Studies Of Polycrystal Microstructure Evolution By Using Phase Field Method

In Polycrystalline material, grain growth is one problem to which be paid more attention by people at all times. Because the size and the shape of grains are closely correlation with properties of metal materials, investigation into the grain growth process is of both theoretical and practical significance. In recent years, computer simulation has become an effective means in the field of materials design and microstructure evolution research. It is helpful to understand the mechanism of solidification and forecast the physical properties of casting by simulation, achieving the aim of materials design at microstructural level. Especially, 3D simulation of grain is more consistent with real solidification process. It will provide credible bases for improvement in microstructure, which has significant utility of design and development of new material. Thus, this thesis simulated three-dimensional grain growth process using the phase field method. Average grain size, grain size distributions and topology evolution mechanism of the quasi-steady state were examined so as to grain growth process can be understood well.In this thesis, convergence of free energy of phase field model was researched. It makes us know the mechanism of solidification. The validity of this method was primarily tested.Simulation of the two-dimensional grain growth process using this model was carried out. Effects of parameters of the phase-field on grain size and grain size distributions are discussed, such as order parameter, grain boundary energy and kinetics coefficient. The simulated results reveal thatâ‘ the grain growth appears to be a quadric power law, and there are little effects of the number of field variables on the grain growth law. The more the number of orientation field variables are, the little the probability of the grain coalescence is, and the more slowly the grains grow. The quasi-steady state grain size distribution for different orientation field variables also was studied. The result shows the distribution has self-similarity.â‘¡Grain boundary energy has relation of the thickness of the grain boundary. The thickness of the grain boundary and size of grain increase with the grain boundary energy increasing. The thickness is proportional to k^1/2.â‘¢If the kinetics coefficient is great, the grain grow quickly.Simulation of the three-dimensional grain growth process using this model was carried out. Average grain size, grain size distributions and topology evolution mechanism of the quasi-steady state were examined. The simulated results of the grain growth follows a quadric power rule. The quasi-steady state distribution of grain size in the 3D grain growth process appears to be Weibull distribution. The topological evolution process of grains on certain cross-section was observed. There are different ways of grain annihilation. The 5-sided grain can annihilate in 4-sided grain way and also in 3-sided grain way.