Heterogeneous nucleation of the primary phase in the rapid solidification of Al-4.5wt%Cu alloy droplet

International audienceThis paper reports on rapid solidification of Al-Cu alloys. A heterogeneous nucleation/growth model coupled with a thermal model of a falling droplet through a stagnant gas was developed. The primary undercooling as well as the number of nucleation points was compared with Al-Cu alloy droplets produced by Impulse Atomization (IA). Based on experimental results from Neutron Diffraction, secondary (eutectic) phases were obtained. Then, primary and secondary undercoolings were estimated using the metastable extensions of solidus and liquidus lines calculated by Thermo-Calc. Moreover, Synchrotron X-ray micro-tomography has been performed on Al-4.5wt%Cu droplets. The undercoolings are in good agreement. Results also evidence the presence of one nucleation point and are in agreement with the experimental observations. 1. Introduction Manufacturing of most metallic alloy products involves solidification at some stage. Mechanical properties of these products are generally related to their solidification microstructures. Depending on the final application of a product, a certain type of microstructure is more appropriate compared to another. For a product that requires directional properties, a microstructure of columnar grains is needed while isotropic properties are satisfied with an equiaxed structure. Generally, post-processing of the solidified materials is required to obtain the final product with desired properties. These post-solidification treatments are generally time-consuming and therefore increase the production cost without fully eliminating solidification related defects such as segregation. Therefore, it is important to understand all the dynamics involved in the formation of solidification microstructures in order to control the properties of the final products. As dendrites growth from an undercooled melt depends a great deal on the nucleation undercooling. Therefore, determination of undercooling and the resulting growth rate, recalescence, microsegregation/phase fraction and grain size is very important. Al-Cu alloys (4.5, 5, 10 and 17 wt% Cu) have been produced by IA and the last three compositions were analysed in our previous papers [1, 2]. IA is a single fluid atomization technique that is capable of producing droplets of controlled size having a relatively narrow distribution and a predictable cooling rate. The alloys (350 to 450g) were melted in a graphite crucible by means of an induction furnace and atomized at 850ºC in an almost oxygen free chamber (10ppm) under Nitrogen, Helium or Argon atmospheres. The atomized droplets rapidly solidify during their fall by losing heat to th

4D synchrotron X-ray tomographic quantification of the transition from cellular to dendrite growth during directional solidification

Solidification morphology directly impacts the mechanical properties of materials; hence many models of the morphological evolution of dendritic structures have been formulated. However, there is a paucity of validation data for directional solidification models, especially the direct observations of metallic alloys, both for cellular and dendritic structures. In this study, we performed 4D synchrotron X-ray tomographic imaging (three spatial directions plus time), to study the transition from cellular to a columnar dendritic morphology and the subsequent growth of columnar dendrite in a temperature gradient stage. The cellular morphology was found to be highly complex, with frequent lateral bridging. Protrusions growing out of the cellular front with the onset of morphological instabilities were captured, together with the subsequent development of these protrusions into established dendrites. Other mechanisms affecting the solidification microstructure, including dendrite fragmentation/pinch-off were also captured and the quantitative results were compared to proposed mechanisms. The results demonstrate that 4D imaging can provide new data to both inform and validate solidification models

Drahtlose Übertragung von Elektrizität

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Direct Simulation of a Solidification Benchmark Experiment

International audienceA solidification benchmark experiment is simulated using a three-dimensional cellular automaton-finite element solidification model. The experiment consists of a rectangular cavity containing a Sn-3 wt pct Pb alloy. The alloy is first melted and then solidified in the cavity. A dense array of thermocouples permits monitoring of temperatures in the cavity and in the heat exchangers surrounding the cavity. After solidification, the grain structure is revealed by metallography. X-ray radiography and inductively coupled plasma spectrometry are also conducted to access a distribution map of Pb, or macrosegregation map. The solidification model consists of solutions for heat, solute mass, and momentum conservations using the finite element method. It is coupled with a description of the development of grain structure using the cellular automaton method. A careful and direct comparison with experimental results is possible thanks to boundary conditions deduced from the temperature measurements, as well as a careful choice of the values of the material properties for simulation. Results show that the temperature maps and the macrosegregation map can only be approached with a three-dimensional simulation that includes the description of the grain structure

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

Numerical Model of Rapidly Solidified Droplets of Al-33 Wt Pct Cu Eutectic Growth

Rapid solidification of Al-Cu droplets of eutectic composition was carried out using Impulse Atomization (a type of drop tube). Two distinct morphologies were observed: an irregular undulated eutectic assumed to form during recalescence, followed by a regular lamellar eutectic. The volume fraction of each morphology was measured and used to deduce the nucleation undercooling based on the hypercooling limit. A model of the eutectic solidification was developed assuming that the kinetics of the undulated and regular regions is the same and follows scaling laws established experimentally. The simulated solid fraction forming during recalescence matches the experimental undulated eutectic fraction. Furthermore, the heat balance confirms the adiabatic nature of the solidification during recalescence. Good agreement is found between the model and experimental measurements of lamellar spacing for the regular eutectic. However, the predicted spacing of the undulated eutectic is much lower than what is observed experimentally. This difference as well as the nature of this morphology is attributed to coarsening during the remaining of solidification of the very fine eutectic formed during recalescence.LSM

Rapid solidification of Al-Cu droplets of near eutectic composition

Near eutectic Al-Cu droplets were rapidly solidified by Impulse Atomization. A wide range of microstructural scales was obtained at different cooling rates and undercoolings. The micrographs of the investigated samples revealed two distinct zones of different structural morphologies: An undulated eutectic morphology developed during recalescence following the single grain nucleation and a regular lamellar eutectic morphology resulting from the solidification of the remaining liquid post recalescence. The volume fraction of each zone was measured as a function of the droplet diameter, and the nucleation undercooling was deduced using the hypercooling limit equation. Scanning Electronic Microscopy imaging and microhardness measurements were used to evaluate the microstructural scale, and mechanical properties

Growth and interaction of dendritic equiaxed grains: In situ characterization by synchrotron X-ray radiography

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Characterization of rapidly solidified Al-Mg-Sc alloys with Li addition

This paper investigate the thermophysical properties of the liquid as well as the rapid solidification ofAl5.0Mg0.2Sc alloys with 3.0 and 6.0 Li wt% by the discharge crucible, and Impulse Atomization techniquesrespectively. The discharge crucible method, allowed a simultaneous determination of density, surface tensionand viscosity as a function of the temperature. While the density and surface tension are found to decrease withLi content, the viscosity increases due to short-range ordering occurring in intermetallic phases. In order todetermine their characteristic temperatures, the alloys powder generated by Impulse Atomization were investigatedusing a Differential Scanning Calorimeter, while microstructural observations and chemical analyseswere conducted using scanning and transmission electron microscopy with energy dispersive X-ray spectroscopy.To study the influence of Li on the microstructural phase formation in these alloys, diffraction patterns analysesin transmission electron microscopy, as well X-ray diffraction using synchrotron measurements were carried out.The conducted measurements and microstructure observations revealed the precipitations of Al3Sc and Al2MgLiphases in both alloy compositions. In addition, AlLi precipitates were observed in Al5.0Mg0.2Sc6.0Li alloy,which is in agreement with the phase diagram

Characterization of a{(Fe60_{60}Co40_{40})75_{75}B20_{20}Si5_{5}}96_{96}Nb4_{4} Impulse Atomized Glassy Powder by Neutron Diffraction and Differential Scanning Calorimetry

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