Physical Model of GB Brittleness in Iridium

It is well known that there are no inherent mechanisms which induce cleavage in f.c.c. metals (8). Iridium may be considered as an exception to this rule since the commercial metal behaves like a brittle crystal and meets some of the formal empirical criteria for cleavage (3, 9). Despite this, no rules can be applied to a metal which shows huge plasticity and simultaneous cleavage (although cleavage takes place in bulk crystals only under tension) (6, 7, 10).

The mechanical behaviour of single crystals of iridium can, however, be explained with the help of three suppositions – all of which have been confirmed by experiment:

The strong anisotropy of the yield stress and work hardening under tension; the highly homogeneous distribution of plastic deformation in the sample; and the distinct slip bands from the primary octahedral plane on the working surface allow us to state that at room temperature plasticity in the crystal is exhausted at the easy slip stage, when a ‘net’ becomes the only permitted dislocation configuration. Because dislocations have low mobility, the net cannot transform into small angle boundaries in spite of the large dislocation density in the iridium material. This is the reason why a neck does not form in iridium single crystals and their fracture surfaces look like brittle transcrystalline fracture (BTF) (3). It seems that the resource of plasticity for single crystal iridium is exhausted when the tensile stress reaches the level at which brittle cracks can appear. For example, iridium crystals fail at 400 to 500 MPa in tensile tests, but under compression, the critical level is not even reached at 2000 MPa (12).

Knowing that GBs have poor cohesive strength can be used as the starting point for discussing brittle fracture in polycrystalline iridium. In commercial metal, only BIF is observed on the fracture surface; this is due to either the segregation of impurities or the weak cohesive strength of the boundaries. The fracture mode of high purity polycrystalline iridium (and its alloys) is a mixture of BIF and BTF, see Figure 1. The proportion of BTF increases as failure proceeds.

For a fine grain aggregate, having maximum plasticity, the ratio between BIF and BTF on the fracture surface is 3:1 (10, 13). Consequently, the cohesive strength of a pure GB could depend on either the type of boundary or its orientation in the field of the applied stress. The former would mean that there are two kinds of GBs (normal and brittle) in iridium, but TEM studies have shown no differences between GBs in iridium and any other f.c.c. metal (14). Therefore the hypothesis of a specific type of GB is not valid as the basis of a physical model. However, an inherent property, which is directly connected with interatomic forces, never depends on macroscopic parameters – such as mechanical stress.

Mixed brittle fracture appears when a critical crack has some flexibility to move either across the grains or along GBs. As mentioned before, BTF becomes possible only in strengthened crystal grains. Where BIF occurs, grains should also be hardened, since polycrystalline iridium fails after a preliminary deformation which is homogeneously distributed in the sample. Indeed, a neck never appears in the samples at room temperature and octahedral slip bands in the primary plane cover the majority of the grains.

This means that all the grains are filled by dislocation nets, where the dislocation density is considerably lower than in failed single crystals, due to the difference in elongations prior to failure (2-6% for the grains, and 20-60% for the failed single crystals). The first value corresponds exactly to the contribution of the easy slip stage to the total plasticity of f.c.c. metal, while the second one is an anomalously high value (15).

In addition, the polycrystalline aggregate looses the ability to deform plastically at the easy slip stage, although each crystal grain still possesses a significant amount of plasticity. Failure happens when the dislocation net in the crystal grain tries to compel newly formed <110> dislocations to move through GBs – which they cannot do. As a result, stress on the GB reaches a critical value when BIF becomes possible.

Experiments have shown that the GB diffusion of impurities occurs in a narrow region around the boundary, where an increased concentration of vacancies is present. For normal f.c.c. metals the width of this so-called diffusion zone (DZ) is about 1–2 interatomic distances. In refractory tungsten, which is also inclined to BIF, the DZ reaches 5, while for iridium the DZ has been estimated as 10–15 interatomic distances (16). Naturally, such a defect-rich layer will hinder the movement of <110> dislocations through the GB and more so as their mobility is very slow. Both these factors make the anomalous width of the DZ the possible cause of GB brittleness in iridium. Clearly, the inclination to BIF is considerably reinforced if non-metallic impurities, such as carbon, oxygen and hydrogen, are situated within the DZ.

In brittle polycrystals, the majority of cracks appear on the GBs, since the number of boundaries is considerably higher than the number of notch-like defects on the surface. A crack advances along a GB as long as its path coincides with the normal-to-tensile component of the applied stress, but it has a tendency to leave the GB when this condition is not met. As a result, the crack will begin to move across the grain. However, it will always return if the main cleavage plane ({100}) does not have a suitable orientation.

Brittle transcrystalline cracks have been found in iridium single crystals after an elongation of 5–10 per cent, whereas deformation prior to failure can reach 20–40 per cent (17). This allows us to state that the presence of transcrystalline cracks in iridium crystals does not mean they lose plasticity. Indeed, the transition of a crack from the GB to the grain may be considered as an effective way to arrest fracture in polycrystalline iridium. The good workability of fine grained work pieces confirms this supposition.

Discussion

A physical model is now proposed to explain brittle fracture in polycrystalline iridium. It is based on the concept of low mobility of the <110> dislocations and their hindered path on GBs. Iridium is a refractory metal, with a f.c.c. crystal structure, having strong interatomic bonds, but also octahedral slip as the dominant mechanism for deformation. The anomalously high yield stress is a result of strong interatomic bonds while octahedral slip causes the material to have high plasticity. There are no channels for the relaxation of the stress, such as transition from a dislocation net to a cellular structure or for dynamic recrystallisation in iridium at room temperature. Therefore, the ability of iridium single crystals to store high density dislocation nets allows practically unlimited growth of the deforming load – without failure.

An inclination to BTF appears when the tensile component of the applied stress reaches the level at which brittle cracks become possible. The opaque GBs for <110> dislocations are the stress concentrators in polycrystalline iridium, so that its total plastic deformation at room temperature is close to zero, in spite of the high plasticity of the crystal grain in the aggregate. The considerable width of the DZ in iridium is a possible reason why the GB does not let the <110> dislocations pass.

Higher temperatures lead to an increase in the mobility of the dislocation: the existing nets begin transforming into a cellular structure and some of the dislocations can pass through the boundaries. Thus, the level of stress on the boundaries is reduced, while the plasticity increases. Indeed, at 400-600°C the elongation prior to failure of fine grain iridium sharply increases (see Figure 2) and necking is observed in the samples; however this does not change the fracture mode (10). An X-ray study of deformed iridium single crystals confirmed the supposition that the mobility of the dislocations starts to grow at 400°C, although recrystallisation only begins at 1000°C (18). At temperatures higher than 1000°C, iridium wire, where no GBs were detected, behaves like a ductile f.c.c. metal and exhibits necking (19), while the neck in a fine grain sample never thins to a point.

The difference in mechanical behaviour may be explained by the assumption that in the first case the high density nets transform to a cellular structure without slowing down on the boundaries (20). The existing GBs hinder this transformation and, therefore, necking to a point does not occur until the temperature is at least 2000°C (21). In addition, the transparency of GBs can increase if they have been doped by heavy metallic impurities, such as tungsten or thorium (22), which reduce the vacancy concentration in the DZ.

An optimised processing scheme for iridium should be able to produce a fine grained metal and be applicable for manufacturing iridium alloys. A few conditions are needed for such a scheme:

One pyrometallurgical scheme for refining iridium allows massive single crystals of high purity metal and some alloys to be grown* and these can be used as initial work pieces (12). However, this scheme cannot be used to manufacture single crystal ingots for modern iridium base materials, as the alloying elements are removed from the matrix during oxidation and electron-beam melting.

After treatment, the ‘single crystalline’ metal is in a severely hardened state and GBs are not present (shown by metallographic means). Sheet and rod processed in this way possess the required workability for manufacturing welded crucibles (with plane or stamped bases) and wires with diameters from a few millimetres down to 10−¹mm. BIF is not a problem in this case, as there are no GBs in the material! Experience has shown that there is no strict requirement for the initial ingot to be a single crystal, because coarse-grain work pieces can also be forged without any problem. Thus, if the tensile component of the applied stress is minimal, GB brittleness can be ignored in polycrystalline iridium.

The Oak Ridge family of alloys (Ir-0.3 wt.% W) are iridium doped with tungsten, thorium and/or cerium (9, 13, 22). Tungsten gives a stable finegrain structure, while the very small additions of exotic elements (such as Th or Ce) suppress the GB brittleness. In view of the fact that the anticorrosion properties of iridium do not depend on the presence of small quantities of metallic impurities, this technology may be considered as another way for solving the ‘iridium problem’.