of increase of micro-hardness in segment BF in the ‘1IP’ curve of Figure 8, and subsequently, the stable β (Mg2
Si) phase forms.
Beginning of Dissociation of Mg Individual-Clusters Point C in Figure 4 Figure 11 is the schematic of a typical sequence of precipitation reaction when incubation was stopped at the beginning of the dissociation of a few Mg individual-clusters, and subsequently, artificial aging was carried out as represented by curve ‘No AA’ until point C, then followed by curve ‘4IP’- see Figure 8.
phase shown by the segment C’F in the ‘4IP’ curve of Figure 8. Subsequently, the β’ (Mg1.8
cantly reduced. Although, additional vacancies would be created by the dissolution of the Mg from the individual-clusters, these new vacancies would be trapped in the self-clusters6
artificial aging, the initial dissolution of the Mg individual-clusters took place by utilizing the va- cancies present in the Al matrix and hence, the amount of available vacancies for the diffusion of the Mg atoms in the SSSS to the Si self-clusters to form the β’ (Mg1.8
phase forms the stable β (Mg2 Si) precipitates are signifi- and would
not be homogenously distributed in the matrix to aid diffusion of Mg to the Si individual-clusters. The amount of un-trapped vacancy for Mg dif- fusion to Si individual-clusters is critical to the kinetics of the precipitation reaction during artifi- cial aging.6
Thus, the kinetics of the precipitation
reaction for the ‘4IP’ curve in Figure 8 is low and the peak hardness reached by this curve is low when compared to the ‘0IP’ and ‘1IP’ curves in Figure 8; this is due to the lower vacancy concen- tration in the primary Al matrix in the ‘4IP’ curve during artificial aging.
Beginning of Formation of Co-Clusters of Si And Mg Atoms Point D in Figure 4 Figure 12 shows a schematic of a typical sequence of precipitation reaction when the duration of incu- bation was stopped at the beginning of the forma- tion of the co-clusters of Si and Mg atoms (GP-zone I) prior to the artificial aging at 155C (311F); this is represented by curve ‘0AA’ until point D followed by curve ‘8IP’ in Figure 8.
28
In this treatment, the Mg atoms at the end of in- cubation form self-clusters; during artificial ag- ing, Mg atoms dissolve back into the SSSS of the primary Al matrix. This is shown by the lack of any significant change in micro-hardness values in the segment C-C’ – see the ‘4IP’ curve of Fig- ure 8. Subsequently, the Mg atoms will diffuse to the Si individual-clusters to form the β’ (Mg1.8
Si) Si)
Si) phase. During
Figure 10. Schematic of a typical sequence for the precipitation reaction in the primary Al phase when the incubation (IP) was stopped at the beginning of the formation of Mg individual-clusters as shown by Point B in Figure 4, and subsequently, artificial aging (AA) was carried out. The sequence of precipitation would be SSSS Individual-clusters of Si atoms β’ (Mg1.8
Si) β (Mg2 Si).
Figure 11. Schematic of a typical sequence for the precipitation reaction in the primary Al phase when the incubation(IP) was stopped at the beginning of the dissociation of few Mg individual- clusters at point C in Figure 7, and subsequently, artificial aging (AA) was carried out. The sequence of precipitation would be SSSS Individual-clusters of Si atoms and a few Individual-clusters of Mg atoms Dissolution of Mg clusters β’ (Mg1.8Si) β (Mg2Si).
International Journal of Metalcasting/Fall 2011
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