The alloys of Ti are extensively used in a number of industries with the α+β alloy Ti-6Al-4V (referred to as Ti64 hereafter) being the most popular. Recently, it was demonstrated that the addition of a small amount of B – as small as 0.04 wt.% – results in an order-of-magnitude reduction in the as-cast grain size. Consequently, there is considerable current interest in understanding the mechanical behaviour of B-modified alloys, with particular emphasis on correlating the microstructural changes with the property variations and the deleterious effects – if any – of TiB particles especially in the context of fatigue. Prior studies have indicated that the addition of 0.1 wt.% B to Ti64 yields the most optimum combination of room temperature properties. The research reported in the current thesis builds further on it, with the objective of exploring the utility of Ti64-B alloys in the engineering applications context. Towards this end, mechanical behaviour of cast and wrought Ti64-B alloys at cryogenic and high temperatures, the possible effect of hydrogenation on the tensile properties, and strain-controlled low cycle fatigue was experimentally evaluated as detailed below. While extensive work is reported on as-cast alloys, the mechanical properties of wrought alloys have not been examined hitherto. Keeping this in view, room temperature tensile and fatigue properties of wrought Ti64-B alloys were investigated. Microstructures of wrought alloys show kinking of the  lamellae and alignment of TiB particles along the flow direction. Marginal enhancement in tensile and fatigue properties upon forging is noted. Decrease in fatigue strength of wrought Ti64-0.04B is observed due to increase in volume fraction of the grain boundary α phase with B addition, which acts as a crack nucleation site. No significant effect of TiB particles on tensile and fatigue properties is observed. Next, strain-controlled fatigue behaviour was investigated. Results show significant softening when the strain amplitudes, ΔεT/2, are ≥0.75%. B addition was found to improve the fatigue life for ΔεT/2 ≤ 0.75% as it corresponds to the elastic regime and hence strength dominated. At ΔεT/2 = 1%, in contrast, the base alloy exhibits higher life as TiB particle cracking due to strain incompatibility renders easy crack nucleation in the B-modified alloys. To examined whether the addition of B to Ti64 is beneficial in enhancing its high temperature mechanical behavior, tensile and creep tests are carried out in the temperature range of 475-550 °C. Experimental results show that the B addition enhances both elevated temperature strength and creep properties of Ti64, especially at the lower end of the temperatures investigated. The steady state creep rate in the B-modified alloys were lower than that in the base alloy, and both the strain at failure as well as the time for rupture increases with the B content. These marked improvements in the creep resistance due to B addition to Ti64 were attributed primarily to the increased number of inter-phase interfaces – a direct consequence of the microstructural refinement that occurs with the B addition – that provide resistance to dislocation motion. Titanium alloys are widely used in various ambient and high temperature applications. However, in some instances these alloys are exposed to hydrogen and low operating temperature environments. Ti64 alloy shows poor ductility in hydrogen and cryogenic environments. Whether the microstructural refinement that occurs with the B addition also improves its relative mechanical performance in such environments is examined. For this purpose, alloys were H charged at 500 and 700 °C for up to 4 h. Microstructures and room temperature tensile properties of the resulting alloys have been evaluated. Experimental results show that charging at 700 °C for 2 h leads to the formation of titanium hydride in the microstructure, which in turn causes severe embrittlement. For shorter durations of charging, a marginal increase in strength was noted, which is attributed to the solid solution strengthening by hydrogen. The mechanical performance of the B modified alloys was found to be relatively better, implying that B addition is beneficial in applications that involve H environment. Finally, the utility of B-modified Ti64 for cryogenic applications is examined through notched and unnotched tensile tests at 77 and 20 K. While the addition of B up to 0.3 wt.% increases the strength at both 77 and 20 K. However, the ductility of the alloys decreases drastically with decrease in temperature. The tensile stress-strain responses of Ti64-B alloys exhibit serrations beyond yielding at 20 K. The extent of serrations were found to be maximum in coarse grained B-free Ti64 alloy, while only one serration could be identified in B-containing alloys. Activation of deformation twinning at 20 K results in the formation of serrations. Three twinning modes were identified in coarse grained B-free Ti64 alloy- {10 ̅2}, {11 ̅1} and {5 ̅1 ̅} while only{10 ̅2}twinning mode was activated in B-containing alloys. Extensive deformation through twinning results in higher ductility of B-free Ti64 alloy at 20 K in comparison to B containing alloys.

Future nuclear fission reactors (GEN IV) are designed to include fast breeder reactor technologies, which can accept transuranics (elements heavier than uranium) as fuel. This has the potential of being more fuel efficient but requires the closing of the nuclear fuel cycle: full recycling of existing and newly generated nuclear waste to extract uranium and transuranic elements which can be reused as fuel. In the UK a system being investigated is electrochemical pyroprocessing which uses molten LiCl-KCl eutectic (LKE), which aims to recover uranium by electrodeposition on an inert (steel) electrode and the transuranics by electrodeposition as alloys with an active metal electrode (bismuth, cadmium or aluminium). Of the three active metal candidates, aluminium has the best separation efficiency of actinides and lanthanides, which is important as lanthanides are neutron poisons and so are not to be extracted. The development of pyroprocessing requires fundamental understandings of electrochemical alloy formation, as well as on-line monitoring tools to ensure the reprocessing occurs safely and efficiently. To that end, this thesis investigates cerium-aluminium alloying (a non-radioactive model system for plutonium-aluminium) on macro- and microelectrodes to understand the limiting factors during the alloying reaction at each electrode scale and also the circumstances under which the Ce3+ concentration can be reliably determined for on-line monitoring. On a bulk aluminium macroelectrode one cerium-aluminium alloying reaction was observed. This reaction was kinetically limited by the phase change from cerium insertion into the aluminium, and resulted in lattice expansion and progressive roughening of the electrode surface. These factors made it difficult to reliably calculate the Ce3+ concentration. Li+ from the solution was also able to reduce and form alloys with aluminium, approximately 0.3 V more negative than the first cerium-aluminium alloying peak. Since lithium atoms are smaller than cerium, and there is an abundance of Li+ in the salt, lithium-aluminium alloy was found to form preferentially to cerium-aluminium alloy at these more negative potentials. By co-depositing Al3+ and Ce3+ together on a tungsten electrode which is inert under these conditions (it does not alloy), the kinetic barrier to alloy formation by cerium insertion was decreased, which is beneficial to studying the thermodynamics of alloying. Studies of pure aluminium plating and pure cerium plating showed each individual reaction was diffusion limited, with an increased contribution of convection to the mass transport at slow scan rates. Co-deposition on macroelectrodes with a low ratio of [CeCl3]:[AlCl3] showed only one cerium-aluminium alloying peak. The co-deposition currents, and ratio of oxidation peaks charges, showed that co-deposition was occurring with both species under diffusion control, resulting in an amorphous alloy with a Ce:Al ratio that smoothly varied with the [CeCl3]:[AlCl3] ratio. This was in contrast to the alloying behaviour of cerium with liquid bismuth, in which co-deposition occurred at specific ratios determined by the crystal phases that could be formed at the applied potentials, with higher co-deposition ratios being achieved at more negative potentials. Co-deposition on macroelectrodes with a high ratio of [CeCl 3]:[AlCl3] could result in up to five cerium-aluminium alloy peaks, corresponding to all five CexAly crystalline phases predicted by the phase diagram. This phase change from amorphous to crystalline was promoted by the high Ce:Al ratio in the amorphous alloy resulting from the high [CeCl3]:[AlCl3] ratio and by plating pure cerium on the surface, which could then insert into the alloy. Charge analysis of these peaks confirmed the expected stoichiometries of the crystal phase from these in-situ measurements which is important for rapid analysis, whereas all previous literature has relied on ex-situ techniques which cooled the alloy, possibly changing its composition and structure. In all circumstances of alloy formation on macroelectrodes, the rate of reduction of Ce3+ was time dependent and sensitive to convection. This significantly complicated analysis of the electrochemical signal, making it very difficult to reliably calculate the concentration of Ce3+, which is required for on-line monitoring. Co-deposition on in-house microfabricated tungsten microelectrodes resulted in steady state currents for both pure aluminium deposition and cerium-aluminium co-deposition (up to the beginning of lithium-aluminium alloying). Thus, unlike on macroelectrodes, the deposition rate occurred at the flux ratio of each species from solution and only one oxidation peak was observed corresponding to the amorphous cerium-aluminium phase, even at high [CeCl3]:[AlCl3] ratios. The steady state alloying current meant that calculating the Ce3+ concentration was relatively simple from co-deposition on microelectrodes. Co-deposition was highly beneficial for studying alloying, however to avoid the addition of Al3+ to the molten salt, in-house microfabricated thin film aluminium microelectrodes were also used to study alloying. Alloying on microfabricated thin film aluminium microelectrodes was hampered by the formation of a native aluminium oxide layer, which prevented cerium insertion into the aluminium. The oxide layer could be disrupted by reduction of lithium, which showed steady state currents (albeit with significant capacitance) could be achieved for alloying by cerium insertion. However, the full surface area of the microelectrode could not be attained and all microelectrodes lost their aluminium layer after multiple lithiation/de-lithiation cycles. These devices need further development to overcome the oxide layer, or prevent its formation, in order to study alloying in greater detail with aluminium microelectrodes to fully realise their advantages for sensing and monitoring in pyroprocessing.

No description available.

The Sn/Pb eutectic alloy system is the most widely used joining material in the electronics industry. In this application, the solder acts as both an electrical and mechanical connection within and among the different packaging levels in an electronic device. Recent advances in packaging technologies, however, driven by the desire for miniaturization and increased circuit speed, result in severe operating conditions for the solder connection. In an effort to improve its mechanical integrity, metallic or intermetallic particles have been added to eutectic Sn/Pb solder, and termed composite solders. It was the goal of this study to investigate the growth and morphology of the two intermetallic phases (Cu6Sn5 and Cu3Sn) that form between a Cu substrate and Sn/Pb solder under different aging and annealing conditions.

No description available.

The aim of the present thesis is to understand the phase transformation behavior of embedded alloy nanoparticles embedded in immiscible matrices. Embedded alloy inclusions have been dispersed in immiscible matrix via rapid solidification method. The present work deals with synthesis of embedded particles, evolution of microstructure, morphology and crystallographic orientation relation relationships among different phases, phase transformation and phase stability behavior of embedded alloy inclusions in different matrices. In the present investigation the systems chosen are Bi-Sn and Bi-Pb in Zn matrix and Cd-Sn in Al matrix. Chapter 1 gives the brief introduction of present work Chapter 2 gives a brief review of nanoscale materials, various synthesis techniques, microstructure evolution, solidification and melting theories. Chapter 3 discusses the processing and experimental techniques used for characterization of the different samples in the present work. Melt-spinning technique used to synthesize the rapidly solidified ribbons. The structural characterization is carried out using X-ray diffraction and transmission electron microscopy. Chapter 4 illustrates the size dependent solubility and phase transformation behavior of Sn-Cd alloy nanoparticles embedded in aluminum matrix. X-ray diffraction study shows the presence of fcc Al, bct Sn, hcp Cd solid solution and hcp Cd phases. Based on Zen’s law, the amount of Sn present Cd solid solution is estimated. Using overlapped sterograms, the orientational relationships among various phases are found. Microscopy studies reveal that majority of the alloy nano inclusions exhibit a cuboctahedral shape with 111 and 100 facets and they are bicrystalline. STEM-EDS analysis shows that both phases exhibit size dependent solubility behavior and for particles size smaller than 18 nm, single phase solid solution could only be observed. Calorimetric studies reveal a depression in eutectic melting point of bimetallic particles. In situ heating studies show that melting initiates at triple line junction corner and melt first grows into the interior of the Sn rich phase of the particle and then later the melt grows into the interior of the Cd phase of the particle. During cooling first Cd phase solidifies later Sn phase solidifies and on further cooling at low temperatures entire particle transforming into complete solid solution phase particle. Size dependent melting studies show that during heating smaller particles melted first, later bigger particles melted. During cooling first bigger particle solidified later smaller particles solidified. High resolution imaging indicates presence of steps across particle-matrix interface that may get annihilated during heating. During cooling, molten particles in the size range of 16-30 nm solidify as solid solution which for molten particles greater than 30 nm solidify as biphasic particle. Insitu heating studies indicates that for solid particles less than 15 nm get dissolved in the Al matrix at temperatures at around 135°C. Differential scanning calorimetry (DSC) studies show in the first heating cycle most of the particles melt with an onset of melting of at 166.8°C which is close to the bulk eutectic temperature of Sn-Cd alooy. The heating cycle reveals that the melting event is not sharp which can be understood from in-situ microscopy heating studies. In the second and the third cycles, the onset of melting observed at still lower temperatures 164.3°C and 158.5°C .The decrease in onset melting point in subsequent heating cycles is attributed to solid solution formation of all small particles whose size range below 30 nm during cooling. cooling cycles exhibit an undercooling of 90°C with respect to Cd liquidus temperature. Thermal cycling experiments using DSC were carried out by arresting the run at certain pre-determined temperatures during cooling and reheating the sample to observe the change in the melting peak position and area under the peak. The areas of these endothermic peaks give us an estimate of the fraction of the particles solidified upto the temperature when the cycling is reversed. Based on experimental observations, a thermodynamic model is developed, to understand the solubility behavior and to describe the eutectic melting transition of a binary Sn-Cd alloy particle embedded in Al matrix. Chapter 5 discusses the phase stability and phase transformation behavior of nanoscaled Bi-Sn alloys in Zn matrix. Bi-Sn alloys with eutectic composition embedded in Zn matrix using melt spinning technique. X-ray diffraction study shows the presence of rhombohedral Bi, pure BCT Sn and hcp Zn phases. In X-ray diffractogram, there are also other new peaks observed, whose peak positions (interplanar spacings) do not coincide either with rhombohedral Bi or bct Sn or hcp Zn. Assuming these new phase peaks belong to bct Sn rich solid solution(based on earlier work on Bi-Sn rapidly solidified metastable alloys) whole pattern fitting done on x-ray diffractogram using Lebail method. The new phase peaks indicated as bct M1(metastable phase1), bct M2(metastable phase2) phases. The amount of Bi present in M1, M2 solid solution is estimated using Zens law. Two sets of inclusions were found, one contains equilibrium bismuth and tin phases and the other set contains equilibrium bismuth and a metastable phase. In-situ TEM experiments suggest that as temperature increases bismuth diffuses into tin and becomes complete solid solution. Melting intiates along the matrix–particle interface leading to a core shell microstructure. During cooling the entire inclusion solidify as solid solution and decomposes at lower temperatures. High temperature XRD studies show that as temperature increases M1, M2 phases peaks merge with Sn phase peaks and Bi phase peak intensities slowly disappear and on further increasing temperature Sn solid solution phase peaks also disappear. During cooling diffraction studies show that first Sn solid solution phase peaks appear and later Bi phase peaks appear. But, the peaks belong to metstable phases not appeared while cooling. Chapter 6 presents morphology and phase transformation of nanoscaled bismuth-lead alloys with eutectic (Pb44.5-Bi55.5) and peritectic (Pb70-Bi30) compositions embedded in zinc matrix. using melt spinning technique. In alloy1[ Zn-2at%(Pb44.5-Bi55.5)] inclusions were found to be phase separated into two parts one is rhombohedral Bi and the other is hcp Pb7Bi3 phase. X-ray diffraction study shows the presence of rhombohedral Bi, hcp Pb7Bi3 and hcp Zn phases in Zn-2at%(Pb44.5-Bi55.5) melt spun sample. The morphology and orientation relationships among various phases have been found. In-situ microscpy heating studies show that melt initially spreads along the matrix–particle interface leading to a core-shell microstructure. And in the core of the core-sell particles, first Bi phase melts later Pb7Bi3 phase will melt and during cooling the whole particle solidify as biphase particle with large undercooling. In-situ heating studies carried out to study the size dependent melting and solidification behavior of biphase particles. During heating smaller particles melt melt first later bigger particle will melt. In contrast, while cooling smaller particles solidifies first, later bigger particles will solidify. Detailed high temperature x-ray diffraction studies indicate there increases first Bi phase peaks disappear later Pb7Bi3 phase peaks disappear and during cooling first Pb7Bi3 phase peaks appear and later Bi phase peaks appear. In alloy2[ Zn-2at%(Pb70-Bi30)] inclusions were found to be single phase particles. X-ray diffraction study shows the presence of hcp Pb7Bi3 and hcp Zn phases in Zn-2at%(Pb70-Bi30) melt spun sample. The crystallographic orientation relationship between hcp Pb7Bi3 and hcp Zn phases. In-situ microscpy heating studies show that melting initiates across the matrix–particle interface grows gradually into the interior of the particle. Three phase equilibrium at peritectic reaction temperature is not observed during insitu heating TEM studies. Size dependent melting point depression of single phase particles is not observed from in-situ heating studies. Detailed high temperature x-ray diffraction studies show that while heating the Pb7Bi3 phase peak intensities start decreasing after 170°C and become zero at 234°C. And during cooling Pb7Bi3 phase peaks starts appearing at 200°C and on further cooling the Pb7Bi3 phase peak intensities increase upto 150°C, below this temperature peak intensities remain constant.

Shape memory alloys (SMAs) exhibit unique combination of structural and functional properties and hence have a variety of current and potential applications. The mechanical behaviour of SMAs, in particular the influence of processing on the microstructure, which in turn influences the performance of the alloy, mechanical properties at the nano-scale, and under cyclic loading conditions, are of great current interest. In this thesis, specific issues within each of these broad areas are examined with a view to suggest further optimize/characterize SMAs. They are the following: (a) For thermo-mechanical secondary processing of SMAs, can we identify the optimum combination of temperature- strain rate window that yields a desirable microstructure? (b) How can indentation be used to obtain information about functional properties of shape memory alloys so as to complement traditional methods? (c) How can the information obtained from indentation be utilized for the identification of the alloy composition that yields a high temperature SMA through the combinatorial diffusion couple approach? Towards achieving the first objective, we study the hot deformation behavior of a cast NiTi alloy with a view of controlling the final microstructure. The “processing maps” approach is used to identify the optimum combination of temperature and strain rate for the thermomechanical processing of a SMA system commonly used in actuators applications (NiTiCu). Uniaxial compressions experiments are conducted in the temperature range of 800- 1050 °C and at strain rate range of 10-3 and 102 s-1. 2-D power dissipation efficiency and instability maps are generated and various deformation mechanisms, which operate in different temperature–strain rate regimes, are identified with the aid of these maps. Complementary microstructural analysis of specimens (post deformation) is performed with the help of electron backscattered diffraction (EBSD) analysis to arrive at a processing route which produces stress free grains. A safe window suitable for industrial processing of this alloy which leads to grain refinement and strain-free grains (as calculated by various methods of misorientation analysis representation) is suggested. Regions of the instability (characterized by the same analysis) result in strained microstructure, which in turn can affect the performance of the SMA in a detrimental manner. Next, to extract useful information from indentation responses, microindentation experiments at a range of temperatures (as the shape memory transformation is in progress) are conducted underneath the Vickers indenter. SME was observed to cause a change in the calculated recovery ratios at temperatures above As. Spherical indentation of austenite and martensite show different characteristics in elastic and elasto- plastic regimes but are similar in the plastic regime. NanoECR experiments are also conducted under a spheroconical indenter at room temperature, where the resistance measured is observed to increase during the unloading of room temperature austenite SMA. This is a signature of the reverse transformation back to austenite during the withdrawal of the indenter. Lastly, recovery ratios are monitored in the case of a NiTiPd diffusion couple before and after heat treatment at different temperature intervals using non- contact optical profilometry. The recovery ratio approach is successfully used to determine the useful temperature and %Pd range for a potential NiTiPd high temperature SMA. The method makes high throughput identification of high temperature shape memory alloys possible due to promising alloy compositions being identified at an early stage.