C - Ni - Si And Ni - Si - Ti System Orderly/disordered Phase Transformation And Phase Equilibrium Thermodynamics Calculation

CALPHAD(CALculation of PHAse Diagram) technology, as a type of materials computation method, has been successfully used in the fields of phase diagram calculation and phase transformation modeling of multi-compound materials. The key parts of CALPHAD technology lie in the suitability of the as-used thermodynamic model and the accuracy of the thermodynamic database. This work will focus on the above two aspects and do basic thermodynamic investigation of selective Ni-based alloy systems. The target systems in this work contain Ni-Si, C-Ni-Si and Ni-Si-Ti. Specific research contents are as follows:1. Analyzing the relationship between the two-sublattice model and the four-sublattice model in describing the fcc ordered/disordered transition. The relationship of the parameters between the two models have been deduced in this work in order to transform the existing database in the form of two-sublattice model into more reasonable four-sublattice model. To reveal the drawbacks of describing the fcc ordered/disordered transition without considering the metastable ordered states, the Ni-Si and Al-Ni systems were investigated. First-principles method was applied to calculate the enthalpy of formation of the fcc and its ordered states in the Ni-Si and Al-Ni systems with a desire to verify the above conclusion. Finally, a new approach to describe both stable and metastable phase equilbria of fcc ordered/disordered phase was established.2. Carrying out critical literature assessment of the Ni-Si system in order to reveal the shortcomings of the previous CALPHAD modeling. New thermodynamic model was applied to describe the Ni2Si_L, NiSi and NiSi2phases in order to improve the description of the thermodynamic properties for these phases. The thermodynamic database of Ni-Si system was established by cooperating the newly suggested approach and CALPHAD technology. One set of self-consistent thermodynamic parameters of the Ni-Si system was obtained.3. Based on the thermodynamic assessment of the Ni-Si system, the thermodynamic calculations of the C-Ni-Si and Ni-Si-Ti systems were carried out. The application of using four-sublattice model to describe the fcc ordered/disordered transition in the ternary system was investigated. The extrapolated Ni3Si-L12ordered phase can be directly formed from the liquid phase when adding Ti, which is in agreement with the literature founding. This indicates the importance of binary system to the thermodynamic investigation of the ternary system. By critically reviewing the literature, the melting behavior of Ni3Si-L12in the ternary system was clarified. First-principles calculations were utilized to compute the enthalpy of formation of the ternary compounds and end-members of the Ni-Si-Ti ternary system at0K to assist CALPHAD modeling. The CALPHAD-type calculated thermodynamic properties of the ternary compounds are in consistent with those calculated by First-Principle method. The thermodynamic properties of other ternary compounds are predicted by present CALPHAD modeling. The calculated phase diagrams are consistent with the experimental results. Moreover, the phase relationship in the Ni3Si-L12rich region and the stable temperature range of τ7are successfully predicted.