The Basal Staking Fault And The Transformation Pathways For Vitual Long Period Stacking Order Structure In Mg:First-Principles Study

It is well known that the microstructure of the material has a great effect on the performance of the material in the steady state. However, the perfect crystal structure would not be existed in practical application. That is, there would be always some defects in the crystal of Material in the actual industrial applications. Generally speaking, the defects in the materials include point defects?dislocation?, line defects?stacking faults? and surface defects, which will affect the mechanical properties of the structure. Dislocation is one of the most basic defects in the material, and the formation of stacking faults is closely related to the slip of the partial dislocations. In the related field, the study for dislocation in materials has reached a considerable level. However, the study for stacking faults would be in exploration. Therefore, in order to further further study the properties of stacking faults in the material, we mainly select Mg as the research object. In this thesis, we mainly study the basic stacking fault and long period stacking order?LPSO? structure of Mg, and the LPSO structures would essentially be the multi-stacking fault structures?MSFS?, which are composed of basic plane stacking faults. And they are novel microstructures that have been committed to explore and research in the theory and experiments. The discovery for the LPSO structures not only improves the mechanical properties of the Mg and its alloy in a large extent, but also promotes and expands their further application in the field of modern industry. The specific contents are as follows:?1? The basic stacking fault and the transformation mechanism of themThe first-principles study based on the density functional theory?DFT? have been used in the work. The basal stacking faults?I₁, I₂, E and T2? have been systematically researched in Mg from the point of view of the theory. The values of the stable stacking fault energy and the unstable stacking fault energy have been calculated. Compared with the previous theoretical and experimental values, our calculated results agree well with them. In addition, we also investigate the transformation mechanisms among the basal staking faults. Through the careful analysis for the generalized stacking fault?GSF? energy curves in the process of transformation and the corresponding schematic diagram of atomic slip, we find that the energy barriers depends on the numbers of the head to head arrangement of the adjacent atoms in the slip process. At the same time, through the change trend of the GSF energy curve, the positive and inverse process of the same transformation would have the different energy barriers. For example, the value of I₁ ? E is 0.073 J / m2, but the value of barriers energy for E ? I₁ is 0.56 J / m2. It means that the phase transformation of E ? I₁ would be more difficult than that of I₁ ? E to achieve.?2? The transformation pathways for the vitual long period stacking order?LPSO? Mg.Based on the first principles study, we mainly study the interaction transformation among the LPSO Mg. Through the careful observation for the stacking sequence of the LPSO Mg and combined with the slip mechanism of hcp crystal structure, we have investigated the specific transformation pathways in theory. In order to clearly reveal the specific slip process of the atoms in the transformation process, we have given the corresponding schematic diagram of atomic slip. At the same time, we also discussed the influence of the vacuum space for the phase transformation to achieve among the LPSO Mg. In addition, two kinds of slip modes, multi-slip-planes mode?M mode? and single-slip-plane mode?S mode?, are proposed by observing the stacking features of the LPSO phase during the transformation process.We have also made the pathway optimization analysis by comparing the energy barriers for the transformation mechanisms containing multiple potential transformation pathways and the optimized transformation pathways can be obtained. The 2H?6H₂, 6H₁?6H₂, 14H₄?14H₂ and 14H₄?14H5 have the same energy barriers under the M mode and S mode. That is, the transformation pathways revealed by M mode and S mode are equivalent in this case. However, the energy barriers of M mode are larger than that of S mode for these transformations with the consideration of vacuum space. It means that the transformation pathway revealed by S mode will be more optimized. For 2H?6H₁ and 14H₁?14H3, the energy barriers of M mode are smaller than that of S mode. The energy barriers of M mode are larger than that of S mode for 18R1?18R2 and 14H₂?14H5. Therefore, for these phase transformations, the influence of vacuum space will not play a decisive role in the transformation process.