Cellular-to-dendritic and dendritic-to-cellular morphological transitions in a ternary al-mg-si alloy

The study is focused on the influence of solidification thermal parameters upon the evolution of the microstructure (either cells or dendrites) of an Al-3wt%Mg-1wt%Si ternary alloy. It is well known that the application properties of metallic alloys will greatly depend on the final morphology of the microstructure. As a consequence, various studies have been carried out in order to determine the ranges of cooling rates associated with dendritic-cellular transitions in multicomponent alloys. In the present research work, directional solidification experiments were conducted using either a Bridgman (steady-state) device or another device that allows the solidification under transient conditions (unsteady-state). Thus, a broad range of cooling rates (dot T), varying from 0.003K/s to 40K/s could be achieved. This led to the identification of a complete series of cellular/dendritic/cellular transitions. For low cooling rate experiments, low cooling rate cells to dendrites transition happens. Moreover, at a high cooling rate, a novel transition from dendrites to high cooling rate cells could be observed for the Al-3wt%Mg-1wt%Si alloy. Additionally, cell spacing λC and primary dendritic spacing λ1 are related to the cooling rate by power function growth laws characterized by the same exponent (-0.55) for both steady-state and unsteady-state solidification conditions529CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP23038.000069/2015-042012/08494-0; 2012/16328-2; 2013/23396-7; 2014/25809-

Strain building and correlation with grain nucleation during silicon growth

This work is dedicated to the grain structure formation in silicon ingots with a particular focus on the crystal structure strain building and its implication in new grain nucleation process. The implied mechanisms are investigated by advanced in situ X-ray imaging techniques during silicon directional solidification. It is shown that the grain structure formation is mainly driven by S3 twin nucleation. Grain competition phenomena occurring during the growth process lead to the creation of higher order twin boundaries, localised strained areas and associated crystal structure deformation. On the one hand, it is demonstrated that local strain building can be directly related to the characteristics of the twin boundaries created during silicon growth due to grain competition. On the other hand, space restriction due to competition during growth can be at the origin of local strain building as well. Finally, the accumulation of all these factors generating strain is responsible for spontaneous new grain nucleation. When occurring, both grain nucleation and subsequent grain structure reorganisation contribute to lower the strain in the growing ingot. It is demonstrated as well that the local distribution of the strained areas created during silicon growth is retrieved after cooling down, from melting temperature to room temperature, on top of an additional larger scale deformation of the sample due to the cooling down only

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

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In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110〉 seeded growth Si

This work is dedicated to the advanced in situ X-ray imaging and complementary ex situ investigations of the growth mechanisms when silicon solidifies on a monocrystalline seed oriented ⟨110⟩ in the solidification direction. It aims at deepening the fundamental understanding of the phenomena that occur throughout silicon crystal growth with a particular focus on mechanisms of formation of defects detrimental for photovoltaic applications. Namely, grain nucleation, grain boundary formation and evolution, grain competition, twining occurrence, dislocation generation and interaction with structural defects are explored and analysed. Nucleation of twin crystals preferentially occurs on {111} facets at the edge of the sample where solid e liquid e vapor triple point lines exist in interaction also with the crucible as well as, at grain boundary grooves at the solid e liquid interface (solid e solid e liquid triple lines), where two grains are in competition, either on the {111} facets of the groove or in the groove. Enhanced undercooling and/or stress accumulation levels are found to act as driving forces for grain nucleation. Additionally, it is demonstrated that twin formation has the property to relax stresses stored in the crystal during the growth process. However, grains formed initially in twin position can undergo severe distortion when they are in direct competition or when they are squeezed in e between grains. Moreover, we show by X-ray Bragg diffraction imaging that on the one hand, coherent S3 ⟨111⟩ grain boundaries efficiently block the propagation of growth dislocations during the solidification process, while on the other hand, dislocations are emitted at the level of incoherent and/or asymmetric S27a ⟨110⟩ at the encounter with either S3 ⟨111⟩ or S9 ⟨110⟩ grain boundaries. Indeed, grain boundaries that deviate from the ideal coincidence orientation act as dislocation sources that spread inside the surrounding crystals

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

Abstract: Synchrotron X-ray imaging has been used to examine in situ the deformation of dendrites that takes place during the solidification of a nickel-based superalloy. By combining absorption and diffraction contrast imaging, deformation events could be classified by their localization and permanence. In particular, a deformation mechanism arising from thermal contraction in a temperature gradient was elucidated through digital image correlation. It was concluded that this mechanism may explain the small misorientations typically observed in single crystal castings

Effects of the interface curvature on cellular and dendritic microstructures

International audienceWe study the solidification of succinonitrile-based alloys contained in cylindrical glass crucibles. In such samples, the solid–liquid interface iscurved as the latent heat generated upon solidification is mainly evacuated through the crucible wall of higher thermal conductivity. This createsradial thermal gradient which drives some important modifications in solute macrosegregation and of the microstructure array. A facility dedicatedto the in situ and real time characterization of solid–liquid interface morphology during directional solidification of bulk samples has been developedby CNES (French Space Agency) in the frame of the DECLIC project. The effects on microstructure dynamics of fluid flow driven by the radialthermal gradient associated to curvature have already been pointed out, but other effects of curvature – function of the type of structure developed– are here presented. For a cellular microstructure, curvature is at the origin of advection that manifests in observation by the gliding of cells fromthe border to the centre of the interface. For a dendritic microstructure, a strong dependence of dendrite growth direction on the curvature appearsthat alters the columnar structure

Al–Fe hypoeutectic alloys directionally solidified under steady-state and unsteady-state conditions

International audienceThe aim of this work is to evaluate the cellular growth, the nature of Al–Fe intermetallic particles and the eutectic arrangement of Al–Fe hypoeutectic samples solidified at growth rates ranging from 0.05 to 2.5 mm/s. The samples grown at higher solidification velocities were obtained using a water-cooled directional solidification apparatus. A Bridgman-type furnace was used to grow samples in the lower range of solidification velocities and an air-cooled mold was used to generate experimental values in between those obtained by the other two techniques of directional solidification. All casting assemblies were set to support upward directional solidification. Based on the present results, a single experimentalpower law seems to be enough to fit all experimental values of cell spacing as a function of cooling rate. The wide range of solidification thermal parameters used in the present study was chosen due to the diversity of foundry processes used for the manufacture of Al–Fe alloys components. For instance, low solidification velocities are typical of sand casting processes while high velocities are typical of direct-chill (DC) castings.In order to investigate the nature of the Al–Fe intermetallics, these particles were extracted from the aluminum-rich matrix by using a dissolution technique. Such phases were then investigated by SEMEDAX microscopy and X-ray diffraction (XRD). It was found that Al3Fe is the predominant intermetallic phase in the Bridgman-grown samples and Al6Fe prevails in the samples grown in the water-cooled solidification apparatus

Grain size reduction by electromagnetic stirring inside gold alloys

The final properties of cast materials depend greatly on the solidification process undergone by the material. In this paper, we study gold alloys dedicated to the watch industry and jewellery in the framework of a research collaboration with the Metalor Company. The aim is to improve the concentration homogeneity of the ingots by controlling the solidification step. It can be achieved by reducing segregations by a decrease in the grain size. For this purpose, we set up a multiphase electromagnetic stirring of the melt to favour the growth of finer grains and improve the homogeneity of the composition. We first design an electromagnetic stirrer by numerical simulation. The stirrer is then implemented on a model experiment. Eventually, the alloys are characterised by metallography and etching to evidence the grain structure. As expected, we obtain a substantial reduction of the grain size although, some work remains to be done to attain the final goal of even finer grains