TY - THES

T1 - Deformation Mechanisms in Metallic Glasses: An Atomistic Study

AU - Moitzi, Franco

N1 - no embargo

PY - 2019

Y1 - 2019

N2 - The influence of composition and temperature on the tensile deformation behavior of amorphous PdSi and CuZr alloys are investigated using large-scale molecular dynamics simulations, potential-energy-landscape saddle point searching and density functional theory simulations. Two distinctive failure mechanisms in tensile samples can be distinguished in the above-mentioned metallic glasses: Highly-localized deformation in one mature shear band parallel to the maximum shear stress and the cracking perpendicular to the loading direction. Only shear banding is commonly seen in simulations of CuZr. For this case, the shear band dynamics, such as deflection and branching, were found to be influenced by long range elastic interactions of the stress fields of shear bands and by local structural inhomogeneities. The structural changes during shear band propagation were visualized by an entropy-based order parameter. In contrast, a cracking-to-shear-banding transition can be achieved upon increasing the temperature or decreasing the amount of silicon for PdSi glass. The Crystal Orbital Hamilton Population analysis based on electronic structure calculation from density functional theory simulations has shown that the difference in chemical bonding is responsible for the observed different deformation behaviors. CuZr shows non-directional metallic-bonding, whereas PdSi has a high amount of directional covalent Si-Si bonds. Sampling of the saddle points on the potential energy surface has revealed that a high fraction of rigid covalent Si-Si bonds increases the energy barriers for atomic rearrangements. These thermally-activated atomic relaxation events change the stress and strain state in the elastic regime and are precursor of local plasticity. High activation energies impede both the stress and strain redistribution and cause cleavage-like cracking due to a delay of the onset of plasticity. On the other side, crack propagation due to void nucleation ahead of the crack tip was seen in a heterogeneous PdSi glass.

AB - The influence of composition and temperature on the tensile deformation behavior of amorphous PdSi and CuZr alloys are investigated using large-scale molecular dynamics simulations, potential-energy-landscape saddle point searching and density functional theory simulations. Two distinctive failure mechanisms in tensile samples can be distinguished in the above-mentioned metallic glasses: Highly-localized deformation in one mature shear band parallel to the maximum shear stress and the cracking perpendicular to the loading direction. Only shear banding is commonly seen in simulations of CuZr. For this case, the shear band dynamics, such as deflection and branching, were found to be influenced by long range elastic interactions of the stress fields of shear bands and by local structural inhomogeneities. The structural changes during shear band propagation were visualized by an entropy-based order parameter. In contrast, a cracking-to-shear-banding transition can be achieved upon increasing the temperature or decreasing the amount of silicon for PdSi glass. The Crystal Orbital Hamilton Population analysis based on electronic structure calculation from density functional theory simulations has shown that the difference in chemical bonding is responsible for the observed different deformation behaviors. CuZr shows non-directional metallic-bonding, whereas PdSi has a high amount of directional covalent Si-Si bonds. Sampling of the saddle points on the potential energy surface has revealed that a high fraction of rigid covalent Si-Si bonds increases the energy barriers for atomic rearrangements. These thermally-activated atomic relaxation events change the stress and strain state in the elastic regime and are precursor of local plasticity. High activation energies impede both the stress and strain redistribution and cause cleavage-like cracking due to a delay of the onset of plasticity. On the other side, crack propagation due to void nucleation ahead of the crack tip was seen in a heterogeneous PdSi glass.

KW - Metallische Glässer

KW - Dichtefunktionaltheorie

KW - Molekulardynamiksimulation

KW - Sattelpunktsuche im Phasenraum

KW - Deformatiosmechanismen

KW - CuZr

KW - PdSi

KW - Simulation

KW - Metallic Glasses

KW - Density Functional Theory

KW - Molecular Dynamics Simulation

KW - Saddle Point Searching

KW - Deformation Mechanisms

M3 - Master's Thesis

ER -