Metallographic Characterization of a Ti-Containing Low-Density Fe-Mn-Al-C Steel in As-Cast Condition
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vs. temperature predicted for Fe-32Mn-7.0Al-2.2C-0.5Ti low-density steel by JMatPro software.
In order to obtain the density value it was used a sample volume of 4.7912 cm3. The value reported here is 6.59 0.03 g/cm3 at room temperature, which is the average of ten independent measurements. The prediction value obtained using JMatPro was 6.7 g/cm3 (Fig. 1). It is worth mentioning that the calculated and experimental values show a very good correlation. The studied LD-Ti steel shows an outstanding reduction of density 16.3% in comparison with conventional steels (7.874 g/cm3) [1]. Microstructure (LOM and SEM-EDS analysis) Fig. 2 shows optical micrographs of the as-cast LD-Ti steel. This microstructure (Fig. 2a) shows large primary dendrites in length and mostly columnar dendritic structures following preferential growth directions. In this case, it is clear that the as-cast low-density steel contains a small percentage of the island-like phase (white-phase) in the ferritic (bcc) matrix (gray-phase), which consists of austenite (fcc), as shown in Fig. 2b. The relative amounts of these two phases are consistent with the prediction of present phases by JMatPro (Fig.3a), which indicate that the major phase is ferrite (59.31%), and the minority phase is austenite (4.87%), as shown in Fig. 3b.
Figure 2. Optical micrographs of the as-cast structure of Fe-32Mn-7.0Al-2.2C-0.5Ti low-density steel. 100
a)
70
90
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50 40
Ferrite Kappa Austenite M(C,N)
59.31%
50
Liquid M(C,N) Austenite Kappa Ferrite
60
Wt% Phase
Wt% Phase
70
b)
30 20 10
40
35.2%
30 20 10
4.87%
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Temperature (°C)
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0.62%
Phase distribution (wt%) at T=20 °C
Figure 3. a) Phase transformation behavior and b) phase fractions of Fe-32Mn-7.0Al-2.2C-0.5Ti low-density steel as a function of temperature predicted by JMatPro.
According to thermodynamic calculations, the liquid phase starts to be solidified as austenite (fcc) at 1346 °C, and gradually decreases with the temperature transforming to a mixture of carbide and ferrite (bcc). The -carbide starts to form at 860 °C, while ferrite phase forms at 500°C. Fig. 4 shows SEM micrographs of the microstructure in the as-cast condition, which reveals that the ferrite (bcc) phase exhibits a special lamellar structure (white-phase); ferrite is clearly the major phase in the microstructure, as shown in Fig. 4a. The chemical composition analysis (SEM-EDS) indicates that in the white-phase the content of Al increases up to 8.6 wt%, which acts as ferrite (bcc) stabilizer (Fig. 4b).
Figure 4. a) SEM image of the microstructure of Fe-32Mn-7.0Al-2.2C-0.5Ti low-density steel in the as-cast condition, and b) Elemental mapping of alloy elements in the system.
Figure 5. a) SEM image of lamellar ferrite (bcc), and b) Line-scan of the alloy elements in the system along the microstructure. In addition to the primary austenite (fcc), precipitation of austenite can be observed surrounding to the lamellar-phase [7], as shown in Fig. 5a. The above suggest
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