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
Microstructural analysis of rapidly solidified aluminium-nickel alloysPowders of Al-50 wt-%Ni and Al-36 wt-%Ni were produced using the impulse atomisation technique, a rapid solidification technique. The molten droplets were cooled in flight by the stagnant helium or nitrogen in the atomising chamber. The resulting powders were sieved into different size ranges. X-ray diffraction and neutron diffraction were used in order to quantify the phase fractions in the samples. Profile refinement, using the computer software GSAS, was used to calculate the weight fraction of the existing phases, namely Al 3 Ni 2 ,A l 3 Ni and Al, as a result of different processing parameters. The Scheil-Gulliver model was applied to investigate the extent to which it can predict phase fractions in the Al-Ni system. In Al-50 wt-%Ni, by increasing cooling rate, the ratio of Al 3 Ni to Al 3 Ni 2 approaches that of Scheil-Gulliver's prediction. Opposite behaviour was observed in Al-36 wt-%Ni. In addition, from the profile refinement, the effect of composition and cooling rate on the lattice parameter of Al 3 Ni 2 was investigated. In Al-36 wt-%Ni, the c/a ratio is significantly smaller than the stoichiometric c/a ratio of Al 3 Ni 2 , and it decreases with increasing cooling rate. On the other hand, for Al-50 wt-%Ni, the c/a ratio is much closer to the stoichiometric value and it increases with increasing cooling rate.On a produit des poudres d'Al-50% en poids de Ni et d'Al-36% en poids de Ni en utilisant la technique d'atomisation par impulsion, une technique de solidification rapide. Les gouttelettes fondues étaient refroidies en vol par l'hé lium ou l'azote stagnants dans la chambre d'atomisation. Les poudres ré sultantes é taient tamisées en diffé rentes gammes de taille. On a utilisé la diffraction des rayons X et la diffraction des neutrons pour quantifier les fractions de phase dans les é chantillons. Le raffinement du profil, obtenu grâ ce au logiciel d'ordinateur GSAS, était utilisé pour calculer la fraction de poids des phases existantes, soit Al 3 Ni 2
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