Sarthok. K. Baruah Sabyasachi Chatterjee Amit Acharya Gerald J. Wang
The application of molecular dynamics (MD) simulations to quasi-static loading is severely limited by the large separation between atomic vibration timescales and experimentally relevant deformation rates. In this work, we employ the Practical Time Averaging (PTA) framework to overcome this limitation and enable atomistic simulations of crystalline solids under quasi-static loading conditions. PTA exploits the intrinsic separation of time scales by defining slow variables as time-averaged observables of the fast atomistic dynamics and their evolution in the slow loading timescale, thereby avoiding explicit integration of the fast dynamics. Using this approach, we simulate uniaxial deformation, in both tension and compression, of (4 to 20) nanometer sized cubic specimens of face-centered cubic Aluminum nanocrystals and applied strain rates approaching quasi-static conditions (10^−4 s−1 − 10^−3 s−1). We define slow variables as the averaged kinetic energy, potential energy and normal stress in the loading direction, and show their evolution in the slow time scale. The stress-strain curves show yield close to the theoretical yield stress for homogeneous nucleation, followed by successive load drops and rise, caused due to dislocation nucleation, motion and exit from free surfaces. The "smaller is harder" effect is evident from the stresss-strain response as well as from the variation of yield stress with the sample size.The serrations in the response are more pronounced for smaller samples. The effects of applied strain rate and initial temperature are studied. An interesting aspect of our study is that it is also able to show the evolution of intricate dislocation microstructures in the slow time scale, by tracking the (fast) time-averaged atomic positions. The PTA framework enables simulations at strain rates several orders of magnitude lower than those accessible to conventional MD, demonstrating significant speedup in computer time, while retaining full atomistic resolution.