Lattice correspondence in deformation twinning in Magnesium
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Authors
Zhang, Qiwei
Issue Date
2018
Type
Dissertation
Language
Keywords
deformation twinning , Hexagonal Close-packed , Lattice correspondence , Magnesium , Molecular Dynamics simulation
Alternative Title
Abstract
Due to their lightweight and high specific strength, magnesium and its alloys have a great potential for a variety of applications. However, compared to face-centered cubic (FCC) metals, magnesium has a limited number of easy slip modes which cannot accommodate the strain along the c-axis, and thus twinning in Mg is an important mechanism of plastic deformation. Although numerous theoretical and experimental studies have been conducted on twinning in magnesium for decades, the mechanisms have yet been understood clearly. There are major discrepancies between theoretical and experimental results. The mainstream models for deformation twinning cannot describe the twinning behavior correctly. In deformation twinning, there is a one-to-one lattice correspondence between the parent and the twin lattices. Thus, this concept of lattice correspondence can be used to resolve the twinning mechanisms in hexagonal close-packed (HCP) metals, but the study of lattice correspondence in deformation twinning in HCP metals only remains on the mathematic level. The concept of lattice correspondence is difficult to be verified by experiments. However, molecular dynamics (MD) simulations can be used to examine lattice correspondence, by tracking the transformation of crystallographic planes of the parent lattice to the corresponding plane of the twin. Thus, twinning mechanisms can be better understood. Although lattice correspondence is a key concept in deformation twinning, as of now, it has often been neglected in the studies of twinning. Consequently, the existing models are unable to account for the phenomena observed in experiments or precisely predict the twinning behavior in Mg, and the twinning mechanisms remain controversial. The purpose of this dissertation is to use molecular dynamics simulations to investigate, in great detail, lattice correspondence in the two major twinning modes, i.e. {10-12}〈10-1-1〉 and {10-11}〈10-1-2〉, in pure Mg. Specific crystallographic planes of the parent are pre-selected and tracked during twinning, and the results are compared with the calculations based on the classical twinning theory. The results obtained from the simulations indicate that, indeed, a unique lattice correspondence exists for each twinning mode. For {10-12}〈10-1-1〉 twinning mode, the corresponding planes obtained in the simulations well agree with the crystallography-based calculations. However, in the simulations, no shear deformation is observed and the twin boundary is extremely incoherent. Lattice transformation is solely achieved by atomic shuffling and no twinning dislocations are involved. For {10-11}〈10-1-2〉 twinning mode, although the twinning elements were correctly predicted by the classical theory, discrepancies are revealed between atomistic simulation results and the crystallography-based calculations. These results unambiguously demonstrate how important lattice correspondence analysis is in resolving twinning mechanisms in complex crystal structures.
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Creative Commons Attribution 4.0 United States