Carbide transformation mechanism of chromium white cast iron under Solidification pressure

Different from equilibrium solidification, the solidification process of alloy melt under pressure is controlled by nucleation and subsequent crystal growth. With the change of solidification conditions, the morphology and phase composition of solidification structure will also change, the fundamental reason is the change of phase stability. The existence and transformation of stable phase and metastable phase will affect the final phase composition under pressure. Previous studies have shown that pressure can reduce the critical nucleation work and the melting entropy of the alloy, which plays an important role in the formation of the phase.

The M7C3 carbide in the solidification structure of chromium white cast iron is a stable carbide formed by replacing the Fe atom in the unstable compound with Cr atom. The transformation between carbides is determined by C element, so the type of carbides is mainly determined by the nucleation stage. The diffusion of Cr atoms will not fundamentally change the type of carbides. Therefore, the formation mechanism of carbide phase can be revealed by stability calculation of fe7c3, fe23c6 and Fe3C carbides.

In recent years, first principles calculation has been paid more and more attention by researchers all over the world. It can predict the microstructure and properties of materials at atomic scale, such as electronic, mechanical and magnetic properties. Carbides in metal materials have attracted much attention due to their excellent physical and chemical properties. In addition, there are many research results on the stability of carbide and the interfacial properties between carbide and matrix. The stability of carbides in chromium white cast iron during cooling is studied by first principles. The volume characteristics of carbides can be analyzed and used to calculate the electronic structure and interface energy of the interface, which provides a theoretical basis for the evolution of carbides during solidification.

Cell structure of different types carbides: (a) Fe3C; (b) Fe7C3; (c) Fe23C6

First principles calculations based on density functional theory are carried out on the VASP software package (Vienna ab initio simulation package). The PBE functional of generalized gradient approximation (GGA) is used to deal with the cross correlation energy. The truncation energy is 420 ev. The projection augmented wave (PAW) method is used to describe the ion potential and valence electron relationship. The MP (monkhorst pack) method with gamma point as the center is used for K sampling in Brillouin zone, and the mesh is divided into 5 parts × five × The convergence criteria of ion relaxation force and electron self consistent iteration are 0.02 EV / Å and 10-4 EV, respectively. As shown in the figure, the Fe3C and fe7c3 models are made up of 1:1 with 10 atoms (6 iron atoms and 4 carbon atoms) and 40 atoms (28 iron atoms and 12 carbon atoms), respectively × It is composed of hexagonal and orthorhombic protocells. The lattice constants are 4.767 Å (Fe3C hexagonal) and 4.537 Å (fe7c3 orthorhombic), respectively. In the process of structure optimization, all atoms are completely relaxed and spin polarization is considered.

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