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강건욱(Keonwook Kang),웨이 사이(Wei Cai) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Bulk silicon is brittle material at room temperature, and an existing crack propagates catastrophically when the material fails under loading. On the other hand, silicon in the form of nanowires whose diameter is a few to a few tens nanometers shows super plasticity under tensile loading and fails in ductile manner, where excessive slip event is observed. The slip is associated with glide motion of dislocations, which is triggered by their nucleation at the nanowire surface. We hypothesized the dislocation nucleation is the tipping event of the following ductile failure of silicon nanowires and calculated the energy barrier of dislocation nucleation using the free-end string method as the nanowire diameter changes. Atomistic calculations reveal that the nucleation energy barrier rapidly decreases as the diameter decreases in the range of a few nanometers. Easier nucleation of dislocation at a thinner nanowire may explain ductile behavior of silicon nanowires.
고변형률 속도에서 단일 Frank-Read source에 의한 Tantalum의 변형 경화 기작에 관한 전위 동역학 연구
문선일(SunIl Moon),강건욱(Keonwook Kang) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
In this study, authors conducted tensile test at several high strain rates in single crystalline BCC tantalum (Ta) using dislocation dynamics (DD) simulations to investigate relationship between strain rate and strain hardening. A single Frank-Read source is introduced for dislocation multiplication in a cubic simulation cell of 14 μm × 14 μm × 14 μm. Periodic boundary condition (PBC) is applied to x-, y- and z-directions in order to describe bulk metals. Tensile tests are carried out with strain rate of ϵ = 10/s, 10²/s and 10³/s along z- direction at 600K. Authors adopted a linear mobility function to describe dislocation behaviors. Simulation results exhibited that yield strength increases as strain rate increases. Furthermore, we quantitatively analyzed the number of junction formation and dislocation density change in time to investigate strain rate dependent strain hardening mechanism in BBC Ta.