Microbehavior Analysis of Cu Nanowires under uniaxial loading by Molecular Dynamics Simulation (MD)

• Main Authors: Dar-Jen Pen, 彭達仁
• Other Authors: Yuan-Ching Lin
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/42926018929411556750

博士 === 國立臺灣科技大學 === 機械工程系 === 96 === This study analyzes mechanical properties and deformation behaviors of Cu nanowires with uniaxl loading states (tension and compression), different strain rates and orientations by molecular dynamics. In this work, the maximum local stress calculated method (MLS) is proposed to validly elucidate the true stress of nanowires under uniaxial loading, in order to improve the problem that that the Virial stress(VS) is easy to undervalue the flow stress during plastic deformation. In addition, the concept of the minimum response time for yielding is proposed to explain the problem that the promotion of strain rate increases the yield stress. The combination of the elastic response time and the required time for cleavage fracture are presented to elucidate the dynamic deformation behavior as the strain rate is above . Moreover, the slipping factor is proposed to evaluate the strain rate sensitivity of yield stress. Further, the effect of lattice distortion and lattice geometrical factor can be used to explain the difference in between the different orientations under tension and compression, respectively. Analysis results demonstrate that slipping factor and lattice geometrical factor can be used to reasonably predict the various behaviors of Cu nanowire with different condition at the strain rate(7*10^7~7*10^9s^-1). The analysis also studies the variation of deformation mechanisms for various orientations and strain rates. At the strain rate of 7*10^7s^-1, the zigzag distribution of partial dislocations is observed in the nanowires of the lower the strain rate sensitivity of yield stress. When the strain rate is 7*10^8s^-1 , twinning occurs in both <100> nanowires and <110>. The variations of lattice orientation caused by twinning can result in geometrical hardening or geometrical softening with distinct loading conditions and orientations. Therefore, the deformation mechanism of two orientations (<100> and <110>) is pseudo skew-symmetry of nanowires under tension and compression. However, twinning is not easy to be operated because of the restriction of rigid body layers and free surface. When the strain rate is 7*10^9s^-1 , many partial dislocations are operated simultaneously for the conditions of higher strain rate sensitivity of yield stress(<100>tension, <111>tension /compression, <110>compression). At the strain rate between 7*10^7s^-1 and 7*10^9s^-1, immediately before fracture, the crystal structure in necking zone loses FCC, so the dislocation slip can not be operated. Therefore, the primary failure mode becomes atomic bond breakage, causing that the MLS stress increases markedly. The simulation results verify the prediction of te and tc, as the pulling speed ranges from 51 to 3000 m/s. When the pulling speed is 51~800m/s, the effect of causes the accumulation of the stress that is applied at the beginning of tensile loading in . The stress rises to the maximum at , and then the stress accumulated in the both ends of nanowires propagate forward the middle of nanowires. As the pulling speed is above 1000m/s, the fracture mode of nanowire is the cleavage fracture and the fracture surface is flat. The maximum tensile stress can be considered as cleavage stress. In this case, tc is definitely smaller than te . When the pulling speed ranges from 800 to 1000m/s, the fracture mode of nanowire is still the cleavage fracture. However, the fracture surface is not flat, because a few atoms in both ends of nanowires slip during cleavage fracture. For the specific strain rate 2.2*10^10s^-1~7*10^10s^-1 (pulling speed 166~512m/s), the stress accumulated in the both ends of nanowires have distinct influnences on dynamic behaviors in different orientation nanowires during the following deformation. For the conditions of the smaller slipping factor (<100>tension, <111>tension/compression, and <110> compression), the propagation of stress waves are obviously observed. The encounter of the stress wave causes constructive interference in the middle of the nanowires, producing the second peak. However, for the conditions whose the slipping factor is larger (<110>tension), stress wave are released by the large plastic deformation in both ends of nanowires before the stress waves propagate forward the middle of nanowires. Interestingly, the phase transformation mechanism that the FCC can be transformed into HCP is observed when the <100> nanowires is under compression. Further, this phase transformation mechanism expresses the characteristic of transitional plastic wave propagation.