To explore an innovative approach to prepare high-performance Mo-based refractory alloys, the rapid solidification mechanisms of undercooled Mo-40%Co hyperperitectic alloy were investigated by both electromagnetic levitation (EML) and drop tube (DT) techniques. During EML experiments, typical peritectic solidification still prevailed even at the maximum undercooling of 252 K (0.14 T-T). The dendrite growth velocity of primary sigma-Mo3Co2 compound varied with alloy undercooling by a power relation, which attained 24.3 mm s(-1). As the bulk undercooling increased, the conspicuous dendrite fragmentation of primary phase took place, resulting in its grain refinement and volume fraction reduction. Meanwhile, the second recalescence degree exhibited a decreasing tendency, which indicated that the peritectic reaction was remarkably suppressed in the undercooled state. Under free fall condition, two critical diameters were determined for tiny alloy droplets and the corresponding undercooling thresholds were calculated as 361 and 443 K, respectively. When the alloy droplet was larger than 707 mu m, the typical peritectic solidification proceeded and the final microstructure was composed of primary sigma-Mo3Co2 dendrites and interdendritic epsilon-Mo6Co7 peritectic phase. With the reduction of droplet diameter, the direct nucleation of peritectic phase was induced. In this case, the epsilon-Mo6Co7 intermetallic compound regions appeared preferentially at the periphery of alloy droplets, whereas the typical peritectic microstructure filled in the center area. Once the droplet diameter decreased to 93 mu m, the peritectic solidification characteristics disappeared completely and the metastable epsilon-Mo6Co7 single phase became the unique growth morphology.

The electromagnetic stirring (EMS) effects on peritectic solidification kinetics of undercooled liquid Fe-Ti alloys have been investigated by electrostatic levitation(ESL) and electromagnetic levitation (EML) methods assisted with in-situ diagnostic techniques. The high-sensitivity pyrometer and high-speed camera were employed to monitor the complete solidification process for levitated liquid Fe59Ti41 alloy in undercooling ΔT range of 0 K to 213 K. Theoretical calculations showed that there existed EMS inside electromagnetically levitated alloy melts, and the internal fluid flow dynamics depended on levitation height and melt undercooling. As ΔT rised, the primary dendrite growth velocity V increased according to a power function. Meanwhile, the peritectic recalescence degree ΔTpr and the peritectic recalescence rate Rpr were enhanced gradually, whereas the peritectic recalescence time tpr and the peritectic solidification time tps were shortened linearly. The comparison between ESL and EML experiments revealed that the EMS resulted in four respects of influences including (1) dendrite growth effect, (2) concentration field effect, (3) peritectic reaction effect and (4) microstructure evolution effect. In contrast with ESL, the V of Fe50Ti50 alloy measured by EML was slightly larger at small undercoolings, indicating the EMS affected dendrite growth processes. The solute concentration $$ C_{\text{L}}^{*} $$ around primary Fe2Ti dendrites for electrostatically levitated liquid Fe59Ti41 alloy deviated far away from original composition, while the EMS homogenized concentration field and the $$ C_{\text{L}}^{*} $$ variation was weak under EML condition. Both tpr and tps in the absence of EMS were longer that those in the presence of EMS, and it was demonstrated that the EMS accelerated peritectic reaction. Except for microstructure refinement, the EMS modulated the microstructure type and also changed the faceted-growth mode of intermetallic compound phases.

The liquid Fe37.5Cu37.5Sn25 peritectic-type alloy was rapidly solidified by glass fluxing, drop tube, and melt spinning techniques. The solidification structures consisted of three phases, including the αFe solid solution and the Cu3Sn and Cu6Sn5 intermetallic compounds under different conditions. In this work, the bulk undercoding ΔT equals the liquidus temperature TL minus the nucleation temperature TN of the primary solid phase. During the bulk undercooling process, a maximum liquid undercooling of 272 K (0.18TL) was achieved. Metastable liquid phase separation was induced if the undercooling exceeded 177 K. Within a moderate undercooling range from 8 to 177 K, the solidified structures showed coarse dendrites, and the dendritic growth velocity increased to 41.5 mm/s following a power relation. As the drop tube technique provided a containerless state, a maximum surface cooling rate of 3.5 × 104 K/s was achieved. The alloy droplets displayed metastable liquid phase separation even at the largest droplet diameter of 944 μm. The cooling rate and thermal Marangoni migration were found to greatly influence the phase-separated patterns of the alloy droplets. Under the melt spinning condition, the microstructures of the Fe37.5Cu37.5Sn25 alloy ribbons were significantly refined and displayed soft magnetic characteristics. Furthermore, the experimental results demonstrated that the grain size of the αFe phase affected the coercivity of the alloy ribbons.

Two types of optimized electromagnetic levitators were designed to achieve the free surface shape control of metallic melts as the combined results of the Lorentz force, sample gravity, and surface tension. The levitation behavior of bulk melts was investigated using computational fluid dynamics (CFD) modeling coupled with high-frequency electromagnetic field analysis and the arbitrary Lagrangian–Eulerian (ALE) method. The difference in the oscillation behavior between the solid and molten samples was explained by the damping of the electromagnetic induction and liquid viscosity. The motion of the mass center, melt shape, flow pattern, and Lorentz force in the levitated melt were determined within a wide excitation current range. With the increase in the applied current, the melt’s centroid position rose sharply in the area with low current but displayed a slow increase in the area with high current. Meanwhile, the stable shape of the bulk melt showed the typical transition from a long taper through a short taper and then into a rhombus. The internal flow pattern transformed from a simple double-loop structure to a complex configuration with three or four loops. The dependence of the deformation on the Bond number was analyzed in the two types of levitators. In addition, the stable shape and swing process of the bulk Al melt within the two types of electromagnetic levitation (EML) systems were quantitatively studied under protective inert gas conditions. The melt contour could be well described by the 10th Legendre polynomial function with a deviation of less than 0.5 pct.

We experimentally and theoretically investigated the rapid solidification and microstructural evolution of freely falling droplets of the ternary Fe45Cu40Co15 peritectic alloy, with the critical undercooling temperature of metastable liquid phase-separation initiation being measured to be 38 K. We found that liquid phase separation occurs when the droplet diameter ranges from 80 to 980 μm, resulting in the formation of either microscopically or macroscopically segregated microstructures. The peritectic solidification microstructure was produced when the droplet diameter either exceeded 980 μm or was below 80 μm. The dispersed morphology of the large alloy droplets possessed nonuniformly dispersive characteristics, i.e., proximity to the droplet surface is negatively associated with the size of Cu-rich globules. With the further reduction of the droplet size, the phase-separated morphology first transformed into a core–shell structure and finally displayed a homogeneously dispersed structure. Our theoretical calculations showed that the residual Stokes motion, Marangoni convection, and surface segregation are the dominant dynamic mechanisms for the phase separation and microstructural evolution under reduced-gravity conditions inside the drop tube.

The evolution kinetics of microgravity facilitated spherical macrosegregation within model Fe-Cu immiscible alloys has been systematically investigated by drop tube experiments and 3D phase-field simulations. In microgravity environment, liquid phase separation of Fe-Cu alloys induced the appearance of spherical macrosegregation patterns with various core-shell structures. The formation probability and evolution characteristics of these phase-separated core-shell morphologies depended on the volume fraction of surface active Cu-rich liquid together with the cooling rate. As the cooling rate decreased, the duration time of phase separation extended and the formed dispersed structures showed a tendency to form as core-shell structures influenced by the effects of Marangoni convection and surface segregation. Meanwhile, core grew larger while the shell became thinner. The occurrence probability of spherical macrosegregation firstly increased and then decreased with the rise in the copper concentration, and core-shell morphologies changed from three-layer to two-layer which was accompanied by core shrinkage and shell thickening.

The rapid solidification kinetics of undercooled hypoperitectic Fe59Th41 alloy was quantitatively investigated by electrostatic levitation (ESL) and electromagnetic levitation (EML) methods Combined with a high-speed photography technique. The maximum undercoolings Delta T obtained by ESL and EML methods were 200 K (0.12 T-L) and 315 K (0.19 T-L), respectively. Double recalescence processes corresponding to the primary dendrite growth and subsequent peritectic reaction were recorded at various undercoolings. The dependence of primary dendrite growth velocity V on the undercooling Delta T satisfied a double exponential relation. A longest incubation time and a highest undercooling of peritectic reaction were experimentally determined at the same critical undercooling of about 86 K. In contrast, the peritectic reaction time decreased linearly with enhanced Delta T. As Delta T rised in ESL and EML experiments, primary Fe2Ti phase successively appeared as well-defined coarse dendrites, greatly refined dendrites and finally evolved into a maze-like morphology composed of vermicular dendrites. Meanwhile, the layer thickness and volume fraction of peritectic FeTi phase remarkably reduced, which suggested that peritectic reaction was suppressed to some extent. The solid solubilities of Fe2Ti and FeTi phases were significantly extended during rapid solidification. As another comparison, drop tube experiment was also conducted to explore the solidification behaviors under larger undercoolings and cooling rates. A mechanism transition of 'peritectic solidification -> metastable coupled-growth between primary and peritectic phases' was observed with decreasing alloy droplet size. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The phase-separation kinetics and microstructure evolution mechanisms of liquid ternary Co43Cu40Pb17 immiscible alloys are investigated by both the drop tube technique and phase-field method. Two successive phase separations take place during droplet falling and lead to the formation of a three-phase three-layer core-shell structure composed of a Co-rich core, a Cu-rich middle layer, and a Pb-rich shell. The Pb-rich shell becomes more and more conspicuous as droplet diameter decreases. Meanwhile, the Co-rich core center gradually moves away from the core-shell center. Theoretical analyses show that a larger temperature gradient inside a smaller alloy droplet induces the accelerated growth of the surface segregation shell during triple-phase separation. The residual Stokes motion and the asymmetric Marangoni convection result in the appearance of an eccentric Co-rich core and the core deviation degree is closely related to the droplet size and initial velocity. A three-dimensional phase-field model of ternary immiscible alloys, which considers the successive phase separations under the combined effects of Marangoni convection and surface segregation, is proposed to explore the formation mechanisms of three-phase core-shell structures. The simulated core-shell morphologies are consistent with the experimental observations, which verifies the model's validity in reproducing the core-shell dynamic evolution. Numerical results reveal that the development of three-phase three-layer core-shell structures can be attributed to the primary and then secondary phase separations dominated simultaneously by Marangoni convection and surface segregation. Furthermore, the effects of droplet temperature gradient on the growth kinetics of the surface segregation shell are analyzed in the light of phase-field theory.

The active control of microstructure evolution is still a challenging factor for the development of advanced immiscible alloys. Here, we make an attempt to modulate the solidification pathways of undercooled Cu62.5Fe27.5Sn10 alloy by glass fluxing and drop tube techniques. Through regulating the liquid undercooling, three types of microstructures, dendrite, dispersive structure and macrosegregation pattern, were formed under the normal gravity condition. Below the first critical undercooling of 15 K, the alloy melt displayed the normal peritectic solidification. At moderate undercoolings above 15 K, the metastable liquid phase separation took place and the solidified microstructure appeared as homogeneously dispersed structure. If undercooling further overtook the second threshold of 107 K, macrosegregation occurred and the bulk alloy separated into an Fe-rich zone and a Cu-rich zone. Under the free fall condition, the alloy droplets with the droplet diameter beyond 805 μm showed the equilibrium peritectic solidification. If the droplet diameter decreased below 805 μm, the metastable liquid phase separation was induced and the microstructural morphology of Cu62.5Fe27.5Sn10 alloy droplet evolved from dendrite into dispersive structure. Furthermore, experimental and simulated results revealed that the temperature gradient had great influence on the size distribution of Fe-rich globules.

The metastable coupled-growth kinetics between the primary Fe2Ti and peritectic FeTi phases of undercooled Fe54.5Ti45.5 alloy was systematically investigated by both electromagnetic levitation and drop tube techniques. Employing a high-speed camera, the rapid crystallization processes of levitated bulk alloy were recorded in the undercooling range of 34–187 K. In small undercooling regime below 143 K, peritectic solidification proceeded and the dependence of primary Fe2Ti dendritic growth velocity V on the bulk undercooling ΔT satisfied a power relation of V = 2.43 × 10−14 × ΔT7.72 (mm s−1). Once liquid undercooling increased beyond 143 K, the metastable coupled-growth was induced and the microstructure was characterized by the Fe2Ti rods embedded in FeTi phase. Furthermore, the coupled-growth velocity decreased linearly with the rise in undercooling according to V = 1.47 × 103-7.44ΔT (mm s−1). In drop tube experiment, peritectic solidification characteristics of small alloy droplets disappeared and the primary and peritectic phases directly nucleated from undercooled liquid and grew cooperatively to form spherical coupled-growth cells if droplet diameter decreased below 481 μm.

Multilayer core-shell structures are frequently formed in polymers and alloys when temperature and concentration fields are well symmetrical spatially. Here we report that two- to five-layer core-shell microstructures were the dominant structural morphology of a binary Fe(50)Sn(50) immiscible alloy solidified under the containerless and microgravity states within a drop tube. Three dimensional phase field simulation reveals that both the uniformly dispersive structure and the multilayer core-shells are the various metastable and transitional states of the liquid phase separation process. Only the two-layer core-shell is the most stable microstructure with the lowest chemical potential. Because of the suppression of Stokes motion, solutal Marangoni migration becomes important to drive the evolution of core-shell structures.

Liquid Fe41.5Sn58.5 alloy was containerlessly solidified under the free-fall microgravity condition inside drop tube. Conspicuous liquid phase separation took place in the broad miscibility gap of 325 K and formed the dispersed pattern microstructures of Fe-rich phase distributed in the matrix of Sn-rich phase. Although the equilibrium phase constitution includes only FeSn and FeSn2 compounds in the phase diagram, the high cooling rate and large undercooling lead to the formation of four new metastable phases alpha Fe, Fe3Sn, (Sn)(1) and (Sn)(2) respectively. The granular patterns of Fe3Sn phase dominates the droplet solidification process. Phase field simulation reveals that the granular microstructure experiences three evolution stages during liquid phase separation: the nucleation, aggregation, or coalescence and Ostwald ripening of secondary liquid phase, which agrees well with the experimental results. Meanwhile, two kinds of Marangoni migrations play an important role to develop the homogeneous granular structures. EDS analysis shows that the solute contents of various phases increase with the decrease of alloy droplet diameter, while the primary aFe phase is found to contain as much as 18.07 at.% Sn at droplet diameter 73 mm owing to the significant solute trapping during rapid solidification. (C) 2016 Elsevier B.V. All rights reserved.

The active modulation and control of the liquid phase separation for high-temperature metallic systems are still challenging the development of advanced immiscible alloys. Here we present an attempt to manipulate the dynamic process of liquid-liquid phase separation for ternary Fe 47.5 Cu 47.5 Sn 5 alloy. It was firstly dispersed into numerous droplets with 66 ~ 810 μm diameters and then highly undercooled and rapidly solidified under the containerless microgravity condition inside drop tube. 3-D phase field simulation was performed to explore the kinetic evolution of liquid phase separation. Through regulating the combined effects of undercooling level, phase separation time and Marangoni migration, three types of separation patterns were yielded: monotectic cell, core shell and dispersive structures. The two-layer core-shell morphology proved to be the most stable separation configuration owing to its lowest chemical potential. Whereas the monotectic cell and dispersive microstructures were both thermodynamically metastable transition states because of their highly active energy. The Sn solute partition profiles of Fe-rich core and Cu-rich shell in core-shell structures varied only slightly with cooling rate.

The phase separation and microstructure evolution of undercooled Fe62Sn34Ge4 immiscible alloy droplets have been investigated by drop tube method and numerical simulations. A conspicuous phase separation has taken place within alloy droplets during free fall. The solidification microstructures are mainly characterized by core/shell and dispersed structures. The cooling rates and microstructure morphologies of undercooled alloy droplets are strongly dependent on droplet size. With the decrease of droplet size, the cooling rate increases and the microstructure morphology transfers from core/shell structure to dispersed structure. A modified Model H, which considers the spinodal decomposition, subsequent coarsening and migration of second-phase globules under the effects of Marangoni migrations, is developed to explore the dynamic mechanisms of liquid/liquid phase separation. Numerical results reveal that the dispersed structures, in which Sn-rich globules are homogeneously dispersed within Fe-rich matrix, are the intermediate and metastable phase separation morphologies before the formation of stable core/shell morphology. The Marangoni migrations are the major dynamic mechanisms responsible for the microstructure evolution from dispersed structure into core/shell structure. The chemical gradient drives the phase separation and microstructure evolution, which lowers the overall energy of alloy melt. Theoretical calculations show that the solutal Marangoni migration contributes to the aggregation and coalescence of neighboring globules, whereas thermal Marangoni migration induces the coalescence between periphery globules and inner globules, and subsequently forces these globules to aggregate at droplet center to form a Sn-rich core. (C) 2015 Elsevier B.V. All rights reserved.

433 Objectives PoleStar m660 is a newly developed high sensitivity PET/CT system with TOF capability. The aim of this study is to characterize the performance of the new system. Methods The PoleStar m660 scanner combines a 64-slice CT with a whole-body PET. The PET subsystem consists of 24 detector buckets, each is organized as 2 detector modules along the trans-axial direction and 3 or 4 detector modules along the axial direction. (A 4-ring configuration was evaluated in this study.) Each detector module contains a 14×14 LYSO array, with a crystal size of 3.63mm×3.63mm×20mm. Performances of the PET subsystem were measured using the NEMA NU 2-2007 standards. A 425-650 keV energy window and a 4.1 ns timing window were used. The timing resolution was measured using a rotating line source that orbits around the center of field-of-view (CFOV) at a radius of 20 cm. Coincidence events from all lines-of-response that pass the CFOV were sorted to create the timing spectrum and to estimate the system timing resolution. Results The transverse (axial) spatial resolutions were 3.76 (3.64) mm and 4.56 (5.29) mm FWHM at 1 cm and 10 cm off the CFOV, respectively. The measured sensitivity was 10.9 kcps/MBq at the CFOV and 10.7 kcps/MBq at 10cm off the CFOV. The peak NECR was 224.6kcps at an activity concentration of 29.0 kBq/ml. For the image quality phantom, the contrast recovery ratios ranged from 70.2% to 94.3%, while the background variability ranged from 7.8% to 2.1%. For the timing resolution, an average of 434 ps FWHM was measured. Conclusions The physical performances of the PoleStar m660 PET/CT system were characterized. Results showed improved system sensitivity, count rate performance, and timing resolution when compared with currently available commercial clinical PET systems. Clinical evaluation of the scanner is currently underway and the results will be presented.

Ternary Fe 48 Cu 48 Si 4 immiscible alloy was rapidly solidified under the containerless microgravity condition inside a drop tube. Liquid phase separation took place in the alloy melt and led to the formation of various segregated structures. The core–shell structure consisting of Fe-rich and Cu-rich zones and the homogenously dispersed structure were the major structural morphologies. Phase field simulation results revealed that the two-layer core–shell was the final structure of liquid phase separation. The solute redistribution of liquid Fe 48 Cu 48 Si 4 alloy experienced the macroscopic solute distribution induced by liquid phase separation, the secondary phase separation within the separated liquid phases and the solute trapping during rapid solidification. Energy dispersive spectroscopy analysis showed that the solute Si was enriched in the Fe-rich zone whereas depleted in the Cu-rich zone. In addition, both α Fe and (Cu) phases in the Fe-rich zone exhibited a conspicuous solute trapping effect. As compared with (Cu) phase, α Fe phase had a stronger affinity with solute Si.

A cadmium-zinc-telluride (CZT) detector with 350 μm pitch pixels was studied in high-resolution positron emission tomography (PET) imaging applications. The PET imaging system was based on coincidence detection between a CZT detector and a lutetium oxyorthosilicate (LSO)-based Inveon PET detector in virtual-pinhole PET geometry. The LSO detector is a 20 ×20 array, with 1.6 mm pitches, and 10 mm thickness. The CZT detector uses ac 20 ×20 ×5 mm substrate, with 350 μm pitch pixelated anodes and a coplanar cathode. A NEMA NU4 Na-22 point source of 250 μm in diameter was imaged by this system. Experiments show that the image resolution of single-pixel photopeak events was 590 μm FWHM while the image resolution of double-pixel photopeak events was 640 μm FWHM. The inclusion of double-pixel full-energy events increased the sensitivity of the imaging system. To validate the imaging experiment, we conducted a Monte Carlo (MC) simulation for the same PET system in Geant4 Application for Emission Tomography. We defined LSO detectors as a scanner ring and 350 μm pixelated CZT detectors as an insert ring. GATE simulated coincidence data were sorted into an insert-scanner sinogram and reconstructed. The image resolution of MC-simulated data (which did not factor in positron range and acolinearity effect) was 460 μm at FWHM for single-pixel events. The image resolutions of experimental data, MC simulated data, and theoretical calculation are all close to 500 μm FWHM when the proposed 350 μm pixelated CZT detector is used as a PET insert. The interpolation algorithm for the charge sharing events was also investigated. The PET image that was reconstructed using the interpolation algorithm shows improved image resolution compared with the image resolution without interpolation algorithm.

A PET insert with detector having smaller crystals and placed near a region of interest in a conventional PET scanner can improve image resolution locally due to the virtual-pinhole PET (VP-PET) effect. This improvement is from the higher spatial sampling of the imaging area near the detector. We have built a prototype half-ring PET insert for head-and-neck cancer imaging applications. In this paper, we extend the use of the insert to breast imaging and show that such a system provides high resolution images of breast and axillary lymph nodes while maintaining the full imaging field of view capability of a clinical PET scanner. We characterize the resolution and contrast recovery for tumors across the imaging field of view. First, we model the system using Monte Carlo methods to determine its theoretical limit of improvement. Simulations were conducted with hot spherical tumors embedded in background activity at tumor-to-background contrast ranging from 3: 1 to 12:1. Tumors are arranged in a Derenzo-like pattern with their diameters ranging from 2 to 12 mm. Experimental studies were performed using a chest phantom with cylindrical breast attachment. Tumors of different sizes arranged in a Derenzo-like pattern with tumor-to-background ratio of 6: 1 are inserted into the breast phantom. Imaging capability of mediastinum and axillary lymph nodes is explored. Both Monte Carlo simulations and experiment show clear improvement in image resolution and contrast recovery with VP-PET half-ring insert. The degree of improvement in resolution and contrast recovery depends on location of the tumor. The full field of view imaging capability is shown to be maintained. Minor artifacts are introduced in certain regions.

We are investigating the feasibility of a high-resolution PET insert device based on a Cadmium Zinc Telluride (CdZnTe) detector with 350 μm anode pixel pitch to be integrated into a conventional animal PET scanner to improve its image resolution to sub-500 micrometer range. In this work, we have used a simplified version of the future 2048-pixel CdZnTe planar detector with 250 μm anode pixel size and 100 μm gap. This simplified 9 anode pixel structure makes it possible to conduct experiments without a complete ASIC readout system (with 2048 channels) that is still under development. We characterized this CdZnTe detector by investigating it charge sharing, spatial resolution, and energy resolution. We imaged a Na-22 point source using the coincidence events between this 350 μm pixelated CdZnTe detector and a lutetium oxyorthosilicate (LSO) based Siemens Inveon PET detector. The reconstructed PET image shows a resolution of 590 μm full width at half maximum (FWHM) by using single-pixel events. When we included double-pixel charge sharing events in the image reconstruction, the image resolution was degraded to 655 μm, but the sensitivity of the coincidence system increased 2.5 to 3 times.