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Format:
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Author:
Deng, Chuang
Dept./Program:
Materials Science Program
Year:
2010
Degree:
Ph. D.
Abstract:
Face-centered cubic (FCC) metal nanowires (e.g., Au, Cu, Ag, Al and Ni) less than 100 nanometers in diameter have attracted considerable attention in recent years as advanced interconnects for nano-electronics and building blocks in nano-electro mechanical systems. FCC metal nanowires have shown strong size effects on mechanical properties, which result in ultrahigh strength in comparison to conventional bulk metals. These unique characteristics may be used to design nanostructures with controlled properties. Moreover, special defects, such as coherent twin boundaries, can be introduced into FCC metal nanowires to obtain enhanced mechanical strength due to recent progress in nanowire synthesis. Therefore, the reliability of devices involving the use of FCC metal nanowires calls for a predictive understanding of the mechanical properties of materials at small length scales.
In this thesis, molecular dynamics simulations with embedded-atom method potentials are used to investigate the mechanisms for yielding and plastic deformation in different FCC metal nanowires under tensile loading. Particularly, Au nanowires with a periodic arrangement of (111) coherent twin boundaries along the axis has been extensively studied. Nanoscale coherent twin boundaries are special grain boundaries with a high degree of symmetry that have been found experimentally to significantly improve both the strength and ductility in bulk metals. Simulation results of this thesis reveal that the addition of nanoscale twins can act to either increase or decrease the yield stress of Au nanowires depending on the twin boundary spacing. Strong size effects have also been found, such that the yield stress of twinned Au nanowires. increases as the nanowire diameter or twin boundary spacing decreases.
Furthermore, it is shown that ultrahigh flow stresses can be enabled in twinned Au nanowires when the ratio of nanowire diameter to twin boundary spacing is larger than 2.14, which is accompanied with a fundamental transition in mechanical behavior from sharp yield and strain-softening to significant strain-hardening. Such a transition in plasticity is also observed when the stacking fault energy of the metal decreases in twinned metal nanowires predicted from different interatomic potentials. Moreover, it is shown that {111} surface faceting can significantly increase the strength of periodically-twinned Au nanowires. This effect results from a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {100} <011> slip systems, instead of the common {111}<112> slip observed in FCC metals. Another important finding is that the ideal strength of Au can be approached in {11 I} faceted nanowires as the twin boundary spacing is decreased below a critical limit.
This thesis provides further fundamental understanding on twin boundary effects in metal plasticity within constrained dimensions, which should motivate new experimental studies in the areas· of nano-structural design and synthesis of metal nanowires with controlled mechanical properties.