AZ91E + Al-5Ti-1B In the study by Liu et al.,12
with the addition of a Mg-TiB2 ed to TiB2
the grain refinement of AZ91D grain refiner was attribut-
at the center of AZ91E grains as shown in Figure 8. The sample in Figure 8 was etched prior to SEM analysis to improve image contrast. The etched surface made the fine TiB2
grains. In the current study, TiB2
particles easier to detect. The apparent “cracks” are a result of varying aluminum concentration in the sam- ple. More detailed explanation about the formation of the “crack like” features of the etched surface can be found in the study by Maltais et al.22
can be an effective nucleant for Mg grains. The results indicated that TiB2
particles providing nucleating sites for the Mg particles were also found
ticles restricting Mg grain growth rather than acting as a site for nucleation. The TiB2
Wang et al.23 by TiB2
tic. To test this hypothesis, a high concentration sample was prepared containing 5 wt.% Al-5Ti-1B. The sample was poured using the same experimental parameters (Ta- ble 4) with no holding time. As shown in Figure 9, large TiAl3
from the liquid front and pushed into the grain boundary regions and become entrapped within the Mg17
agree with Wang et al.23 the Mg17
and fine TiB2 Al12
particles would be rejected Al12
eutec-
noted that the grain refinement of Mg alloys particles could possibly be attributed to TiB2
par-
particles were found entrapped within eutectic at the grain boundaries. The results Therefore, the addition of Al-
5Ti-1B grain refiner can also reduce the AZ91E Mg alloy grain size by grain growth restriction.
(b) (a)
Figure 8. SEM image of AZ91E + 1.0 wt.% Al-5Ti-1B showing TiB2 TiB2
nucleating particle and (b) EDX analysis of TiB2 particle. nucleating particle (a)
(b) (a)
Figure 9. SEM Image of AZ91E + 5.0 wt.% Al-5Ti-1B (a) TiAl3 and TiB2 eutectic, (b) EDX analysis of TiAl3
and (c) EDX analysis of TiB2 International Journal of Metalcasting/Spring 11 . particles entrapped within Mg17 Al12
(c)
35
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