diameter of the Si atom at 0.222 nm is less than the vacancy size of 0.236 nm (diameter of the Al atom), whereas the di- ameter of the Mg atom at 0.290 nm43
is larger than the size
of the vacancy created in the matrix; and the binding energy of the Mg atom to the vacancy is far less than that for the Si atom with the vacancy.6
Hence, the Si individual-clusters
form more rapidly than the Mg individual-clusters. The Mg individual-clusters begin to form at ‘Point B’ in Figure 4. Subsequently, since the Mg individual-clusters are far less stable than their Si counterparts, at ‘Point C’ some of the Mg individual-clusters begin to dissociate and these Mg atoms rendered back into the SSSS of the Al phase matrix. The dissolution occurs between points C and D in Figure 4. The Mg atoms in the SSSS of the matrix subsequently diffuse to the Si individual-clusters and forms co-clusters (GP zone I) and this stage begins at ‘Point D’ in Figure 4. These co- clusters of Si and Mg atoms grow and stabilizes to form the β”-Mg5
a monoclinic crystal structure oriented along the <100> di- rection in the Al matrix. The micro-hardness value remains constant after the formation of β” precipitates, thus, indicat-
Si6 phase precipitate, which are needle shaped with
ing the end of the precipitation reaction sequence during this process. This proposed hypothesis only identifies the critical stages in the precipitation sequence during incubation and do not present a complete and comprehensive understanding of the various possible meta-stable cluster/phase formations during this reaction. The critical stages during this precipita- tion reaction are shown in Figure 5.
Figure 6 (a), (b) and (c) show the typical micro-hard- ness measurements during the incubation of the samples quenched in water at 80C (176F), water at room tempera- ture (23C [73F]) and mixture of anti-freeze with dry ice at -40C (-40F), respectively. Further, Table 3 presents the significant observations from the results shown in Figure 6 (a), (b) and (c). The upper and lower limits of 95 % confidence interval for the data in Figure 6 (a) is shown in Figure 6 (d) where it is observed that the micro-hardness value obtained during the incubation process is quite reli- able. The 95 % confidence intervals for the micro-hard- ness data in Figure 6 (b) and (c) were similar to Figure 6 (d). Further, the critical stages in the hypothesis presented
(a)
(b)
(c)
(d)
Figure 6. Typical micro-hardness data during incubation at room temperature in samples quenched in (a) water at 80C (176F), (b) water at room temperature (23C [73F]), (c) anti-freeze and dry ice at -40C (-40F), respectively and (d) Data shown in (a) along with the 95% confidence interval of the average data.
24 International Journal of Metalcasting/Fall 2011
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