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Abstract: In situ synchrotron X-ray radiography of columnar dendritic and eutectic growth under directional solidification of Al–30wt%Cu has been carried out with nominal spatial and temporal resolutions of 1.4 μm and 150 ms, respectively. The images can be processed to reveal spatiotemporal co-ordinates of the solid–liquid phase fronts for both α-dendrites and the proceeding eutectic. Also estimates for local solute boundary layers ahead of and in the mushy zone can be extracted from image processing. Steady-state growth could not be realized in the experiments. Presumably this relates to a combination of: (a) insufficient time available for initial instabilities to evolve into a steady state; (b) primary α-branches may interact through solute diffusion in the confined sample volume; (c) the inevitable presence of thermo-solutal flow; and (d) a long-term sample position dependent drift in the furnace to sample heat transfers. Nevertheless, the data collected contain information that may be valuable for microstructure modelling of dendritic growth. [Copyright &y& Elsevier]

An in-situ study on the directional solidification of an inoculated Al-20 wt%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out using a microfocus X-radiography set-up. The influences of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also promotes preferential growth of dendrite arms along the temperature gradient direction, and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is revealed that the propagation velocity of the nucleation front in directional solidification castings is approximately equal to the ratio between cooling rate Ṫ and temperature gradient G . Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’ (INZ) and ‘active nucleation zone’ (ANZ). The model has been applied to simulate the present in-situ solidification experiments. A good agreement was achieved between the predicted grain number density and the experimental measurements, showing the importance of including the temperature gradient effect on heterogeneous nucleation. Furthermore, the present model also has the capability to predict the temperature gradient necessary for the transition from equiaxed to columnar grain growth. [ABSTRACT FROM AUTHOR]

Bulk nanostructured (NS) metallic materials usually have high strength but extremely low ductility. In this study, we successfully prepared a bulk NS binary Al–7 Mg alloy possessing both high strength and high uniform ductility, approaching ∼600 MPa and ∼14%, respectively, by utilizing room temperature equal channel angular pressing (ECAP) combined with inter-pass annealing. The mechanism that governs the high ductility in the alloy and especially the role of the high Mg solute concentration was explored in terms of microstructure analysis. [ABSTRACT FROM AUTHOR]

Achieving high strength and uniform deformation simultaneously in Al alloys processed by severe plastic deformation (SPD) remains a challenging task as these alloys usually possess high strength but very limited uniform deformability, i.e.<∼4%. In the present work, however, high strength (∼507 MPa) and large uniform elongation (∼11%) were achieved simultaneously in a binary Al alloy by using room-temperature equal-channel angular pressing (ECAP) and a high solute Mg content. Detailed characterization by electron backscattered diffraction (EBSD), conventional transmission electron microscopy (TEM) and delicate ASTAR-TEM orientation imaging has been carried out to reveal the microstructure evolution. It was found that it was difficult to form well-developed substructures in Al–7 Mg deformed by 1 pass or 2 passes of ECAP, in contrast to the pure Al and dilute Al–Mg alloys deformed under similar conditions, where well-aligned subgrains and cells are frequently formed. As a result, a bimodal grain structure, i.e. ultrafine/fine grains with mean sizes of < ∼500 nm were developed accompanied by micron-sized grains having a high dislocation density after 3 passes. The high strength is due to a combination of strengthening by the high density of dislocations, ultrafine grains and high solute Mg content. The high uniform elongation is due primarily to the enhanced work hardening resulted from the high solute Mg content and the bimodal grain structure, while the dynamic strain aging effect is supposed also to contribute to the high ductility. Most importantly, the present work revealed that inhomogeneous deformation during SPD introduced by high solid-solution contents can be utilized as a strategy to generate desirable bimodal grain structure, gaining both a high strength and high uniform ductility. [ABSTRACT FROM AUTHOR]

The formation of primary Fe-rich intermetallic compound crystals and the effect of TiB 2 and TiC inoculation during solidification of a hypereutectic Al-Fe alloy has been investigated. Both horizontal, near isothermal conditions and vertical, directional thermal gradient conditions were studied using laboratory and synchrotron X-ray radiography. TiB 2 and TiC inoculation enhanced formation of Fe-rich intermetallics under all experimental conditions. Near isothermal solidification resulted in the highest intermetallic number density and average formation rate, whilst increasing the thermal gradient reduced both. A model for intermetallic crystal formation is proposed to explain the effect of thermal gradient and cooling rate on the intermetallic number density and average formation rate. [ABSTRACT FROM AUTHOR]

The present work revealed that equal-channel angular pressing (ECAP) deformation combined with appropriate annealing could represent as an efficient route to modify immiscible as-cast microstructure of a monotectic Al–8Bi alloy by a complete redistribution of immiscible Bi particles and grain refinement of alloy matrix. Especially, we found that the mean size of Bi particles in the 4-pass ECAPed Al–8Bi sample decreases after annealing at 200 °C for 8 h, due to inverse coarsening driven by the reduction of elastic energy and interfacial energy induced by morphology change and spatial distribution of Bi particles. [ABSTRACT FROM AUTHOR]

The current work represents the first implementation of Zernike phase contrast for compound refractive lens based x-ray microscopy, and also the first successful Zernike phase contrast experiment at photon energies above 12 keV. Phase contrast was achieved by fitting a compound refractive lens with a circular phase plate. The resolution is demonstrated to be sub-micron, and can be improved using already existing technology. The possibility of combining the technique with polychromatic radiation is considered, and a preliminary test experiment was performed with positive results. [ABSTRACT FROM AUTHOR]

Analytical expressions for the transmission cross-coefficients for x-ray microscopes based on compound refractive lenses are derived based on Gaussian approximations of the source shape and energy spectrum. The effects of partial coherence, defocus, beam convergence, as well as lateral and longitudinal chromatic aberrations are accounted for and discussed. Taking the incoherent limit of the transmission cross-coefficients, a compact analytical expression for the modulation transfer function of the system is obtained, and the resulting point, line and edge spread functions are presented. Finally, analytical expressions for optimal numerical aperture, coherence ratio, and bandwidth are given. [ABSTRACT FROM AUTHOR]

A comprehensive study on the heterogeneous nucleation and grain growth of Al-10 wt.%Cu alloys inoculated with Al-5Ti-1B was carried out. To further reveal the solute segregation stifling mechanism, in-situ near-isothermal melt solidification experiments with constant cooling rates and greatly suppressed melt convection were realized by in-situ microfocus X-radiography study. The kinetics of heterogeneous nucleation and grain growth under the isolated influence of cooling rate and addition level of inoculant particles has been quantitatively studied. Moreover, novel image processing and analysis approaches have been proposed, to determine the maximum nucleation undercooling and solid volume fraction at nucleation ceasing. To better understand the heterogeneous nucleation and grain growth behaviors under the in-situ experiment conditions, a new grain size prediction model in which both pure globular growth kinetics and dendritic growth kinetics including spherical/globular to dendritic transition (GDT) has been developed. The quantitative agreements between the simulation results and experimental results in terms of grain size, maximum nucleation undercooling and solid fraction at nucleation ceasing, have confirmed the validity of the solute segregation stifling mechanism for castings without recalescence. Furthermore, it is demonstrated that globular growth kinetics is an acceptable approximation for grain size prediction purposes of well grain-refined aluminum alloys. However, for poorly inoculated aluminum alloys with well-developed coarse dendritic grains, an application of dendritic growth kinetics significantly improves the grain size prediction power of the model. [ABSTRACT FROM AUTHOR]

Morphology and growth of primary α-Al(FeMnCr)Si intermetallics have been studied in-situ during solidification of a commercial secondary aluminum alloy employing X-radiographic imaging combined with deep-etching. The α-Al(FeMnCr)Si intermetallics were found to nucleate primarily on surface oxides, and the continued growth yielded both rhombic dodecahedrons and elongated rod-like morphologies. Both morphologies were observed as hopper and massive types, where the hopper intermetallics had the higher growth rates. The growth rate, which determines the type, appears to be linked to nucleation frequency; higher nucleation frequency promoted massive types and lower nucleation frequency promoted hopper intermetallics. [ABSTRACT FROM AUTHOR]

• The superplastic response is studied in an Al–7Mg alloy with multi-scale microstructural heterogeneities. • The Al–7Mg exhibits enhanced thermal stability and superplasticity of ∼523% at 573 K and 1 × 10−3 s−1. • Enhanced thermal stability owes to cooperative effects of bimodal grain structure and mg segregation along GBs. • The high superplasticity is due to a cooperated mechanism involving dislocation slip accommodated by CDRX and GBS. Achieving high superplasticity in single-phase Al alloys remains a challenge, since the fine-grained structure required for superplastic deformation coarsens rapidly in the absence of dispersed second-phase particles during tensile deformation at elevated temperatures. This paper concentrates on the superplastic response of a high solid solution Al–7Mg alloy processed by equal-channel angular pressing (ECAP) under uniaxial tension. The ECAP-processed Al–7Mg alloy features multi-scale microstructural heterogeneities including a bimodal grain structure and Mg solute segregation along grain boundaries (GBs) of nano/ultrafine grains. To identify effects of multi-scale microstructural heterogeneities on superplastic deformation behavior of the high solid solution Al–7Mg alloy, microstructural evolutions are studied systematically by combing electron backscatter diffraction (EBSD), ASTAR-transmission electron microscopy (TEM) orientation imaging and atom probe tomography (APT). During deformation at the optimal tensile condition of 573 K and 1 × 10−3 s−1, the heterogeneous microstructure evolves to a stable uniform fine grain structure via continuous dynamic recrystallization (CDRX), and impressive superplasticity of ∼523% elongation is achieved. The high superplasticity is discussed in terms of the cooperated mechanism by dislocation slip accommodated by CDRX at the early tensile deformation stage and grain boundary sliding (GBS) at the late deformation stage. Our findings show that the evolution of microstructural heterogeneities in high solid solution Al–Mg alloys can be regulated, favoring for superplastic deformation, which offers an alternative strategy for developing low-cost Al alloys for enhanced mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Real-time observation of structure evolution in a hypereutectic Al-Si(-Cu) alloy under near-isothermal melt solidification condition was conducted by using in-situ micro-focus X-radiography. The nucleation, growth rate and morphological development of primary Si particles (PSPs) were studied at different cooling rates. It is found that an increase of cooling rate increases nucleation rate, reduces the crystal growth and extends the nucleation temperature range of primary Si particles, which all help to refine primary Si particles. The minimum nucleation undercooling also increases with increasing cooling rate. Based on the experimental findings, it is proposed that Si atom clusters act as the nuclei of Si crystals. In addition, cooling rate does not alter the branching growth mechanism of PSPs but only shorten the branch length and reduce the development of side plates. Higher cooling rate may increase the peak growth velocity of PSPs due to the higher nucleation undercooling. Further, electron backscatter diffraction (EBSD) characterization of the post solidified in-situ sample confirms that twinning in primary Si occurs in the nucleation stage or during the growth process, and the growth of twined branches and plates follows the twin plane re-entrant edge (TPRE) mechanism. Twinning during growth may facilitate the branching. • An in-situ quantitative study on the kinetics of nucleation and growth of facetted primary Si particles has been realized. • The nucleation rate, minimum nucleation undercooling, and growth rate, as a function of cooling rate have been quantified. • The quasi-3D morphology evolution, such as branching and development of side plates, has been captured. • Twins form in both the nucleation and growth stages. [ABSTRACT FROM AUTHOR]