Drop-tube investigation of anomalous eutectic formation in undercooled melts

The rapid solidification of the Ag − 40at %Cu alloys was studied by the drop tube technique, aiming to investigate the formation of anomalous eutectic and morphology transitions at different cool- ing rates. The droplets of different diameters with 38 ∼ 850 μm droplets were obtained. Several characteristic techniques were used to analyse the as-solidified samples, including SEM and DTA.
The drop-tube processed Ag − 40at %Cu alloys formed anomalous eutectic as the dominant phase in all ranges of droplets, with either lamellar eutectic or lamellar eutectic and single phase dendrite being present. Among those droplets, certain large degrees of undercooling can theoretically be at- tained, either by the avoidance or at least the reduction of heterogeneous nucleation on container walls, or by the great amount of cooling rate to hinder the nucleation event in response. Three typ- ical morphologies were observed: (1) lamellar and anomalous eutectic at undercooling 10 ∼ 60 K; (2) lamellar eutectic and single phase dendrite at > 60 K; and (3) lamellar and anomalous eutectic and single phase dendrite at ∼ 60 K.
Unfortunately, the drop tube technique has not yet been developed a proper experimental mea- surement of neither the nucleation temperature nor the undercooling directly. Therefore the indi- rect method by using heat transfer modelling is used to quantitatively predict the thermal history with regard to time and the size of the as-solidified droplets. Furthermore, the 2D unsteady con- vection heat transfer model is developed to better numerically estimate the thermal extraction sit- uation during the solidification, in which minimum value of Nusselt number Nu no longer stays at 2 as a portion of the heat transferred from the leading part of sphere is return to the rear part. Thus, the Nu changing along the surface of sphere gives rise to the temperature gradient inside droplet, regardless of its size.
To further prove the temperature gradient existed in droplet, the ImageJ macro scripting is de- veloped to analyse the BSE images quantitatively. It is found that the distribution of anomalous eutectic due to temperature gradient, is changed with the droplet size. With the increase of droplet size, the greater amount of anomalous eutectic is retained in the centre of droplet than the surface of droplet.