Time-resolved and <i>In-situ</i> Observation of <i>δ</i>–<i>γ</i> Transformation during Unidirectional Solidification in Fe–C Alloys

Abstract

Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that a massive-like transformation, in which the γ phase was produced through the solid-solid transformation and partitioning of substitute elements such as Mn and Si at the δ-γ interface was negligible, was selected in the unidirectional solidification of 0.3 mass% C steel at a pulling rate of 50 μm/s. The massive-like transformation produced fine γ grains near the front of the δ-γ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δ-γ front and the advancing front of coarse γ grains was as short as 200 μm. Namely, the fine γ grains disappeared within 10 s owing to growth of coarse γ grains. In addition, the observation of the δ-γ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like γ growth occurred at a growth velocity of 5 μm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.

Transformation from Ferrite to Austenite during/after Solidification in Peritectic Steel Systems: an X-ray Imaging Study

“…A selection of the massive-like transformation was confirmed in 1-mm-thick bulk specimens. 62) Undercooling for nucleation of the γ phase was smaller compared with undercooling in 100-μm-thick specimens. Thus, the specimen dimension influences undercooling for γ-phase nucleation, but a massive-like transformation was still selected and undercooling was required for nucleation of the γ phase even in the bulk specimens.…”

“…In the early stage, solidification in Sn, Al and Mg alloys was observed in-situ by X-ray transmission imaging (twodimensional imaging). [29][30][31][32][33] Later, the further-developed apparatus allowed for the observation of Cu, Ni, and Fe alloy solidification above 1 300 K. 21,[34][35][36][37][38] Transmission imaging was combined with X-ray diffraction (XRD) measurements. Figure 1(a) shows a typical setup for transmission imaging with XRD.…”

“…Figure 6 shows radiography snapshots during the unidirectional solidification of 0.3C steel (0.3 C, 0.6 Mn and 0.3 Si in mass%) at a pulling rate of 50 μm/s. 34,38,63) The γ phase was intended to exist behind the liquid and δ phases before pulling down (out of the observation area). The vertical axis in the specimen was defined to track the interfaces, as shown in Fig.…”

Time-resolved and <i>In-situ</i> Observation of Solidification in TiAl Alloys

“…20) In addition, the massive-like transformation can be selected from the surface to the center in the continuous casting of Fe-based alloys. 21) Therefore, mechanism of the massive-like transformation is an interesting phenomenon. The isotopic transformation from α phase to γ phase in Ti-based alloys and the isotopic transformation from δ phase to γ phase in Fe-based alloys are essentially the same cannot be concluded from the present observations.…”

“…It started with the observation of low-melting-point alloys such as Al, Sn, and Zn alloys, [8][9][10][11][12] and now it is possible to observe a wide range of alloys with high melting points such as Fe-based alloys. [13][14][15][16][17][18][19][20][21] For example, it is found that the peritectic solidification, which has been considered to be the conventional is not selected in Fe-C alloys, 16,17) which increases the significance of insitu observation of solidification from both academic and industrial perspectives.…”