Understanding the structure of dislocations using an innovative simulation method
Plasticity is a key characteristic of metals. Metals therefore have a high potential for plastic deformation. This property may however vary strongly – from very high to very low ductility, similar to glass. Many alloys with body-centred cubic crystal structure, e.g. many steels, tungsten and molybdenum, may become brittle under unfavourable conditions and break without deformation.
The way in which materials deform and break depends on their atomic structure. It determines the cohesion of atoms within a crystal and between the crystals of a solid, while also influencing the movement of dislocations which act as deformation carriers. Dislocations may be lattice defects which move through the crystal in metals subjected to sufficiently high stresses. The simultaneous movement of millions of dislocations leads to a macroscopic deformation of the crystals.
The dislocation structure determines the sliding and deformation properties of metals
The sliding properties of dislocations are determined by their geometrical structure. Atomistic calculations are the only way to describe and predict the structure of dislocations in detail. Density functional theory (DFT) calculations in particular provide a valuable insight into dislocation structures.
Materials scientists are keen to understand how alloying elements influence the movement of dislocations. Several factors play a role in this context: the critical stress required to move dislocations, the question along which planes the dislocations can move and how easily they can change from one slip plane to another. Detailed knowledge of the atomic structure of a dislocation is required to be able to directly modify this structure and develop materials characterised by high plasticity and ductility and thus high break resistance. Tungsten-rhenium alloys have been used to show that the structure of the dislocation core and plasticity can be modified by adding rhenium to tungsten.
Atomistic modelling: the key to alloy design at the atomic level
MCL places a special focus on the "atomistic" description of alloys. This includes efficient modelling of alloys, lattice defects such as vacancies, dislocations and interfaces and their effects. The main aim is to advance the spin-wave method for the description of paramagnetic states. Research and development activities in the field of atomistic modelling are complemented by research into the functional properties of materials.
These novel theoretical methods enable completely new insights into the atomic structure of materials and their properties at this length scale and thus provide the basis for new development approaches to enhance the plasticity and break resistance of alloys. The MCL already uses atomistic modelling for the design of steels and refractory metals.