Probing the dynamics of quasicrystal growth using synchrotron live imaging

The dynamics of quasicrystal growth remains an unsolved problem in condensed matter. By means of synchrotron live imaging, facetted growth proceeding by the tangential motion of ledges at the solid-melt interface is clearly evidenced all along the solidification of icosahedral AlPdMn quasicrystals. The effect of interface kinetics is significant so that nucleation and free growth of new facetted grains occur in the melt when the solidification rate is increased. The evolution of these grains is explained in details, which reveals the crucial role of aluminum rejection, both in the poisoning of grain growth and driving fluid flow

Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

International audienceDuring casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements

Modelling of the transition from a planar faceted front to equiaxed growth: Application to photovoltaic polycrystalline silicon

International audienceRecent experiments under X-ray examination have shown that a transition from planar front to equiaxed growth is likely to occur in the case of faceted interfaces. Such a transition is suspected to be at the origin of the sudden occurrence of deleterious small grains, among large columnar grains, observed in photovoltaic silicon ingots.A model is presented for the occurrence of equiaxed grains observed ahead of a planar faceted interface. Simple expressions are obtained which predict when the equiaxed structures should develop, in the case of rough (thermal dendrite) and of faceted equiaxed grains. These models provide a semiquantitative basis to discuss the Faceted front to Equiaxed structure Transition (FET). Then, it is applied to the case-study of photovoltaic silicon. Further developments are proposed to improve the model

Modelling of the transition from a planar faceted front to equiaxed growth : application to photovoltaic polycrystalline silicon

International audienceRecent experiments under X-ray examination have shown that a transition from planar front to equiaxed growth is likely to occur in the case of faceted interfaces. Such a transition is suspected to be at the origin of the sudden occurrence of deleterious small grains, among large columnar grains, observed in photovoltaic silicon ingots.A model is presented for the occurrence of equiaxed grains observed ahead of a planar faceted interface. Simple expressions are obtained which predict when the equiaxed structures should develop, in the case of rough (thermal dendrite) and of faceted equiaxed grains. These models provide a semiquantitative basis to discuss the Faceted front to Equiaxed structure Transition (FET). Then, it is applied to the case-study of photovoltaic silicon. Further developments are proposed to improve the model

Effects of the Interface Curvature and Dendrite Orientation in Directional Solidification of Bulk Transparent Alloys

International audienceThe properties of structural materials are to a large extent determined by the solid microstructure so that the understanding of the fundamental physics of microstructure formation is critical in the field of materials engineering. A directional solidification facility dedicated to the characterization of solid-liquid interface morphology by means of optical methods has been developed by CNES in the frame of the DECLIC project. This device enables in situ and real time studies on bulk transparent materials. The aim of the project is to perform experiments in microgravity to eliminate the complex couplings between solidification and convection and to get reliable benchmark data to validate and calibrate theoretical modeling and numerical simulations. Presently, ground experiments are performed to finalize the design and the experimental procedures and to guarantee the accuracy of the measurements. These experiments also provide reference data for the study of solidification microstructure dynamics in the presence of buoyancy-driven natural convection. Recent progress is presented concerning the control of the interface shape (critical for pattern analysis), the selection of single crystal of defined orientation (critical for dendritic growth) and the analysis of the dendrite shape

Microstructural development during transient directional solidification of hypermonotectic Al–Bi alloys

International audienceDirectional unsteady-state solidification experiments were performed with hypermonotectic Al–5.0 wt%Bi and 7.0 wt%Bi alloys. Thermal parameters such as the growth rate (v) and the thermal gradient (G) were experimentally determined by cooling curves recorded along the casting length. The predominant Bi-rich phase was characterized by droplets embedded in the aluminum matrix. Both the interphase spacing (k) and the Bi-rich particles diameter (d) were measured along the casting length. These microstructural features were correlated to the solidification thermal parameters: growth rate, cooling rate and thermal gradient. An experimental law expressing k as a function of both G and v was found to better represent the growth of hypermonotectic Al–Bi alloys. Moreover, it was found that the interphase spacing decreases with increasing alloy bismuth content

Determination of the average nucleation undercooling of primary Al-phase on refining particles from Al–5.0wt% Ti–1.0wt% B in Al-based alloys using DSC

International audienceThe understanding and control of the transition from columnar to equiaxed growth (CET) is an essential issue according to the desired usage properties. In Al-based alloys, refiners are routinely added to favor equiaxed growth. In that case, equiaxed grains nucleate preferentially on the refiners and can compete with columnar grains. To predict CET, two main parameters have been studied in models: the particle density,N0 and the nucleation undercooling of the refining particles (TN).We have used DSC successfully to measure the critical value of the nucleation undercooling. Thanks to these results, the efficiency of the specific refining particles (Al–5.0 wt% Ti–1.0 wt% B) in refined Al–3.5 wt% Ni alloy has been proved.Additionally, these measurements clearly point out the lowest efficiency of the same particles in refined Al–7.0 wt% Si which is generally admitted to be due to a Si-poisoning effect. More generally, the nucleation undercooling measured can be used in models dedicated to the quantitative prediction of CET

Microstructural development during transient directional solidification of hypermonotectic Al-Bi alloys

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Directional unsteady-state solidification experiments were performed with hypermonotectic Al-5.0 wt%Bi and 7.0 wt%Bi alloys. Thermal parameters such as the growth rate (upsilon) and the thermal gradient (G) were experimentally determined by cooling curves recorded along the casting length. The predominant Bi-rich phase was characterized by droplets embedded in the aluminum matrix. Both the interphase spacing (lambda) and the Bi-rich particles diameter (d) were measured along the casting length. These microstructural features were correlated to the solidification thermal parameters: growth rate, cooling rate and thermal gradient. An experimental law expressing). as a function of both G and upsilon was found to better represent the growth of hypermonotectic Al-Bi alloys. Moreover, it was found that the interphase spacing decreases with increasing alloy bismuth content. (C) 2010 Elsevier Ltd. All rights reserved.311045844591FA-PESP (The Scientific Research Foundation of the State of Sao Paulo, Brazil)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAEPEX - UNICAMPConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Twinning occurrence and grain competition in multi-crystalline silicon during solidification

International audienceMulti-crystalline silicon solidification is investigated by performing directional solidification experiments. Twinning phenomenon has been identified and observed in situ and in real time during the solidification using X-ray synchrotron imaging techniques: radiography and topography. The radiography observations give information on the formation, birth localized at the interface and evolution of the twins during solidification. The topography results give further information on the grain arrangement and on new grains in twinned position and grain growth competition. We have evidenced two twinning mechanisms: the first is the multiple twin formations during the growth of one grain. The second is the nucleation of a grain in twinned position at the bottom of a grain boundary groove