Growth mechanism of Tin crystal in gray cast iron

Many gray particles can be observed in the SEM picture of Fig. 1 (a). Therefore, the cross-sectional component surface scanning test is carried out. The results are shown in Fig. 1 (b). From this picture, it can be observed that the distribution of Ti, N, Mn and S elements will overlap. According to the binding ability between the elements, we judge that there are a certain amount of tin and MNS and the particles growing together on this section.

The purpose of adding a certain amount of Mn to gray cast iron is to control the content of S element. The binding energy between Mn and S is high, which is very easy to form MNS particles and has the effect of removing s. As shown in Figure 2, MNS starts nucleation and crystallization when the molten iron is cooled to 1570 ° C.

The formation of MNS particles not only controls the s content in cast iron alloys, but also provides heterogeneous cores for the nucleation of graphite and some second phases. Fig. 3 shows the micro morphology and element composition distribution of tin with MNS as the core in gray cast iron. Obviously, during the crystallization of cast iron melt, the heterogeneous nucleation process of tin with MNS as the core occurred. According to the research, there are a lot of oxides and sulfides in the cast iron liquid, and the number of particles in 1cm3 is more than 40-50 million, which provides very favorable conditions for the non spontaneous nucleation of tin. Since MNS is also a NaCl type crystal structure, and its lattice constant is 4.02a, the mismatch degree with tin lattice is slightly greater than 0.05 (the upper limit of lattice mismatch degree for the two phases to form a coherent interface). When the temperature is 1570 ° C (1843k), MNS particles begin to crystallize in molten iron, which is higher than the temperature at which tin begins to crystallize. Therefore, the non spontaneous nucleation of tin is often based on MNS particles, which conforms to the principle of minimum energy. Therefore, MNS particles in molten iron play a catalytic role in promoting the non spontaneous nucleation of tin.

It is found that the shape of tin particles in gray cast iron is mostly regular quadrilateral. According to the Bravais rule in crystal growth, the closely aligned surface (the smallest surface energy) – the surface network with the largest density will have a major impact on the growth and shape of crystals. Tin crystal is face centered cubic structure. According to its growth mechanism, when equilibrium crystallization is carried out, the shape of face centered cubic crystal is composed of {100} and {111}. If it is surrounded by the crystal plane family, its model diagram is shown in Fig.4. At the same time, combined with Curie Wulf principle, since the crystallization or crystallization process of tin in gray cast iron takes place under non-equilibrium conditions, the morphology of tin will not grow completely according to the equilibrium state, but the growth shape of tin is still determined by the crystal plane with the lowest specific surface energy (generally {111} crystal plane family), Therefore, the shape of tin should be a regular polyhedron, so the shape of tin section is mostly a regular quadrilateral.

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