Quantification of Primary Dendritic and Secondary Eutectic Nucleation Undercoolings in Rapidly Solidified Hypo-Eutectic Al-Cu Droplets

Abstract

International audienceThis paper reports on the quantification of primary dendritic and secondary eutectic nucleation undercoolings during rapid solidification of impulse atomized hypo-eutectic Al-Cu droplets. The procedure consists in determining the eutectic fraction of each investigated droplet from the fraction of intermetallic Al2Cu obtained by Rietveld refinement analysis of neutrons scattering data. The corresponding eutectic nucleation undercooling is then deduced from the metastable phase diagram of the alloy. The primary dendritic nucleation undercooling is subsequently determined using semi-empirical coarsening models of secondary dendrite arms. The two nucleation undercoolings are finally used as input variables to run a microsegregation model for binary alloys. The fractions of eutectic computed by the microsegregation model compare very favorably with the experimental results

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

“…The eutectic fraction is estimated by extending the solidus and liquidus lines of the Al-rich corner of Al-Cu phase diagram using Thermo-Calc. A detailed description of this analysis is given in [1,2]. However, it is worth noting that the time/temperature variation of the coarsening parameter was not taken into account in [1,2] whereas this variation is considered in the present case.…”

“…Considering the correlation proposed by Whitaker [3] and a correction term (due to the definition of Nu at Re=0) [11], equation (2) can be rewritten as…”

“…This provides us the nucleation temperature (T L -ΔT) and so the undercooling. In a second time and only if the nucleation is allowed, a run is done on the meshes contained in the droplet (due to memory issue, only a box of 10×10×10 µm 3 is considered for the number of clusters and not for the entire droplet). All the meshes are investigated.…”

“…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.…”

“…Subsequently, the solid droplets are washed, dried and sieved into different sizes. A detailed description of the IA process is given in [3]. A solidification model of a droplet falling through a stagnant gas, based on heterogeneous nucleation/growth was developed.…”