Platinum Metals Rev., 1994, 38, (1), 12
Plastic Deformation and Fracture of Ruthenium Single Crystals
At the present time ruthenium, with its hexagonal close packed lattice, does not find wide application in industry when compared with platinum and iridium, due to its poor workability and its unsatisfactory resistance to corrosion at elevated temperatures (1 – 3). However, its high melting point, 2250°C, and relatively low cost suggest that some ruthenium-based compounds might possibly be considered as high temperature materials for aerospace applications (4).
Ruthenium is of interest to materials science since it has a low value for the ratio of the lattice constants c:a of 1.582 (1). In general hexagonal close packed metals obey an empirical rule, according to which structures with c:a > √(8/3) display basal slip behaviour, while those with c:a < √(8/3) show prismatic slip behaviour under deformation. The value of c:a = √(8/3) is determined from geometric considerations of the atomic stacking in the hexagonal unit cell (5). The value for ruthenium lies between the c:a ratio for titanium (1.587) and for beryllium (1.567) (1). Titanium is deformed at room temperature by prismatic slip while beryllium is deformed by means of basal slip, which is an exception to the empirical rule (5). Thus it would be very interesting to discover what is the main deformation mechanism in ruthenium single crystals at room temperature. This information may be helpful in specifying optimal regimes for processing ruthenium work-pieces. Plastic deformation and fracture of ruthenium single crystals are considered here at room temperature.
Large single crystals of ruthenium have been grown by electron beam melting at the Ekaterinburg Plant for Processing of Non-Ferrous Metals, which is also the method for growing iridium crystals (6). They had sizes: 100 mm in length and 10 mm in diameter. The amounts of metallic impurities in compacted ruthenium workpieces for growing crystals and in the single crystals are given in the Table. X-ray analysis has shown that ruthenium crystals are grown along the c axis. Samples of single crystal ruthenium for use in mechanical tests have been cut from a large ruthenium crystal by means of spark erosion.
Crystals for tensile testing were parallelepipeds of size 10-15 × 2 × 0.5 mm, while crystals for compression were of size 3 × 2 × 2 mm. Mechanical tests were carried out at room temperature on Instron test machines; the traverse speed being 1 mm/min and hydraulic grips were used in tensile tests. Fracture surfaces of the crystals were examined by scanning electron microscopy.
The mechanical behaviour of single crystal samples with two orientations has been studied under both tensile and compression tests. The first orientation was favourable for basal slip and thus unsuitable for prismatic slip, while the second orientation was favourable for prismatic slip and unsuitable for basal slip. The crystal geometry of the samples is shown in Figure 1.
Results and Discussion
Deformation curves of tensile testing and compression for single crystal ruthenium samples are given in Figures 2 and Figures 3, respectively. It is clear that crystals under tension, with orientation favourable for prismatic slip, fail without any preliminary plastic deformation. However, for crystals of orientation favourable to basal slip the deformation prior to failure can reach 20 per cent. In both cases, the fracture mode of the ruthenium crystals was attested as cleavage or brittle transcrystalline fracture, see Figures 4, and the morphology of the ruthenium crystal fracture surfaces resembled the morphology of the fracture surfaces of iridium single crystals (7). Crystal geometry analysis shows that the fracture surface of these ruthenium crystals is macroscopically parallel to one of pyramidic planes.
Similar behaviour takes place during compression. Samples of ruthenium crystals with orientation favourable for basal slip can be deformed by up to 80 per cent of their original size. It should be noted that a number of small cracks appeared on their surfaces under loading, but this did not lead to single crystals being separated. Crystal samples with an orientation favourable for prismatic slip changed the direction of their compression after the application of load: they began to crack and fold, after which the crystals were deformed in the direction favourable for basal slip. Their fracture mode was also attested as cleavage or brittle trans-crystalline fracture.
The results obtained may be briefly summarised as follows. Ruthenium crystals with an orientation favourable for basal slip are deformed quite well under both tensile and compression tests, while the plasticity of single crystals having an unsuitable orientation for basal slip is poor. When doing this, “plastic” ruthenium samples fail by cleavage under tension and crack under compression. In addition, is should be pointed out that the “re-orientation” of the crystals from unsuitable to favourable for basal slip direction of compression, takes place during compression tests.
Similar mechanical behaviour can be observed in single crystals of hexagonal close packed metals, such as zinc, which are deformed by basal slip and cleave under tension. Therefore, these findings provide a basis for stating that at room temperature the main deformation mode in ruthenium single crystals is by basal slip*. However, some insignificant contributions to their plasticity may be brought about by alternative deformation mechanisms, such as mechanical twinning and both prismatic and pyramidic slip. Consequently, ruthenium is the next metal to beryllium in the Periodic Table with hexagonal close packed structure and a low c:a ratio, which is deformed by basal slip.
This fact, contrasted with the above discussed empirical rule, will be the subject of further work.
At the present time, the physical mechanisms of cleavage in single crystals of hexagonal close packed metals are unclear; they may be connected with both features of crystal structure and the influence of impurities. As a rule, cleavage fracture is observed in metals having basal slip as the main deformation mode (5). Usually, brittle fracture in crystals of these metals is connected with an insufficient quantity of slip planes in the lattice. In agreeing with these arguments it may be concluded that the inclination to cleavage fracture is an inherent property of ruthenium single crystals. Indeed, experiments have shown that highly pure ruthenium single crystal workpieces crack during treatment at room temperature. In so doing, the poor workability of polycrystalline ruthenium compacted from powder is caused by of different impurities.
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