Sequence of Precipitation Reaction During Artificial Aging at 155C (311F)
The transformations in the precipitation reaction during the artificial aging at high temperature depended on the extent of incubation at room temperature carried out prior to increasing the aging temperature. Artificial aging could be initiated at any time during the incubation process at room temperature between points A to E in Figure 4. In addition to the hypoth- esis for the precipitation reactions proposed during the in- cubation treatment, there are two additional precipitates that formed during the artificial aging process at 155C (311F): β’ - Mg1.8
Si precipitates à β - Mg2 Si stable precipitates.
Figure 8 shows the micro-hardness data obtained during ar- tificial aging carried out after various durations (0, 1, 4, 8 and 10 hours) of incubation as show by points A to E in Fig- ure 6 (a) for samples quenched in water maintained at 80C (176F). The ‘No AA’ curve in Figure 8 is identical to the curve shown in Figure 6(a). In Figure 8, suppose the incuba- tion treatment was stopped after 8 hours at room temperature and the sample was immediately subjected to artificial aging at 155C, then the micro-hardness data would have followed the ‘No AA’ curve until point D at 8 hours time and then continued to follow the ‘8IP’ curve. Similar, interpretations of the curves in Figure 8 would be applicable for the incuba- tion durations of 0, 1, 4 and 10 hours at room temperature prior to the artificial aging treatment at 155C (311F).
The following sub-sections present the hypothesis for the se- quence of precipitation in artificial aging shown in Figure 8, after various durations of incubation stopped at points A, B, C, D or E as shown in Figure 4.
BF in the ‘0IP’ curve of Figure 8, and subsequently the stable β (Mg2
stable phase precipitate.
Beginning of Formation of Mg Individual-Clusters Point B in Figure 4 Figure 10 is the schematic of a typical sequence of precipitation reaction when the incubation was stopped at the beginning of the formation of the Mg individual-clusters as shown by point B in Figure 4;
International Journal of Metalcasting/Fall 2011
Figure 9. Schematic of a typical sequence for the precipitation reaction in the primary Al phase with no incubation (IP) and artificial aging (AA) was carried out immediately after quenching the alloy subsequent to solution heat treatment. The sequence of reaction is SSSS Individual-clusters of Si atoms β’ (Mg1.8
Si) β (Mg2 Si). 27
No Incubation Point A in Figure 4 Figure 9 is a schematic of the precipitation reaction sequence when artificial aging was carried out im- mediately after quenching subsequent to solution heat treatment. No incubation was carried out in this case as represented by the curve ‘0IP’ in Figure 8.When there was no incubation, individual clusters of Si at- oms formed during the initial stages of artificial aging as evidenced by the high rate of increase of hardness in segment AB in the ‘0IP’ curve of Figure 8. Subse- quently, the Mg atoms primarily in the SSSS of the Al matrix would diffuse to these Si individual-clusters and form the β’ (Mg1.8
Si) phase shown by the segment Si) phase would form as the final
and subsequently, artificial aging was carried out as repre- sented by curve ‘No AA’ until point B followed by curve ‘1IP’ in Figure 8.
When incubation was terminated quickly such that only Si individual-clusters and very few Mg individual-clusters formed, then the majority of the Mg atoms would be in the SSSS of the primary Al matrix, and would diffuse during the initial stages of the artificial aging to Si individual-clusters to form the β’ (Mg1.8
Si) phase. This is shown by the high rate
Figure 8. Typical micro-hardness data during artificial aging at 155C (311F) following various durations of incubation at room temperature for samples quenched in water at 80C (176F) after solution heat treatment. The notation “IP” and “AA” in the graph stand for “Incubation process” and “Artificial Aging”, respectively. The curve “0AA” is identical to the curve in Figure 6 (a).
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