Twin-Slip Interaction in Plastic Deformation of Magnesium
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Authors
Chen, Peng
Issue Date
2019
Type
Dissertation
Language
Keywords
{10-12} twinning , dislocation , hardenning , magnesium , twin-slip interaction
Alternative Title
Abstract
Magnesium (Mg) alloys are the lightest structural metals and have received tremendous attention in recent years. However, because of their low-symmetry hexagonal close-packed (hcp) crystal structures, the plastic deformation behavior is very complicated and not well understood. Deformation twinning and dislocation slip are the most important mechanisms in determining the mechanical properties of crystalline metals, i.e., strength and ductility. {101 ̅2}〈101 ̅1〉 twinning is the most common twinning mode in Mg and other hcp metals, and it can be activated by tension along the c-axis or compression perpendicular to the c-axis. During plastic deformation involving both twinning and dislocation slip, twin-slip interaction occurs. However, how such interaction influences the deformation behavior and the interaction mechanisms remain unclear. Solving these fundamental problems is of great significance to understand the physics of deformation mechanisms in Mg and to design and processing of Mg alloys, as well as to validate numerical models.The research in this dissertation aims at a better understanding of the {101 ̅2} twin-slip interaction in Mg. Experiments were conducted to quantitatively investigate how extension twinning contributes to the plastic strain in highly texture Mg alloys and how twin-slip interaction in Mg alloys affects the hardening behavior. Atomistic simulations were also performed to simulate twin-slip interaction.The experimental results show that, at the very beginning of yielding, basal dislocation slip is the main contributor to the plastic strain, but twin nucleation is already activated. The contribution from twinning rapidly increases as the plastic strain increases. After the plastic strain exceeds 1.0%, extension twinning contributes 80~90% of the plastic deformation until twinning is saturated.By taking advantage that twinning and slip can be activated separately in highly texture Mg alloys, twin-slip interaction was studied experimentally. Dislocations of different densities were first introduced by prestraining in tension of an extruded Mg-3Al-1Zn alloy to 5% and 10% of total strain, respectively. Then, the loading direction was reversed to compression such that deformation twinning is activated. The results show that the hardening rate for different prestrains remains nearly unchanged, indicating that twin-slip interaction produces minimal effect on strain hardening.The simulation results show that when the Burgers vector of a matrix dislocation is parallel to the zone axis of an extension twin, the matrix dislocation was transformed into a dislocation in the twin and the slip planes before and after transformation are corresponding planes well predicted by the classical twinning theory. In contrast, when the Burgers vector of a matrix dislocation is non-parallel to the zone axis, the matrix dislocation was absorbed by the twin boundary which acts as a dislocation sink.The twin-slip interaction mechanisms revealed by the simulations can be well correlated to the experimental observations. Because the majority of the matrix dislocations can be absorbed by twin boundaries, and only those dislocations whose Burgers vectors are parallel to the zone axes of twins can be transformed to dislocations in the twin, twin-slip interaction should contribute insignificantly to strain hardening.