cipitate forms. Dissolution of the Mg individual-clusters will result in the annihilation of vacancies in the primary Al ma- trix and the resultant diffusion of the Mg atoms to join the Si individual-clusters will be more sluggish than the ‘0IP’ and ‘1IP’ curves shown by the lower slope and peak hardness of the ‘8IP’ curve. However, since, no vacancy was anni- hilated in the artificial aging step; the kinetics of precipita- tion shown by ‘8IP’ curve would be greater than the ‘4IP’ as shown by the higher slope and peak hardness of the ‘8IP’ curve.
rate of increase of micro-hardness in segment DF in curve ‘8IP’ of Figure 8. Subsequently, the β (Mg2
After the Formation of Stable β”(Mg5
Point E in Figure 4 Si6) Precipitates
formed in the primary Al matrix; this was followed by artificial aging as represented by the curve ‘No AA’ until point E at 10 hours, and then followed by curve ‘10IP’ for artifi- cial aging - see Figure 8.
Figure 13 shows the schematic of the sequence of precipitation when incubation process was stopped after stable β” (Mg5
Si6 ) precipitates were
phase precipitates and hence, during the initial stages of artificial aging, Mg atoms dissolve back into the SSSS of the Al matrix as observed by the lack of any significant in- crease in micro-hardness – see seg- ment E-E’ in curve ‘10IP’ of Figure 8. This is followed by the diffusion of Mg atoms to the Si-individual- clusters to form β’(Mg1.8
In this treatment, Mg atoms are mostly present in the β’’ (Mg5
Si6 Si) phase
precipitates observed by the in- crease in micro-hardness value in segment E’F of curve ‘10IP’ of Figure 8. Finally, the terminal and stable β (Mg2
Si) phase precipitates
forms. The rate of increase of mi- cro-hardness during artificial ag-
Figure 12. 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 for co-clusters of Si and Mg as shown by point D in Figure 4, and subsequently, artificial aging (AA) was carried out. The sequence of precipitation reaction would be SSSS Individual-clusters of Si and Mg atoms Dissolution of Mg clusters β’ (Mg1.8Si) β (Mg2Si).
International Journal of Metalcasting/Fall 2011 29 )
When incubation was stopped at the beginning of the for- mation of the co-clusters of Mg and Si atoms (GP-zone I), most of the Mg atoms would have dissolved back into the SSSS of the Al matrix during the segment CD in the `No AA` curve of Figure 8. Since, Mg atoms at the beginning of artificial aging (curve ‘8IP’) would be in the SSSS of the Al matrix, the initial stages of artificial aging would be the diffusion of the Mg atoms to the Si self-clusters to form the β’ (Mg1.8
Si) phase precipitates. This is evidenced by the high Si) phase pre-
would be created within the co-clusters during the dissolu- tion of Mg, these vacancies are useful for the subsequent diffusion of Mg in the Al matrix.6
ing shown by the low gradient of curve ‘10IP’ in Figure 8 is due to the lack of vacancies for diffusion of Mg atoms to form the β’ (Mg1.8
Si) precipitates. Although, vacancies
The review of this hypothesis giving the sequence of precipi- tation reaction in the primary Al phase matrix during various combinations of incubation at room temperature and artificial aging at 155C (311F) shows that the location of Mg atoms in the Al matrix at the end of incubation is critical. Assuming quenching rate is constant after solution treatment, if the Mg atoms are in the SSSS of the Al matrix at the end of incuba- tion, the rate increase of hardness (kinetics of the reaction) will be high and the peak hardness value will also be high. This is shown by curves ‘0IP’ and ‘1IP’ of Figure 8. In curve ‘8IP’ of Figure 8, the kinetics of the precipitation reaction and
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