A method for analyzing nanomechanical deformation of nanocrystalline Ni at higher timesteps than is possible in classica
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0978-GG06-03
A method for analyzing nanomechanical deformation of nanocrystalline Ni at higher timesteps than is possible in classical molecular dynamics Vikas Tomar Aerospace and Mechanical Engineering, University of Notre Dame, 376 Fitzpatrick Hall, Notre Dame, IN, 46556
ABSTRACT A majority of computational mechanical analyses of nanocrystalline materials have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (~108 s-1) rarely observed in experiments. In this investigation a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain rate dependent) atomistic mechanical deformation of nanocrystalline structures at higher timescales than currently possible using MD is established. In this method there is no restriction on the size of MD timestep except that it must be such that to ensure a reasonable acceptance rate between consecutive Monte-Carlo (MC) time-steps. For the purpose of comparison HMC analyses of a nanocrystalline Ni sample at a strain rate of 109 s-1 with three different timesteps, viz. 2 fs, 4fs, and 8 fs, are compared with a corresponding analysis using MD simulation at the same strain rate and with a MD timestep of 2 fs. MD simulations of nanocrystalline Ni reproduce the defect nucleation and propagation results as well as strength values reported in the literature. In addition, HMC with timestep of 8 fs correctly reproduces defect formation and stress-strain response observed in the case of MD simulations with permissible timestep of 2 fs (for the interatomic potential used 2 fs is the highest MD timestep). Simulation time analyses show that by using HMC approximately 4 times saving in computational time can be achieved bringing the atomistic analyses closer to the continuum timescales. INTRODUCTION A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Except in a very limited number of cases, such as experimental verification of the MD results of [1] on deformation twinning in nanocrystalline Al, results of MD simulations have not been directly verified using experiments. The primary reason for this discrepancy has been the limitations on timescale (of the order of nanoseconds) of MD simulations. Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (~108 s-1) rarely observed in experiments. Solutions to this problem are methods that allow the use of larger time-steps in MD simulations with simultaneous resolution of atomic level vibrations, such as MD time-acceleration methods, see [2], and hybrid-Monte Carlo (HMC) method (cf. e.
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