The properties of titanium alloys are based on alloy compositions and microstructures that consist of mixtures of the two allotropic modifications of titanium, the low temperature α (hcp) and the high temperature β (bcc) phases. This thesis deals with the hot working behaviour of three commercial titanium alloy compositions designated IMI834, Ti17 and Ti5553 with a focus and detailed analysis of the Ti5553 alloy. These alloys represent the differing uses of titanium alloys in the aerospace industry. IMI834 is a near α alloy used in high temperature creep resistant applications as compressor discs and blades in aeroengines. Ti17 is a high strength alloy α+β used at intermediate temperatures in fan and compressor discs of aeroengines, while Ti5553 is a high strength-high toughness metastable β alloy used in the undercarriages of aircraft. The three alloys have widely differing β transus temperatures (related to α phase stability) and compositions. Titanium alloys are vacuum arc melted and thermomechanically processed. This process involves ingot breakdown in β (bcc) phase, and subsequent thermomechanical processing in two-phase α+β (hcp+bcc) region at temperatures that typically involve volume fractions of α in lath or plate form ranging from 15% to about 30%. The thermomechanical processing breaks down lath α to spheroidal particles, a process known as globularisation. Chapter I of this thesis reviews the current understanding of the hot working of titanium alloys and microstructure evolution during the hot working process. Chapter II summarises the main experimental techniques used: the hot compression test, and subsequent microstructure and microtexture analysis by scanning electron microscopy and related electron back scattered diffraction techniques (EBSD), transmission electron microscopy and related precession electron diffraction techniques (PED) for orientation imaging. The starting structure in the α+β domain of hot work is generally not a random distribution of the 12 variant Burgers Orientation Relationship (BOR) between the α and β phases, (11̅0)β || (0001)α and <111>β || <112̅0>α . A variety of morphologies and distributions ranging from the typical colony structures of near α and α+β alloys to the fine distributions of variants arranged in a triangular fashion are observed with specific growth directions and habit planes. Chapter III describes a quantitative evaluation of α distribution that are typical of some of the starting structures for the hot working conditions used in this thesis, specifically in the Ti5553 alloy. For this purpose, a Matlab based script has been developed to measure the spatially correlated misorientation distribution. It was found that experimental spatially correlated misorientation distribution varies significantly from a random frequency for both pair and triplet wise distribution of α laths. The analysis of these structures by established techniques of analysis of self-accommodated structures based on strain energy minimisation shows that the observed variant distribution arise from the residual strain energy accommodation of the semi-coherent α plates. The hot working process has been examined through hot compression tests of the 3 alloys at strain rates ranging from 10-3 s-1 to 10 s-1 over a temperature range designed to maintain constant volume fractions of the α and β phases during deformation ranging from about 30% α to a fully β structure. Since extensive prior work has been carried on the processing of titanium alloys, Chapter IV focuses on a comparative study of hot deformation behaviour of the three alloys with an emphasis on isolating microstructural and other effects. The macroscopic flow behaviour has been analysed in terms of conventional rate equations relating stress, strain, strain rate and temperature. The three alloys show very similar features in their stress-strain behaviour. β phase deformation exhibits yield points whose magnitude varies with strain rate and temperature. The flow stress curves are typical of materials undergoing dynamic recovery and recrystallization processes. The stress-strain behaviour in the α+β domain of hot work exhibits significant flow softening in the early stages of deformation with a subsequent approach to steady-state behaviour at true strain of about 0.5. Activation energy analysis of the steady state condition suggests that the rate controlling mechanism is related to recovery in the β phase in both α+β and β processing. Zener-Hollomon plots of the flow stress in the three alloys indicate that their flow stress can be normalized to a temperature-compensated strain rate and they differ only in the slopes of the plots that are related to the stress exponent. Empirical constitutive models were developed for a predictive understanding of the flow stress as a function of strain, strain rate and temperature using conventional rate equations for the flow stress Chapter V and VI examine the evolution of microstructure and microtexture in detail during hot deformation and subsequent heat treatment in Ti5553. A combination of EBSD (micron and submicron scale) and PED (nano meter scale) is used in orientation imaging to examine the globularisation process of the α phase and the recovery and recrystallization in the β phase in both supertransus and subtransus hot compression. The understanding of these processes is enhanced by tracking the same starting β grain through the deformation process. The effect of strain, strain rate and temperature on the evolution of subgrains in α and its fragmentation into spheroidal α is quantified. In the absence of shear bands, the globularisation process is seen to evolve from a strain driven Raleigh instability of the plate α, by subgrain formation in α and β phases. The related microtexture evolution is analysed. The analysis of recovery and microtexture evolution in the β phase described here has not been attempted earlier in the literature. The overall evolution of structure and texture is seen to result from the complex interplay between recovery and recrystallisation in the α and β phases in substranus deformation. While the Burgers orientation relationship between α and β is lost in the early stages of deformation, it appears to be restored at large strains as a consequence of ‘epitaxial’ recrystallisation processes that seem to result from the discontinuous nucleation of recrystallization of either phase at interphase interfaces in the Burgers orientation. The effect of substranus deformation on β texture following supertransus post deformation heat treatment is also examined and compared with β textures resulting from alternative strain paths such as friction stir processing. Finally Chapter VII summarises these results and the new insights into the evolution of structure and microtexture during hot deformation of titanium alloys and suggests directions for future work.

Sintering is a primary particulate manufacturing technology to provide densification and strength for ceramics and many metals. A persistent problem in this manufacturing technology has been to maintain the quality of the manufactured parts. This can be attributed to the various sources of uncertainty present during the manufacturing process. In this work, a two-particle phase-field model has been analyzed which simulates microstructure evolution during the solid-state sintering process. The sources of uncertainty have been considered as the two input parameters surface diffusivity and inter-particle distance. The response quantity of interest (QOI) has been selected as the size of the neck region that develops between the two particles. Two different cases with equal and unequal sized particles were studied. It was observed that the neck size increased with increasing surface diffusivity and decreased with increasing inter-particle distance irrespective of particle size. Sensitivity analysis found that the inter-particle distance has more influence on variation in neck size than that of surface diffusivity. The machine-learning algorithm Gaussian Process Regression was used to create the surrogate model of the QOI. Bayesian Optimization method was used to find optimal values of the input parameters. For equal-sized particles, optimization using Probability of Improvement provided optimal values of surface diffusivity and inter-particle distance as 23.8268 and 40.0001, respectively. The Expected Improvement as an acquisition function gave optimal values 23.9874 and 40.7428, respectively. For unequal sized particles, optimal design values from Probability of Improvement were 23.9700 and 33.3005 for surface diffusivity and inter-particle distance, respectively, while those from Expected Improvement were 23.9893 and 33.9627. The optimization results from the two different acquisition functions seemed to be in good agreement with each other. The results also validated the fact that surface diffusivity should be higher and inter-particle distance should be lower for achieving larger neck size and better mechanical properties of the material.

Photovoltaics provide a feasible route to fulfilling the substantial increase in demand for energy worldwide. Solution processable organic photovoltaics (OPVs) have attracted attention in the last decade because of the promise of low-cost manufacturing of sufficiently efficient devices at high throughput on large-area rigid or flexible substrates with potentially low energy and carbon footprints. In OPVs, the photoactive layer is made of a bulk heterojunction (BHJ) layer and is typically composed of a blend of an electron-donating (D) and an electron-accepting (A) materials which phase separate at the nanoscale and form a heterojunction at the D-A interface that plays a crucial role in the generation of charges. Despite the tremendous progress that has been made in increasing the efficiency of organic photovoltaics over the last few years, with power conversion efficiency increasing from 8% to 13% over the duration of this PhD dissertation, there have been numerous debates on the mechanisms of formation of the crucial BHJ layer and few clues about how to successfully transfer these lessons to scalable processes. This stems in large part from a lack of understanding of how BHJ layers form from solution. This lack of understanding makes it challenging to design BHJs and to control their formation in laboratory-based processes, such as spin-coating, let alone their successful transfer to scalable processes required for the manufacturing of organic solar cells. Consequently, the OPV community has in recent years sought out to better understand the key characteristics of state of the art lab-based organic solar cells and made efforts to shed light on how the BHJ forms in laboratory-based processes as well as in scalable processes. We take the view that understanding the formation of the solution-processed bulk heterojunction (BHJ) photoactive layer, where crucial photovoltaic processes take place, is the one of the most crucial steps to developing strategies towards the implementation of organic solar cells with high efficiency and manufacturability. In this dissertation, we investigate the mechanism of the BHJ layer formation during solution processing from common lab-based processes, such as spin-coating, with the aim of understanding the roles of materials, formulations and processing conditions and subsequently using this insight to enable the scalable manufacturing of high efficiency organic solar cells by such methods as wire-bar coating and blade-coating. To do so, we have developed state-of-the-art in situ diagnostics techniques to provide us with insight into the thin film formation process. As a first step, we have developed a modified spin-coater which allows us to perform in situ UV-visible absorption measurements during spin coating and provides key insight into the formation and evolution of polymer aggregates in solution and during the transformation to the solid state. Using this method, we have investigated the formation of organic BHJs made of a blend of poly (3-hexylthiophene) (P3HT) and fullerene, reference materials in the organic solar cell field. We show that process kinetics directly influence the microstructure and morphology of the bulk heterojunction, highlighting the value of in situ measurements. We have investigated the influence of crystallization dynamics of a wide-range of small-molecule donors and their solidification pathways on the processing routes needed for attaining high-performance solar cells. The study revealed the reason behind the need of empirically-adopted processing strategies such as solvent additives or alternatively thermal or solvent vapor annealing for achieving optimal performance. The study has provided a new perspective to materials design linking the need for solvent additives or annealing to the ease of crystallization of small-molecule donors and the presence or absence of transient phases before crystallization. From there, we have extended our investigation to small-molecule (p-DTS (FBTTh2)2) fullerene blend solar cells, where we have revealed new insight into the crucial role of solvent additives. Our work has also touched upon modern polymers, such as PBDTTPD, where we have found the choice of additives impacts the formation mechanism of the BHJ. Finally, we have performed a comparative study of the BHJ film formation dynamics during spin coating versus wire-bar coating of p-DTS(FBTTh2)2: fullerene blends that has helped in curbing the performance gap between lab-based and scalable techniques. This was done by implementing a new apparatus that combines the benefits of rapid thin film drying common to spin coating with scalability of wire-bar coating. Using the new apparatus, we successfully attain similar performance of solar cell devices to the ones fabricated by spin coating with dramatically reduced material waste.

In the present work, we studied the effect of liquid Bi on the microstructure evolution of ultrafinegrained Cu at elevated temperatures.

The diffuse-interface phase-field model is a powerful method to simulate and predict mesoscale microstructure evolution in materials using fundamental properties of thermodynamics and kinetics. The objective of this dissertation is to develop phase-field model for simulation and prediction of interdiffusion behavior and evolution of microstructure in multiphase binary and ternary systems under composition and/or temperature gradients. Simulations were carried out with emphasis on multicomponent diffusional interactions in single-phase system, and microstructure evolution in multiphase systems using thermodynamics and kinetics of real systems such as Ni-Al and Ni-Cr-Al. In addition, selected experimental studies were carried out to examine interdiffusion and microstructure evolution in Ni-Cr-Al and Fe-Ni-Al alloys at 1000°C. Based on Onsager’s formalism, a phase-field model was developed for the first time to simulate the diffusion process under an applied temperature gradient (i.e., thermotransport) in single- and two-phase binary alloys. Development of concentration profiles with uphill diffusion and the occurrence of zeroflux planes were studied in single-phase diffusion couples using a regular solution model for a hypothetical ternary system. Zero-flux plane for a component was observed to develop for diffusion couples at the composition that corresponds to the activity of that component in one of the terminal alloys. Morphological evolution of interphase boundary in solid-to-solid two-phase diffusion couples (fcc-γ vs. B2-β) was examined in Ni-Cr-Al system with actual thermodynamic data and concentration dependent chemical mobility. With the instability introduced as a small initial compositional fluctuation at the interphase boundary, the evolution of the interface morphology was found to vary largely as a function of terminal alloys and related composition-dependent chemical mobility. In a binary Ni-Al system, multiphase diffusion couples of fcc-γ vs. L12-γ′, γ vs. γ+γ′ and γ+γ′ vs. γ+γ′ were simulated with alloys of varying compositions and volume fractions of second phase (i.e., γ′). Chemical mobility as a function of composition was employed in the study with constant gradient energy coefficient, and their effects on the final interdiffusion microstructure was examined. Interdiffusion microstructure was characterized by the type of boundaries formed, i.e. Type 0, Type I, and Type II boundaries, following various experimental observations in literature and thermodynamic considerations. Volume fraction profiles of alloy phases present in the diffusion couples were measured to quantitatively analyze the formation or dissolution of phases across the boundaries. Kinetics of dissolution of γ′ phase was found to be a function of interdiffusion coefficients that can vary with composition and temperature. The evolution of interdiffusion microstructures in ternary Ni-Cr-Al solid-to-solid diffusion couples containing fcc-γ and γ+β (fcc+B2) alloys was studied using a 2D phase-field model. Alloys of varying compositions and volume fractions of the second phase (β) were used to simulate the dissolution kinetics of the β phase. Semi-implicit Fourier-spectral method was used to solve the governing equations with chemical mobility as a function of compositions. The simulation results showed that the rate of dissolution of the β phase (i.e., recession of β+γ twophase region) was dependent on the composition of the single-phase γ alloy and the volume fraction of the β phase in the two-phase alloy of the couple. Higher Cr and Al content in the γ alloy and higher volume fraction of β in the γ+β alloy lower the rate of dissolution. Simulated results were found to be in good agreement with the experimental observations in ternary Ni-CrAl solid-to-solid diffusion couples containing γ and γ+β alloys. For the first time, a phase-field model was developed to simulate the diffusion process under an applied temperature gradient (i.e., thermotransport) in multiphase binary alloys. Starting from the phenomenological description of Onsager’s formalism, the field kinetic equations are derived and applied to single-phase and two-phase binary system. Simulation results show that a concentration gradient develops due to preferential movement of atoms towards the cold and hot end of an initially homogeneous single-phase binary alloy subjected to a temperature gradient. The temperature gradient causes the redistribution of both constituents and phases in the two-phase binary alloy. The direction of movement of elements depends on their atomic mobility and heat of transport values.

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The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects. / Thesis / Master of Applied Science (MASc) / The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects.

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