Platinum

While early work (3–6) with platinum indicated that air weakening took place at various temperatures, other and largely more recent work (Fig. 1) particularly in the temperature regime above 1200°C, has shown that neither air-weakening nor strengthening is observed on testing both sheet and wire specimens (7–12). The air weakening effects that were recorded can probably be attributed (7, 13) to the presence of oxidisable impurities. Several mechanisms have been suggested, most of which, according to a recent review (14), involve either the intrusion of environmental atoms which interact with impurities or the loss of impurities. Thus, the appropriate control of impurity levels gives reproducible high temperature strengths for platinum and its alloys, and rupture curves of the type presented in Figure 1 can be used with confidence to predict the performance of these materials.

Palladium

Stress-rupture data reported by several sets of workers and obtained using palladium wire specimens tested in air at temperatures above 1000°C are summarised in Figure 2. Material which was 99.9 weight per cent pure displayed an air-strengthening effect at 1100 and 1250°C (15). Material of a higher purity, 99.95 weight per cent, while being stronger than the 99.9 weight per cent material (as is usually the case if the impurity level is decreased) nonetheless showed an air-weakening effect at 1150°C (16). Moreover, it can be inferred that this relatively pure material was no stronger than 99.9 weight per cent material tested at 1200°C (17). On the basis of these results there would appear to be a complex relationship between the impurity content and the stress-rupture behaviour of palladium.

In order to separate these impurity effects from a possible generic effect it is necessary to examine material of even higher purity than that tested elsewhere. Accordingly, high purity palladium sponge was argon arc melted, swaged and cold drawn without intermediate annealing to 1 mm diameter wire. Lengths of the wire were annealed in air at 1200°C for fifteen minutes before being stress-rupture tested, four at a time, by hanging them under dead-weight loading conditions in stagnant air contained within a vertical tube furnace held at 1200°C. Spectrographic analyses of the wire before testing gave the following average impurity levels in ppm by weight:

The results of the stress-rupture testing are plotted in Figure 2. It can be seen that the rupture properties of our high purity material at 1200°C were similar to those of the impure material. Moreover, the rupture curve for the high purity palladium displayed no significant discontinuity or at the very most a weak air-strengthening effect. A pronounced air-weakening effect similar to that observed (16) for 99.95 weight per cent material tested at 1150°C was not detected. The results of the present study therefore indicate that pure palladium, like platinum but unlike nickel, is essentially insensitive to air exposure at high temperatures. Moreover, previously reported data showing both air-strengthening and air-weakening effects can probably be attributed to the presence of oxidisable impurities.

Discussion

It has been argued (1) that the high temperature embrittlement of nickel is caused by the pinning of grain boundaries following exposure to oxygen. This pinning prevents the movement of grain boundaries to remove the build up of dislocation debris which takes place when grain boundary sliding during creep is accommodated by slip. An inherent or generic effect arises if oxygen which is segregated at grain boundaries is able to pin the boundaries. This effect may be reinforced by additional pinning brought about by the segregation and oxidation of impurities at the boundaries. Alternatively, the effect can apparently be suppressed if boron is allowed to segregate to the boundaries (18). Nickel oxide is stable throughout the temperature range within which embrittlement occurs. It is therefore reasonable to suppose that the segregation of oxygen modifies the structure and properties of nickel grain boundaries to give a generic embrittlement effect.

Palladium and platinum oxides on the other hand are unstable at high temperatures so it is less likely that any oxygen which is segregated at grain boundaries can restrict the movement of these boundaries. The conclusion is that pure platinum and palladium and their alloys, unlike nickel, do not suffer from a generic form of high temperature embrittlement or weakening on exposure to environments containing oxygen. Furthermore, by carefully controlling impurity levels it is possible to ensure that these two platinum group metals will provide consistent and reliable performance, when employed as structural components in high temperature process equipment.