Molecular Dynamic Simulation of Structures and Interfaces in Amorphous/Ordered Composites.

This thesis describes molecular dynamics simulation studies of the structure-property relationships of molecular network systems, including inorganic and organic bulk amorphous systems, as well as two different amorphous polymers at the interface with ordered substrates. A series of soda lime silicate glasses were simulated, with up to 50% total modification and varying ratios of sodium and calcium. The clustering of cations and second-neighbor connectivity affect vibrational modes and the compressibility vs. pressure behavior. Mean-field theory is unable to account for mixed modifier effects in soda lime silicates. The structure and tensile behavior of a dynamically reacted bulk epoxy network were studied, demonstrating an improved polymerization method for continuously monitoring properties as a function of network growth, including volumetric shrinkage and internal stresses. A bifunctional epoxy resin is reacted with two aliphatic amines at room temperature, comparing simulation size, amine functionality, and stoichiometry. The elastic properties change by only 1-2 GPa during the growth of the network within the achieved degree of conversion. Tensile strength increases by ~100 MPa. Systems with surplus amine hardener reach higher degrees of epoxide conversion, but lag in formation of an infinite network. As a simple model system for amorphous/ordered interfaces, a thin alkane film was placed onto a metallic substrate. The ordered substrate creates a layered polymer configuration within the adjacent 10 Å, as shown by density profiles, pair correlation functions, and monomer orientation statistics. This structural change also affects the mechanical properties, as the elastic moduli of nanoconfined alkane systems are higher than would be expected for a simple laminate composite, based on extrapolating from the bulk properties of the two materials. Lastly, epoxy/carbon laminate systems were investigated, comparing different epoxy layer thicknesses and amine functionality. The cure and shrinkage behavior mimic the bulk epoxy, though the percolation of an infinite cluster is delayed. Post-annealed structures show a nearly uniform decrease in both the elastic modulus and tensile strength. Local heterogeneity is important in predicting nanoscale mechanics for all systems investigated. Larger system size provides better accuracy in determining mechanical properties of simulated highly cross-linked network polymers.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111417/1/kabeck_1.pd

Alloy Partitioning Effect on Strength and Toughness of κ-Carbide Strengthened Steels

Alloy partitioning during heat treatment in a lightweight precipitation hardened steel was investigated using transmission electron microscopy and atom probe tomography. The mechanical properties are discussed as a function of the effect of solution treatment temperature and aging time, giving rise to variations in chemical modulation. A wrought lightweight steel alloy with a nominal composition of Fe-30Mn-9Al-1Si-1C-0.5Mo (wt. %) was solution-treated between 1173–1273 K and aged at 773 K. Lower solution treatment temperatures retained a finer grain size and accelerated age hardening response that also produced an improved work hardening behavior with a tensile strength of −1460 MPa at 0.4 true strain. Atom probe tomography indicated these conditions also had reduced modulation in the Si and Al content due to the reduced aging time preventing silicon from diffusing out of the κ-carbide into the austenite. This work provides the framework for heat-treating lightweight, age hardenable steels with high strength and improved energy absorption