The Investigations Of Polytypic Transformations Of Ca（Mg,Al）₂ And TiCr₂ Laves Phases As Well As Effects Of Nonmetallic Solutes On Stacking Faults In α-Ti

Laves phase intermetallic compounds in Mg alloys and Ti alloys have been developed as a new high-temperature structural materials. Because the mechanical properties are closely associated with their structural transformations, the studies of deformation and structural transformation plays a vital role in improving the mechanical properties of materials. In this paper, the polytypic transformations of Laves phases Ca(Mg1-x,Alx)2 and TiCr2 as well as the basal slip and stacking fault of a-Ti including nonmetallic solute atoms have been study based on density founctional theory calculations. The investigation work is summarized as follows:Firstly, the theoretical study for polytypic transformations of Laves phases Ca(Mg1-x,Alx)2 have been carried out based on DFT(Density Functional Theory) calculations. The results indicate C14â†'C36â†'C15 polytypic transformations of Ca(Mg1-x,Alx)2 with an increase in Al contents. The electronic structures demonstrate that stronger Al-Ca bonds first mainly enhance the vertical interaction between atomic layers, then the horizontal bonding within layer is strengthened by stronger Al-Al and Mg-A1 bonds. With increase of A1 contents, the bonding of Ca(Mg1-x,Alx)2 is relatively uniform between vertical and horizontal directions when the A1 contents reaches higher level, intrinsically promoting the hexagonal C14 or C36â†'cubic C15 structure evolution. The variation of bonding electron numbers exhibits C14â†'C36â†'C15.Secondly, the model of C14â†'C36â†'C15 polytypic transformations is constructed based on synchroshear mechanism, especially an original C36â†'C15 model (Model B) is proposed. Based on these models, the energy barriers and electronic structure of TiCr2 Laves phase is systematically studied during polytypic transformations. The results show original C36â†'C15 model (Model B) is a feasible for TiCr2. Electronic structures show the Cr-Ti bonds between layers nearby synchroshear plane are related to the polytypic transformation for TiCr2 Laves phase.Thirdly, the effects of nonmetallic solute atoms including C, H, O and N on <a> basal slip and stacking faults in a-Ti is studied by first-principles method, and generalized stacking fault energies are calculated. The results indicate that nonmetallic solutes C, O and N atoms enhance unstable stacking fault energy and result in the lower mobility of (0001)[1120] slip. And nonmetallic solutes C, H, O and N atoms decrease GSFE of the (0001)[1010] basal slip. In particular, reduced intrinsic stacking fault energies facilitate the formation of deformation stacking faults I2. The electronic structures demonstrate that stable Ti-X (X= C, H, O and N) covalent bonds play an essential role in basal slipping process.