Atomic scale modeling of metallic nanoparticles embedded in nanoscale carbon structures
Final Report Abstract
Rapid development of nanotechnology has brought about a need to understand the mechanical properties of strained nanoscale volumes of matter and to develop models and predictions of the mechanical performance and reliability of nanomaterials. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength, it is well established that the macrocopic strength of metals can be pushed towards their theoretical limit, if characteristic internal feature sizes, like film thickness or average grain size, enter the nanometer regime. In this project, we have studied the deformation of confined metallic nanocrystallites embedded in carbon nano-onions. These nano-onions contract by electron irradiation induced defect formation and can exert forces in GPa range to the encapsulated system. A hole punctured during the pressurisation allows the material to flow out after a threshold pressure, depending on the material properties, is reached ("nanoextrusion"). Our results reveal that dislocation nucleation and accumulation in such nanograined materials is the key deformation mechanism. Dislocation pileup leads to the formation of dislocation dendrites inside the particle, causes strain hardening and limits the plastic flow. We identified novel dislocation reactions activated by the high pressure and dislocation density, and low dislocation length in the nanoparticle, which are unprecedented in bulk material. The dislocation activity becomes supressed as the extrusion hole radius is reduced and the mode of plasticity is transferred from displacive to surface amorphisation, which is not observed for bulk materials. Our simulations have also shown that twinning faults inside the nanoparticle is affecting the nanoextrusion. For Cu and Pd particles we find that the presence of twin planes reduces the yield stress, since they provide addtional nucleation sites for dislocations. By employing a dislocation extraction algorithm, we could furthermore show, that the presence of twin planes reduces the accumulated dislocation length as well as the number of dislocations inside the nanocrystal during for a given timestep during deformation. This is not due to a change in deformation mechanism, but to a more confined dislocation motion restricted to the twin planes, which results in less dislocation-dislocation interactions and thus reduces dislocation density inside the nanocrystal, since no or less dislocation locks are formed than in the single-crystalline particle.
Publications
- Dislocation interactions in metal nanoparticles under high pressure. Materials Science and Engineering, Darmstadt, 2012
Anti Tolvanen, K. Albe
- Plasticity of Metal Nanoparticles in Nanoextrusion. TMS Spring Meeting, Mechanical Behavior at Nanoscale, Orlando 2012
Anti Tolvanen, K. Albe
- Plasticity of Cu nanoparticles: Dislocation-dendrite-induced strain hardening and a limit for displacive plasticity. Beilstein J. Nanotechnol. 4, 173 (2013)
Antti Tolvanen, Karsten Albe
(See online at https://doi.org/10.3762/bjnano.4.17)
