for the ZE41A castings. In addition, the AZ91D castings from all foundries had grain structures that contained both small spherical grains and larger irregu- lar grains. Both these phenomena are likely due to the relatively weaker and less abundant Mn–Al refining particles in AZ91D as compared to often excess zirconium added to ZE41A.
Fractography Figure 3 shows optical macrographs
of the fractured tensile samples of ZE41A cast by Foundries A and C with corresponding SEM micrographs of the observed inclusions (indicated by the arrows). Inclusions are known to act as stress risers, and their presence on a fracture surface can indicate their role in fracture initiation. Figure 3a shows an inclusion that likely initiated failure in a sample collected at the start of the experimental trials. Analysis of the inclusion using EDX indicated it was rich in magnesium, zinc, and oxygen. Te inclusion is likely a Mg–O-based inclu- sion with zinc contributions from the alloy matrix. Te lack of any iron in the analysis eliminates the possibility of the inclusion being an iron-based interme- tallic. Te inclusion in Figure 3a also has a fold or crack defect at its interface with the magnesium matrix. Similar results were observed with the samples from Foundry C, as shown in Figure 3b where a Mg–O-based inclusion (indicated by the arrow) was observed with poor interfaces with the magnesium matrix. Te Mg–O inclusions appeared mainly as films sitting atop the fracture surfaces. Tese Mg–O films accumulate during the production run, becoming entrapped in the molten metal during sampling, pouring, and holding. Tis defect indi- cates that the observed Mg–O inclusion was weakly bonded to the magnesium matrix, making it a likely source of failure during tensile loading. Figure 4 shows optical macrographs
of the fractured tensile samples of AZ91D cast by Foundry A and B, that was collected at the start of the experi- mental trials. Te corresponding SEM micrographs are also shown in Figure 4. Figure 4a shows an inclusion-free frac- ture surface. Te corresponding SEM image of the fracture surface depicts dimple-like features and confirms the absence of inclusions on the surface. Tese dimples usually indicate good
casting ductility. Samples from Foundry B were similar to those from Foundry A with no inclusions evident on the fracture surface, and their microstruc- ture does not contain any noticeable cleavage planes. Samples from Foundry D from the start of the experimental trials were also free of inclusions.
Inclusion Assessment Some of the fracture bars from
Foundries A and B were virtually inclu- sion free, while the maximum inclusion areas were under 2%. If tensile samples were prepared, Foundry B likely would produce castings with mechanical properties very similar to those of Foundry A. On the other hand, the castings from Foundry C contained the highest median inclusion area and had a maximum inclusion area of about 9%. Tis can be attributed to the fact that
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March 2017 MODERN CASTING | 49
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