68 Prakt. Met. Sonderband 38 (2006) 
Table 1: Composition of main components [wt.%] and casting conditions of the investigated alloys 4. ı 
Alloy Mg | si Fe | Tor | Toe  SDAascast | SDAremen” Casting Solidifi- Fi 
designation +Mn | [°C] | [°C] cooling rate | cooling rate conditions cation rate Ig. 
AG5.7S1.9 | 5.68 622 | 592  5-15um | 30-40um cont. cast bars ca. 60g/s by 
AG5.782.4 | 5.71 | 2.39 | 0.7 | 620 | 594 40+10K/s? | 2+1K/s  45x90mm% SAG, " fica 
AG5.552.5 502  5-15um | 30-40um  cont.cast 5mm thick| ca.100g/s (sec 
AG7.3S3.5 | 7.29 | 3.45 | 0.5 | 607 | 593 40+10K/s” | 2+1K/s  strip/ Alcan CR, " tinu 
AG5.8S5.6| 5.8 | 5.6 558 20-30 um a lab. gravity cast | ca. 5g/s rate 
AG4.988.4 | 4.9 | 8.4 | 0.6 | 591 | 558  6+3 Kis , 150x20x10mm® sho 
" as well remelted and gravity cast samples; * cooling rate estimated acc.[9, oe 
ibi 
- Fig.1: The Al corner of ider 
| + MgySi + Si | the AI-Mg-Si system The 
AISI8.4M94.9 below solidus tem- and 
perature [5] indicating stru 
Co stoechiometric 50°C the phase separations coa 
is AIMg5 85i5 67 ossrines . at 550, 500 and hoje 
5 ec 300°C, where the chin 
: RE os are 
3 : ; vesti 
9 A indicated (white stars 
AIMg5.7511.9 with excess of Mg, 
1g ve grey stars with excess 
a m= — er : . of Si with respect to 
> 4 > 36 ; 8 ° - the stoechiometry of 
wi. % Mg Mg>Si ). 
3. METALLOGRAPHIC METHODS 
The samples were cut along transverse and longitudinal planes with respect to the casting 
direction. No additional contrasting was applied to the surfaces ground and polished as 
described in Tab.2. Several LOM pictures at 20x magnification were processed for image 
analysis by AxioVision Rel.4.4 (Zeiss) to yield the area (=volume) fraction of the dendritic 
o-Al and of very fine structured eutectics, if present. The phases were distinguished by 
back scattered electron images (BSE); Si at accelerating voltage <15 keV. The qualitative 
analysis of elements was executed using energy dispersive spectroscopy (EDX). 
Samples of AG5.8S5.6 and AG4.9S8.4 were turned to cylinders of 1 mm in diameter for 
tomographic recordings at ESRF ID19 [10]. This synchrotron beam line provides high 
resolution with a voxel size of (0.7 um)*. The evaluation of the 3D grey level images was 
executed using the “ImageJ” software. Only the discrimination of Mg,Si and AlFeSi phases 
is reported here, for which 3D images are presented. In order to bias the results of the 
volume fractions, only particles bigger than 3x3x3 voxels were taken into account. 
Table 2: Steps of specimen preparation for light and scanning electron microscopy 
Support disc MD-Piano | MD-Largo MD-Mol MD-Chem 2a, 
Grinding/polishing medial Diamond DP-Suspension Magnesiumoxide Ea 
Grain size (number) 220 Sum | 6 um 3 um 0,4 um Fig. 
Lubricant Water DP-Lubricant ‘blue’ DP-Lubr.red’ . Aqua dest. 
Speed [U/min] 300 150 a volu 
Pressure [N] 150 7180 180 150 60 sph 
Time required [min] until plane 2.5 as 2-3 ca. ?