It is of great importance for a materials scientist both from fundamental and applicability aspects to have better understanding of solid-state phase transformations and its kinetics responsible for micro-/nano-structure development in alloys and corresponding physical and mechanical properties. Transformation kinetics can be analyzed by various experimental techniques such as thermal analysis, laborious electron microscopy combined with extensive image analysis or by measuring changes in electrical resistivity, specimen volume and relative intensities of diffraction lines caused by the phase transformation. Beyond these conventional techniques, this dissertation provides a novel magnetic monitoring approach to study the isothermal kinetics of phase transformations in multicomponent alloy systems involving measurable changes in overall magnetic moment as the transformation proceeds. This dissertation focuses on understanding the microstructural evolution, macro- and micro-alloying behavior, magnetic properties, thermal characteristics, mechanical properties and kinetics of solid-state transformations, i.e. nanoscale precipitation and nanocrystallization, in nickel aluminides and Fe-based bulk amorphous alloys. Microstructural characterization of alloys was done by X-ray diffraction, scanning electron microscopy and transmission electron microcopy techniques. Magnetic properties were analyzed by vibrating sample magnetometry whereas thermal characteristics were evaluated by differential scanning calorimetry. Mechanical properties of alloys were determined by microhardness measurements and compression tests. The influence of Fe macroalloying and 3d transition metal microalloying on the microstructure and properties of Ni-Al-Fe alloys were studied for as-cast and annealed states and it is shown that desired microstructure and related properties can be obtained by proper selection of the type and concentration of macro- or micro-alloying elements together with an appropriate annealing procedure. Thermomagnetic characterization reveals the nanoscale precipitation of a ferromagnetic second phase with annealing. In conjunction with saturation magnetization dependence on annealing, an optimum temperature is identified where nanoscale precipitates impart the highest extent of precipitation strengthening. The isothermal kinetics of ferromagnetic second phase precipitation reveals invariant Avrami exponents close to unity, indicating that nanoscale precipitation is governed by a diffusion-controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. Appropriate annealing of the Fe-based bulk amorphous alloy precursor produced by suction casting demonstrated extremely fine microstructures containing uniformly distributed and densely dispersed nanocrystals inside a residual amorphous matrix. In order to have better understanding of nanocrystallization mechanisms, kinetic parameters were determined via isothermal magnetic monitoring and non-isothermal differential scanning calorimetry where excellent agreement was obtained in Avrami exponent and activation energy. Analyzing the local kinetics, the nanocrystalline phase was found to evolve through distinct transformation regimes during annealing which were discussed on the basis of transformation kinetics theory and microscopical investigations on each characteristic transformation regime.

Ni-base superalloys are widely used in hot sections of gas turbine engines due to the high resistance to fatigue and creep at elevated temperatures. Due to the demands for improved performance and efficiency in applications of the superalloys, new and improved higher temperature alloy systems are being developed. Constitutive relations for these materials need to be formulated accordingly to predict behavior of cracks at notches in components under cyclic loading with peak dwell periods representative of gas turbine engine disk materials. Since properties are affected by microstructure at various length scales ranging from 10 nm tertiary γ' precipitates to 5-30 μm grains, hierarchical multiscale modeling is essential to address behavior at the component level. The goal of this work is to develop a framework for hierarchical multiscale modeling network that features linkage of several fine scale models to incorporate relevant microstructure attributes into the framework to improve the predictability of the constitutive model. This hierarchy of models is being developed in a collaborative research program with the Ohio State University. The fine scale models include the phase field model which addresses dislocation dissociation in the γ matrix and γ' precipitate phases, and the critical stresses from the model are used as inputs to a grain scale crystal plasticity model in a bottom-up fashion. The crystal plasticity model incorporates microstructure attributes by homogenization. A major task of the present work is to link the crystal plasticity model, informed by the phase field model, to the macroscale model and calibrate models in a top-down fashion to experimental data for a range of microstructures of the improved alloy system by implementing a hierarchical optimization scheme with a parameter clustering strategy. Another key part of the strategy to be developed in this thesis is the incorporation of polycrystal plasticity simulations to model a large range of virtual microstructures that have not been experimentally realized (processed), which append the experimentally available microstructures. Simulations of cyclic responses with dwell periods for this range of virtual (and limited experimental) polycrystalline microstructures will be used to (i) provide additional data to optimize parameter fitting for a microstructure-insensitive macroscopic internal state variable (ISV) model with thermal recovery and rate dependence relevant to the temperatures of interest, and (ii) provide input to train an artificial neural network that will associate the macroscopic ISV model parameters with microstructure attributes for this material. Such microstructure sensitive macroscopic models can then be employed in component level finite element studies to model cyclic behavior with dwell times at smooth and cracked notched specimens.

Nickel-base superalloys obtain high temperature mechanical properties through formation of precipitate phases formed via heat treatment. The precipitate microstructure evolves with heat treatment or thermal exposure, which can lead to degrading mechanical properties. This project focuses on the use of electrical resistivity as a non-destructive testing method to monitor the precipitate phase in Waspaloy (a polycrystalline nickel-base superalloy). The evolution of the precipitate microstructure is characterized throughout the volume of the specimens using both small angle neutron scattering (SANS) and ultra small angle X-ray scattering (USAXS) measurements. These measurements are also aided by microscopy and X-ray diffraction measurements.

Advanced nano-structured ferritic alloys (NFAs) containing a high density of ultra-fine (2-5 nm) nanoclusters (NCs) enriched in Y, Ti, and O are considered promising candidates for structural components in future nuclear systems. The superior tensile strengths of NFAs relative to conventional oxide dispersion strengthened (ODS) ferritic alloys are attributed to the high number density of NCs, which may provide effective trapping centers for point defects and transmutation products generated during neutron irradiation. This study consists of production, irradiation, and characterization of an advanced NFA, designated 14YWT, currently being developed at Oak Ridge National Laboratory (ORNL), in Oak Ridge, Tennessee. The purpose of this study was to characterize the tensile and fracture toughness properties of 14YWT produced during this project at ORNL before and after irradiation to evaluate it's resistance to radiation-induced changes in mechanical properties. Another alloy, designated 14WT, was produced during this project using identical production parameters used for 14YWT but without the Y2O3 addition during ball milling required for NC formation. Tensile and fracture toughness specimens were produced from both alloys and irradiated in small "rabbit" capsules in the High Flux Isotope Reactor (HFIR) at ORNL. Five other structural alloys that are currently being evaluated for applications in nuclear environments were irradiated and tested during this project to serve as comparison materials. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, transmission electron microscopy, and atom probe tomography. Tensile strengths for 14YWT were found to be far superior to the other alloys for both irradiated and unirradiated conditions, with yield strength for 14YWT decreasing from ~1,450 MPa at 26°C to ~700 MPa at 600°C. Moderate radiationinduced hardening (50-200 MPa) and reduction in ductility was observed for 14YWT for all irradiation conditions and test temperatures. Fracture toughness results showed 14YWT in the unirradiated condition had a fracture toughness transition temperature (FTTT) around -150°C and upper-shelf K[subscript JIc] values around 175 MPa m. Results from irradiated 14YWT fracture toughness tests were found to closely mirror the unirradiated data and no shift in FTTT or decrease in K[subscript JIc] values were observed following neutron irradiation to 1.5 dpa at 300°C. Master curve analysis of the fracture toughness data show 14YWT to have a T[subscript o] reference temperature of -188 and -176°C in the unirradiated and irradiated condition, respectively, which is unprecedented for a high-strength dispersion strengthened ferritic alloy. The results from this study show 14YWT to be resistant to radiation-induced changes in mechanical properties and a promising candidate for structural applications in future nuclear systems. / text

A systematic study of the effect of sulphidizing atmosphere on the High Temperature Low Cycle Fatigue (HTLCF) properties of coated and uncoated unidirectionally solidified MARM002 nickel base superalloy was performed at 870°C. The coating systems investigated were, aluminide coating, three types of platinum modified aluminide coatings, and platinum coating. The creep-plasticity mode of the strain range partitioning method was used for creep-fatigue loading. A constant loading regime (Strain range 6.6 x 10-3 ) was used to test the samples in argon, air and Ar + 5%S02 and a lower strain range of3.8 x 10-3 was used to investigate the creep-fatigue properties in Ar + 5%S02 only. The results were analysed using scanning electron microscopy including spot analyses (SEM-EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The synergistic effect of sulphidizing environment and the creep fatigue loading (Strain range - 0.66%) resulted in accelerated failure in all the materials systems tested, except for the TYPE I platinum aluminide coated sample. This coating displayed a "self-healing" mechanism which enhanced its fatigue life under sulphidizing conditions. In general, the coatings had an adverse effect on the fatigue properties of the material systems. This was due to the poor mechanical properties of the coating. The mechanical properties of the coating was influenced by the coating microstructure and the chemical composition. The modification of the NiAI zone with platinum in the platinum aluminide coatings improved the fatigue properties of the coating by altering the crack propagation mechanism in the NiAl zone. The higher the platinum content in this region the more brittle it became. The platinum modified aluminide coating showed an improvement in the corrosion fatigue properties in the S02 containing environment at the higher strain range when compared with the uncoated, aluminide coated and platinum coated samples. However, at the lower strain range all the coating systems performed worse than the uncoated alloy. This was mainly due to the brittle failure of the coating. The platinum modified aluminides performed the worst due to the presence of brittle platinum aluminide phases. The interdiffusion and interaction of platinum with the substrate alloying elements, resulted in this coating being ineffective for corrosion protection. The resultant coating layer produced poor corrosion-fatigue properties. Although the coating systems did show evidence of resistance to sulphidation and oxidation there were relatively ineffective under the combination of sulphidizing environment and fatigue loading due to their poor mechanical properties. The mechanism of sulphidation was consistent for all the material systems tested with oxidation proceeding first and sulphidation proceeding at the corrosion scale/substrate interface. The crack propagation in the coating and substrate was controlled by the sulphidation attack at the crack tip and failure of the oxide scales formed in the cracks. / Thesis(M.Sc.Eng.)- University of Natal, Durban, 1998.

No description available.

Face milling machinability investigation of two difficult-to-machine nickel-based superalloys, namely Inconel 718 and Waspaloy, has been carried out with four different types of tungsten carbide tools under various cutting conditions. The tools comprised of one double-layer CVD-TiCN+Al2O3 coated (KC994M), two PVD-TiN coated (KC720 and KC730) and one uncoated (KMF) tungsten carbide tools. The objectives of the study include investigation of tool performance, failure modes and wear mechanisms under the cutting conditions employed. In addition, surface integrity of the machined surfaces, with regard to surface finish, subsurface microhardness and metallographic examination of the subsurface microstructure, was investigated. CVD-coated KC994M gave the best overall performance in terms of tool life at low and high cutting conditions on both workpieces. The second best-performing tool was the uncoated KMF grade which gave as high tool lives as KC994M at lower cutting speeds. However at higher cutting speeds, KMF was generally outperformed by PVD-TiN coated tools. Short tool lives were obtained at higher cutting speeds of 75 and 100 m/min due to premature failure by chipping. Tool wear at low cutting speed range was due to a combination of progressive microchipping and plucking through a fracture/attrition related wear mechanism associated with cyclic workpiece adhesion and detachment and abrasion/diffusion-related flank wear. Plucking and microchipping were the dominant wear mechanisms. Coating layers on the rake face of both CVD and PVD coated tools were almost completely removed within the first few seconds of cutting at all cutting speeds tested, thus becoming ineffective. On the flank face, however, they remained intact for a longer period and hence increasing tools performance at the medium cutting speed range. Analysis of the subsurface microstructures and microhardness measurements showed that plastic deformation was the predominant effect induced onto the machined surface, the degree of which influenced by the cutting speed, tool wear and prolonged machining. In addition surface irregularities in the form of tearing and embedded hard particles were found to occur which was mainly associated with the chipping dominated wear mode.

The climb of dissociated dislocations in FCC materials is known to be complex: Observations of climb under HVEM irradiation in CuAl suggest that it, proceeds via the nucleation of interstitial loops directly onto individual partials. In silver, however, dissociated dislocations appear to constrict at an early stage in the irradiation, and dense vacancy cluster damage is seen to form in their vicinity. This thesis presents results and analysis of a study aimed at the determination of the interaction of HVEM generated interstitials and vacancies with pre-existing dislocations in a range of Fe-17Cr-Ni ternary alloys (with the nickel content varying between 15 and 40%). Two quaternary alloys (Fe-15%Ni-17%Cr-l%Si and Fe-15%Ni-17%Cr-2%Mo) arc also studied. As with earlier studies in CuAl and Ag, pre-existing dislocations in a (111) orientated foil were first characterized at subthreshold voltages employing the weak-becim technique, then irradiated with IMeV electrons at temperatures in the range 400-430°C and finally returned to the low voltage microscope for postirradiation characterization of the observed damage. Analysis of the post-irradiation microstructures indicates that interstitial climb only occurs at particularly favourable sites, such as pre-existing jogs: For the ternary alloys, constrictions are removed along edge and mixed dislocations, whilst zig-zagging of screw and near-screw dislocations may also be attributable to jog climb. After the annihilation of constrictions evidence of climb is not seen and pipe diffusion is thought to be occurring. The precipitation of small clusters, many of which are identifiable as vacancy SFT, is reminiscent of observations in silver. The addition of silicon to the matrix apparently leads to the creation of favourable sites for interstitial climb, as evidenced by the formation of high densities of new jogs after irradiation. Loops are seen to precipitate close to dislocations in the Mo-doped material. The origin of these loops is unclear at this stage. The relevance of the results to the phenomenon of void swelling is discussed.

Thesis (Ph.D.)--McMaster University (Canada), 1987. / Source: Dissertation Abstracts International, Volume: 49-07, Section: B, page: 2799. Supervisors: D. A. Thompson; W. W. Smeltzer; J. A. Davies.

Thesis (Ph.D)--Materials Science and Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Meilin Liu; Committee Member: Arun Gokhale; Committee Member: Christoper Summers; Committee Member: Preet Singh; Committee Member: Tom Fuller. Part of the SMARTech Electronic Thesis and Dissertation Collection.