Evaluation And Calculation Of The High-pressure Phase Diagram Of Pure Metallic Elements

High-pressure phase diagram is the diagram of material under pressure which reveals the relationship between substance under high pressure in a particular state and equilibrium. Substances under high or ultrahigh behavior has attracted a lot of research work such as solid state physics, geography, chemistry, astrophysics, materials science and metallurgy. High-pressure phase diagram studies can provide useful information to work in many areas. Therefore, phases relationships and description of the phase diagram under conditions of high pressure is of great significance. SGTE database for pure elements as the basis for calculating the thermodynamic database describes 78 kinds of pure elemental Gibbs free energy of different phases with the temperature changes and shows the relationship between phases under different temperatures. However, the description about change in Gibbs free energy under different pressures is little, so basing on the SGTE database for pure elements, utilizing the appropriate theoretical models, exploring and researching to describe the Gibbs free energy under different pressures is necessary.This work utilized the CALPHAD method which used to describe the Gibbs free energy of substances, and combined different equations of state to complete the assessments of high-pressure phase diagram of pure metallic elements. Also, this work improved the SGTE pure elements database in describing the Gibbs free energy of Ti, connected the Murnaghan theoretical model to complete the describing of the P-T phase diagram of Ti, the triple point which in the P-T phase diagram is close to the experimental values. This work combined with recent experimental or calculated data, re-assessed the parameters under high pressure of Mo,W and Cu, completed the describing of P-T phase diagram by using Murnaghan theoretical model, compared with previous findings, the degree of fitting is well. This work tested the accuracy of Murnaghan theoretical model in the experimental range. Each elements in models were obtained by the method of least squares fitting. The software used to describing the P-T phase diagram of substances in this work is MATLAB.In the CALPHAD method, this work applied the Mie-Grüneisen theoretical model in the calculation and evaluation for the P-T phase diagram of the pure metallic elements. The Mie-Grüneisen theoretical model was applied to solve the integral on volume which in the Gibbs free energy, through the method of changing the limitations of integration, this work completed the describing of Gibbs free energy under high pressure. Also, this work demonstrated the feasibility of combining the Mie-Grüneisen theoretical model with the SGTE database for pure elements in describing the P-T phase diagram. Preliminarily explored the relationships and transition of phases of pure Be contains hcp, bcc, fcc and liquid phases under high temperature and high pressure, also, predicted the P-T phase diagram of Be. Finally, using the Mie-Grüneisen theoretical model can predict a greater range in temperature and pressure for the pure elements.