Doctor of Philosophy
School of Mechanical, Materials and Mechatronic Engineering
Zhang, Liang, Atomistic simulation of plasticity mediated by grain boundary and stacking fault tetrahedron in FCC metals, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2015. http://ro.uow.edu.au/theses/4551
Nanocrystalline material has been the subject of widespread research over the past couple of decades. When the grain sizes of crystals are down to nanoscale, the so-called nanocrystalline material can exhibit distinct physical properties, unlike their conventional counterparts. The strength and plastic deformation of nanocrystalline material were among the most broadly investigated properties from the mechanical and material perspective. But since the rapid increases in computational power, atomistic simulation has been used extensively to study the mechanical properties of nanocrystalline material from which enormous progress has been made in computational simulation to understand the deformation mechanisms at an atomic scale. In this thesis, molecular dynamics (MD) simulations were carried out to study two common types of crystal defect induced plasticity in nanocrystalline metallic materials, i.e. grain boundary (GB) and stacking fault tetrahedron (SFT).
The first part of this thesis focuses on symmetric GBs where MD simulations were carried out to study dislocation nucleation from a number of 〈1 1 0〉 tilt GBs that covered a wide range of misorientation angles (θ). The results indicated that the mechanical behaviour of GBs and the energy barrier of dislocation nucleation from GBs were closely related to the lattice crystallographic orientation, GB energy, and the intrinsic GB structures. An atomistic analysis of the nucleation mechanisms provided details of dislocation nucleation and emission from the GBs.
The second part of the thesis focuses on the structure and mechanical property of asymmetric GBs, with the results showing that the structure of Σ5 and Σ11 asymmetric GBs with different inclination angles (Ф) consisted of structural units that are closely related to their corresponding symmetric GBs. Tensile deformation was carried out on the bicrystal models with Σ5 GBs under either 'free' or 'constrained' boundary conditions, and the results indicated that the stress state can play an important role in the dislocation nucleation mechanisms. Different deformation mechanisms were reported due to the Σ11 GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, or nucleation of dislocations and stacking faults from the GB.
In the third part, MD simulations were used to investigate the atomic mechanisms of SFT induced plasticity in Cu single crystal. The mechanical response and deformation mechanisms of SFT depended mainly on the crystal orientation and loading direction. The structural transformation of SFT was prevalent under the applied loading; this resulted in a different reduction of yield stress in compression and tension, and also caused a decreased or reversed compression/tension asymmetry. Compressive stress can result in the unfaulting of Frank loop in some crystal, and the process of unfaulting was closely related to the size of the dislocation loop and the stacking fault energy according to the elastic theory of dislocation.
The research in this thesis provides a fundamental understanding of grain boundary and stacking fault tetrahedron induced crystal plasticity at nanometer size, and it can help to enrich the theoretical basis for improving the performance of nanocrystalline materials.