In contrast with the DTA records obtained from the no- CHG material, those obtained for with-CHG show fea- tures apparently varying with the cooling rate. Considering firstly the records for the slower and higher cooling rates for the with-CHG material, it is seen they appear similar to those for the no-CHG material, though extended over a slightly larger temperature range. Comparing the corre- sponding microstructures illustrated in Figure 8 (column to the left), it is noted that the with-CHG samples present a higher number of nodules than no-CHG samples. For the intermediate cooling rate of 10ºK/min, the DTA record on the with-CHG material also shows two peaks during the eutectic reaction, but in this case of similar amplitude (in contrast with all other records). Looking at the correspond- ing micrograph in Figure 8 (note that the scale is not the same as for the other samples), it is seen that these two peaks may be easily associated with a two-step solidifica- tion of the sample, the outer shell with lamellar (under- cooled) eutectic and the inner part with nodular/vermicular eutectic. As for the other cooling rates, it should be stressed that the nodule count in the central area is much higher for the with-CHG material than for no-CHG.
The reproducibility of the DTA traces recorded at a cool- ing rate of 10ºK/min was checked with the additional trial mentioned previously. For both the with-CHG and no-CHG materials, it was found that the record for the first cooling was similar to that after rapid melting and immediate cool- ing, while the second record after holding was similar to the one of the first series of DTA runs which includes a high temperature holding. This underlines the peculiarity related to the 10ºK/min cooling rate, namely that the difference of the DTA records for with-CHG and no-CHG materials at that particular cooling rate, is reproducible.
(a)
(b)
(c)
Figure 6. Comparison of the DTA traces recorded during the first and second coolings of the additional trial in the case of the no-CHG material. The vertical dashed line represents the calculated temperature of the stable eutectic.
International Journal of Metalcasting/Winter 2012
Figure 7. Comparison of the DTA records obtained upon cooling of with-CHG (solid lines) and no-CHG (dotted lines) materials after very short holding in the liquid state. Cooling rates were (a) 5 K/min, (b) 10 K/min and (c) 15 K/min. The vertical dashed line represents the calculated eutectic temperature.
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