one or more refractory group metals can be explained by the Lewis acid based stabilisation theory. If this theory is applied to the formation of oxides (3) in rhodium alloys and those rhodium alloys which contain other platinum group metals, it can explain the improvement in the stress rupture properties of dilute solutions of refractory group metals in binary and ternary alloys. It shows that the improvement in creep strength of ternary alloys of refractory group metals is produced by the large binding energies of the AB compounds between these metals. Metals from Groups III and IV, such as scandium and zirconium, produce the largest binding energies with rhodium, and also in ternary alloys containing these elements. The effect of the addition of a second platinum group metal, such as iridium or osmium, is to promote the formation of even more stable AB compounds, such as IrHf 3 and Zrlr3. Also the theory predicts extremely strong bonding of osmium with hafnium, niobium and zirconium; and in those ternary and quaternary alloys of rhodium which contain combinations of two or more of these refractory group metals with osmium some of the longest lives at 345 bar at 1200°C occurred.
At high loadings the creep properties of certain rhodium alloys are superior to those of ZGS platinum, ZGS 10 per cent rhodium-platinum, nickel and cobalt superalloys. At loadings of 345 bar the principle mode of failure is by slip, which is easily made more difficult by internal oxidation of refractory group metals to a fine dispersion of oxides. At a lower loading of no bar failure is initiated by grain boundary sliding and cavitation. Under these conditions the easy diffusion of oxygen along the grain boundaries of rhodium alloys promotes the formation of large oxide particles. These large oxides prevent dynamic recovery and act as crack initiators promoting rapid failure of the alloys. Under conditions of low stress this investigation could not find any rhodium alloys which had longer stress rupture lives than ZGS platinum or nickel and cobalt superalloys.
The addition of one or more elements to binary alloys of rhodium can improve the properties of the binary alloys. The formation of a stable protective oxide coat can prevent liquid glass from being coloured. The inherent grain boundary weakness of rhodium will limit its usefulness as a high temperature material. The presence of small additions of non-metallic elements significantly reduced the stress rupture life of rhodium alloys. The longest creep lives in rhodium based alloys were obtained from alloys containing mixtures of either refractory group metals, or rare earths and refractory group metal, or refractory group metal with the addition of osmium or iridium.
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- 2 J.J deBarbadillo Trans. AIME, 1983, 14A, 329
- 3 J.K Gibson, L. Brewer K.A Gingerich Trans. AIME, 1984, 15A, 2075