the peak hardness value in artificial aging are high but not as high as in the curves ‘0IP’ and ‘1IP’ due to the lower vacancy concentration. In curves, ‘4IP’ and ‘10IP’, both the kinet- ics of reaction and peak hardness values are the lowest due to the annihilation of vacancies during the initial stages of artificial aging caused by the dissolution of Mg atoms from clusters/precipitates to the SSSS of the Al matrix. In Figure 8, lower peak hardness values indicate lower amounts and distribution of the final β (Mg2
Si) phase precipitates. Uni-Axial Tensile Properties
Fracture at the end of the test was nearly at the middle of the gauge section in all the samples showing the integrity of the casting process and reliability of the ten- sile test data.
Ultimate Tensile Strength (UTS), Yield Strength (YS) and percent elongation (%el), respectively, for artificial aging of 1.5 hours after various duration of in- cubation are given in Figure 14 (a), (c) and (e). Figure 14 (b), (d) and (f) present the UTS, YS and %el, respectively, for 5, 8 and 10 hours of artificial aging after various durations of incubation. Table 5 presents the data from the tensile testing as shown in Figure 14.
A quick review of Figure 14 reveals that there is a significant difference in the YS and %el for different durations of incu- bation and the same duration of artificial aging; this suggests that the effect of in- cubation is pronounced on the resultant mechanical properties. Further, the varia- tion in UTS for the different durations of incubation is not significant and this sug- gests that the sequence of precipitation reaction during the aging treatment did not affect the UTS significantly. Seye- drezai37
also observed that the changes
in the sequence of precipitation reaction during aging did not have a pronounced effect on the resultant UTS of the Al-Si- Mg wrought alloy samples. Hence, our discussion on the effect of the precipi- tation reaction on the tensile properties would be limited to the consideration of only the YS and %el and not the UTS.
In Figure 14, for all the duration of in- cubation, YS increased with increasing duration of artificial aging as observed by the continuous increase in the micro- hardness, Figure 8. As explained in the
30
previous section, the location of Mg in the primary Al matrix at the end of the incubation treatment plays a critical role on the sequence of precipitation reaction during artificial aging. The rate of precipitation reaction during artificial aging was high resulting in a peak micro-hardness value for samples in which Mg is mostly in the SSSS of the primary Al matrix at the end of the incubation (IPà0 and 1 h).This is explained by the large number of precipitates at the end of artificial ag- ing. This resulted in high YS and low %el as reflected by the samples in which the incubation was carried out for 0 and 1
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 Formation of β’’ (Mg5
Si6 of some β” β’ (Mg1.8Si) β (Mg2Si).
Figure 13. Schematic of a typical sequence for the precipitation reaction in the primary Al phase when the incubation (IP) was stopped after the formation of stable β”(Mg5
Si6) Dissolution
) precipitates as shown by Point E in Figure
International Journal of Metalcasting/Fall 2011
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