Materials scientists at the Max-Planck-Institut für Metallforschung, Stuttgart have achieved significant plastic deformation in strontium titanate (SrTiO₃), an oxide ceramic material hitherto believed to be extremely fragile and brittle at room temperature. These results will change some of the concepts with which ceramic materials are treated as engineering materials today.

Materials scientists at the Max-Planck-Institut für Metallforschung, Stuttgart have achieved significant plastic deformation in strontium titanate (SrTiO3), an oxide ceramic material hitherto believed to be extremely fragile and brittle at room temperature. These results, reported in the August 20 issue of "Physical Review Letters" and in the May issue of the "Journal of the American Ceramic Society", may change some of the concepts with which ceramic materials are treated as engineering materials today.

Strontium titanate is a prominent representative of the group of ceramic oxides, which crystallise in the cubic perovskite structure. At ambient temperatures, perovskites behave as most of the other ceramic oxides, which include the usual household ceramics as well as most of the rock-forming minerals in the crust and the mantle of the earth: they are brittle and shatter like glass. This is believed to be due to the difficulty with which dislocations move through the crystalline structure of these materials. Dislocations are defects of the regular crystal structure and serve as the elementary vehicle of permanent plastic deformation in most crystalline materials. When a dislocation moves through the crystal, it shears the crystal along its plane of motion (slip) by a well-defined displacement vector, like a wave in the carpet helps to move it across the floor. The ductility of metals can directly be attributed to the ease of motion of these dislocations. In contrast, the ionic and covalent nature of the bonding in ceramic oxides normally makes this slipping process difficult and the dislocations are essentially immobile up to high temperatures of the order of 1000°C.

The lack of plastic deformation of strontium titanate was the feature which the researchers at the Max-Planck Institute for Metals Research (Max-Planck-Institut für Metallforschung) in Stuttgart wanted to make use of when calibrating new mechanical testing equipment. It came as an enormous surprise to them that this seemingly hard single crystal deformed plastically at flow stresses as low as 120 MPa (comparable to aluminium or copper alloys) and that it reached strains of up to 7% at room temperature. The researchers immediately launched a thorough investigation of this behaviour and discovered that strontium titanate single crystals tested in compression not only display the usual transition from ductile behaviour at high temperatures (above 1000°C) to brittle behaviour below but also a second transition back to ductile behaviour below 600°C. The measured flow stresses at various temperatures for compression along two different orientations of the strontium titanate crystals are shown below.

Detailed microscopical analysis revealed that the deformation in both the high-temperature and the low-temperature regime are carried by the same type of dislocations (at least in the [100] oriented specimens). The researchers conclude that these dislocations in strontium titanate exist with two different inner (core) structures. Since the core structure of dislocations is largely dictated by symmetry and crystal structure, this observation strongly suggests that such different inner structures of the dislocations should also exist in other perovskites.

The researchers in Stuttgart now want to extend these studies to other deformation modes and application oriented questions. Having shown that the paradigm of immobile dislocations in ceramic oxides at room temperature does not hold, it appears to be worth reconsidering some of the engineering concepts connected with the use of ceramics. It certainly remains true that they are brittle - after all the crystals break if you drop them on the floor. However, it might well be possible to do some limited amount of forming at low (or even cryogenic) temperatures. Similarly, sand blasting, which in metals introduces an increased dislocation density associated with a beneficial compressive stress state into the surface layer, might also work in ceramics. These and other ideas are currently under investigation together with further microscopic studies which hopefully clarify the details of the as yet unknown core transformation of the dislocations.

Contact:

• Dieter Brunner
• Max-Planck-Institut für Metallforschung
• http://wwwmf.mpi-stuttgart.mpg.de/
• d.brunner@mf.mpg.de
• Phone: +49-711-2095-220; FAX: +49-711-2095-320

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