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Roles of nanoclusters in shear banding and plastic deformation of bulk metallic glasses

Description: During the course of this research we published 33 papers in various physics/material journals. We select four representing papers in this report and their results are summarized as follows. I. To study shear banding process, it is pertinent to know the intrinsic shear strain rate within a propagating shear band. To this aim, we used nanoindentation technique to probe the mechanical response of a Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass in locality and found notable pop-in events associated with shear band emission. Using a free volume model and under the situation when temperature and stress/hardness are fixed result in an equation, which predicts that hardness serration caused by pop-in decreases exponentially with the strain rate. Our data are in good agreement with the prediction. The result also predicts that, when strain rate is higher than a critical strain rate of 1700 s^-1, there will be no hardness serration, thereby no pop-in. In other words, multiple shear bandings will take place and material will flow homogeneously. The critical strain rate of 1700 s^-1 can be treated as the intrinsic strain rate within a shear band. We subsequently carried out a simulation study and showed that, if the imposed strain rate was over , the shear band spacing would become so small that the entire sample would virtually behave like one major shear band. Using the datum strain rate =1700 s^-1 and based on a shear band nucleation model proposed by us, the size of a shear-band nucleus in Au-BMG was estimated to be 3 × 10^6 atoms, or a sphere of ~30 nm in diameter. II. Inspired by the peculiar result published in a Science article “Super Plastic Bulk Metallic Glasses at Room Temperature”, we synthesized the Zr-based bulk metallic glass with a composition identical to that in the paper (Zr64.13Cu15.75Ni10.12Al10) and, subsequently, ...
Date: July 31, 2012
Creator: Nieh, T.G.
Partner: UNT Libraries Government Documents Department

Computational Study of Dislocation Based Mechanisms in FCC Materials

Description: Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 µm under uniaxial tension was simulated using an improvised model by integrating dislocation pile-up mechanism at grain boundaries has been performed. This study showed that for a same initial dislocation density, the Hall–Petch relationship holds well at small grain sizes (0.5–2 µm), beyond which the yield strength remains constant as the grain size increases.
Date: August 2014
Creator: Yellakara, Ranga Nikhil
Partner: UNT Libraries

Glass Forming Ability, Magnetic Properties, and Mechanical Behavior of Iron-Based and Cobalt-Based Metallic Glasses

Description: Lack of crystalline order and microstructural features such as grain/grain-boundary in metallic glasses results in a suite of remarkable attributes including very high strength, close to theoretical elasticity, high corrosion and wear resistance, and soft magnetic properties. In particular, low coercivity and high permeability of iron and cobalt based metallic glass compositions could potentially lead to extensive commercial use as magnetic heads, transformer cores, circuits and magnetic shields. In the current study, few metallic glass compositions were synthesized by systematically varying the iron and cobalt content. Thermal analysis was done and included the measurement of glass transition temperature, crystallization temperature, and the enthalpies of relaxation and crystallization. Magnetic properties of the alloys were determined including saturation magnetization, coercivity, and Curie temperature. The coercivity was found to decrease and the saturation magnetization was found to increase with the increase in iron content. The trend in thermal stability, thermodynamic properties, and magnetic properties was explained by atomic interactions between the ferromagnetic metals and the metalloids atoms in the amorphous alloys. Mechanical behavior of iron based metallic glasses was evaluated in bulk form as well as in the form of coatings. Iron based amorphous powder was subjected to high power mechanical milling and the structural changes were evaluated as a function of time. Using iron-based amorphous powder precursor, a uniform composite coating was achieved through microwave processing. The hardness, modulus, and wear behavior of the alloys were evaluated using nano-indentation.
Date: December 2014
Creator: Veligatla, Medha
Partner: UNT Libraries

Final Technical Report of project: "Contactless Real-Time Monitoring of Paper Mechanical Behavior During Papermaking"

Description: The early precursors of laser ultrasonics on paper were Prof. Y. Berthelot from the Georgia Institute of Technology/Mechanical Engineering department, and Prof. P. Brodeur from the Institute of Paper Science and Technology, both located in Atlanta, Georgia. The first Ph.D. thesis that shed quite some light on the topic, but also left some questions unanswered, was completed by Mont A. Johnson in 1996. Mont Johnson was Prof. Berthelot's student at Georgia Tech. In 1997 P. Brodeur proposed a project involving himself, Y. Berthelot, Dr. Ken Telschow and Mr. Vance Deason from INL, Honeywell-Measurex and Dr. Rick Russo from LBNL. The first time the proposal was not accepted and P. Brodeur decided to re-propose it without the involvement from LBNL. Rick Russo proposed a separate project on the same topic on his side. Both proposals were finally accepted and work started in the fall of 1997 on the two projects. Early on, the biggest challenge was to find an optical detection method which could detect laser-induced displacements of the web surface that are of the order of .1 micron in the ultrasonic range. This was to be done while the web was having an out-of-plane amplitude of motion in the mm range due to web flutter; while moving at 10 m/s to 30 m/s in the plane of the web, on the paper machine. Both teams grappled with the same problems and tried similar methods in some cases, but came up with two similar but different solutions one year later. The IPST, GT, INL team found that an interferometer made by Lasson Technologies Inc. using the photo-induced electro-motive force in Gallium Arsenide was able to detect ultrasonic waves up to 12-15 m/s. It also developed in house an interferometer using the Two-Wave Mixing effect in photorefractive crystals that showed good promises ...
Date: August 30, 2005
Creator: Lafond, Emmanuel; Ridgway, Paul; Jackson, Ted; Russo, Rick; Telschow, Ken; Deason, Vance et al.
Partner: UNT Libraries Government Documents Department

Ruthenium Aluminides: Deformation Mechanisms and Substructure Development

Description: Structural and functional materials that can operate in severe, high temperature environments are key to the operation of a wide range of energy generation systems. Because continued improvements in the energy efficiency of these systems is critical, the need for new materials with higher temperature capabilities is inevitable. Intermetallic compounds, with strong bonding and generally high melting points offer this possibility for a broad array of components such as coatings, electrode materials, actuators and/or structural elements. RuAl is a very unusual intermetallic compound among the large number of B2compounds that have been identified and investigated to date. This material has a very high melting temperature of 2050?C, low thermal expansion, high thermal conductivity and good corrosion resistance. Unlike most other high temperature B2 intermetallics, RuAl possesses good intrinsic deformability at low temperatures. In this program fundamental aspects of low and high temperature mechanical properties and deformation mechanisms in binary and higher order RuAl-based systems have been investigated. Alloying additions of interest included platinum, boron and niobium. Additionally, preliminary studies on high temperature oxidation behavior of these materials have been conducted.
Date: May 11, 2005
Creator: Pollock, Tresa M.
Partner: UNT Libraries Government Documents Department

Long-Term Mechanical Behavior of Yucca Mountain Tuff and its Variability, Final Technical Report for Task ORD-FY04-021

Description: The study of the long term mechanical behavior of Yucca Mountain tuffs is important for several reasons. Long term stability of excavations will affect accessibility (e.g. for inspection purposes), and retrievability. Long term instabilities may induce loading of drip shields and/or emplaced waste, thus affecting drip shield and/or waste package corrosion. Failure of excavations will affect airflow, may affect water flow, and may affect temperature distributions. The long term mechanical behavior of rocks remains an elusive topic, loaded with uncertainties. A variety of approaches have been used to improve the understanding of this complex subject, but it is doubtful that it has reached a stage where firm predictions can be considered feasible. The long term mechanical behavior of "soft" rocks, especially evaporites, and in particular rock salt, has been the subject of numerous investigations (e.g. Cristescu and Hunsche, 1998, Cristescu et al, 2002), and basic approaches towards engineering taking into account the long term behavior of such materials have long been well established (e.g. Dreyer, 1972, 1982). The same is certainly not true of "hard" rocks. While it long has been recognized that the long term strength of ?hard? rocks almost certainly is significantly less than that measured during "short", i.e. standard (ASTM D 2938), ISRM suggested (Bieniawski et al, 1978) and conventionally used test procedures (e.g. Bieniawski, 1970, Wawersik, 1972, Hoek and Brown, 1980, p. 150), what limited approaches have been taken to develop strategies toward determining the long term mechanical behavior of "hard" rock remain in the early research and investigation stage, at best. One early model developed specifically for time dependent analysis of underground "hard" rock structures is the phenomenological model by Kaiser and Morgenstern (1981). Brady and Brown (1985, p. 93) state that over a wide range of strain rates, from 10^-8 to 10^2/s the difference ...
Date: March 20, 2006
Creator: Daemen, Jaak J.K.; Ma, Lumin & Zhao, Guohua
Partner: UNT Libraries Government Documents Department