The Investigation Of Phase Relations In Zn-Nb-(Fe,Ti) Ternary System

The most common alloying elements in IF steels are Ti and Nb. In the research of surface coatings in IF steels, using hot-dip galvanizing and post-dip annealing approach to generate appropriate intermetallic layer, which could improve the powdering, flaking tendency of galvanneal coatings during forming processes and corrosion resistance of IF steels. The content of alloying elements of Nb and Ti in IF steels and the annealing time of GI steels are closely related to the structure of coatings. Thus, the Zn-Fe-Nb ternary system has been thermodynamically assessed with CALPHAD technique. And the phase relations of Zn-Nb-Ti ternary system has been investigated combining experimental measurements. Based upon the phase relations of Zn-Nb-Ti ternary system, the thermodynamic database of this ternary systems was established using CALPHAD method. Thus, it is possible to provide scientific foundation for the design of composition in IF steels and optimization of technological parameters in galvanizing process.The formation enthalpies for Nb Zn2, NbZn3 and NbZn15 were calculated by ab initio methods.The Nb-Zn binary system has been thermodynamically assessed using CALPHAD approach with Thermo_calc software. Incorporated with the reported two isothermal sections at 723 K and 873 K, the Zn-Fe-Nb ternary system were thermodynamically optimized. Reasonable agreement between the calculated and experimental data was achieved and a set of thermodynamic parameters for describing the Zn-Fe-Nb ternary system was obtained. Furthermore, liquidus projection was predicted accordingly.The equilibrium alloy method and diffusion couple approach have been used in this work. The samples of equilibrium alloy of the Zn-Nb-Ti ternary system have been observed by scanning electron microscope(SEM) and detected by X-ray diffraction analysis(XRD), and the isothermal section of the Zn-Nb-Ti ternary system at 723 K has been measured. The experimental results show that the system exists a continuous solid solution(Nb,Ti)Zn3 in the Zn-rich corner at 723 K. Three three-phase regions and seven two-phase regions have been detected, i.e. L + TiZn16 + NbZn7, Ti3Zn22 + TiZn16 + NbZn7,(Nb,Ti)Zn3 + Ti3Zn22 + NbZn7,(Nb,Ti)Zn3 + Ti3Zn22、Ti3Zn22 + TiZn16,(Nb,Ti)Zn3 + NbZn7、NbZn7 + Ti3Zn22, NbZn7 + TiZn16, L+TiZn16 and L+Nb Zn7, and Liquid. On the basis of the thermodynamic parameters of the three sub-systems, the 723 K isothermal section of Zn-Nb-Ti ternary system was thermodynamically optimized with CALPHAD method.