|M.Sc Student||Chen Sagi|
|Subject||The Dependence of Mechanical Properties on the Nanograins|
Distribution in HCP Metals
|Department||Department of Mechanical Engineering||Supervisors||Professor Dan Mordehai|
|Professor Daniel Rittel|
|Full Thesis text|
Adiabatic shear failure is a dynamic ductile failure mechanism that is characterized by the development of a narrow band (ASB). ASB is the result of intense shear localization casing noticeable material softening, followed by catastrophic failure. The observation of dynamic recrystallization (DRX) and formation of nanograins (NG) in the ASB’s has led to new hypotheses about the origin of ASB’s, in particular, whether NG’s form during deformation in the ASB or are they at the origin to the ASB formation. It was found experimentally that the deformed microstructure of Ti6Al4V comprised NG’s prior to ASB formation [1, 2]. Accordingly, a microstructural model was suggested ASB induce failure: NG’s islands form first, followed by a coalescence phase of those regions into a fully developed shear band.
In this work, we present a detailed molecular dynamics (MD) study to examine how the presence of NGs in the crystal may give rise to formation of ASBs. Mg lattices (Hexagonal Crystal Packed structure) with different distributions of NGs were constructed and a tensile strain was applied to the computational cell. Four distinct stages appear in the calculated stress-strain curves: The first stage corresponds to the elastic regime. In the second stage, the tensile stress remains constant during the deformation. This is accomplished through dislocation nucleation at the grain boundaries and their evolution within the NG’s. During the third stage the material hardens, which corresponds to the nucleation of dislocations on the grain boundaries and their glide outside the NG’s into the crystal. However, this stage is terminated when the nucleated dislocations are absorbed in the neighboring NG’s. Following this event, noticeable softening occurs (fourth stage), since the absorption of the dislocations promotes the nucleation of new ones. Consequently, NG's which are aligned along active slip planes enhance the transition between hardening and softening and a clear localization of the deformation is observed on these slip planes. Finally, with large scale MD simulations, we show that the strain at the onset of the softening stage is decreasing with an increase in the number of NG islands per unit volume of material. We concluded that the distribution of NG’s may lead to localized plastic deformation, which will eventually leads to the formation of an ASB.
 D. Rittel, Z. G. Wang, M. Merzer PRL 96, 075502 (2006)
 D. Rittel, P. Landau, A. Venkert, PRL 101, 165501 (2008)