| 其他摘要 | Titanium alloys and nickel-base superalloys are the commonly used high-temperature structural alloys, and they have been the key materials in the aeroengine due to their excellent mechanical properties. However, the further developments of high-temperature engineering, such as aeronautic and astronautic industry, set higher requirements for these two kinds of alloys, and it is urgent to develop the new products with better properties and cheaper cost. If one wants to improve the properties of these alloys, it is very necessary and important to understand deeply the formation mechanisms and evolution laws of the microstructures in alloys.
Recently the rapid development of computational material sciences has provided a powerful tool for the research of materials science and engineering. Besides theories and experiments, computer simulation can be used to resolve the various problems in materials. In this study, we employ the mesoscopic phase field model to study the microstructural evolution in titanium alloy and nickel-base superalloy.
Firstly, the formation and growth of alpha sideplates in titanium alloys are explored. The phase field model of beta-alpha diffusional transformation in Ti-6Al-4V alloy is established. Without taking into account the elastic energy, the effects of interfacial energy anisotropy and heat-treatment temperature on the morphology of alpha sideplates are studied, respectively. The results show that the interfacial energy anisotropy can accelerate the formation of sideplates, and the morphology of alpha colony can change from rod to plate with the different interface energy anisotropy. The heat-treatment temperature also can influence the growth velocity and plate space of alpha colony. When the temperature is high, the sideplates can not be obtained even if the phase transformation still occurs.
The dissolution kinetics of gamma prime particles in Ni-Al binary alloy are studied using quantitative phase field model. The dissolution of single particle in one-dimensional (1D) and 3D space are simulated firstly, and the kinetics results are compared with analytical solution and numerical solution obtained from sharp-interface model. The multi-particle dissolution in 3D is then simulated, and the effects of initial particle size distribution (PSD) on the dissolution kinetics are studied. The simulation results show that the change of average particle size depends strongly on the initial PSD, and the particle volume fraction decreases with time exponently in spite of the initial PSD, but the dissolution rate is relative to the initial PSD.
The grain growth in the particle-contained system is studied by using phase field method. The phase field model of particle pinning (also called as Zener pinning) is built, and the Zener pinning in 2D and 3D are simulated, respectively. A method is proposed to convert the experimental SEM image into the data file which can be the input of phase field model. In 3D case, the minimum particle size in the phase field model is determined firstly. Then the 3D simulations in big system show that the pinning force in 3D is weaker than that in 2D, and that the particle size distribution has effect on the final stable grain size. A new parameter is introduced to characterize the Zener effect, and a modified Zener equation is proposed. Based on the simulations of gamma prime dissolution and Zener pinning, the first fast acting model (FAM) is proposed to predict the grain growth kinetics in René 104 alloy.
The second model of grain growth in René 104 alloy includes the effects of forging. The possible factors, such as stored energy, gamma prime dissolution rate, and initial grain size distribution, are examined, respectively. The simulation results indicate that both stored energy and gamma prime dissolution rate seem to have little influence on the grain growth kinetics. But the initial grain size distribution can change the kinetics obviously. A mean field statistical model of grain growth is proposed, and the grain growth in three cases are studied, including without pinning force, with constant pinning force, and with decreasing pinning force. It is found that, when the pinning force decreases with time, the abnormal grain growth may occur, which will lead to a final stable grain size beyond the Zener limit. Based on these results, the second FAM is proposed to predict the grain growth kinetics during hot deformation and heat treatment. |
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