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Gamma Prime Precipitation Mechanisms and Solute Partitioning in Ni-base Alloys

Description: Nickel-base superalloys have been emerged as materials for gas turbines used for jet propulsion and electricity generation. The strength of the superalloys depends mainly from an ordered precipitates of L12 structure, so called gamma prime (γ’) dispersed within the disorder γ matrix. The Ni-base alloys investigated in this dissertation comprise both model alloy systems based on Ni-Al-Cr and Ni-Al-Co as well as the commercial alloy Rene N5. Classical nucleation and growth mechanism dominates the γ’ precipitation process in slowed-cooled Ni-Al-Cr alloys. The effect of Al and Cr additions on γ’ precipitate size distribution as well as morphological and compositional development of γ’ precipitates were characterized by coupling transmission electron microscopy (TEM) and 3D atom probe (3DAP) techniques. Rapid quenching Ni-Al-Cr alloy experiences a non-classical precipitation mechanism. Structural evolution of the γ’ precipitates formed and subsequent isothermal annealing at 600 °C were investigated by coupling TEM and synchrotron-based high-energy x-ray diffraction (XRD). Compositional evolution of the non-classically formed γ’ precipitates was determined by 3DAP and Langer, Bar-on and Miller (LBM) method. Besides homogeneous nucleation, the mechanism of heterogeneous γ’ precipitation involving a discontinuous precipitation mechanism, as a function of temperature, was the primary focus of study in case of the Ni-Al-Co alloy. This investigation coupled SEM, SEM-EBSD, TEM and 3DAP techniques. Lastly, solute partitioning and enrichment of minor refractory elements across/at the γ/ γ’ interfaces in the commercially used single crystal Rene N5 superalloy was investigated by using an advantage of nano-scale composition investigation of 3DAP technique.
Date: August 2014
Creator: Rojhirunsakool, Tanaporn
Partner: UNT Libraries

Solid Lubrication Mechanisms in Laser Deposited Nickel-titanium-carbon Metal Matrix Composites

Description: A Ni/TiC/C metal matrix composite (MMC) has been processed using the laser engineered net shaping (LENS) process from commercially available powders with a Ni-3Ti-20C (atomic %) composition. This processing route produces the in-situ formation of homogeneously distributed eutectic and primary titanium carbide and graphite precipitates throughout the Ni matrix. The composite exhibits promising tribological properties when tested in dry sliding conditions with a low steady state coefficient of friction (CoF) of ~0.1 and lower wear rates in comparison to LENS deposited pure Ni. The as deposited and tribologically worn composite has been characterized using Auger electron spectroscopy, scanning electron microscopy (SEM), X-ray diffraction, high resolution transmission electron microscopy (HRTEM) with energy dispersive spectroscopy (EDS), dual beam focused ion beam SEM (FIB/SEM) serial sectioning and Vickers micro-hardness testing. The evolution of subsurface stress states and precipitate motion during repeated sliding contact has been investigated using finite element analysis (FEA). The results of FIB/SEM serial sectioning, HRTEM, and Auger electron spectroscopy in conjunction with FEA simulations reveal that the improved tribological behavior is due to the in-situ formation of a low interfacial shear strength amorphous carbon tribofilm that is extruded to the surface via refined Ni grain boundaries.
Date: December 2012
Creator: Mogonye, Jon-Erik
Partner: UNT Libraries

Perceptions of Disaster Professionalism in Mexico: Adding a New Public Management Perspective to Emergency Management

Description: This study investigated the perceptions of emergency managers regarding the degree of emergency management professionalism in Mexico and how it can be improved. The disaster of the Mexico City earthquake of 1985 was used as the starting point for this case study, as the prospects for more-frequent and more-intense disasters lend credence to the need for improved professionalism and, thus, effectiveness among emergency managers in the future. An expansive framework of emergency management professionalism mechanisms (or characteristics) and an additional compilation of new public management components (or values) were devised from the extant literatures found within the respective emergency management and public administration fields. The theory advanced by this study is that by integrating new public management components with emergency management mechanisms, professionalism in Mexico will improve and, thus, emergency managers will become more effective. ualitative field research was the methodology employed and it included interviews with 35 emergency managers in Mexico in corroboration with documentary evidence, to ascertain emergency managers' perceptions of professionalism in Mexico. The findings of this study determined that emergency managers in Mexico are implementing many of the mechanisms of professionalism but fewer new public management components. This study posits that by integrating new public management components with emergency management professionalism mechanisms, professionalism in Mexico will improve and will increase emergency managers' effectiveness.
Date: August 2010
Creator: Urby, Heriberto, Jr.
Partner: UNT Libraries

Thermodynamics, Kinetics, and Mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) Rearrangements (silox = tBu3SiO; M = Nb, Ta)

Description: Article discussing the thermodynamic, kinetic, and mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) rearrangements (silox = tBu3SiO; M = Nb, Ta).
Date: March 10, 2005
Creator: Hirsekorn, Kurt F.; Veige, Adam S.; Marshak, Michael P.; Koldobskaya, Yelena; Wolczanski, Peter T.; Cundari, Thomas R., 1964- et al.
Partner: UNT College of Arts and Sciences