Strengthening Platinum-Palladium-Rhodium Alloys by Ruthenium and Cerium Additions
Strengthening Platinum-Palladium-Rhodium Alloys by Ruthenium and Cerium Additions
Effects on Mechanical Properties at Room and High Temperature
The effects of cerium and ruthenium additions on the mechanical properties of Pt-15 Pd-3.5 Rh alloy (wt.%) at ambient and high temperature are examined. The mechanical properties of the alloy were improved by adding cerium (≤ wt .%) or ruthenium (≤ 0.5 wt .%) solute, with the cerium additions giving a better strengthening effect. Higher concentrations of cerium and ruthenium did not visibly increase the strength properties of the Pt-15Pd-3.5Rh alloy and even reduced some of the mechanical properties at high temperature. Increasing the palladium content in the Pt-Pd-Rh alloy could enhance the alloy strengths at room temperature but damaged the mechanical properties at high temperatures. Observations of the morphologies of fracture sections of alloy samples after creep-rupture tests at high temperature showed ductile fracture for alloys with lower contents of palladium, cerium or ruthenium and brittle fracture for alloys with higher contents ofpalladium, cerium or ruthenium. The different strengthening mechanisms of palladium, cerium and ruthenium additions to Pt-15Pd-3.5Rh alloy are discussed.
Platinum-palladium-rhodium (Pt-Pd-Rh) alloys are the main catalytic materials used for the preparation of nitric acid by the ammonia oxidation reaction. As early as the 1940s, scientists in the U.S.S.R. had developed an alloy catalyst comprising 92.5 per cent Pt, 4 per cent Pd and 3.5 per cent Rh (Pt-4Pd-3.5Rh). The alloy catalyst was based on a Pt-Rh alloy containing 92.5 per cent Pt and 7.5 per cent Rh with Pd being substituted for part of the Rh. The experience gained using this catalyst over many years in ammonia oxidation reactors, under pressures of 0.1-0.9 MPa and temperatures of 800-900°C, showed that the Pt-4Pd-3.5Rh alloy catalyst was as good as the Pt-Rh alloy catalyst.
This success encouraged the researchers in the U.S.S.R. to continue their investigations and, some 25 years later, another catalyst alloy was developed with lower Pt content, of composition 81 per cent Pt, 15 per cent Pd, 3.5 per cent Rh and 0.5 per cent ruthenium (Pt-15Pd-3.5Rh-0.5Ru). Compared to the Pt-4Pd-3.5Rh alloy, it was reported that the Pt- 15Pd-3.5Rh-0.5Ru alloy had 20 to 25 per cent higher mechanical strength and other advantages when used as a catalyst in ammonia oxidation reactors (1). Despite all of this, no data about the effects of the Ru content on the mechanical properties of Pt-Pd-Rh alloy were reported in the literature.
The Pt-4Pd-3.5Rh alloy was introduced into China from the U.S.S.R. in the 1960s to use as the catalyst for the production of nitric acid. Extensive research, undertaken at the Kunming Institute of Precious Metals in China, starting in the 1970s (2), concerning the influence of rare earth metals on the properties and structure of precious metals, indicated that rare earth additions produced a good strengthening effect on platinum (3-5) and palladium (6, 7).
As part of an attempt to develop a new catalyst alloy with higher palladium content, work was undertaken to find a suitable rare earth metal as a fourth component in the Pt-Pd-Rh alloys. In this paper, the strengthening effects of cerium additions on the Pt-15 per cent Pd-3.5 per cent Rh (Pt-15Pd-3.5Rh) alloy are studied. In view of the lack of information about the influence of Ru additives on the mechanical properties of Pt-Pd-Rh alloy, a parallel study was carried out on the mechanical strength of Pt-Pd-Rh-Ru alloys with different Ru additions, and with different Ce additions, to compare the strengthening effects.
Elements Pt, Pd, Rh and Ru having 99.95% purity and Ce with 99.9% purity were used to prepare alloys. The elements, at the nominal contents listed in Table I, were melted and alloyed in a high frequency induction furnace and poured into a water-cooled copper mould under an argon atmosphere. Ingots of the alloys were made into wires of diameter (ϕ) 0.2 mm by forging, rolling and drawing. The amounts of Pt, Pd, Rh and Ru in the wires were determined by chemical analysis, and the Ce content was determined by quantitative spectral analysis (ICP-AES). The nominal and actual contents of various alloy components are listed in Table I. The large difference between the nominal and actual content for cerium in the cerium-containing alloys was found to be due to the large amount lost in the melting process.
Tensile strengths of the alloys were obtained using a FPZ-100 type electron tensile tester. A creep-rupture apparatus (8) was used to determine creep-rupture values and the lasting strength at high temperatures. Lasting strength is defined here as the stress to produce failure in a particular time at a particular temperature, in this case in 100 hours at 900°C. Alloy microstructures were observed by transmission electron microscopy (TEM) (JEM-2000EX). The fracture sections of the alloy samples, studied after creep-rup- ture testing at high temperatures, were observed by scanning electron microscopy (SEM).
Tensile Strengths at Ambient
Temperature for Annealing Alloys
In Figure 1, the curves (a) showed the dependence of the tensile strength at room temperature on the annealing temperatures for Pt- 4Pd-3.5Rh (PPR-1) and Pt l5Pd-3.5Rh (PPR-2) alloys. The strength curves are roughly parallel for both alloys. Over the range of annealing temperatures from 750 to 950°C, PPR-2 has ∼ 20 per cent higher tensile strength and ∼ 2-5 per cent higher elongation than PPR-1. The curves show that increasing the palladium content in Pt-Pd-Rh alloy does indeed enhance the strength properties at room temperature.
Figure 2 shows the effect of different amounts of Ce and Ru additions on the tensile strengths and elongations of Pt-15Pd-3.5Rh alloy, with Ce and Ru substituted for Pt. All samples were annealed at 900°C for 10 minutes. When the Ru content is increased, the tensile strengths of the PPR-3 to PPR-5 alloys increased almost linearly, the elongation also increased slightly.
For the alloys containing Ce, having codes PPR-6 to PPR-8, the tensile strengths clearly increased when the Ce content was less than 0.1 wt.% Ce; then increased slowly as the Ce content increased beyond 0.1 wt.% Ce. The change in the elongation with increasing Ce content had essentially the same tendency as the tensile strength. From Figure 2 it can be clearly seen that the tensile strengths and elongations of the Ce-containing alloys are much higher than those for the Ru- containing alloys.
Tensile Strength at High Temperature
The tensile strengths at high temperatures for all the alloys decreased different amounts as temperature increased. Above ~ 780°C, PPR-2 alloy has lower tensile strength than PPR-1 alloy, see Figure 1(b). Figure 3 shows the dependence of the tensile strengths, at 900°C, of Pt-15Pd-3.5Rh- Ru(Ce) alloys on the amounts of Ru and Ce additions.
The tensile strength of the alloys containing Ru increases linearly with increasing Ru content, however, for the Ce-containing alloys, the tensile strength clearly increases with the increase of Ce when Ce content < 0.1 wt.%, but then did not obviously increase when the Ce content > 0.1 wt.%. It was again found that the mechanical properties of Pt-15Pd-3.5Rh-Ce alloys at high temperature are much higher than those of Pt- 15Pd-3.5Rh-Ru alloys. For example, the tensile strengths for Pt-15Pd-3.5Rh-0.05Ce and Pt-15Pd-3.5Rh-0.5Ru alloys are 146 and 78 MPa, respectively.
Creep Rupture at High Temperature
The creep-rupture curves of the alloys at 900°C are shown in Figure 4. The curves are broken lines comprised of two straight lines with different slopes. The creep-rupture curves of the PPR-1 and PPR-2 alloys and of Ru-containing PPR-3 to PPR-5 alloys, turned downwards after the turning points, while those of the PPR-6 to PPR-8 alloys, containing Ce, turned upwards after the turning points. Alloy PPR-1 has longer creep-rupture times than PPR-2. This means that increasing the palladium content in the Pt-Pd-Rh alloy shortens the creep-rupture life during the creep process at high temperatures. The Ce and Ru additives gready enhanced the creep-rupture times and the lasting strengths of the Pt-15Pd-3.5Rh alloy. Moreover, the creep-rupture times and lasting strengths of the Ce-containing alloys are much higher than those of the Ru-containing alloys, especially at lower load.
The dependence of the creep-rupture time (τ) on the stress (σ) and the temperature (T) can be expressed as follows:
A1 and A2 are constants, Q is the creep activation energy, n is a stress-sensitive factor and R is the gas constant.
The effects of the amount of Ce and Ru additions on the creep-rupture time (τ), the creep activation energy (Q), the lasting strength (σ100h900°C)and the elongation to rupture (δ) are shown in Figures 3, 4, 5 and 6.
Under a load of 29.4 MPa and at a temperature of 800-1000°C, adding Ru in amounts of up to 0.5 wt.% to the Pt-15Pd-3.5Rh alloy obviously increased its creep-rupture time, activation energy, lasting strength and elongation. However, above Ru contents of 0.5 wt.%, although the τ and σ100h900°C) values do not obviously decrease as the Ru content increases, the Q and δ values do decrease. As far as the creep property of the alloy is concerned, Ru additions above 0.5 wt.% are not always favourable to the Pt-Pd-Rh alloy.
With Ce additions up to 0.1 wt.%, the creep- rupture time, lasting strength, activation energy and elongation of Pt-15Pd-3.5Rh alloy greatly increase. However, with Ce contents greater than 0.1 wt.%, the values for σ100h900°C and δ no longer increase (remain constant), while the value for Q decreases as the Ce content increases. It can be seen from Figure 4 that the Ce-containing alloys possess higher strength properties and longer creep-rup- ture times at lower stress than at higher stress. So, low Ce contents (up to 0.1 wt.% Ce) and lower stress loads are very favourable for a longer creep- rupture life and higher lasting strengths in Ce-containing alloys.
Table II compares the mechanical properties of PPR-1, PPR-2, PPR-3 and PPR-6 alloys. It is clear that the PPR-2 alloy has lower mechanical properties at high temperatures, although it has higher tensile strength at room temperatures than PPR-1. Pt-15Pd-3.5Rh-Ce(Ru) alloys containing Ru and Ce additions possess much higher strength properties than PPR-2 alloy (with no Ce or Ru) especially alloys containing Ce. The strength properties of all the Ce-containing alloys are higher than those of the Ru-containing alloys. For example, the creep- rupture time (τ) and the lasting strength of the PPR-6 alloy which actually contains 0.05 wt.% Ce are about one order of magnitude and nearly twice as high, respectively, as those of PPR-3 alloy containing 0.45 wt.% Ru (actual content), so a dilute Ce addition to Pt-15Pd-3.5Rh has a much higher strengthening effect than a larger Ru addition.
|Alloy code||Tensile strength||Creep-rupture time (900°C, 30 MPa), τ, h||Lasting strength σ100h900°C, MPa||Activation energy (800-1000°C), Q**, kJ mol-1||Elongation δ,%|
|σh, RT*||MPa, 900°C|
Microstructures of the Alloys
According to electron diffraction patterns, alloys PPR-1, PPR-2 and PPR-5 are single solid solutions. For the Pt-15Pd-3.5Rh-Ce alloys, PPR-6 alloy is a solid solution, but small amounts of particles of a second phase occurred in PPR-7 and PPR-8 alloys (see Figure 7). The sizes of the second phase particles in the PPR-8 alloy are ~ 50-150 nm in the grains and ~ 300 nm at the grain boundaries. Through energy spectrum analysis, the second phase is found to be a quartemary compound containing Ce; its composition is 81.88 wt.% Pt, 8.57 wt.% Ce, 6.44 wt.% Pd and 3.11 wt.% Rh, which roughly corresponds to the formula Pt(Pd, Rh)11Ce. The compound is clearly a precipitated phase.
Strengthening Mechanism of Ce and Ru Additions to Pt-Pd-Rh Alloy
Strengthening and Brittleness of Pd in Pt-Pd-Rh Alloy
It can be seen from the above experimental results that increasing the palladium content in the Pt-Pd-Rh alloy increased the strength properties of the alloy at room temperature, but decreased the strength properties and elongation to rupture at high temperatures. SEM observations of the morphology of the fracture sections caused by creep rupture under a load of 29.4 MPa at 900°C for PPR-1 and PPR-2 alloys showed that the fracture of PPR-1 was ductile while that of PPR-2 was brittle (Figure 8). The alloy with the higher palladium content should contain higher oxygen content as oxygen has a higher solid solution in palladium than in platinum. Therefore, the alloy containing the higher oxygen content should contain a higher content of PdO due to the internal oxidation of Pd below 870°C, or should develop fine porosity due to the dissociation of PdO above 870°C (9).
On the other hand, the increase in oxygen content in the Pt-Pd-Rh alloy also led to the formation of more of the non-volatile oxide Rh2O3 by the diffusion of oxygen along grain boundaries. Analysis of XPS results indicated that the content of Rh2O3 on the surface layer of PPR-2 alloy (∼ 15%) was about 50% higher than the Rh2O3 content on the surface layer of PPR-1 alloy (∼ 10%). The brittleness of the PPR-2 alloy may be due to the higher content of PdO and Rh2O3 distributed along grain boundaries as well as to the formation of strings of fine porosity.
Strengthening of Pt-15Pd-3.5Rh Alloy by Ru Additions
A Ru addition brings a good solid-solution strengthening effect to Pt-Pd-Rh alloy, but the strengthening effect weakens with increasing temperature. The affinity of Ru for O2 and the vaporisation rate of the Ru oxide are higher than those of Pt, Pd and Rh. The preferential oxidation and vaporisation of the Ru addition in Pt-Pd-Rh alloy during the creep process at high temperatures could protect and prevent the alloy matrix from oxidation and reduce the amount of Rh2O3 formed along grain boundaries. The content of Rh2O3 in PPR-3 alloy was determined by XPS and is ~ 10%, which is lower than in PPR-2 alloy. When the Ru content is low, for example < 0.5 wt.%, the Ru should give protective and thus strengthening effects to the Pt-15Pd-3.5Rh alloy matrix.
The protective and strengthening effects in Pt- Rh alloys due to the preferential oxidation and evaporation of small additions of, for example, Mo, W and other elements with higher vapour pressures have been reported (10, 11). When the Ru content is higher, for example > 1.0 wt.% Ru, the rapid vaporisation of the Ru and its oxide should make a large number of fine porosities. The interaction of the porosities with dislocations makes the time taken from forming groups or strings of porosity to rupture shorter, which makes the creep life and the elongation-to-rupture at high temperature decrease. SEM observations of the morphology of the fracture sections showed that PPR-3 alloy was ductile and PPR-5 alloy was brit- de in the creep-rupture test (Figure 9). Large cavities were found in the PPR-5 alloy. Thus, large amounts of Ru additions are not desirable for Pt- Pd-Rh alloy.
Strengthening of Pt-15Pd-3.5Rh Alloy by Ce Additions
Ce additions have a greater solid solution strengthening effect on Pt-15Pd-3.5Rh alloy than Ru because Ce has a larger atomic radius and a lower solid solubility in the alloy. The larger atomic radius means a larger atomic radii difference between the Ce solute and the matrix of the alloy and larger lattice distortion in the alloy. In fact, a large number of petal-like dislocations were observed in the micro-defects of Pt-15Pd-3.5Rh- Ce alloys, which means that the stacking fault energy had decreased. In theory, solute metals with higher valence and a known solid solubility will make the stacking fault energy of f.c.c. metal solvent decrease. The Ce additive of higher valance (III) introduced into Pt-15Pd-3.5Rh alloy was found to decrease the stacking fault energy from 115 x 10-7 J cm-2 for PPR-2 alloy to 65 x 10-7 J cm-2 for PPR-7 alloy (12). The decrease of the stacking fault energy means the width of the stacking fault increased. It was of great importance to increase the high temperature creep-rupture strength of the alloy because a wider stacking fault made the slip and climb of dislocation more difficult. On the other hand, the lower solid solubility of Ce in the alloy means the precipitate phase separated out even at lower Ce concentrations. This is responsible for the precipitate strengthening that occurs in the Ce-containing alloys.
When the Ce addition is in low concentration, for example < 0.1 wt.%, the precipitated phase containing Ce is fine and dispersed, which attached itself to the dislocation lines as shown in Figure 7a. So, it is also a strengthening factor. However, in alloys containing higher Ce contents, the particles of the precipitated phase clearly grew along the grain boundaries. This growth should damage the strength properties of the alloy and especially the ability to resist creep deformation.
The fracture sections of PPR-6 and PPR-7 alloys were ductile but the fracture of PPR-8 alloy was brittle, which was noted in alloy samples by SEM observations of the morphology of the fracture sections after creep-rupture tests. This is the reason why the strength properties of Pt-15Pd- 3.5Rh alloys containing high Ce concentrations do not carry on increasing and might even decrease. So, the strengthening mechanisms of the dilute Ce additive on Pt-15Pd-3.5Rh alloy should indude at least the mechanisms of the solid solution and of the predpitated phase.
On the other hand, the creep rupture for alloys containing Ce was affected by the stress put on the alloy samples. Figure 10 shows the morphologies of fracture sections of PPR-7 alloy under different loads. The fracture was ductile under higher stress due to the shorter creep time, and brittle under lower stress due to the longer creep time. During the longer creep process particles of the predpitated phase grow and cerium oxide is also formed by internal oxidation along grain boundaries.
Increasing the Pd content in Pt-Pd-Rh alloys can increase the strength properties of the alloys at room temperature, but reduce them at high temperatures; the tendency to brittleness in the alloys at higher temperatures should also increase.
Adding a small amount of Ru (for example ≤ 0.5 wt.%) has a good strengthening effect on the Pt-15Pd-3.5Rh alloy from solid solution strengthening and a protective effect from the Ru solute in the alloy matrix being preferentially oxidised and vaporised. Higher Ru concentrations could decrease the mechanical properties at high temperature and increase the tendency to brittleness due to the formation of a large number of porosities and porosity groups or strings.
Ce additions to Pt-15Pd-3.5Rh alloy have a much bigger strengthening effect than Ru additions whether at ambient or high temperatures. Adding a small amount of Ce (for example ≤ 0.1 wt.%) can greatly increase the mechanical properties of the alloy by strengthening mechanisms in the solid solution and secondary phases. Ce additions of higher concentrations are not desirable for strengthening Pt-15Pd-3.5Rh alloy because of the growth of secondary phase particles along grain boundaries.
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Yuantao Ning is a Professor of Physical Metallurgy at Kunming Institute of Precious Metals, China. His main research interests are related to the principles of alloying and to new materials based on precious metals, particularly the platinum metals and their alloys, including ones modified by the rare earth metals. He has worked in the field for more than 30 years. He has published around 200 papers in national and international periodicals and won national prizes for his scientific achievements.
Xin Hu is a Senior Engineer at Kunming Institute of Precious Metals, China. He has a Master’s Degree of Physical Metallurgy. His research interests are mainly in developing and producing new materials for industrial application based on precious metals, including catalytic and catchment gauzes based on platinum and palladium alloys. He has published several papers and won national prizes for his work.