First-principles study on ferrite/TiC heterogeneous nucleation interface
Graphical abstract
Highlights
► Interface stability of ferrite (1 0 0)/TiC (1 0 0) was studied. ► The effectiveness of TiC as the heterogeneous nuclei of ferrite was analyzed. ► Ti-termination and C-termination are the two binding modes for ferrite/TiC interface. ► Interfacial energy of the Ti-termination is larger than that of the C-termination. ► On C-termination, ability of TiC promotes ferrite heterogeneous nucleation is strong.
Introduction
With high yield strength, excellent intergranular corrosion resistance and low crack tendency, ferrite steel has been widely used in the modern manufacturing industry [1], [2], [3]. After being in service for a period of time, the workpieces manufactured by ferrite steel usually failed because of excessive wear and corrosion [4]. The shape and size of the failed workpieces can be restored by means of remanufactured technologies, in which, the surface welding (surfacing) is one of the best effective methods [5], [6], [7], [8].
During the surfacing process, because the molten pool is cooled slowly, the ferrite in surfacing metal tend to grow into columnar crystal, which results in grain coarsen. Lee [9] showed that, if effective heterogeneous nuclei exist in the molten pool before the solidification, a large number of small ferrite equiaxed grains were formed in molten pool, which can replace the inherent columnar crystal and the center segregation can be reduced largely. Wang et al. [10], [11], [12] found that, by adding an appropriate amount of alloy element Ti in the surfacing metal, TiC grain which can be the effective heterogeneous nucleation of ferrite was formed obviously, thereby ferrite was refined. Cantor and Kim [13], [14] indicated that, in essentially, heterogeneous nucleation is a process, in which, nucleation phase atoms continuous growing up by adsorbed on the surface of the nucleation core.
First-principles calculation is an important microscopic research method, which is developed in recent years and has been widely used in modern science. Han et al. [15], [16] calculated the microscopic properties of TiB2 particle and analyzed the mechanism of TiB2 as the heterogeneous nuclei of the phase in Al-alloy by first-principles. Wang et al. [17], [18] researched the preparation of the Fe–Ti–N master alloy using first-principles, in which the ferrite can be refined by TiN. Liu et al. [19] studied the best geometric structure, thermodynamic property and electronic structure of NiAl (1 1 0)/Cr (1 1 0) interface based on a first-principles density functional plane-wave ultrasoft pseudopotential method, besides identified the interface stable structure thermodynamically and analyzed bonding characteristics among the interface atoms. Arya and Cater [20] analyzed the process of forming film between TiC and ferrite by first-principles, and showed that with increasing their packing densities, the stabilities of both TiC and ferrite surfaces are (1 1 0) < (1 1 1) < (1 0 0) for TiC and (1 1 1) < (1 0 0) < (1 1 0) for ferrite. However, the research on TiC as the heterogeneous nucleus of ferrite by first-principles has not been reported.
Therefore, the electronic structure combined power and interfacial energy between TiC (1 0 0) surface and ferrite (1 0 0) surface were calculated within atomic and electronic size scales using a first-principles density functional plane-wave ultrasoft pseudopotential method, which can provide theoretical basis for TiC as the heterogeneous nuclei of ferrite grain.
Section snippets
Calculation method
DFT (density functional theory) with ultrasoft pseudopotential is employed in the CASTEP (Cambridge Sequential Total Energy Package) mode, which utilizes plane-wave pseudopotential to perform first-principles quantum mechanics calculations [21], [22]. LDA (Local Density Approximation) with the CAPZ (Ceperley–Alder–Perdew–Zunger) functional and GGA (Generalized Gradient Approximation) with the PBE (Perdew–Burke–Ernzerhof) functional are employed as exchange–correlation functionals.
In order to
Bulk and surface calculations
The crystal structures of ferrite and TiC are body-centered cube and face-centered cube respectively, and their space group structures are IM–3M and FM-3M. Each ferrite cell contains two unit cell (Z = 2) and TiC cell contains four unit cell (Z = 4). The crystal structures of ferrite and TiC are shown in Fig. 1.
Establishment of the interface of ferrite/TiC
On the basis of calculated results above mentioned, the model of the ferrite/TiC interfaces uses a superlattice geometry, in which a 5-layer slab of ferrite (1 0 0) is placed on a 5-layer TiC (1 0 0) slab. The free surfaces of ferrite and TiC are separated by at least 12 Å vacuum. In addition, for interfacing the (1 0 0) planes, the slabs were oriented about an axis normal to the interface so as to align the [1 0 0] directions, resulting in a “cube-on-cube” orientation relationship [33]: ferrite [1 0 0] (1
Conclusion
Overall, a LDA-CAPZ study of the interface structure, cohesive energy, electronic structure, and bonding of the ferrite/TiC interface has been performed, in order to analyze the mechanism of TiC as the effective heterogeneous nuclei of ferrite. TiC bonding is dominated by the C-2p, C-2s and Ti-3d electrons. In essence, it is a mix bond composed by small portion of metallic bond and large portion of covalent bond and exhibited high covalency.
Structural relaxation of the TiC (1 0 0) and ferrite (1 0
Acknowledgments
The authors would like to express their gratitude for projects supported by Program for National Nature Science Foundation of China (51271163) and Key Project of Science and Technology of Hebei Province (09215106D).
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