Prakt. Met. Sonderband 38 (2006) 69 
alloys 4. METALLOGRAPHIC RESULTS 
Es | Fig.2 shows the structure of the AG5.7S1.9 as cast bars: the dendritic a-Al is surrounded 
00s by a fine eutectic structure with interlamellar distances of 1-2 ym. Fig.3a shows a signi- 
ficantly higher volume fraction of the eutectic region in the as cast AIMg5.7Si2.4 bar. SDA 
100g/s (secondary dendrite arm spacings) of 5um are observed close to the surface of both con- 
tinuously cast bars increasing to 15 um in the centre. In table 1 the corresponding cooling 
; PI rates acc. to [9] are given. After solution treatment the lamellar structure spheroidizes as 
| shown in Fig.3b forming roughly two size classes of particles (<1um / >1.5 ym). The 
— average composition of the eutectic corresponds to Mg,Si. Some individual particles ex- 
hibit a higher Si content than stoechiometric Mg,Si, but separate Si phases could not be 
yrner of identified by SEM/EDX. Lengthy AlFeSi particles (<3um) are revealed by BSE. 
system The structure of these materials changes completely when remelted and gravity cast. Fig.4 
 tem- and 5 show larger SDA of the o-Al dendrites (see table 1) and “chinese script” Mg,Si 
dicating structures with interconnected lamellae spaced about 10 um. Fig.5b also shows some 
arations coarse nodes and very fine structured eutectic regions (features < 1um). The different 
) and heterogeneities can be clearly recognised in gravity cast AG5.8S5.6 in Fig.6: in addition to 
a the chinese script Mg,Si and the fine structured eutectic, some AlFeSi needles are observed. 
he in- 
ys are 
e stars 
of Mg, 
excess 
pect to 
etry of 
casting 
hed as 
image : . : : 
endritic Fig.2: LOM of continuously cast AG5.7S1.9 showing primary o-dendrites and at higher 
hed by magnification fine lamellar eutectic structures in between 
alitative 
eter for 
3s high 
es was 
phases 
; of the 
bi 
xide 
Fig.3: Samples of continuously cast AG5.7S2.4: a) LOM of primary a dendrites and a high 
volume fraction of eutectic structure. b) BSE-SEM image of a solution treated sample with 
spheroidized Mqg»Si (dark) of different size classes and AlFeSi-phases (white).