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.

Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified. The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was done using a statistical approach of analyzing the "pop-in" load marking incipient plasticity. The strain rate sensitivity of nanocrystalline Al0.1CoCrFeNi alloy was determined by in situ compression of nano-pillars in a Pico-indenter. The nanocrystalline alloy demonstrated a yield strength of ~ 2.4 GPa, ten times greater than its coarse grained counterpart. The nanocrystalline alloy exhibited high strain rate sensitivity index of 0.043 and activation volume of 5b3 suggesting grain boundary dislocation nucleation.

Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy. Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading to formation of fine crystallites/grain (~32 nm) of α-(Fe,Si) as the major component and Fe2B as the minor component. The structural relaxation, crystallization fractions of 5.6–8.6 Vol% with α-(Fe,Si) as the main component, and crystallite/grain size of the order of 12 nm obtained in laser fluence range of 0.39-0.49 J/mm2 had minimal/no influence on tensile behavior of the laser treated Fe-Si-B metallic glass foils. An increase in laser fluence led to progressive increase in crystallization fractions with considerable amounts of Fe2B (2-6 Vol%) and increase in grain size to ~30 nm. Such a microstructural evolution severely reduced the strength of Fe-Si-B metallic glass. Moreover, there was a transition in fracture surface morphology of laser treated Fe-Si-B metallic glass from vein pattern to chevron pattern. Tensile loading lacked any marked influence on the crystallization behavior of as-cast and structurally relaxed laser-treated metallic glass foils. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallized metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.