Synergetic Effect Of H And He Impurities In Ti₃AlC₂:first Principles Calculations

In the past 20 years,much research was conducted on a class of ternary layered compounds,Mn+1AXn(MAX)phases,where n = 1,2,3 or 3,M is an early transition metal,A is a group 3A or 4A element,and X is C or N.Due to the structural characteristics,MAX phases exhibit some outstanding properties combining merits of both ceramics and metals,such as low density,high strength and modulus,good high temperature stability,high thermal and electric conductivity,excellent thermal shock resistance and high temperature oxidation resistance.Therefore,the MAX phase compounds have a wide range of possible technological and engineering applications.Particularly,MAX phases could become a candidate for high temperature structure materials and protective coatings in future nuclear reactors.In the present work,we have performed first principles calculations to investigate the synergetic effect of H and He impurities with vacancies in Ti3AlC2.The structure and energies of Hn-He-VA1 complexes(n?4)and the He-He/He-H/H-H interactions have been studied.The impurity H atom prefers to occupy the tetrahedral site(Itetr-3)with the lowest formation energy-3.51eV,while the He atom prefers to occupy the octahedral site(Ioct-4)with a formation energy of 2.95eV.In the Ti-Al open space layer,the formation energies of interstitial H atoms follow the sequence of Eftetr<Efoct<Efhex1<Efhex2,but an anomalous sequence will occur for interstitial He atoms with Efoct<Eftetr<Efhex1<Efhex2In Ti3AIC2 with a pre-existing A1 vacancy,the most favorable site for a He atom is close to tetr-site,meanwhile the H atom preferentially deviates from the vacancy center with a separation of 1.3 A along the direction.Inside the A1 vacancy,both He-H(2.25 A)and H-H(2.67 A)distances are obviously greater than that of He-He(1.93 A).He-H and He-He show a weakly attractive interaction,but weak repulsion occurs in the H-H interaction.The He-VA1 complex can effectively stabilize a few H atoms leading to the formation of a H-He hybridized bubble,while the He atom subsequently suppress further aggregation of H atoms and blocks the hydrogen embrittlement and volume swelling caused by the H-He hybridized bubble.This result originates from two aspects.On the one hand,He prefers to bind with defects(particularly a vacancy)in Ti3AlC2 to form a He-defect cluster which blocks the formation of H2 molecules,and effectively suppress the H bubble formation.On the other hand,a strong attraction between H and a He-defect cluster in Ti3AlC2 drives the segregation of H atoms towards the He atom,leading to the accumulation of H atoms surrounding the He atom in the near-surface region,which limits the permeation depth of the H atom into the bulk.Both different mechanisms contribute to suppressing H blistering in Ti3AlC2.As a final statement,the synergetic effect will occur when the impurity H and He atoms are produced by transmutation reactions in a fusion reactor.The formation of a H-He hybridized bubble can block the hydrogen embrittlement and volume swelling Our theoretical results are not only helpful to illustrate the nucleation mechanism of H/He bubbles and the origin of H/He embrittlement of Ti3AlC2 under irradiation,but also helpful to the design and improvement of Ti3AlC2-based materials in the nuclear industrial field.