Phase Field Study On The Kinetics In Shock-induced Phase Transitions

The kinetic of Shock-Induced Phase Transitions (SIPT) is a fundamental topic in many research fields, such as shock wave physics, impact dynamics and material science. It’s research results have important application value in the material synthesis, national defence engineering.Much SIPT phenomenon belongs to first-order phase transition, its dynamic process is consist of a series of nucleation and growth events. Nowadays peoples can not directly diagnose this fundamental dynamic process in experiment, so the measurement and interpretation of wave profile has become an important means for understanding the kinetic of SIPT. Theoretically, the interpretation of wave profile is usually based on the macro mixed phase phenomenological model, which do not consider any of microscopic and mesoscopic mechanisms of phase transition. In this sense, the usefulness of experimental wave profiles as an important window for studying SIPT is largely unexplored.The phase change process of SIPT is consisted of a series of the propagation and interaction of micro phase boundaries; therefore, it is very important to establish a theoretical model based on the micro phase boundaries description. In recent years, phase field model has achieved great success in computational material science, it can describe complex micro-structural evolution based on the diffusion boundary thought, but phase field model has not been applied to the area of SIPT. In this dissertation, a theoretical model is developed to study the kinetics of phase transformation based on the phase field description of nucleation and growth, and reveal the basic laws of the propagation and interaction of micro phase boundary, and reveal the direct link between a wave profile and the nucleation and growth process. The relevant research methods and understanding is important for the design of experiment, accurate interpretation of experimental data. The main conclusions of this paper are as follows:1) A theoretical model is developed to study the kinetics of phase transitions based on the microscopic phase boundary description. Single and multi phase field model are established, the constitutive equation of phase transformation is deduced, combined with the phase field evolution equation and the hydrodynamic equations, we established a complete theoretical frame based on the micro phase boundary description the, and can be applied to study the kinetics of SIPT.2) A numerical method is introduced to coupling solve the phase field dynamic equation, the constitutive equation of phase transformation and the hydrodynamic equations. A finite element program is developed within the framework of DYNA2D program to solve those equations, which can applied to study the propagation of phase boundary and the interaction between the phase boundary and stress wave. According to the idea of V&V, we examine the calculation results and accuracy of the program.3) The basic law of the propagation and interaction of phase boundary in one dimension is revealed. Firstly, the traveling wave solution characteristics of the phase boundary propagation in dynamic problems is verified by the mathematical analysis and phase field simulation, and further point out the existence of the maximum phase boundary velocity and how to calculate this maximum velocity; then, based on the solution of Riemannn problem, establish the dynamic relationship of the propagation of phase boundary; based on the wave analysis and the numerical solution, it is indicated that the dynamic relationship of the propagation of phase boundary play a key rule in the interaction between the phase boundary and the stress wave, and further puts forward the conception of critical phase transition rate, and summarizes the basic laws of the interaction between stress wave and phase boundary interaction.4) Using the proposed model, the phase transformation kinetics process of some typical experiment is investigated under the viewpoint of nucleation and growth:b) it is indicated that the affect of individual nucleation and growth can be neglected for for larger scale samples (thickness millimeter), and the collective nucleation density and distribution will effect the wave profile. The main conclusions are as follows:1) by comparing the relationship between the propagation of phase boundary dynamics of metal bismuth and iron, it shows that the phase transition rate of bismuth is more faster, and also points out that the material has a more steep maximum velocity curve is a necessary condition of higher phase transformation rate; 2)through the interpretation of the experimental wave profile and combined with microscopic analysis of recovered sample, derive the close connection between the twins and deformation of martensitic transformation, it is worthy of further in-depth study; 3) through the research on the overshoot single wave loading and unloading process, it is indicated that the reverse transformation during the unloading process will inevitably lead to the separation of wave structure, rather than the existence of the "overshoot" phenomenon in loading process, so the reverse transformation will occur in sequence along the unloading path.