From the point of view of thermodynamics, primary carbides should only exist in hypereutectic alloy. But in fact, it can also be observed in the near eutectic alloy 2 #. According to the previous research results, this may be due to the left shift of eutectic point caused by pressure, which makes the carbide nucleate preferentially and obtains the solidification structure with hypereutectic characteristics.
The growth of primary carbides and eutectic carbides are in completely different conditions. Primary carbides grow in lower stress field, most of them are hexagonal polygon, while eutectic carbides grow in clusters in higher stress field. Therefore, the effect of pressure casting on two different carbides needs to be discussed separately. In order to study the effect of pressure on primary carbide and eutectic carbide, it is necessary to distinguish the two carbides in metallographic images. When the casting pressure is higher than 100MPa, the size difference between primary carbide and eutectic carbide is large, so it is easy to identify. The larger polygon is primary phase, and the smaller one is eutectic phase. However, when the pressure is less than 100 MPa, the size difference between the two carbides is not obvious, and it is not easy to distinguish. However, the morphology of eutectic carbides is mostly lath like with large aspect ratio, while primary carbides are polygonal in spite of small size. At the same time, the primary carbides are surrounded by eutectic carbides, and the direction of the lath points to the primary phase.
It can be seen that the morphology of primary carbide has changed obviously by analyzing the solidification structure of alloy 2 under different pressures. Firstly, with the increase of pressure, the number of primary carbides increases from sporadic to obvious. Secondly, the size of primary carbide is counted. As shown in Figure 1, when the pressure is less than 100MPa, the size of primary carbide gradually decreases with the increase of pressure, but the decrease is not large. When the pressure increases to 130 MPa, the size of primary carbide continues to increase for a time. Only when the pressure reaches 160 MPa, the size of primary carbide decreases obviously and the microstructure is refined obviously. The results show that small casting pressure can promote the nucleation of primary carbides, but the nucleation rate is not high. The pressure can promote the growth of carbides and increase their size rather than continue to nucleate. Only when the pressure is high enough, the primary carbide will be refined obviously.
Fig. 2 SEM photos with high magnification are mainly primary carbides. Because primary carbides nucleate and grow preferentially, we usually distinguish primary carbides from eutectic carbides by their size difference. This is because primary carbides grow in liquid phase, there is no competition between austenite and primary carbides, external temperature is higher, alloying elements are more active, and diffusion coefficient is less affected by pressure. Ignoring the size difference of primary carbides under different pressures, the shape difference of carbides can be observed from SEM. The carbides in Figure 2 are refined and fragmented with the increase of pressure. The carbides in Fig. 2 (a) present lath + dendrite morphology, the primary carbides in Fig. 2 (b) become short and thick with strip like protrusion at the edge, and the carbides in Fig. 2 (c) present closed ring. With the pressure up to 160MPa, the primary carbide not only closed completely, but also became a solid polygon. Through comparison, it is found that except for the coarse strip shape of primary carbides under normal pressure, although the closure condition of primary carbides is different, and the size of internal hollow area is also different, the large external edge still presents the outline of polygon, and the boundary of polygon is relatively smooth, This is in sharp contrast to the changing process of eutectic carbide from lath to dendrite and then to equiaxed.