Molecular Dynamics Simulation of Deformation Behavior of Nanocrystalline Al and CNT Reinforced Nanocrystalline Al Nanocomposites

Abstract

A new class of nanocomposites called metal matrix nanocomposites with modest amounts of nanoparticles with large specific surfaces as reinforcement exhibit exceptional physical and mechanical characteristics and have potential uses in various fields. These reinforcements interact more intensely with the metal matrix, improving the nanocomposites' mechanical, thermal, structural, and physical properties. Metal matrix composites reinforced with carbonaceous nanofillers such as carbon nanotubes (CNT) display exceptional performance even in harsh environmental circumstances. Material scientists have always been fascinated by the lack of structural flaws in carbon nanotubes, which have unmatched mechanical capabilities, for their possible inclusion in a metal matrix to produce exceptional mechanical stability. The primary contribution of this thesis is to provide a detailed knowledge of the mechanical performance and deformation behaviour of nanocrystalline (NC) materials and CNT reinforced NC Al NCs subjected to random, uniform, and columnar grains under various loading situations and operating parameters at atomic level using molecular dynamics simulation. It is challenging to conduct experimental research on the underlying deformation mechanisms for NC materials and CNT reinforced NC nanocomposites since it is costly and time-consuming. MD simulation is a trustworthy and efficient method to pinpoint the underlying deformation mechanism at the nanoscale. The present study starts with assessing the mechanical performance of NC Al and CNT reinforced NC Al nanocomposites subjected to random grains in metal matrix material at two strain rates, three armchair chirality’s, and three operative temperatures (10 K, 300 K, 681 K) using MD simulations. The grain size effect on the considered specimens has been analysed with respect to the volume fraction of CNT and temperature variation. In addition, the underlying deformation mechanism has also been investigated for NC Al and CNT-NC Al nanocomposites. The hybrid potentials (EAM, AIREBO, and LJ) have been considered to carry out the tensile deformation on considered nanocomposite specimens. The structural variations and defect evolution have been investigated during the deformation. An enhancement in both strength and ductility is observed in the CNT embedded NC Al specimens with respect to NC Al specimens. Such improvement is significant in case of (30,30) CNT embedded NC Al specimens. Further, emphasis was given to studying the underlying deformation mechanism of random grains in predetermined parallel and perpendicular cracks in NC Al and CNTs reinforced NC Al nanocomposites under uniaxial tensile loading using molecular dynamics simulations with a mechanical performance at three different operative temperatures and three armchair chiralities. The stacking faults interaction with various dislocations, twin boundary, CNT-matrix interface, and grain boundary widening has been elucidated in detail with atomic snapshots during the tensile deformation of NC Al and CNT-NC Al nanocomposites. The next objective is to investigate the creep-ratcheting behavior of NC Al and CNT-NC Al nanocomposite specimens using molecular dynamics simulations at three armchair chiralities and temperatures. The influence of deformation temperature on creep-ratcheting behavior has been studied and associated with underlying mechanisms based on the structural evolution of the material identified. The vacancy concentrations and dislocation densities have been evaluated at the end of each stage of the creep-ratcheting process for two ratcheting stress ratios, three different temperatures, and chiralities. The predominant stages involved in the creep-ratcheting deformation process are cyclic hardening and cyclic softening. Conversely, the primary, steady, and tertiary regions are observed from the creep-ratcheting plots. Finally, it is seen from the dislocation analysis that the Shockley partial and full dislocations are the driving dislocations for the creep-ratcheting deformation process. Then this work is extended to study the creep-ratcheting behavior of columnar NC Al at different temperatures. The cyclic hardening and cyclic softening phenomena are examined during the creep-ratcheting process. The grain boundary-based deformation mechanisms of columnar NC Al are elucidated in detail. The effect of dislocation density and types have been studied along with structural analysis. Finally, the deformation behavior of NC Al and CNT reinforced NC Al nanocomposite specimens (CNT-NC Al NCs) have been inspected under torsional loading. The evolution of CNT-NC Al NCs at the nanoscale, the changes in crystal structure at the atomic scale, and their correlation with defects are investigated at different stages of torsional deformation. The fracture path for NC Al and CNT-NC Al NCs is seen along the grain boundary, and the CNT fracture process in torsion is also studied. In summary, the work presented in this thesis offers a fundamental understanding of the mechanical performance and deformation mechanism of different grains (random, uniform, and columnar) of NC Al and CNT-NC Al nanocomposites during the tensile, creep-ratcheting, and torsional processes. Additionally, this cutting-edge simulation method helps to comprehend the impact of temperature, volume fraction, and operating parameters on the behaviour of nanofiller based nanocomposites subjected to mechanical properties, structural analysis, and defect evolution.