Experimental and Computational Investigations of Structure and Plastic Flow in Bulk Metallic Glasses

Download or read book Experimental and Computational Investigations of Structure and Plastic Flow in Bulk Metallic Glasses written by Matthew James Lambert and published by . This book was released on 2005 with total page 260 pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Thermo-mechanical processing of metallic glasses has been shown to change the free volume and have some effect on the mechanical properties. In order to better quantify these effects, we have studied the homogeneous flow of metallic glasses and the subsequent changes in atomic ordering, in terms of free volume and x-ray measurements, and mechanical properties via nanoindentation. The internal free volume of the specimens, as measured through changes in the specific heat by differential scanning calorimetry, has been shown to increase with increasing levels of total strain in tension and in compression. X-ray diffraction of amorphous specimens in the as-cast, annealed, and homogeneously deformed state also showed a change in the short-range atomic order of the alloy. The shape of the x-ray patterns for the as-cast and annealed states were nearly identical, while a decrease in intensity of x-rays was seen at high angles of 2[theta] in the homogeneously deformed samples. These high angles describe a change in the state of the short-range order. Nanoindentation has shown slight changes in the elastic properties and density of serrations in amorphous materials with changes in the free volume. Using available embedded atom method potentials, molecular dynamics simulations of several Cu-Zr binary, a Cu-Zr-Al ternary, and a quinternary Zr-Cu-Ni-Al-Ti alloy systems at multiple quench rates from the liquid were performed. Using an annealing/quenching technique that allows the simulation of even extremely slow quench rates, glassy structures of these alloys were produced at varying quench rates and their nearest-neighbor coordination examined. Radial distribution functions of the modeled systems show excellent agreement with experimental data, suggesting that the predicted atomic structure should have realistic features similar to real materials. The glass transition temperatures of several of these alloys were determined through a simulated dilatometry technique. Comparing the features and changes in the nearest-neighbor order and the simulated glass transitions with available experimental data, we found a novel set of criteria to predict the effect of changes in alloy composition on glass forming ability. The change in the distribution of nearest neighbors with quench rate, as compared in alloys with different experimentally determined glass forming abilities, did only provide some insights into the formation of glasses. However, the measurement of the fraction of atomic pairs that exhibited icosahedral-like short-range order was found to be directly related to the relative glass forming abilities of the alloys simulated.