Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3748. Adviser: Armand J. Beaudoin. Includes bibliographical references (leaves 161-166) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3752. Adviser: Charles L. Tucker, III. Includes bibliographical references (leaves 106-110) Available on microfilm from Pro Quest Information and Learning.

Thesis (M.S.)--Lehigh University, 2009. / Adviser: Richard P. Vinci.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3754. Adviser: Shiv G. Kapoor. Includes bibliographical references (leaves 206-223) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3756. Adviser: Narayana R. Aluru. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--Lehigh University, 2009. / Adviser: Robert P. Wei.

<p> Developing light metals that have desirable mechanical properties is always the object of the endeavor of materials scientists. Magnesium (Mg), one of the lightest metals, had been used widely in military and other applications. Yet, its relatively poor formability, as well as its relatively low absolute strength, in comparison with other metals such as aluminum and steels, caused the use of Mg to be discontinued after World War II. Owing to the subsequent energy crisis of the seventies, recently, interest in Mg development has been rekindled in the materials community. The main focus of research has been quite straight-forward: increasing the strength and formability such that Mg and its alloys may replace aluminum alloys and steels to become yet another choice for structural materials. This dissertation work is mainly focused on fundamental issues related to Mg and its alloys. More specifically, it investigates the mechanical behavior of different Mg-based materials and the corresponding underlying deformation mechanisms. In this context, we examine the factors that affect the microstructure and mechanical properties of pure Mg, binary Mg-alloy (with addition of yttrium), more complex Mg-based alloys with and without the addition of lanthanum, and finally Mg-based metal matrix composites (MMCs) reinforced with ex-situ ceramic particles. More specifically, the effects of the following factors on the mechanical properties of Mg-based materials will be investigated: addition of rare earths (yttrium and lanthanum), in-situ/ex-situ formed particles, particle size or volume fraction and materials processing, effect of thermal-mechanical treatment (severe plastic deformation and warm extrusion), and so on and so forth. </p><p> A few interesting results have been found from this dissertation work: (i) although rare earths may improve the room temperature ductility of well-annealed Mg, the addition of yttrium results in ultrafine and un-recrystallized grains in the Mg-Y alloy subjected to equal channel angular pressing (ECAP); (ii) the reverse volume fraction effect arises as the volume fraction of nano-sized ex-situ formed reinforcements is beyond 10%; (iii) nano-particles are more effective in strengthening Mg than micro-particles when the volume fraction is below 10%; (iv) complete dynamic recovery and/or recrystallization is required to accomplish the moderate ductility in Mg, together with a strong matrix-particle bonding if it is a Mg-based composite; and (v) localized shear failure is observed in all Mg samples, recrystallized completely, which is attributed to the reduced strain hardening rate as a result of the exhaustion of twinning and/or dislocation multiplication.</p>

<p> A series of mechanical tests was performed on a matrix of pressure vessel alloys to establish correlations between shear punch tests (SPT), microhardness (Hv), and tensile data. The purpose is to estimate tensile properties from SPT and Hv data. Small specimen testing is central to characterization of irradiation-induced changes in alloys used for nuclear applications. SPT have the potential for estimating tensile yield and ultimate strengths, strain hardening and ductility data, by using TEM disks, for example. Additional insight into SPT was gained by performing finite element analysis (FEA) simulations. </p>

<p> The penetration of granular materials is of interest to a variety of different fields, and is an active area of research. The objective of this project is to gain understanding of the dynamics of a projectile penetrating into a granular material. To do this, experiments were run and a numerical model was created. </p><p> A dart gun was used to accelerate an aluminum dart to velocities around 100 m/s, which then impacted a target tank filled with Ottawa sand. The dart flew along a view window, which allowed for a recording of the penetration event using a high speed camera. Pressure gauges inserted into the target tank measured the timing and magnitude of the compaction wave created by the dart. In these penetration events a two wave structure was discovered; a compaction wave and a fracture wave. The fracture wave is characterized by a white cone around the nose of the dart, which is created by increased reflectance from the newly created fracture surfaces in the grains of sand. </p><p> An experiment was conducted in which single grain of sand was crushed. From this experiment it was discovered that the phenomenon that creates increased reflectivity is the creation of fractures faces in the sand, and is not triboluminescence. Stress-strain data for the sand was also gathered, to be used in the numerical simulation. An ultrasonic pulser/receiver was used to gather data on the longitudinal and shear wave sound speeds through "as poured" Ottawa sand; 263 m/s and 209 m/s respectively. It was determined that the compaction and damage wave speeds were not related to either the longitudinal or shear wave speeds. </p><p> A numerical model was created using an EMU Peridynamic code. This code utilizes integral rather than differential equations, which allows for the modeling of crack propagation and fracture. The numerical simulations run were two-dimensional and on a smaller scale than the penetration experiments. The numerical simulation showed evidence of a compaction wave, force chain creation, and grain fracture, all of which were also observed in the penetration experiments.</p>

<p> A new method for the sintering of ceramics will be presented in detail. This method called Flash-Sintering was first reported in 2010 by Cologna <i> et al.</i> In Flash-Sintering an electric field is applied across a "green" sample with a pair of electrodes and the sintering is measured as a function of the field and temperature. The electric field is shown to remarkably enhance densification. Both the sintering time to achieve near full density and the temperature required are reduced substantially. These changes allow for sintering of 3m% yttria stabilized zirconia at furnace temperatures below 850&deg;C in a matter of seconds. </p><p> The objective of this dissertation is to understand the phenomenological behavior of flash-sintering. This new method is a highly non-linear event which occurs at a particular temperature for a given applied field and sintering is accompanied by an abrupt rise in the conductivity. The development of relationships between the electrical control parameters, the sintering behavior, and the evolution of the microstructure are the principal themes of this doctoral research. </p><p> The present work covers the following topics: (i) The influence of uniaxial pressure applied in combination with electrical field on sintering and superplastic deformation, which show an equivalence between mechanical and electrical driving forces, (ii) A shift of the flash to a higher temperature with increasing particle size of the ceramic powders, (iii) The influence of the electric field on the incubation time for the onset of the flash in experiments carried out at isothermal furnace temperatures, and the effect of the current density immediately following the flash on densification, (iv) The relationship between electrical parameters on microstructure (grain size) evolution, (v) A comparison with the microstructure and mechanical strength of specimens prepared by conventional sintering, and (vi) Measurement of luminescence spectra, which lies in the visible range, that accompanies flash sintering. </p><p> The unusually low processing temperatures and short sintering times portend a potential for a new era in ceramic manufacturing. The energy savings can be substantial, and the tooling costs can be significantly lower. The short sintering times offer the possibility of continuous rather than batch processing of ceramics. These ideas outside the entire scope of this thesis, however, they point toward the potential for broad impact of this research.</p>