from ~5000, to ~13000 in the first five minutes of immer- sion (Figure 7a). These changes all occurred while the sam- ple was heating up, and at this time the metal temperature was 480C (896F) (Figure 7c). The corresponding aspect ratio of the Si was decreased from 2.24 to 2.1 (Figure 7d), and the average Si particle size decreased from 2.8 µm2 to 1.2 µm2
sion, the Si grew by Ostwald ripening to an average 1.7 µm2
(Figure 7a). In the next ten minutes of immer- and continued to spheroidize further, resulting in the
microstructure that was observed after 15 minutes of im- mersion (Figure 6b). The number of particles (in the fixed area) was ~10000, and the average aspect ratio decreased to 1.9. For the central regions, the same trends were ob- served. In the first ten minutes of immersion, the number of particles increased from ~2600 to ~5000, the average size of the particles decreased from 4.4 µm2
to 2.6 µm2 (Figure
7b) but the average aspect ratio only changed slightly from 2.6 to 2.5 (Figure 7d). This result reflects the fact that many of the larger, longer particles did not fragment as signifi-
, their number decreased to ~4000, and the average aspect ratio reduced further to 2.4.
cantly, rather they tended to become more rounded. In the five minutes that followed, the Si particles grew to a size of 4.5 µm2
Fragmentation, spheroidization and Ostwald ripening therefore changed the size, shape and aspect ratio of the Si phase, in both edge and centre regions. Although the actual time of immersion was limited due to the need to avoid blister formation, the changes which occurred were significant during the 15 minutes immersion. These changes will influence the onset of cracking of the Si par- ticles, as well as influencing crack propagation. As a re- sult, they will also influence tensile ductility. Cracking of the Si phase represents the (early) formation of new voids which later become a portion of the fracture surface, meaning that control of the size, shape and contiguity of the Si in the HPDC also directly influences mechanical properties.
(a)
(c)
(b)
(d)
Figure 7. Changes to Si particles during solution treatment of the A380 HPDC alloy. Note that blisters form from 20 minutes immersion and onwards. Images a) and b) (after ref. 6 Si present in an area of 122063 μm2
) show the mean Si area and the number of particles of , for edge and centre regions respectively. Image c) presents the corresponding thermal cycle, and d) shows the changes in Si aspect ratio as the samples are solution treated. 54 International Journal of Metalcasting/Fall 2011
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