The High Temperature Stress-Rupture Properties of Platinum and Palladium
The High Temperature Stress-Rupture Properties of Platinum and Palladium
The Effect of Environment and Composition on Service Performance
The industrial applications of platinum and palladium in wrought form derive, in large measure, from the unique ability of these metals to maintain their mechanical properties over long periods in oxidising environments at high temperatures. Published data on the strengths of these metals and their alloys is, however, somewhat confusing and often contradictory. A critical appraisal of the data, including some results from experimental work carried out recently in this laboratory, is presented so that the creep-rupture strengths of high purity platinum and palladium can be specified and discussed in terms of environmental sensitivity and of the role played by impurities.
Stress-rupture curves are often used to specify the high temperature performance of structural materials. These curves give the load dependence of the times to failure for a specified testing environment and they generally display a discontinuity if the material being tested is environmentally sensitive. A discontinuity corresponding to a transition to longer rupture times as the applied load is decreased is commonly referred to as a strengthening effect. Conversely, weakening corresponds to a transition to shorter rupture times.
Environmental sensitivity may arise as a result of interactions between either the metal or its impurities and the testing environment. Interactions between the metal itself and the environment are referred to as generic effects because they are an inherent property of the pure metal and as such they may place fundamental limitations on its practical application. While stress-rupture testing, which manifests the combined effects of creep deformation and crack growth, cannot be employed to characterise environmental effects—generic or otherwise—directly, it can be a sensitive indicator of their occurrence. The embrittlement and intergranular fracture that arise as a result of exposing nickel and its alloys to air at high temperatures (1) appears to be a characteristic feature of environmental degradation in these materials at high temperatures, and is accompanied by an air-strengthening effect (2) clearly discernable in the stress-rupture curves. It is therefore reasonable to use stress-rupture data to monitor the effects of environment on a particular metal or alloy.
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.
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.
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.
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