First-principles Investigation Of The Atomic Diffusion In Hexagonal Close-packed Titanium Alloys

Titanium alloys find wide applications in the field of aerospace due to their high specific strength and good corrosion properties.It is urgent to improve the mechanical properties of titanium alloys and develop novel titanium alloys to satisfy the continuously developing requirements.Atomic diffusion is very important for the microstructure evolution as well as the mechanical properties such as the high temperature creep resistance of the titanium alloys.It follows that,understanding the diffusion mechanism and obtaining high quality diffusion data are crucial for the development of high performance titanium alloys.The diffusion coefficients from experimental measurements are largely scattered because it is highly sensitive to the impurities containing in the sample.First-principles methods avoid the interference of the impurities by assuming an ideal atomic environment and calculate the diffusion coefficient at the atomic scale.Recently,the diffusion of several solutes in a-Ti has been investigated by using the first-principles method.However,systematic investigation on the atomic diffusion in titanium is still absent such that the diffusion mechanisms and basic regularities are still to be elucidated.With the above background,in this thesis,a first-principles method is adopted to investigate systematically the atomic diffusion in a-Ti alloys.We first investigated the influences of some key factors such as plane-wave cutoff energy,k-mesh,supercell size,and geometric optimization schemes on the migration energy barrier and its anisotropy for the self-diffusion in metals with hexagonal close-packed structure.The supercell size affects heavily the migration energy barrier and its anisotropy for Ti,Zr,and Hf but not for Mg and Zn because the interaction between the valence d electrons of Ti,Zr and Hf are longer ranged than that between sp electrons in Mg and Zn.This result explains the discrepancy of the migration energy barriers and its anisotropies of Ti,Zr,and Hf reported in literature.With the migration energy barrier calculated with large supercell(4×4×3),the self-diffusion coefficients were evaluated within the framework of the transition state theory.The obtained self-diffusion coefficients are in good agreement with the experimental values.Secondly,the migration energy barriers of the solute atoms in a-Ti as well as the influences of the solute on the migration energy barrier of the host Ti atom and the vacancy formation energy were investigated.Generally,the migration energy barrier of the 3d,4d,5d transition metals increase with increasing atomic number.The anisotropy of the migration energy barriers for the transition metal solutes are very weak.The migration energy barriers of noble metal and simple metal solutes,with high anisotropy,are close to or higher than that for the self-diffusion of Ti atom.The transition metal solutes(except the elements with number of d electrons of 2)decrease heavily the migration energy barriers of their nearest neighboring host Ti atoms whereas the influences of the noble and simple metal solutes on the migration energy barriers of their nearest host Ti atoms are very weak.The solute migration energy barrier are mainly determined by the atomic size and the chemical bonding.Generally,the solutes with small atomic size or strong chemical bonding migrate with high energy barrier.Most of the solutes decrease the vacancy formation energy,i.e.,they attract the vacancy.Al,Si,Cu and Sn are repulsive to their nearest neighboring vacancy and increase the vacancy formation energy.Both the distortion induced by the atomic size mismatch between the solute and host Ti atoms and the chemical bonding control the solute-vacancy interaction energy.Then,the preferential energy for the solute atoms occupying the high symmetry interstitial site to the substitutional site;as well as the migration energy barrier of the solute atoms along the pathways between the high symmetry interstitial sites are calculated.The preferential site occupancy energy are positive for all the solute atoms,indicating that substitutional site is more stable than the interstitial site.The 3d transition metal solutes Mn,Fe,Co and Ni have relatively small preferential energy.The preferential energy decrease with increasing temperature.The most stable interstitial site is not necessarily relevant directly to the size of the interstitial site.Both the elastic distortion and the chemical interaction affect the relatively stability of the interstitial configurations.All the solutes have relatively low interstitial migration energy barriers.Along the diffusion pathway between the high symmetry interstitial sites,we find three stable low symmetry interstitial configurations with low formation energy,indicating that treat the energy difference between the high symmetry interstitial configurations as the interstitial migration energy barrier as reported in literature are not reliable enough.Finally,we calculated the solute diffusion coefficients under both mono-vacancy mediated and the dissociated interstitial mechanisms.The mono-vacancy mediated diffusion coefficients calculated with the 8 frequency model are within 3?4 orders of magnitude higher or lower than the self-diffusion coefficients at 1000 K,exhibiting normal diffusion behavior.Among all the solutes,Si have the slowest mono-vacancy mediated diffusion rates(2?4 orders of magnitude lower than that of self-diffusion).The calculated mono-vacancy mediated diffusion coefficients for most of the solutes agree well with the experiment measurements.However,the calculated mono-vacancy mediated diffusion coefficients of Cr,Mn,Fe,Co and Ni are 5?7 orders of magnitude lower than the experimental values.With this regard,we calculated the diffusion coefficients of 3d transition metal,Al,Sn and Si with the dissociated interstitial mechanism.We showed that the diffusion coefficients of Mn,Fe,Co and Ni under dissociated interstitial mechanism are 7-11 orders of magnitude higher than that of the self-diffusion of Ti,exhibiting ultra-fast diffusion behavior in ?-Ti,in agreement with the experimental findings.Our calculations are expected to provide clues for the design of high temperature titanium alloys with improved creep resistance.