Structural and compositional variations in aluminium alloys rapidly quenched from the melt

Rapid quenching of aluminium alloys by meltspinning results in ribbons showing a heterogeneous solidification structure [1, 2] . This structural heterogeneity is expected to result from local differences in cooling rate. Thus, compositional variations due to differences in solidification conditions may be expected. Therefore, the relation between the local concentration of the alloying element and the microstructure has been investigated. Ribbons of A1Si and A1Mg alloys were prepared by applying a melt-spinning technique ([3]; in the present investigation the circumferential velocity was 31 .4msec-1) . The hypo-eutectic alloys used were prepared from 99.994 wt% A1, 99.99 wt % Si and 99.99 wt % Mg. The thickness of the ribbons obtained varied between 40 and 80 /an . Cross-sections were obtained by preparation of the ribbons along their longitudinal axis. After mechanical polishing and etching in Keller and Wilcox's reagent the cross-sections were examined with a Neophot 2 (Carl Zeiss Jena) optical microscope. X-ray microanalysis was performed with an ARL electron microprobe. The content of alloying element was determined near the wheel side, near the upper side and in the centre as an average value obtained from tracks with a length of about 45/am parallel to the outer surfaces. Cross-sections of melt-spun ribbons are shown in Fig. 1. Three regions can be distinguished. 1. A very finely grained (sometimes "featureless") zone at the wheel (chill) side. 2. A zone of columnar grains. 3. A zone of equiaxed grains at the upper side. The results of the X-ray microanalysis are gathered in Table I. It is concluded that the concentration o f alloying element is highest in the centre and lowest near the upper side, whereas the concentration near the wheel side takes an intermediate value. The results obtained suggest a close relation