Nanocrystalline metals and alloys have average grain sizes smaller than 100 nm. Owing to the reduction in grain size, they display remarkable high strength compared to bulk alloys. However, the small grain size creates large grain boundary areas and therefore high interfacial energies, which tends to lead to grain size coarsening. Thus, the substantial gains in strength come at the expense of losses in other mechanical properties such as creep resistance.
Recently, a new class of Cu-Ta alloys has been developed and shown to be thermodynamically stable even at high temperatures. This means that grain boundaries in them remain pinned up to a significant portion of the melting temperature. This suppresses grain boundary motion. As a consequence, the thermal and strain rate sensitivity of the Cu-Ta alloy appears to be very low even at high strain rates. In principle, this would mean that the Cu-Ta alloy ought to display a lower resistance to the formation of adiabatic shear bands. However, experiments have shown this is not the case: despite their low strain rate sensitivity, stable nanocrystalline alloys have much better resistance to shear localisation than their bulk counterparts. Equally so, they do not display thermal softening up to almost the melting point.
This project aims to investigate the causes of this phenomenon from a theoretical standpoint. It is postulated that this behaviour is mediated by elastodynamic dislocation plasticity. Dislocations are generated at a grain boundary and, in the absence of obstacles, they quickly accelerate and reach the other opposite grain boundary, where they get trapped. In conventional plasticity, the stress field of these static pinned dislocations is sufficient to significantly influence grain boundaries only in the immediate surrounding of their own pinning site. This means that a given trapped dislocation would only be responsible for the generation of a small number of additional dislocations. This behaviour would result in a coordinated slip transfer across nearby grain boundaries, which ought to lead to shear localisation due to high local stresses at nearby grains.
However, if the description of dislocation plasticity is altered to account for inertial effects in the material’s response, it is possible to show that, if the dislocations reach velocities of the order of 50% of the speed of sound in the metal, then their stress fields are able to influence a considerably larger area of grain boundaries, and trigger an increase number of dislocation sources at remote grain boundaries. As a result, shear localisation would be impeded by a diffuse slip transfer across many grain boundaries. This elastodynamic effect offers a promising rationale to understanding why the strain rate sensitivity of the material does not grow whilst shear localisation remains weak.
The candidate will have at least a 2:1 class degree in Materials Science, Physics, Applied Mathematics, Engineering, or other relevant discipline. A background or interest in computer programming and computer modelling would be advantageous. For more information or informal enquiries, please contact Dr Beñat Gurrutxaga-Lerma at [Email Address Removed]
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