After cooling to room temperature, the TA cups were sec- tioned and polished for metallographic examination. The samples were ground using 180, 320, and 600 grit SiC sand paper. Rough polishing was done with 6 µm and 1 µm poly- crystalline diamond compounds. A 0.05 µm colloidal silica compound was employed for the final polishing. Lastly, the samples were etched with Curran’s etch (10g FeCl3 HCl, 120 mL H2
, 30 mL O).
results and discussion 304 results
The undercooling results for the 304 castings are shown in Fig- ure 3. All the powder additions resulted in a reduction in the un- dercooling for initiating solidification indicating that they were effective heterogeneous nuclei. NbO and TiN had the lowest and most consistent reduction in undercooling. The MgO ad- dition samples displayed more variability than the other addi- tions. This might have been a result of some MgO powder float- ing on top of the TA cups. MgO on top of the TA cup surface would have decreased the cooling rate of the sample.
Figures 4 and 5 provide representative images of the micro- structure of the 304 TA cup samples. The matrix was fully austenitic in all the samples. Some carbides were present at the dendritic boundaries.
Measurement of the SDAS was conducted on the samples to determine how the additions affected the final microstructure. SDAS measurements were accomplished by measuring the to- tal distance between several secondary dendrite arms and divid- ing by the number of dendrite arms. These measurements were conducted using an open source image analysis package. The SDAS results are illustrated in Figure 6. There was no differ- ence in SDAS for most of the samples. The only exception was the MgO samples which had a higher average SDAS.
The lack of SDAS decrease is contrary to the observations of other experiments with heterogeneous nuclei additions in mul- tiple alloy systems. It has been reported that SDAS decreases with the addition of a grain refiner.2,3,4, 8,9, 10
However, the un-
dercooling measurements indicate that all of the added powders decreased undercooling, and therefore assisted with heteroge- neous nucleation. The cause of this discrepancy could be re-
Figure 3. Undercooling measurements for the 304 samples.
Figure 4. Micrograph of the 304 sample with no addition.
Figure 5. Micrograph of the 304 sample with a NiAl addition. International Journal of Metalcasting/Winter 2012
Figure 6. SDAS measurements for all of the 304 samples. 31
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