טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKositski Roman
SubjectThe Strength of Nanoparticles with Body-Centered Cubic
(BCC) Lattice Structure
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Dan Mordehai
Full Thesis textFull thesis text - English Version


Abstract

With the advancement of technology and the growing use of micro and nano-devices it is important to understand the mechanical behavior of material at the small scale. It has been established that materials can change mechanical properties when the sample size is reduced to the micro and nano-scales. Developing the understanding of plasticity at the small scale will improve the overall understanding of plastic deformation and will help develop and design better microsystems.

In recent years, the research efforts focused on studying the deformation of face centered cubic (FCC) metals. In this research we study the deformation of α-Fe specimens, thin-films and nanoparticles, which are of body centric cubic (BCC) lattice structure. The nanoparticles obtain a Winterbottom faceted shape, which minimizes their surface and interface energies and in this work we refer to nanoparticles that are formed experimentally via the dewetting method. With the help of Molecular Dynamics simulations, we study the dislocation mechanisms controlling the plastic deformation during two different load methods: nanoindentation of thin-films and nanoparticles and compression of nanoparticles.

In the nanoindentation simulations of nanoparticles and thin-films, the specimens are single crystalline and pristine from defects a-priori, and the onset of plasticity is dictated by dislocation nucleation. We found that after nucleation, dislocations are not depleting easily on the free surfaces and that the mechanism controlling the deformation is the depinning of dislocations rather than their nucleation and annihilation. As a result, the indentation curves are nearly unaffected by the specimen lateral dimension, as opposed to FCC specimens.

In the case of compression loading, we observed dislocation nucleation on two upper vertices and expansion into the nanoparticle along the <111> direction on the {110} slip planes. The dislocations accumulate inside the particle resulting in two non-interacting edge dislocation pile-ups. At a certain stress level a sudden and significant strain burst is observed as a result of a “collapse” of the pile-up. We associate this strain burst to a novel rapid conservative athermal mechanism, by which the arrested edge dislocations split into two other edge dislocations that glide on two different crystallographic planes. This discovered mechanism, for which we coined a term “cross-split of edge dislocations”, is a unique and collective phenomenon, which is triggered by an interaction with another same-sign pre-existing edge dislocation.

In both cases, indentation and compression, we use the MD simulations as a qualitative tool to observe the mechanisms dominating the deformation and we show how the mechanisms reported in this Thesis rationalizes experimental observations in α-Fe thin-films and nanoparticles.