Molecular Dynamics Simulation of Deformation Behavior of Al90Sm10 Metallic Glass and Al-Al90Sm10 Crystalline-amorphou Nanolaminate.

Abstract

Metallic Glasses (MGs) have gained significant attention from scientific and technological community due to their unique properties such as corrosion resistance, high specific strength, ultrahigh fracture toughness and superior biocompatibility. However, lack of ductility in MGs have hindered its structural applications immensely despite possessing combination of attractive properties. According to literature suitable patterning of amorphous MGs with its crystalline counterpart has proven to overcome the ductility barrier. However, an understanding of the structural evolution at the crystalline-amorphous (C/A) interface at the nano-scale level under the subjugation of different modes of loading is still an open question. In this thesis, the glass forming process of Al90Sm10 MG and deformation behaviour of Al-Al90Sm10 MG C/A nanolaminate under different loading conditions by employing molecular dynamics (MD) at nano-scale is investigated. MD has proven to be an invaluable tool in overcoming the difficulties that has been countered in experimental investigations at atomic- scale, owing to its ability to provide detailed atomic trajectories and dynamic characterization under different conditions. One of the contributory attributes associated with this work is to provide an insight of the structural evolution and underlying physics associated with the glass formation process Al90Sm10 MG and deformation behaviour of Al-Al90Sm10 MG C/A nanolaminate under different modes of loading. Firstly, this thesis constitutes of a detailed study of the role of hydrostatic pressure on the glass transition behavior of Al90Sm10 MG. The glass transition temperature rises with increasing values of applied hydrostatic pressure by virtue of increasing population of distorted ICO-like structures. Secondly, the shock compression behavior of Al-Al90Sm10 MG C/A nanolaminate has been investigated with variation of shock intensities, shock direction and architectural variation in the crystalline counterpart. The shock profile behavior is found to be dependent of shock intensities with transition from elastic to plastic behavior with increasing values of shock intensities. The shock profile behavior is also influenced by the architecture of the crystalline counterpart in C/A nanolaminate, elastic behaviour is apparent for single crystal and columnar grained nanolaminate whereas plastic behaviour dominates in nanocrystalline nanolaminate. Shock induced martensitic phase transformation is apparent in single crystal and nanocrystalline nanolaminate. Rarefaction waves are generated at the crystalline-amorphous interface under the subjugation of shock compression. Third, shock response and spall behavior of different configurations of C/A Al Al90Sm10 MG nanoporous nanolaminate has been thoroughly analyzed. The martensitic transformation in the crystalline counterpart follows KS relationship. The void collapse behavior is hardly affected by the configurations of the NP nanolaminate specimen. Fourth, the torsion behavior of Al90Sm10 MG and C/A Al-Al90Sm10 MG have been studied with intricate details. Shear transformation zones (STZs) nucleate homogenously near the vicinity of the clamped regions, grow and coalesce into the wide shear bands (SB) that grow along the interior of the NW specimen with increasing degree of rotation in case of Al90Sm10 MG. The amelioration of ICO-like Voronoi polyhedral (VPs) with increasing torsional angle possessing liquid like character encourages collective shear transformation and thereby enhancing plastic deformation under torsion load. The localization of dislocation density rings induces the formation of dislocation substructure in Al/Al90Sm10 nanolaminate under torsional loading leading to torsional buckling. The C/A interface serves as a nucleation site for the generation STZs in Al90Sm10/Al nanolaminate and serves free surface and encourages the formation of such dislocation substructure Al/Al90Sm10 nanolaminate. Finally, nano-indentation behaviour of Al2O3 coated Al and Al-Al90Sm10 nanolaminate with variation in test temperature and indentation speed has been investigated with intricate details. With the increasing nano-indentation test temperature, load bearing capacity as well as calculated hardness of Al2O3film deposited Al becomes less for a particular depth of indentation. The nano-indentation behaviour as a consequence of dislocation activity and VP population exhibits a strong dependency on the orientation of the C/A nanolaminate. To sum up, the work done in this thesis can serve as a platform for understanding the fundamentals associated with the role of C/A interface C/A nanolaminate and deformation mechanism of MG under different modes of loading.