טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShreiber Koren
SubjectModeling the Strength of Ni3Al Nanocubes Using Molecular
Dynamics Simulations
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Dan Mordehai
Full Thesis textFull thesis text - English Version


Abstract

Materials can drastically change mechanical properties when their size is reduced to the nano -scale. This is mainly because of an increase in the surface to volume ratio and of lowering the amount of dislocations, which are line defects in the lattice structure. In recent years, there is a growing interest in employing mobile-dislocations free nano-particles as specimens to study their strength at the nano-scale. These specimens yield at very high stresses, an indication that plasticity commences by dislocation nucleation. This observation provides us with a unique opportunity to study dislocation surface nucleation and how it is affected by the size of nano -particles. However, in experiments, we can only measure macroscopic properties and cannot identify deformation mechanisms during the deformation.

In this work, we perform a molecular dynamics (MD) simulation study, to obtain insights on dislocation mechanisms which dominate the deformation of Ni3Al nanocubes. In conjunction with experimental results that exist in literature, we provide a detailed description on how these nanocubes deform under compression. In order to interpret our results, a method was developed to identify and visualize defects in alloys. This method is parameterized in order to identify the defects in Ni3Al, which is in L12 crystal structure. The nanocubes deform elastically and reach compressive stresses of about 7 GPa before yielding abruptly. The nanocubes yield by nucleating Shockley partial dislocations at the vertices on {111} planes, leaving a symmetric structure of complex stacking faults. The mechanical response was found to be size independent, which we attribute to the cubical shape of the nanoparticles and the lack of stress gradients at its vertices.

Our simulation results, in combination with macroscopic observations in the experiments, provided us with the tools to understand the mechanical behavior of Ni3Al nanocubes. This understanding brings us one step forward towards design of specimens with improved mechanical properties