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63 Mat. Res. Soc. Symp. Proc. Vol. 398 ©1996 Materials Research Society

EXPERIMENTAL Droplets of Cu - 30 at.% Ni were prepared using containerless processing by electromagnetic levitation [11]. Solidification was triggered at selected undercoolings (in the range 60 - 243 K) by touching the droplet with a needle. For convenience, the point at which the needle touches the droplet is termed the 'south' pole. After solidification, each droplet was sectioned along the 'North' - 'South' axis and mounted in resin. Each section was polished to a 1/4 [tm finish and then etched in a mixture of 5 g FeC13, 2 ml HCI (conc.) and 95 ml ethanol. Optical metallography was performed on these sections to reveal the grain structure. A Philips-'X-pert' texture goniometer was used with CuKa radiation (Ni-filter) to obtain (220) pole figures (contour maps). The experiments were performed with sample oscillation, so that the analysed area extends over the complete droplet sections. To study changes in texture with undercooling more quantitatively, linear scans are used. For these, a given maximum on the pole figures is first analysed at higher resolution to determine its exact position. A linear scan of reflected intensities is then performed through the peak with a resolution of 0.2* per step by rotation on the 0-axis. RESULTS Introduction Previous work on the microstructures of levitated Cu-Ni droplets has demonstrated that dendritic microstructures appear in a range of undercoolings between critical values designated AT,* and AT2 *. For concentrated alloys, the values of AT 1* and AT2* are weakly dependent on composition; for example for Cu - 30 at.% Ni, AT,* = 75 K and AT2* = 214 K. For droplets with undercoolings greater than AT2* the microstructures are fully grain-refined, whereas at undercoolings less than AT1 *, the microstructures consist of a mixture of fine grains and fragments of dendrites. This work focuses on three droplets which exhibit the characteristic microstructure for each undercooling range; their undercoolings are 60 K (AT < AT 1 *), 195 K (AT,* < AT < AT2 *) and 228 K (AT > AT 2 *). AT= 195 K The overall microstructure of the droplet solidified at 195 K is dendritic (Figure le). The contour plot has clearly defined poles (Figure le, inset) which show [12] the main growth direction to be near [100]. Each pole on the contour plot was examined in more detail using linear scans; an example is given in Figure 2a. The parameters analysed for each pole were the relative intensity (I/Io) and the full width at half maximum (FWHM) which provides information on the spread of orientations at each undercooling. Each pole has a different tilt angle V", but since there is no dependence of /lo or FW.HM on V, the data from different maxima on one contour plot can be combined for each undercooling. AT= 228 K The microstructure of this droplet is grain-refined throughout, with the grain size at the poles being larger than at the centre of the droplets (Figures la and lb). Perhaps associated with reduced partitioning for solidifica