Molecular Dynamics Simulation Of Plastic Yield And Deformation Mechanisms Of Metal Nanowires Under Tension, Compression, Torsion And Combined Loading

Employing molecular dynamics method based on the embedded atom method (EAM) potential, this thesis investigated the elasto-plastic properties and the deformation mechanisms of single crystalline face-centered-cubic (FCC) metal nanowires (Au and Cu) with the diameter less than100nm under tension, compression, torsion and combined loading. It places the emphases on the size effect due to free surface, the crystal orientation effect on mechanical properties, and the plastic yield and plastic deformation mechanism of nanowires under combined stress states. Main contents and conclusions include:(1) The size effect and free surface effect of the round-section single crystal [111] orientation gold nanowires with varying diameter (2.4nm-16.3nm) under tension and compression are investigated numerically. By dividing the nanowire into the surface layer, the near-surface layer, the sub-surface layers and the core layer, the non-uniform distribution of initial stress within the nanowire without externally applied stress are revealed due to free surface. The elastic modulus of the surface layer does not change with a decreasing nanowire size whereas the elastic modules of core layer decrease with a decreasing wire size, leading to the fact that the overall elastic modulus of nanowires decreases as the diameter decreases. The tensile yield strength of nanowires has not obvious size effect for the examined nanowires. The compressive yield strength increases with increasing diameter upto6nm, then decreases with increasing diameter. The non-symmetry of yield strength under the tension and compression is observed and explained.(2) Mechanical properties and deformation mechanisms of single crystal copper nanowires with three orientations of [001],[110] and [111] are studied separately under the tension (squared section) and torsion (round section). The mechanical properties and evolution of atomic structure of metal nanowires controlled by crystal-orientation are analyzed under different loading conditions. The [001] orientation nanowire has the highest torsional yield strength and the largest twist angle to yield. The shear elastic modulus calculated from twisted wires indicates:G[001]> G[111]> G[110]. The [111] orientation nanowire has the highest tensile yield strength, the [110] orientation the smallest, and the [001] orientation the in-between. The tensile elastic modulus calculated from stretched wires indicates: E[111]>E[110]>E[001].When the three nanowires are stretched the partial dislocation is nucleated first at free surface and pass through the slip plane, leading to stepped surface. When the nanowires are twisted the twist boundary is formed in the [111] and [001] orientated nanowires whereas the co-axial dislocation is nucleated in [110] orientated nanowires.(3)Plastic yield and microstructure evolution of the circular section single crystal [001] copper nanowires under combined axial-torsi onal loading are studied. We find that plastic deformation mechanism varies with torsion/tension (compression) ratio kε, resulting in the different dislocation nucleation modes. A transition in both yield locus and dislocation pattern with change in torsion/tension ratio is shown. Yielding of the wire is provoked by nucleation of partial dislocation of (111)/[112] type with lower torsion/tension ratio (kε<1.48) and by nucleation of partial dislocation of (111)/[121] type with higher torsion/tension ratio (kε>1.48). The dislocations organize into a stacking fault structure-dominated configuration in naowires loaded with lower torsion/tension ratio. The dislocations organize into a perfect dislocation network-dominated pattern in nanowires with higher torsion/tension ratio. It is suggested that Schmid’s law along with the critical resolved shear stress can be used to predict the yield strength of the nanowires under combined tension-torsion loading. The solid circular nanowire undergoes an unstable deformation process after yield. This suggests that solid nanopillars could be used as experimental specimens to measure the yield locus of nanowires under multiaxial loading.