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TRODUCTION

UNDERCOOLING is observed to various extents in practically every alloy that undergoes solidification processing. Indeed, microstructure development occurs into an undercooled melt after primary phase nucleation. The growth rate depends on the degree of undercooling.[1] The higher the primary phase nucleation undercooling (DTP), the faster is the growth / solidification rate of the primary phase. Fast dendrite growth induced by a high DTP is, however, accompanied by a large release of latent heat. Therefore, an alloy with a low specific heat will show strong recalescence characterized by a temperature rise which affects the homogeneity of the resulting microstructure and may result in partial re-melting of already solidified dendrites.[2–4] Similar undercooling phenomena and recalescence accompany the nucleation of secondary phases for the formation of the eutectic. This nucleation is termed secondary eutectic nucleation undercooling (DTE). DTP and DTE in rapid solidification yield materials with improved mechanical and chemical properties due to the resulting microstructure refinement and reduction of microsegregation.[5] In order to control microsegregation and microstructure distributions in hypoeutectic alloys, it is important to understand how DTP and DTE relate to processing conditions for a specific alloy.

ABDOUL-AZIZ BOGNO, Postdoctoral Fellow, POOYA DELSHAD KHATIBI, Graduate Student, and HANI HENEIN, Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2G6 Canada. Contact email: bogno@ ualberta.ca CHARLES-ANDRE´ GANDIN, Professor, is with MINES ParisTech, Centre de Mise en forme des matO˜riaux, CNRS UMR 7635, CS10207, 06904, Sophia Antipolis, France. Manuscript submitted December 15, 2015. Article published online June 29, 2016 4606—VOLUME 47A, SEPTEMBER 2016

Impulse Atomization (IA) is one of the drop tube types of rapid solidification techniques. With IA, the cooling rate is controlled by the heat exchange between the stagnant gas and the atomized droplets. This heat exchange is a function of upon the nature of the gas in the atomization chamber (helium or nitrogen in the present study), the velocity of the droplets and their size, which depend on atomization operating parameters.[6,7] Due to practical difficulties, in-situ measurements of nucleation temperatures cannot be achieved during IA experiments; consequently a post-mortem method of determining the nucleation temperatures, TP, of the primary phase and TE of the secondary phases for the formation of eutectic has to be developed. Only in the electromagnetic levitation (EML) system, direct measurements of undercoolings can be made. However, due to the large size of droplets used in EML (6mm), most of the microstructure of the solidified droplet forms post-recalescence and under conditions of low cooling rate. The aim of this paper is to consider experimental results obtained by Rietveld refinement analysis of Neutron diffraction (ND) data to determine the eutectic fraction (FE) and DTE. Then,