where, (hkl)s = a low-index plane of the substrate, [uvw]s = a low-index direction in (hkl)s, (hkl)n = a low-index plane in the nucleated solid, [uvw]n = a low-index direction in (hkl) n,
n = the interatomic spacing along [uvw]n direction, d[uvw]s
= disregistry between (hkl)s and (hkl)n planes, d[uvw] =
the interatomic spacing along [uvw]s direction, and θ = the angle between the [uvw]s and [uvw]n directions.
Table 2 lists the disregistries between possible nucleating materials and austenite. While TiN and TiC have the lowest disregistries, they are not as low as is generally agreed on from other alloy systems for nucleation.1-6
However, analyz-
ing the disregistry between TiN and TiC and δ-ferrite re- veals a much closer correlation (See Table 3). These results are in agreement with Bramfi tt’s earlier work.5
The lattice parameters of TiN and TiC are larger than δ-ferrite’s, but the calculated disregistry is low. The disreg- istry between TiN and TiC with δ-ferrite are calculated based on the distance between the titanium atoms in the unit cell and not the entire length of the unit cell (See Figure 15). While only TiN inclusions were observed, the lattice disregistry analysis does point to the possibility of TiC inclusions acting as nuclei. This was observed in Wallace’s work.6,7,8
It is pos-
sible that TiC inclusions were not detected because of the car- bon coating placed on the sample for the SEM investigation.
Using the steel chemistry for Samples 2 and 3, thermodynam- ic calculations were conducted using ThermoCalc©
to predict
the phases formed during solidifi cation. δ-ferrite is predicted to begin forming at 1494°C while austenite would not begin
Table 2. Lattice Parameters and Calculated Disregistries with Austenite17
Table 3. Disregistries Between Selected Phases and δ-ferrite and Their Crystallographic Information17
3.060
Figure 15. Crystal structure of TiC depicting the measurement of space between Ti atoms (Ti -light grey and C –dark grey).
International Journal of Metalcasting/Summer 10
Figure 16. Schematic method for determining undercooling from a cooling curve in steel.
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