Simulation Of Nanocrystalline Grain Growth And Abnormal Grain Growth For AZ31 Mg Alloy By Phase Field Methods

Researches of nanomaterials have been popular for recent thirty years due to their extraordinary physical and mechanical properties. Therefore, it will be a very interesting research subject to fundamentally understand the mechanism of nanocrystalline grain growth. As a rapid progress in the computer technology, the simulations of microstructure evolution and property have become an important issue in the state of art materials science. On the other hand, as one of environmental-friendly engineering materials in the 21st century, magnesium alloys have great potential applications in various aspects of industry. The nanocrystallization may be the potential solution to break the bottleneck of magnesium alloy applications because of poor performance of aging strengthening. As far as the authors know, there is no report of simulation on nano-scale polycrystalline grain growth at a constant temperature in real alloys. So, the grain growth in nanostructural AZ31 magnesium alloy at different temperatures has been studied by phase field method in this research.A phase field model to simulate grain growth at nano-scale has been developed based on our previous phase field model for polycrystalline grain growth at micro-scale in realistic spatial and temporal scales. The expression of local free energy density function is modified to simulate the grain growth from nano-scales to micro-scale in polycrystalline AZ31 Mg alloy. The physical values of the model parameters are properly redefined respectively due to different scales of grain growth. It is shown that the grain boundary range in nano-grains should obtain the maximum value without losing the physical meaning where the term of grain boundary range is to explain the physical backgrounds of the order parameter gradients at grain boundary. Therefore, the range is suggested to set between 40 nm to 400 nm which covers no more than two adjacent grains to systematically investigate the microstructure evolution, in contrast with that in micro-grains with a constant of about 1.2 ?m.It is found by simulation that the grain boundary energy in nanocrystalline alloy is lower than that in microcrystalline alloy. The speed of grain growth in nano-scales is also much slower than that in micro-scale. The time exponent in the kinetic equation is varied from 5 to 2 associated with the nano-structure to micro-structure. Some of those findings can be proved by experimental results found in the literatures. The mechanism of grain growth is discussed by the low grain boundary mobility and low grain boundary energy in nanostructure.The segregation of solute atoms at grain boundaries in nano-structure and micro-structure has also been investigated in the research. The simulated results indicate that solute atoms would like to segregate at the grain boundaries much more severely in nano-structure than that in micro-structure. However, the effect of temperatures on segregation is the same in both structures. The severe segregation may lead to lower boundary mobility in the nano-structure. The effect of Al atom diffusion factor has been examined on the segregation of grain boundary and the process of grain growth in the nano-and micro-structural AZ31 alloy. The simulation conclusions are as follows:the distribution of Al composition along straight grain boundary is more uniform than that along curved grain boundary. The larger the diffusion mobility, the severer the segregation concentration of Al element takes at the grain boundaries. When the diffusion mobility is large enough, the further increase has little impact on average grain size after a grain growth time.It is found that the fluctuation of grain size after grain growth is more intense in nano-grains than that in micro-grains. Therefore, our researches suggest that to introduce some independent large-sized grains in nano-grains by local abnormal grain growth during annealing may improve plasticity of alloy while maintaining high strength. This might be a new way to develop advanced nanomaterials. The abnormal grain growth in nano-structural AZ31 Mg alloy is simulated under different micro-conditions. Through examining these simulation results, we find that the expected mixture with nano-structure might be obtained by controlling local three primary micro-factors:strain restored energy, interface energy, and interface mobility, respectively. The simulations show that the local variations of these three factors must be greater than the critical values. The local restored energy should be 1.6 times higher than the average value of the matrix, or the local interface energy 0.76 times lower than the average value, or the local interface mobility 3 times higher than the average value, respectively. Also the change of local diffusion mobility and initial mixed-grain cannot cause local abnormal grain growth.Finally, the grain growth processes of AZ31 magnesium alloy containing spherical particles with different sizes and contents are simulated in both spatial scales. It shows that the role of pinning effect of the second phase particles during grain growth in nano-structure is different with that in micro-structure. There is a critical particle size to affect the grain growth in nano-structure. If the size of particles is lower than the critical value, the effect of pinning for grain growth will be increased with further decreasing the size. If the size is larger than the critical value, the particles nearly have no pinning effect. Different with the critical particle size in micro-structure, the larger size of the particles will result in a greater pinning effect during grain growth when the particle size is lower than the critical size; however the effect goes opposite if the particle size is larger than the critical value. In the nano-structural polycrystalline, the critical value is 200 nm when the content of particles is 10%, and the critical value is decreased when their content is increased. With the size of particles is 30 nm, the time exponent of grain growth increases when the content of particles is increasing so that the pinning effect of particles on the grain growth is increased as well.