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Only the no powder addition micrographs are presented since there was not a significant difference in structure. Grain size comparison between samples was done through visual inspection.


data for this sample did find a higher yield strength than the baseline 1010 material.


O3


The 1010 0.2% RE silicide sample had no evidence of grain refinement (Figure 12). Considering that this sam- ple had an average yield strength of 177 MPa, the re- sults are consistent with grain size being the dominant strengthening mechanism in the 1010 samples. The lack of refinement (despite this sample having the highest rare earth content) seems inconsistent with the formation of heterogeneous nuclei.


The 1010 samples with 0.1% RE silicide had a smaller over- all grain size. This is consistent with the Hall-Petch grain size strengthening mechanism. A reduction in grain size pro- duces an increase in strength. The grain size reduction could be due to the formation of RE2


type oxides.8,9,10 Tensile


In the 1010 sample with 0.1% MM, the author observed sev- eral regions of large grains and smaller grains. This can be observed in Figure 13, where the center of the image con- tains several small grains and the upper left portion contains a larger grain. The large variation in grain size would indi- cate regions of high and low nuclei density. Regions with a low nuclei density developed larger grains. The cause of this may be related to the low TRE. During the ladle addition of the misch metal, the sample is exposed to oxygen from the atmosphere which can react to form RE oxides. The low TRE content indicates significant oxidation loss occurred. At such a high oxidation loss, it is likely “huge” oxides formed. The “huge” oxides would have a significant buoy- ancy force resulting in flotation. The oxides also could have formed on the melt surface. In either case, they would have become part of the ladle slag and not the melt. Some of the RE content could have also oxidized into small particles that were mixed into the melt and remained there during pouring. These smaller particles may have clumped near each other and produced a region of high nuclei density, which then developed a small grain size region in the casting.


Figure 10. Micrograph of baseline as-cast 1010 sample.


Figure 12. Micrograph of 1010 with 0.2% RS sample.


Figure 11. Micrograph of 1010 0.1% RS sample. International Journal of Metalcasting/Spring 2012


Figure 13. Micrograph of 1010 with 0.1% MM sample. 57


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