In situ observations of the rapid solidification for undercooled Al 30 Si 70 alloy melt
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solidification of Al30Si70 alloy was studied via electromagnetic levitation technique. The solidification kinetics and the morphology of the solidification front of the Si phase were analyzed in situ by using a high-speed video camera and subsequent microstructural analysis of assolidified samples. It shows that solidification of the sample always starts from one point. After that, nucleation continues to proceed at the interface front during growth. The morphology of primary Si transforms from faceted wafer to nonfaceted equiaxed grain and the grain size decreases with increase of undercooling. At small undercooling, the growth velocity of primary Si decreases with time and the floated Si wafers have a trend to agglomerate, while at large undercooling, the nucleation rate decreases with time, which are explained by the fact that silicon content, undercooling and density at the solid–liquid interface change with time in solidification. Finally, the nucleation rate and growth velocity were discussed in combination of classical theory.
I. INTRODUCTION
Levitation techniques are important to study the rapid solidification of undercooled melts because it removes the crucible wall which is the preferential site of heterogeneous nucleation.1 In particular, electromagnetic levitation (EML) is one of the suitable techniques for studying the solidification behaviors of metallic or semiconductor systems,2,3 especially for some materials which react with quartz crucible, such as Al–Si alloys. Accordingly, undercooled solidifications of many alloys have been realized by EML technique.4,5 In those reports, the microstructure evolutions are rich,2–5 but experiment observations of undercooled dendrite growth processes are rarely seen. Most of theories always assumed the behaviors of interface for undercooled solidification similar to that of directional solidification which has been studied deeply.6 Jackson and Hunt discovered a group of transparent organic compounds that solidify like metals, which have been used widely to model the growth of metal alloys.7,8 With the help of the transparent materials, the solidification picture can be taken by looking through the glass plate, so that the dendrite growth of directional solidification process can be studied thoroughly.6–10 However, the dendrite growth in undercooled melt cannot be observed directly by transparent materials, so it is not yet understood clearly. Most of the
Contributing Editor: Eric A. Stach a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.401 222
J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
studies consider the growth process similar to the directional solidification, i.e., the velocities of growth interface, the content in solid–liquid interface and the undercooling in solidification process remain constant.11 Actually, from the present experiment, the pre-existing assumption should not be all right. The high speed video (HSV) can often be used to in situ observe the solidification interface as well as to measure the growth veloc
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