Platinum Metals Rev., 1995, 39, (4), 167
Microstructure and Properties of Some Dispersion Strengthened Platinum Alloys
The Influence of Yttrium and Zirconium Additions
The effect of adding very small amounts of yttrium and zirconium to platinum alloys has been investigated. The results indicate that a platinum alloy which contains bath elements has a higher recrystallisation temperature and superior mechanical properties than a platinum alloy containing only zirconium. In addition, the platinum alloy which contains both yttrium and zirconium has a stable structure in which regular second phases have been found both in the grains and on the grain boundaries.
Platinum has excellent properties when used at high temperatures in oxidative atmospheres, although its industrial application is restricted by its relatively low strength. In order to improve its strength, rhodium was initially used as a strengthening element at contents less than 20 to 25 per cent (1). Since the 1970s, ZGS platinum (2) and ODS platinum have been developed; these being strengthened by well dispersed additions of zirconium oxide, ZrO2 and yttrium oxide, Y2O,, respectively. The ZGS platinum and ODS platinum alloys have much improved properties, but their processing technologies have to some extent limited their possible applications.
For this reason, we have studied the effects that two alloying elements, yttrium and zirconium, have on the properties of platinum. The alloys were made by the metallurgical method described below and the results were encouraging.
AUoy Preparation and Experimental Methods
A mixture of platinum with yttrium and zirconium, having purities of 99.95, 99.9 and 99.9 per cent, respectively, were electron-beam melted in vacuum. The total weight fraction of zirconium, or of yttrium and zirconium, was less than 0.7 weight per cent.
The ingot sized platinum-zirconium and platinum-yttrium-zirconium alloys were cold forged and rolled into sheets suitable for microstructural examination and also drawn into wires for mechanical testing. No annealing was done during these forming processes. However, to facilitate rolling it may be better to include an anneal.
The alloys were then annealed at various temperatures and later etched in a mixture of hot hydrochloric and nitric acids for measurement of the grain diameters.
Microstructure and Stability of the Materials
The properties of these materials are determined by their microstructures. As shown in Figure 1, which is typical of many metals, the hardness of the platinum, platinum-zirconium and platinum-yttrium-zirconium alloys varied with the annealing temperature (annealing lasted for 0.5 hours) and decreased rapidly over the temperature range in which recrystallisation took place.
The hardness and also the recrystallisation temperature of the platinum-yttrium-zirconium alloy have higher values than those of platinum-zirconium. The hardness, Hv, of the platinum-yttrium-zirconium alloy annealed at over 1200°C is 72, and its recrystallisation temperature is approximately 450°C higher than that of pure platinum.
Microscopical analysis indicates that after annealing at 800°C for 0.5 hours recrystallisation of the platinum-zirconium alloy occurs, with a very small grain size but with the elongate structure resulting from rolling still visible in some areas; the platinum-yttrium-zirconium alloy does not show any recrystallisation. After annealing at 1000°C for 0.5 hours, the grain size in the platinum-zirconium has grown significantly, and the platinum-yttrium-zirconium alloy has begun to recrystallise, but its grain size is still relatively small, see Figure 2.
When annealed at 1200°C for longer periods, the grain size of the platinum-zirconium changes substantially while the platinum-yttrium-zirconium grain size is little changed, as shown in Figure 3.
Therefore, it can be deduced that alloying platinum with both yttrium and zirconium gives superior structural stability compared with alloying platinum with zirconium alone. This is very important for materials employed at high temperature. Additionally, it is considered that platinum-yttrium-zirconium is more stable than platinum-zirconium; the initial tendency to change is apparent in platinum-zirconium, but not in platinum-yttrium-zirconium.
The distribution of a second phase in platinum-zirconium and platinum-yttrium-zirconium alloys annealed at 1200°C is illustrated in Figure 4. The second phase in the platinum-zirconium, which exists mainly on the grain boundaries, was analysed using an Energy Analysing Electron Microscope and found to be a zirconium compound. The main compounds in the platinum-zirconium alloy system are PtZr1 Pt2Zr and Pt3Zr, and these are formed when the platinum alloy contains more than 3 per cent zirconium (4).
However, as the amount of zirconium in the platinum-zirconium alloy which we were investigating was less than 0.7 per cent, the compound was not likely to be any of these three platinum-zirconium compounds. It may be reasonable to deduce that the zirconium compound in our platinum-zirconium alloy is zirconium oxide formed during processing. In addition to this zirconium oxide on the grain boundaries, the platinum-yttrium-zirconium alloy also has some regular shaped particles of an yttrium and zirconium compound distributed within the grain.
These particles, which are only tens of nanometres in size, have not yet been studied, but are likely to improve creep resistance and high temperature stability. This is to be the next step of our work.
Physical Properties of the Materials
The resistivities of platinum-yttrium-zirconium and platinum-zirconium alloys are a little higher than that of pure platinum, while their elongations are slightly less. However, their hardnesses and strengths are gready improved compared to platinum, especially the high temperature strength and the creep resistance of the platinum-yttrium-zirconium alloy, see the Table.
|Resistance at 20°C, μcm||10.6 (3)||12.46||12.42||18.4 (3)|
|TCR*/°C; mean 1-100°C||0.0039 (3)||0.0017||0.0011||0.0017 (3)|
|Tensile strength, UTS, MPa (annealed)||124 (3)||278||283||330 (3)|
|Elongation, per cent, (annealed)||40 (3)||36||35||35 (3)|
|Hardness, Hv (annealed)||40 (3)||65||72||75 (3)|
|UTS at 1200°C, MPa||35.2 (5)||40.0||44.6||58.8 (5)|
|Rupture time, at 1200°C, 25 MPa, h||11.5 (5) (at 4.9 MPa)||3||7.6||307 (5) (at 4.9 MPa)|
The values for hardness given in the Table were measured on sheet; all the other values in the Table were determined using wire samples. The strength of the alloys increases with increases in the total volume fraction of yttrium and zirconium, but the elongation decreases.
The stress-rupture life for platinum-yttrium-zirconium at 1200°C and 25 MPa is similar to that of platinum-10 per cent rhodium alloy at 1000°C Further work on stress rupture and creep data is presently being undertaken.
The following three factors are believed to result in the properties of the platinum-yttrium-zirconium alloy being superior to those of plat-inum-zirconium.
[a] Solution Strengthening
The difference in the atomic radii between platinum (1.38Å) and zirconium (1.579Å) is less than that between platinum and yttrium (1.797Å), so the platinum-yttrium-zirconium lattice is more distorted than the platinum-zirconium lattice, which results in increased hardness.
The difference in electronegativity between platinum (2.2) and zirconium (1.4) is less than that between platinum and yttrium (1.2), so the bond forces and the polarisation effect in platinum-yttrium-zirconium are greater; this improves die high temperature strength (6). This is a special effect that occurs in alloys containing very small additions of alloying elements.
The interaction between zirconium and rare earth yttrium dissolved in platinum increases the alloy strength more than yttrium alone in the platinum-yttrium alloy.
[b] Dispersion Strengthening
The second phase in platinum-yttrium-zirconium, distributed on the grain boundaries and also in the grains, improves the high temperature strength and the creep resistance.
[c] Boundary Strengthening
Compared with zirconium, yttrium is more inclined to deposit on the grain boundary, causing impaired boundary diffusion and migration (7). Therefore, it is more effective for platinum to be alloyed with small amounts of zirconium and the rare earth yttrium, than it is to be alloyed with the same amount of zirconium alone.
The addition of very small amounts of yttrium and zirconium to platinum makes the platinum alloy structure more stable and greatly improves the recrystallisation temperature. The second phase distributed on the grain boundaries and within the grains improves the high temperature strength and creep resistance.
These additions of yttrium and zirconium improve the properties of platinum at both room and elevated temperatures, and alloying with the two elements is more effective than alloying only with zirconium.
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