Molecular Dynamics Study Of Super-Plasticity And Pseudo-Elasticity In Hexagonal-Close-Packed Metallic Nanowires

Plasticity is important to the mechanical behavior of materials, since it directly influeces the quantity of deformation. Increasing the plasticity is always a critical issue to structural materials. Pseudo-elasticity is one important property of shape-memory materials, since a higher recoverable strain means more mechanical energy that the shape-memory materials can absorb during deformation.Compared with coarse-grained materials, nanowires feature much higher plasticity. Via stress-induced structural transformation or deformation twinning, the elongation of nanowires can reach about 50%, realizing the so-called ?super plasticity‘. Moreover, due to the surface effect, nanowires can spontaneously recover to their original configuraions, realizing the pseudo-elasticity. Thus, nanowires become a possible solution to new materials with super plasticity and ultra-high recoverable strain. Up to now, the researches on nanowires mostly focus on face-centered-cubic(FCC) and body-centered-cubic(BCC) structures, while there is fewer reports on hexagonal-close-packed(HCP) structures. Since there is limited number of slipping systems, HCP metals are thought to be less ductile than FCC and BCC metals. The elongation of coarse-grained HCP metals is usually less than 10%. Nevertheless, whether HCP nanowires are less ductile than FCC and BCC ones or not is still to be discovered.In this work, molecular dynamics simulation was carried out in order to predict the feasibility of super plasticity and pseudo-elasticity in HCP nanowires. The simulation contained uni-axial tens ile and free relaxation process. Two types of HCP systems, one has stress induced phase transformation and the other has deformation twinning, were chosen for the simulation. Cobalt and Cobalt-Iron nanowires were the systems for the former type, and Magnesium nanowires for the latter one. The main achievments of this work were:(I) In Cobalt nanowires, the elongation can reach about 80% via two-step HCPâ†'FCCâ†'HCP structural transformations during tension process. The deformed nanowire can recover to its original configuration, achieving pseudo-elasticity and an ultra-high recoverable strain about 71%. It was proved by molecular static calculation of energy barrier that two-step structural transformation is an optimal deformation path since it decreases general energy barrier. Such model was further improved in order to describe the sequence of phase transformations, and succeeded in predicting deformation mechanism the in Co-Fe nanowires.(II) In Magnesium nanowires, the elongation can reach about 60% via secondary twinning. The pseudo-elasticity was also observed via de-twinning during recovery. The twinning mode of secondary twinning was {11(?)1}, which is rarely observed in Mg, and resulted from local stress concerntration. The non-symmetric configuration was observed in {11(?)1} twin boundary, so that further work was conducted in order to study the <1(?)00> symmetric tilt grain boundaries. Two new configurations were discovered:(1) {11(?)3} twin boundary was to be one of the base structures;(2) Re-orientation phenomenon was observed in {11(?)6} twin boundary. The energy map of <1(?)00> symmetric tilt grain boundaries was then revised based on the two new configurations discovered here.In summary, molecular dynamics simulation was carried out in this work to explore possible deformation paths to achieve high plasticity and recoverable strain in HCP structure. This work will shed light on the study of deformation mechanisms in materials as well as the development of new materials with high ductility.