DEFECTS IN METALS AND SIMULATION OF MECHANICAL ...

Chapter 4 Dislocations effect on Mechanical Properties during nanoindentation
As in chapter two we studied the effect of vacancies on the mechanical properties of Fe, in this
chapter a study on the effect of dislocations on mechanical properties of Fe is presented.
Simulations of nanoindentation were performed on single crystal Fe with similar conditions used
in chapter one for vacancies except that the defects introduced in the simulation cell were
dislocations.
The concept of a dislocation was first introduced by Orowan [ 45 ] and Taylor [ 46 ] to explain the
discrepancy between the observed and theoretical shear strength of metals, they showed that the
motion of dislocations through a crystal lattice requires less stress than the theoretical stress.
Two basic types of dislocations are edge dislocations and screw dislocations, the shear
displacements associated with plastic deformation occurs primarily by the movement of
dislocations. The slip planes and direction are those of highest atomic density.
It is worth to note previous works that involved nanoindentation and dislocations, in fact H.G.M.
kreuzer et al. [ 47 ] simulated nanoindentation in discrete dislocation (DD) by the motion of
statically distributed discrete dislocations and found that the pre-existing dislocations caused a
decrease in the hardness with increasing indentation depth. Other scholars have investigated the
effect of dislocation on yielding, in fact Qizhen Li [ 6 ] studied the effect of dislocation source
length on yield strength of nanostructured metallic multilayer thin films and the general
conclusion was that dislocations do lower the yield strength with a focus on the length of the
dislocation as a variable.
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