Further detail was sought by examining oxide surfaces in greater detail, and an image of the defect in Figure 12 is presented at higher magnification in Figure 13. In particu- lar, Figure 13b shows that there were ductile regions dis- persed on the surface of the defect, and dimple rupture was observed. These dimples formed during deformation, and indicate that some ductility was present in the immediate vicinity of the interface, (albeit far less than the general microstructure). This indicates that some microstructural yielding must have occurred immediately adjacent to the oxide flake defect prior to cracking, but this was limited. It would however be reasonable to assume that stress locali- sation leading to cracking and failure was very likely to be initiated from this feature. One additional consequence of this observation is that stress localisation at the edge or tip of the flake defect, would not actually happen until after decohesion of the metal-oxide interface occurred, opening the defect into a void.
The appearance of ductile zones at the oxide interface was found to be common where planar oxide films were pres- ent on fracture surfaces. (It should also be noted that there were instances where ductile zones were not observed). It has been suggested that oxides in aluminium castings have a “wetting side”, corresponding to that which was in contact with the liquid metal, and a “non wetting side”, which corresponds to that side which was previously ex- posed to the atmosphere.e.g.12
However, the appearance of
the defect presented in Figure 13 suggests that the actual situation could be more complex. The presence of these do- mains suggests either the liquid aluminium partially wets the oxide in these regions, bonding to it, or alternately, the oxide was discontinuous and some metallurgical bonding was present across the interface. Possible explanations are 1) the oxide flake is bent, folded or cracked during cast- ing and turbulent metal flow, allowing metal to penetrate across the interface; 2) partial wetting is facilitated by oth-
reaction with Mg present in the melt. Combinations of the abovementioned effects, and others, are also plausible. It is particularly important to note that these thin, oxide films or flakes are assumed to be extremely friable and easily ruptured. It is also quite possible some may originate from cold flakes or externally solidified particles, present in the molten metal. Further work to accurately assess the true origins of these features is underway.
er means, such as by pressure exerted by the molten metal on the oxide, or by partial reduction of the (Al2
O3
For the samples produced in the solution treated condition, the two lowest values (again present at below q=0.2) also ex- hibited planar oxide defects present on the fracture surface. These samples were able to still achieve reasonable levels of tensile elongation. In these cases, it is possible that the ma- terial is capable of withstanding a larger defect size before critical fracture begins; that is, the surrounding aluminium matrix deforms more prior to the onset of cracking. This is an important observation because it suggests that aluminium HPDC alloys could be developed that may be capable of withstanding, or minimizing, the effects of the planar oxide film or flake defects generated during casting.
For the T6 treated material, none of the samples dis- played visible planar oxide defects present on the fracture surface. Although oxides are reasonably assumed to be present in the material in the same quantities as all other samples, those present may not have been large enough to noticeably influence tensile properties in the same man- ner as in the solution treated and T4 treated samples. This also helps to explain the results showing that the equiva- lent defect fractions for the T6 material (Figure 10) were reduced and Weibull modulus increased, compared to the other two heat treated conditions. As mentioned in the earlier section, microstructural yielding and failure as- sociated with the Si particles in the plastic zone of the
) oxide by
(a)
(b)
Figure 13. The defect presented in Figure 12, at higher magnification (secondary electron mode). a) shows the region examined, outlined in white, and b) shows the highlighted region at higher magnification. Ductile zones at the oxide-metal interface are widely distributed, and dimples are observed. See text for details.
International Journal of Metalcasting/Fall 2011 59
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