Computional Studies On Interactions Between Hydrogen And The (11(?)1) Twin Boundary In Hexagonal Close-packed Titanium

Titanium(Ti)is an important structural metal.At room temperature,titanium takes a phase with the hexagonal close-packed(hcp)structure.Titanium alloys may be often under stress in applications,while twinning is the main plastic deformation mode of hcp systems.Therefore,it is important to study the twinning mechanism of Ti and defect effects in twin boundary(TB)area.The {11 2 1}<11 26>twin system is one of the most common twin systems in hcp-Ti,and the {11(?)1} TB is formed solely by basal dislocations glide along every c lattice.It is reported to have a very small TB shift energy barrier.On the other hand,Ti has a good affinity to hydrogen(H),as H concentration increases,phase transition from hcp to cubic or tetragonal structure may be caused,and titanium hydrides would nucleate,even possibly cause hydrogen embrittlement.As far as we know,the theoretical researches on Ti-H systems are mostly based on bulk systems,but less on the interactions between H and twin boundaries.And the discussion about a larger amount of H atoms existing near TB is not involved in literature.In this paper,we employed first-principles calculations and ab initio molecular dynamics to study the interactions between H and TB.We constructed the(11(?)1)TB model of hcp-Ti,and studied the solution sites of H with different amount.We investigated the kinetics of H in the(11(?)1)TB structures as well as the TB.Moreover,we simulated the tensile process of TB in the presence of H,taking into account the effect of H concentration.The main results are as follows:(1)H solutes to interstitial sites of Ti host.One H atom prefers to occupy an octahedral interstitial site in bulk Ti.In the(11(?)1)TB structure of Ti,a small amount of H tends to occupy the octahedral interstitial sites in bulk Ti,while more H atoms tend to occupy the tetrahedral interstitial sites.Both of their solution energies in TB structure are lower than in bulk,which indicates that H prefers to stay near TB.(2)In the lowest energy configuration,when H jumps from the initial position to the adjacent interstitial cite,its energy barrier must be higher than that of its reverse process to recover the lowest energy configuration.Most of the energy barriers that H jumping far away from the TB are higher than the reverse process,which indicates that H prefers to approach TB rather than get far away.In most cases,the H diffusion energy barrier near TB is lower than in the distance.(3)The pristine(11(?)1)TB shift energy barrier is very small,numerically 2.25 mJ/m2.In the H containing twin structures,H may prohibit the movement of TB.As the H concentration increases,the TB shift barrier increases,in other words,TB moves more difficultly.(4)As to the uniaxial tension normal to TB plane simulated in the fully relaxed mode,both the perfect bulk phase and the twin structures show complex structural changes under tensile strain.The hcp?fcc phase transitions occur in the test,and the ? phase appears at a higher H concentration,which increases the maximum tensile stress during the entire process.