Molecular Dynamics Simulation of Deformation Behavior During Nanoscale Rolling

Abstract

Cost-effective processing of textured nanoscale metallic systems is extremely sought after, as tuning the crystallographic orientations can significantly impact their mechanical properties. In this regard, the metal rolling process is known to have considerable influence on the structural properties of nanomaterials. Previous literature studies have shown that manipulating the processing parameters during the rolling of bulk metallic systems can alter the morphology, internal stresses, texture, and defect density of the system. However, comprehension of the deformation mechanism at the nanoscale level during the rolling process is still unclear due to constraints in instrument setup and the high cost involved. The present thesis is a thorough investigation of the nanoscale rolling process of various metallic systems using molecular dynamics (MD) simulations. MD simulations have effectively analyzed the underlying physics behind the nanoscale deformation mechanisms, phase transformation, defect evolution, and texture. The original contribution of this work is to provide insights into the atomistic mechanisms during the nano-rolling process of crystalline, amorphous, and nanolaminate metallic systems and apprehend the deformation behavior and structural evolution during the rolling process. First, this thesis includes investigating the effect of initial crystallographic orientation, stacking fault energy (SFE), and processing parameters on the rolling behavior of single-crystal (SC) metallic systems. Nano-rolling of SC Al and Cu have shown variation in the defect generation and strain distribution due to their difference in the SFE. Whereas the evolution of new grains with enhancement in ultimate tensile strength with respect to higher roller speed is observed in SC Mg. Moreover, the lattice distortion and the grain rotation phenomenon are analyzed through the virtual characterization method for SC Fe. Second, the nano-rolling process has been implemented to investigate the deformation behavior, stress distribution, and orientation evolution in nanocrystalline (NC) metallic systems. Nanoscale rolling of NC Ni has revealed that the compressive stresses accumulate at the grain boundaries, whereas the triple junctions accrete the tensile stresses. The effect of rolling temperature is also analyzed, which demonstrated that cryo- and cold-rolling processes facilitated the formation of sub-grain boundaries and grain refinement in NC Ni and W specimens. Moreover, the grain refinement along with the twinning phenomenon aided in the texture weakening in NC Mg specimen during the nano-rolling process. After investigation of the crystalline systems, this work is then extended to explore the structural transformation in metallic glass (MG) during the nano rolling process. The low-temperature rolling process has shown dense and concentrated primary shear bands (SBs) in Cu-Zr MG. Whereas comparatively dispersed and thicker SBs are formed in the MG during the hot-rolling process. Finally, the influence of processing parameters such as the rolling speed and temperature on the atomic-level deformation mechanism and structural evolution is studied during the accumulative roll bonding process of Cu and Zr crystalline nanolaminates. Also, the effect of residual stress and defect density after the nano-rolling process on the shock deformation behavior is examined for the crystalline Cu /amorphous Cu-Zr nanolaminates. In summary, the work done in this thesis provides a fundamental comprehension of the deformation mechanism during the nano-rolling process. Moreover, this novel simulation technique aids in understanding the influence of initial crystallographic orientation and operating parameters on structural evolution, mechanical properties, and texture.