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Variable Energy Cyclotron Centre, Kolkata

Study of the dynamics of dislocation by molecular dynamics simulation techniques

A. Lattice resistance at the nanoscale: Mechanical and carrier transport properties of thin films are extensively dependent on the structure, distribution and dynamics of dislocations present in them. Particularly in nanomaterials, the surface should play a vital role in dictating this dynamics as the dislocations are too close to the surfaces. A dislocation can move under the influence of a shear load, thereby causing the plastic strain in the crystal. Dynamics of dislocations are governed by drag forces including lattice resistance and other drags, although the effect of surfaces on mobility of dislocations has not been in focus of study so far. 

We propose a model [PRL 101, 115506 (2008)] that reveals a prominent change in the velocity of a dislocation due to the presence of a free surface in the proximity of the dislocation line in a finite nanoscale crystalline solid. This effect has been attributed to the altered lattice resistance to dislocation motion in different system configurations. To verify this finding, MD simulations for an edge dislocation in bcc Molybdenum (Mo) are performed and the results (Fig. 1) are found to be in agreement with the numerical implementations of this model. The reduction in this effect at higher stresses and temperatures, as revealed by the simulations, confirms the role of lattice resistance behind the observed change in the dislocation velocity. We think that such an alteration in the velocity of dislocation would play a significant role in deciding the mechanical properties in thin films.

Fig. 1 : MD simulation output with fitted profile  

B. Study of dynamics of dislocation pinning: The phenomenon of dislocation pinning is vital to the process of plastic deformation and dictates the mechanical strength of a crystalline solid. A moving dislocation can get pinned upon its interaction with obstacles like point defects, voids, precipitates and other dislocations. In this context, a substantial amount of research in dislocation science explicitly deals with the process of depinning and its relation with the nature of the obstacles. Owing to its effective mass, a moving dislocation is associated with certain momentum and kinetic energy, which is known to dissipate after it gets pinned. However, the dynamics of pinning at ultrafine scales of length and time are still unexplored. 

We harness the techniques of MD simulations to reveal the dynamics of dislocation pinning and report for the first time, observations of damped oscillations [Phys. Rev. B 82, 184113 (2010)] of pinned dislocation segments under static shear load (refer Figure 2). Studies are performed in two different systems, namely Molybdenum (Mo) and Tungsten (W). We have also used Koehler’s vibrating string model as a mathematical tool to analyze the simulation output. The combined approach of MD simulation and analytical solution reveal significant features of dislocation dynamics. Our MD results reveal that the relation between the oscillation frequency and the link length significantly differs from that predicted using the framework of the celebrated Koehler-Granato-Lücke theory, which leads us to modify the model assuming coupled oscillations of dislocation segments in contrast to the conventional idea of independent oscillations. We would like to point out that the reported oscillations may solve the longstanding mystery of electromagnetic emission occurring during the plastic deformation and earthquakes. 

 Fig. 2 : Snapshots of dislocation void interactions with oscillations  

C. Study on void induced dislocation climb (VIC) : The phenomenon of dislocation climb is important in context of irradiation creep and existing theories of climb are formulated in the framework of diffusion based kinetics. With advent of simulation techniques, a new kind of climb by the gliding dislocations at nanovoids has been recently observed in both molecular statics (MS) and molecular dynamics (MD) simulations and this process seems to be much faster. This is known as void induced climb (VIC) and we attempt to explore the underlying mechanism of this void-induced climb (refer Figure 3) in bcc systems.       

 Fig. 3 : A segment of dislocation if found to climb at a nano-void

In this work, we present a novel simulation strategy, which estimates the energies associated with the void-induced climb of dislocations. The results highlight that the curvature of the pinned dislocation segment plays a key role in mediating this climb. The lowering of critical depinning load and the effect of thermal assistance to void-induced climb is also explained. Our study reveals that the kinetics of this climb process is fundamentally distinct from the conventional diffusion-controlled climb. [Acta Materialia 60, 3789 (2012)] 


 Contact person : Dr. Mishreyee Bhattacharya