2) Dissolution of the cu Bearing Phases
Figure 8 shows backscattered electron (BSE) images of the as-cast and (T4) heat treated conditions for the A380 alloy. The discrete white phases (those with high atomic contrast) present in the as-cast microstructure were one of three types of hard intermetallic particle present in the solidified eutec- tic. Two of these were Fe-bearing phases, either the poly- hedral α-Al15 β-Al5
Zn-rich aluminium may have been present in the solidified eutectic and would also appear bright in a BSE image, al- though it is expected that the amount out of solution would be low since the maximum solubility of Zn in Al is very high (i.e. ~83 wt% at 382C [720F]).
solution treatment. The third of these hard particles was re- sidual θ-Al2
FeSi. Neither of these was influenced directly by the Cu, formed during eutectic solidification. Some
(Fe,Mn)3 Si2
What was especially noticeable from the SEM images in Fig- ure 8 was the reduction in the high atomic contrast (white) phases present in the microstructure following solution treat- ment. During solution treatment, the Cu from the θ-Al2
, or the needle/ platelet shaped
ways is a semi-continuous, brittle skeleton. As may be ap- preciated, decreasing the contiguity of the solidified eutectic through solution treatment causes more of the brittle skeleton to be eliminated or dispersed. The Si present fragments and spheroidizes during solution treatment, decreasing contiguity (e.g. Figure 6). The dissolution of the eutectic θ-Al2
Cu simul-
taneously decreases the fraction of the brittle intermetallic phases (Figure 8). As a result, the contiguity of all of the hard, brittle phases present in the solidified eutectic was reduced, and the relative microstructural quality improved.
3) Homogenisation of the Aluminium grains Cu
present in the solidified eutectic was dissolved into solution within the aluminium grains, leaving only the Fe-bearing in- termetallic phases. These Fe-bearing intermetallics in them- selves must still constitute a portion of the defect structure and many will appear on the fracture surface,1
but what is most
important was that the total fraction of hard, brittle intermetal- lic particles present within the solidified eutectic was reduced. As may be seen from a comparison of Figure 8a with 8b, or from a comparison of Figure 8c with 8d, the contiguity of the brittle intermetallics (white phases) substantially decreased, a feature that would also be expected to improve quality.
In regards of the above discussion, it is interesting to consider further the contiguity of the solidified eutectic, which in many
When solution treatment takes place, not only are the prior eutectic Cu-bearing phases dissolved into solution, hetero- geneous precipitation that has already formed within the structure during cooling after solidification, either within the aluminium grains or along grain boundaries (dispersoids), are also dissolved into the aluminium solid solution. Solute con- centration profiles within the aluminium grains may also be expected to be variable in the as-cast condition, such as would result from coring, for example. Solution treatment should therefore homogenise all of the (free) soluble elements (Cu, Zn, Si, Mg). Precipitation that then follows by heat treatment is relatively homogeneous (e.g. Figure 2). In the as-cast con- dition, heterogeneous precipitation is common, and examples are shown in Figure 9. As a result, deformation may also be more heterogeneous in the as-cast conditions.
4) Elimination of residual stresses
Stresses present in the casting arising from solidification, are expected to be eliminated by solution treatment. Such residual stresses are not expected to be significant in the cur- rent samples however, due to their relatively small size (i.e. gage diameter 5.55±0.1 mm [0.218±0.004 in.]). Quench-
(a)
(b)
(c)
(d)
Figure 8. Backscattered SEM images showing edge (a & b) and centre regions (c & d) either as-cast or heat treated. Images a) and c) show the as-cast condition, b) and d) show the heat treated T4 condition. Solution treatment for b) and d) was 15 minutes at 490C (914F) prior to water quenching. During solution treatment, the Cu-bearing phases (mostly θ-Al2
FeSi). International Journal of Metalcasting/Fall 2011
Cu) present in the solidified eutectic are dissolved, leaving only the Fe bearing phases within the microstructure (polyhedral α-Al15(Fe,Mn)3Si2 and the needle-like or platelet β- Al5
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