Deformation mechanism of complex metals discovered
IFF-News March 1, 2010
by Marc Heggen, Lothar Houben, and Michael Feuerbacher
ER-C scientists have discovered a new deformation mechanism in complex metallic alloys. Their findings explain why such materials are deformable and do not break when put under pressure. The new deformation mechanism is based on two neighbouring, structurally similar phases existing in the phase diagram of the investigated material. It wasdemonstrated for the first time that there is a connection between the plasticity of a material and the specific properties of the associated phase diagram. This knowledge can be used for targeted material design in the future.
For the above purpose, T-Al-Mn-Pd, a typical material from the large group of complex metallic alloys (CMA), which is known to have some technologically interesting properties, was investigated. These metallic alloys have highly complex lattice structures with up to several tens of thousands atoms per elementary cell. An elementary cell is what physicists call the smallest recurring unit of atoms in a crystal. The crystal is formed by numerous elementary cells arranged next to each other and on top of each other in chains. The high complexity of CMAs meant that until the end of the 1990s is was impossible to investigate their physical properties in detail. It was only the tremendous improvements in experimental and computing facilities in recent years which changed this situation and brought CMAs to the attention of the scientific community.
What is surprising about CMA materials is that they can be deformed by pressure even though the deformation rules of simple crystals cannot apply for them and that therefore, one would expect them to break easily. Using high-resolution scanning transmission electron microscopy, it was observed what happens during the deformation of T-Al-Mn-Pd at the atomic level. In general, deformation is conveyed by dislocations. What is so special about the dislocation observed here is its extremely high structural complexity. Its core comprises several hundred atoms. During the deformation process, the dislocation moves through the lattice structure and forms a bow wave - similar to a ship in water - which prepares the atomic restructuring by forming auxiliary escort defects. Each time this happens, several thousand atoms must exchange places in a coordinated manner. Even though the mechanism is extremely complex on the atomic level, it can be described by a simple model in which geometric elements interlock like pieces of a jigsaw. Once the dislocation has slid through, the restructuring is complete. It is demonstrated that the mechanism utilises the presence of a neighbouring, structurally related phase in the alloy system as an additional degree of freedom for the plastic deformation, which prevents the material from breaking.
Marc Heggen, Lothar Houben, and Michael Feuerbacher: Plastic deformation mechanisms in complex solids, Nature Materials (2010) | DOI:10.1038/NMAT2713