First-principles Investigations On Electronic Structure And Structure-property Relationships Of Zr-based Alloys

Structure–property relationships established at each characteristic length scale are among the cornerstones upon which modern materials sciences are based. These relationships form the basis for all design, including computational materials design. Without this knowledge, material development has to proceed by trial and error, which is both costly and time consuming. Most material properties are closely related to electronic properties. As to Zr-based alloys, understanding their unique and outstanding properties in an electronic structure level is of fundamental importance to rationally bottoms-up design of new alloys. From literature and our own calculation results, we noticed that in many Zr-based alloys there exists an intriguing pseudoatom bonding phenomenon which probably is the origin of their unique behaviors. This dissertation is devoted to exploring the existence and its mechanism of pseudoatom in Zr and its intermetallics and the involved structure-property relationships through first-principles calculations and Bader’s electron density topological analysis.Firstly, we compared the electron density topologies of three common phases（α, β and ω） of Zr and find that with an increase in distortion of Zr-tetrahedron configurations, the Zr-Zr bonding changes from pseudoatom bonding（α phase） to bifurcated bond paths（ω phase） and to common straight bonding（β phase）. In different hcp metals, the pseudoatom nature increases with decrease in electronegativity. Therefore, the occurrence of pseudoatom phenomenon is controlled by atom packing and electronegativity of constituents. These special bongding types could render Zr-based alloys special properties in elastic anisotropy, structure of dislocation cores, dislocation slip, and so on.A comparison of ω-Zr and the ordered ω phase Zr₂ Al indicates that the ordered Al substitution for Zr doesn’t result in pseudoatom bonding and both phases contain Zr-Zr banana bonds. However, their mechanical properties are dramatically different: Zr₂ Al possesses considerably higher elastic moduli and Vikers hardness and lower elastic anisotropy. Al substitution for Zr is found to not only increase the bond directionality and electron density at bond critical points but also decrease the bond anisotropy. The present study is of importance since it provides clues to tuning properties of omega phase and hence of related alloys by alloying.We also found pseudoatom in tetragonal Zr₂ Cu, which under pressure collapses to common straight bonding. This electron density topological transition results in a special isosymmetric phase transition. As this phase transition has features of both conventional isosymmetric phase transition and structural phase transition, we suggest it belongs to a new kind of phase transition, quasistructural phase transition.Our electron density topological study on three competing phases in Zr₂ Cu reveals that with an increase in distortion of Zr-tetrahedron configuration, the Zr-Zr bonding changes from pseudoatom bonding（C11b phase） to bifurcated bond paths（C16 phase） and to common straight bonding（E9₃ phase）. These results have implications for the Zr（Ti, Hf）-rich metallic glasses, where close-packed Zr（Ti, Hf）-tetrahedra are ubiquitous and favor the unconventional pseudoatom or bifurcated bonding, which in turn plays a role in the behavior of the metallic glasses.The distinct H absorption behavior of the two similar intermetallic compounds Zr₂ Pd and ZrPd₂ has been highly confusing, provided their common Mo Si2-type structure and hydride-forming constituents. Zr₂ Pd readily absorbs hydrogen and forms a series of hydrides, while ZrPd₂ does not form a hydride even under pressure. Our study addresses that their extended structures（electron density topologies） are actually different, which causes their different properties.