hour, respectively, in Figure 14. The samples in which the du- ration of incubation was carried out for 4 and 10 hours would have Mg in either the individual-clusters or co-clusters at the beginning of artificial aging and this will lead to a low rate of precipitation during artificial aging and result in low micro- hardness values at the end of artificial aging. The sample incubated for 4 and 10 hours ex- hibits a low YS and high %el due to less number of precipitates at the end of artificial aging as shown in Fig- ure 14. The samples for which the dura- tion of incubation was 8 hours in Figure 14, show a relatively high YS and low %el for 1.5 hours duration of artificial aging which can be attributed to the high rate of precipita- tion at the initial stag- es of artificial aging as shown by segment DF of curve ‘8IP’ in Fig- ure 8. Subsequently, the rate of precipitation is slow after point F in curve ‘8IP’ of Figure 8 because of the lack of high vacancy concen- tration to affect a high diffusion rate of Mg at- oms to form the β’ and β precipitates. This is reflected in the relative- ly lower YS and higher %el in Figure
(a) (c)
Tensile samples were larger than the samples (cylindrical shape) used for the micro- hardness measure- ments to understand the sequence of precip- itation. This difference in size results fewer vacancies at the be- ginning of incubation. Hence, tensile samples undergo a slower rate
of precipitation reaction than in the smaller cylindrical samples. Therefore, the exact correlation of tensile properties shown in Figure 14 to the hypothesis of the sequence of precipitation shown by the hardness curve in Figure 8 may not be feasible.
(b)
(d)
(e)
(f)
Figure 14. Tensile properties of A356.2 for various combinations of incubation and artificial aging conditions after solution treatment and quenching in water at 80C (176F). (a) to (f) - Ultimate Tensile Strength (UTS), Yield Strength (YS) and Percent Elongation (%el) for various aging conditions. ‘IP’ and ‘AA’ refer to the expressions incubation process and artificial aging, respectively.
International Journal of Metalcasting/Fall 2011 31
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