Research On Effect Of Hydrogen In Titanium Crystal By First-Principles Calculation And Molecular Dynamics Simulation

Recently a great deal of attention from many material researchers has been given to the studies on thermohydrogen processing (THP) of titanium alloys and its theory. A great lot of investigations on the procedures of THP and mechanical properties, hot workability and microstructure of titanium alloys after hydrogenation have been performed and a large number of research findings devoted to those aspects have been obtained. However, the research work on the mechanisms of hydrogen-induced thermoplasticity of titanium alloys has lagged behind the THP technology, and especially the basic research on the effects of hydrogen in titanium from the electric and atomic scale which can reveal the mechanism of hydrogen-induced thermoplasticity from physical principle is scarce. In the present work, the crystal structures, energy properties, atomic diffusion characteristics, elastic moduli and mechanical properties of Ti-H system were investigated systematically by the first-principles plane-wave pseudopotential method and molecular dynamics (MD) simulations, and the mechanism of effects of hydrogen in titanium crystal is clarified from the electric and atomic scale.The occupation of hydrogen in Ti crystal is the basis of constructing Ti-H crystal model to calculate its properties, so firstly it was investigated by the first-principles method. The results show that the lattice distortion and volume expansion of bothα-Ti-H andβ-Ti-H crystals with hydrogen atoms at the octahedral sites are lower than the ones with hydrogen atoms at the tetrahedral sites, and the absolute values of heat of solution of hydrogen inα-Ti-H andβ-Ti-H crystals with H atoms at the octahedral sites are higher than the ones with hydrogen atoms at the tetrahedral sites, signifying that hydrogen is inclined to occupy the octahedral site in bothα-Ti andβ-Ti crystals. It can be found that in the case of hydrogen occupying the octahedral site inα-Ti, there is a charge transfer from 3p orbit of Ti atoms which are the nearest neighbors of hydrogen to 1s orbit of hydrogen atom, and the bonds between hydrogen atom and its nearest neighboring Ti atoms are covalent. Hydrogen atom changed the density of state of Ti atoms nearest neighboring it, resulting in reduction in the interaction between Ti atoms. In the case of hydrogen occupying the octahedral site inβ-Ti, H atom changed the density of state of Ti atoms around it and enhanced the interaction between Ti atoms. The results indicated that hydrogen weakened the interatomic bonding inα-Ti and enhanced the interatomic bonding ofβ-Ti.Interatomic potentials are the foundation and key element for the molecular dynamics simulation. The EAM (embedded-atom method) model for Ti-H system was established based on original EAM and combined with Johnson's analytic EAM model. The expressions of potential functions and model parameters for Ti were determined according to the analytic EAM of Johnson. The new potential function expressions for H were provided and the model parameters were determined by fitting to the lattice constant, binding energy and bulk modulus of fcc metal hydrogen and the volume of dissolution and heat of solution of hydrogen inα-Ti.The hydrogen diffusion inα-Ti andβ-Ti were simulated by MD with EAM potentials for Ti-H system. The results show that the hydrogen diffusion inα-Ti is anisotropy and the calculated activation energy of hydrogen diffusion along c axis is smaller than that in the basal plane. The indirect O-T-O mechanism is most favorable for hydrogen diffusion in the basal plane ofα-Ti. The simulated values of activation energy and pre-exponential factor for hydrogen diffusion inα-Ti were in good agreement with the experimental data. The direct O-O mechanism is most favorable for hydrogen diffusion inβ-Ti and the calculated activation energy and pre-exponential factor for hydrogen diffusion inβ-Ti were in agreement with the experimental data.The effect of hydrogen on the Ti self-diffusion characteristics inα-Ti andβ-Ti were simulated by MD with EAM potentials for Ti-H system. The calculated activation energy for Ti self-diffusion inα-Ti-H is lower than that inα-Ti, and the calculated activation energy for Ti self-diffusion inβ-Ti-H is larger than the value inβ-Ti. The results indicated that hydrogen atom decreased and increased potential barriers for Ti self-diffusion inα-Ti andβ-Ti, resulting in the enhancement and weakening in the diffusivity for Ti inα-Ti andβ-Ti, respectively.The elastic moduli of Ti and Ti-H crystals were calculated. The results showed that the addition of hydrogen increased the bulk modulus and decreased the Young's and shear moduli ofα-Ti. The calculated bulk molulus, Young's modulus and shear modulus ofβ-Ti-H crystals with different hydrogen were all higher than those ofβ-Ti, that is, the addition of hydrogen increased elastic moduli ofβ-Ti.The deformation behaviors of Ti and Ti-H crystals were simulated by MD method. The results show that the calculated theoretical breaking strength along c axis and the theoretical yield strength ofα-Ti-H crystals with different hydrogen concentration were all lower than the values of pureα-Ti, and moreover, they decreased with increasing hydrogen concentration and temperature. For different simulation temperatures, the critical stress for phase transition inβ-Ti-H is higher thanβ-Ti. The results indicated that hydrogen improve the stability ofβ-Ti. The effect of hydrogen on the theoretical yield strength ofβ-Ti related to the temperatures. Hydrogen enhanced the theoretical yield strength ofβ-Ti at higher temperature.