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

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

SEM Characterization of Al₃Ni Intermetallics and its Influence on Mechanical Properties of Directionally Solidified Hypoeutectic Al-Ni Alloys

International audienceRod-like Al3Ni intermetallic structures have been widely studied by Bridgman techniques of solidification. However, there is a lack of experiments conducted under unsteady-state solidification conditions. Such conditions are very close to the industrial reality since the thermal solidification variables (tip cooling rate, tip growth rate and thermal gradient) are freely changing as solidification progresses. In this research, Al3Ni structures found in hypoeutectic Al-Ni alloys were characterized under transient solidification conditions. Two Al-Ni alloys (1.0 and 5.0 wt%Ni) were directionally solidified. SEM (Scanning Electron Microscope) micrographs were obtained along the casting length (P). It was possible to observe with adequate magnifications the distribution of rod like Al3Ni particles along the interdendritic regions. In order to emphasize the examination ofmorphology and distribution of such particles, the aluminum-rich matrix was dissolved by immersion of the sample in a fluoride acid solution (0.5%HF + 99.5% H2O). The effects of nickel content, dendritic arrangement and Al3Ni distribution on mechanical properties were investigated by tensile tests

Microstructure and mechanical properties of Sn–Bi, Sn–Ag and Sn–Zn lead-free solder alloys

International audienceThe aim of this paper is to develop a comparative evaluation of mechanical properties of as-cast Sn–Bi, Sn–Ag and Sn–Zn alloys as a function of the resulting microstructural arrays with a view to application as solder materials. The resulting microstructures, ultimate and yield tensile strengths and elongations of Sn–40wt.%Bi,Sn–3.5wt.%Ag and Sn–9wt.%Znalloys were examined and compared with the corresponding results of the traditional Sn–40wt.%Pbsolder alloy. Tensile tests were carried out using specimens selected from specific positions along the length of the directionally solidified castings and Hall–Petch type correlations have been proposed relating the resulting tensile properties of each alloy to a parametric scale of the microstructure. It was found that all Pb-free alloys examined comply with a compromise between compatible mechanical strength and environmental protection. The Sn–Bi alloy has the highest ultimate tensile strength of all alloys examined, while both Sn–Ag and Sn–Zn alloys are lighter and exhibited higher ductility than the Sn–Pb and Sn–Bi alloys

Simulation of grain evolution in solidification of silicon on meso-scopic scale

International audienceWe present a cellular automata model for computing the grain evolution during directional solidification of silicon on a meso-scopic scale. Firstly, the method is applied to test cases with different shapes of the melt/crystal interface. In a second step we compute the case of an experiment with in-situ observation of the interface shape evolution [1]. Here we also include the effect of twinning. The interchanging appearance of two twins could be revisited by our calculations. The probabilities used correspond to those which were analytically derived for an undercooling of ∆T = 0.6 K [29]. This undercooling is a typical value for a groove with 111 facets [34]

Modeling and experimental characterization of the microstructure and grains structure of Al-7wt%Si directionally solidified

International audienceControlling the solidification microstructure of metallic materials is the main objective of several researches developed nowadays due to its strong influence on the mechanical properties. The natural convection in the bulk liquid caused by the action of the gravitational field during solidification process can modify the final structural morphology of the materials. The objective of this study is to develop a numerical and experimental approach to verify the evolution of the grain morphology, segregation of the eutectic phase and predictions about the action of the natural convection over the final grain structure of an Al-7wt%Si alloy in the presence of refining particles (0,5wt% Al-Ti-B). The experiments were carried out by using Bridgmann technique and the numerical analysis was done by using the cellular automaton with finite element model. The experimental results presented good agreement with the simulations

Modeling and experimental characterization of the microstructure and grains structure of Al-7wt%Si directionally solidified

International audienceControlling the solidification microstructure of metallic materials is the main objective of several researches developed nowadays due to its strong influence on the mechanical properties. The natural convection in the bulk liquid caused by the action of the gravitational field during solidification process can modify the final structural morphology of the materials. The objective of this study is to develop a numerical and experimental approach to verify the evolution of the grain morphology, segregation of the eutectic phase and predictions about the action of the natural convection over the final grain structure of an Al-7wt%Si alloy in the presence of refining particles (0,5wt% Al-Ti-B). The experiments were carried out by using Bridgmann technique and the numerical analysis was done by using the cellular automaton with finite element model. The experimental results presented good agreement with the simulations

Distributions of structures and solute in directionally solidified Al - 7 wt % Si

International audienceBridgman experiments for various withdrawal rates were conducted in an Al - 7 wt % Si alloy. The dendrite arm spacing and the grain size of the primary dendritic aluminium-rich phase were measured, together with the spatial distribution of the interdendritic eutectic micro structure. A fine grain structure is observed for large pulling rates at constant temperature gradient, corresponding to a uniform distribution of the eutectic structure very close to the Gulliver-Scheil approximation. When decreasing the cooling rate, the average fraction of the eutectic structures decreases as it is expected based on back-diffusion. However, the distribution of the eutectic structure progressively looses uniformity and the grain size increases. Detailed analyses of these observations are conducted using a cellular automaton - finite element model. It allows the simulation of the local solidification path for an open system coupled with a solution of the heat and solute mass balances in the presence of fluid flow for the entire Bridgman sample. Evolutions of the size and density of structures are retrieved when increasing the cooling rates. This demonstrates the key role played by the thermosolutal buoyancy forces and their interaction with the solidifying dendritic grain structure to explain the distributions of structures and solute segregation