Theoretical Study Of Stacking Faults In MgZn₂,Cr₂X（X=Nb, Zr, Hf） And Dislocation In B2-MgRE,L1₂-Al₃X（X=Sc, Mg）

Alloys which have intermetallic compounds as matrix, have been developed as a new type of metal materials. With the development of science and technology, the requirements of the material property are higher and higher. More and more attention has been paid to the research on the intermetallic compounds. At present, the development of common intermetallic compound materials can be divided into two categories. One is the development of new lightweight material, which has low density, high specific strength and good corrosion resistance, such as magnesium-based alloys; the other one is the development of high-temperature structural material, which has high melting point, high creep resistance, good corrosion resistance and oxidation resistance, such as Laves phase chromium-based alloys. Although magnesium-based and chromium-based alloys gain the recognition and application owing to their superior properties, they have a common drawback--brittleness at low temperature. This drawback hinders the further development and the application of magnesium-based and chromium-based alloys.Brittleness at low temperature is closely related to plastic deformation characteristics which are been controlled by micro-defects structures, such as the stacking faults, antiphase boundary and dislocations. Through the study of micro-defects structures of the intermetallic compounds, revealing their plastic deformation characteristics and deformation mechanisms as well as the related mechanical properties, thereby designing materials having excellent mechanical properties, which is the key subject of this study.Based on density function theory, the stacking faults and deformation mechanism in C14Laves phase MgZn2and C15Laves phases Cr2X (X=Nb, Zr, Hf) have been studied by first-principles calculations. The charge density distribution, interaction of atoms and chemical bonding have been analyzed in detail to reveal the intrinsic mechanism for the formation of stacking faults in C14Laves phase MgZn2and C15Laves phases Cr2X (X=Nb, Zr, Hf). Furthermore, the structure and properties of<111>{110} superdislocations in B2-MgRE (RE=La-Er) and<110>{111} superdislocations in L12-A13X (X=Sc,Mg) have been investigated using the Peierls-Nabarro model in combination with generalized stacking fault energies. The thesis is mainly divided in to four parts:(1) Based on synchroshear mechanism, the formation of intrinsic stacking fault I2and twin-like stacking fault T2in C14Laves phases has been modeled in details and the generalized stacking fault energy curve of I2and T2for C14Laves phase MgZn2has been calculated from first-principles. The results demonstrate that the unstable stacking fault energy of I2by synchroshear is still very large, implying that the formation of I2SF in MgZn2is difficult. Upon the pre-existing I2configuration, the T2stacking fault can be further formed by following a next synchroshear. The unstable and stable stacking fault energies of T2are only slightly larger than those of I2, implying that the formation of T2may be essentially similar to that of I2. From the obtained generalized stacking fault energy, the relevant deformation mechanism of MgZn2is also discussed. The result suggests that deformation is performed by partial dislocations in C14Laves phase MgZn2. Finally, the electronic structure during synchroshear process is further studied to unveil the intrinsic mechanism for the formation of I2and T2in C14Laves phase MgZn2.(2) Based on the synchroshear model, the formation of stacking fault and twinning fault in C15Laves phases is modeled, then the generalized stacking fault energy curves and deformation mechanism in C15Laves phases Cr2X (X=Nb, Zr, Hf) alloys are investigated by ab initio calculations based on the density functional theory. The results demonstrate that the unstable stacking fault and twinning fault energies of C15Laves phases Cr2X (X=Nb, Zr, Hf) by the synchroshear are still large while the stable stacking fault and twinning fault energies are low, and the deformation modes by extended partial dislocation and twining are feasible in C15Laves phases Cr2X (X=Nb, Zr, Hf). Moreover, the Cr2Nb has the largest deformation twinning tendency, followed by Cr2Zr and Cr2Hf. The evolution of electronic structure during the synchroshear process is further studied to unveil the intrinsic mechanism for the formation of stacking fault and twinning fault in C15Laves phases Cr2X (X=Nb, Zr, Hf). (3) The structure and properties of<111>{110} superdislocations in B2-MgRE (RE=La-Er) intermetallics are investigated using the Peierls-Nabarro model in combination with generalized stacking fault energies. The results demonstrate that the<111>{110} superdislocations in B2-MgRE have dissociated into two collinear partials bound to anti-phase boundary. For the same material system, the dislocation dissociated width of screw is narrower than that of edge, while the Peierls energy and stress are larger. Among the B2-MgRE, MgEu has the highest Peierls energies and stresses for screw and edge superdislocations, while those for MgSm are the lowest, indicate that movement of<111>{110} superdislocation in MgEu is the most difficult, while it is the easiest in MgSm. With increasing of atomic number, the dislocation dissociated width, Peierls energy and stress in B2-MgRE decrease for both early lanthanides from La to Sm and late lanthanides from Eu to Er.(4) The generalized stacking fault energy curves along the<110> direction on{111} slip plane for L12Al3SC and Al3Mg are calculated within framework of density functional theory, and the anti-phase boundary energies are obtained. Then the structures and properties of collinear dissociated<110>{111} dislocations in Al3Sc and Al3Mg are studied using Peierls-Nabarro model combined with generalized stacking fault energies, the obtained dislocation dissociation width of the<110>{111} edge dislocation in Al3Sc is in agreement with the available experimental value. In comparison with Al3Mg, the dislocation dissociation widths of both screw and edge in Al3Sc are narrower and the Peierls energies and stresses are lower. Furthermore, for both Al3Sc and Al3Mg, the dislocation dissociation width of screw dislocation is smaller than that of edge dislocation, while the Peierls energy and stress of screw dislocation is slightly larger.