In-situ TEM Research On The Irradiation Behavior And Mechanism Of Palladium During Hydrogen And Helium Irradiation

Owing to its high resistance to oxidation,wide plateau of pressure-composition isotherms,fast rate of hydrogen absorption and desorption,palladium(Pd)is widely applied in tritium engineering.However,tritium is radioactive,and will decay into 3He,which can interact with hydrogen isotopes and seriously deteriorate Pd service life(i.e.,tritium aging effect in Pd).Therefore,the study of the interaction and synergistic effects of He and hydrogen isotopes in Pd can not only deeply understand the tritium aging mechanism of Pd,but also provide an important reference for the development of tritium engineering materials,which is of fundamental theoretical and high technological importance.In this thesis,ion beams of He+,H2+,He+&H2+ were used to irradiate Pd to simulate the interaction and mechanism of helium and hydrogen isotope in service.The nucleation,growth,migration and aggregation of bubbles,dislocation loops and the bubble-loop complexes at different temperatures,thicknesses,doses and concentrations of hydrogen&helium were observed by in-situ transmission electron microscopy(TEM).On the basis of qualitative and quantitative analysis of the size,quantity and distribution characteristics of these defects,the interaction mechanism of H and He was deeply explored and the tritium aging behavior of Pd was revealed.Frank dislocation loops(FDLs)with b=1/3<111>and the perfect dislocation loops(PDLs)with b=1/2<110>were induced by He+,H2+,He+&H2+ irradiation.However,most of loops were FDLs at low irradiation fluence.With the increase of irradiation fluence,the size of dislocation loops increased,but loop volume number density remained almost constant until dislocation loops merged and evolved into dislocation network.The thickness of TEM foil obviously affected the size and number density of dislocation loops,and the characteristics of bubble-loop complexes.The low density of loops with large size presented in a thin foil.The loop growth rate showed a linear relation with the production rates of Frenkel pairs,and it was dominated by the sample surface at the onset of irradiation.Afterwards,it encountered a slight drop-down due to the unfavorable energy for the growth of a large-size faulted loop in the presence of a large number of vacancies(or vacancy-clusters)at a higher dose.With the single implantation of He+,the ratio of 1/3[111]and l/3[111]loops,1/3[111]loops,and 1/3[111]loops were 75%,12.5%,and 12.5%,respectively,in the region of 80 nm thickness;but 100%loop were 1/3[111]loops at the monitored region with 50 nm thickness.The bubble-loop complexes could be seen only when the irradiation fluence was up to 5.0 × 1015 ions/cm2 at the monitored region with a thickness of 50 nm.However,lower ion fluence was needed for the appearance of bubble-loop complexes at the sample thickness of 80 nm.Irradiation temperature obviously affected the morphology,density,and size of dislocation loops.At lower temperatures(RT and 473 K),the self-climbing movement of dislocations caused clusters of SI A to aggregate,thereby forming high-density small-size irradiation defects,whereas the formation of large-size bubble-loop complexes was observed at 573 K.Most of loops determined as 1/3<111>FDLs had quasi-circular shape under low dose irradiation at 573 K;while the 1/2<110>PDLs transforming from 1/3<111>FDLs exhibited wavy and polygonal shape at high dose.The shape,distribution and size of bubbles were obviously affected by irradiation temperature.Small and spherical bubbles were observed at 473 K,whereas large and non-spherical bubbles with non-uniform sizes were present inside bubble-loop complexes at 573 K.The heterogeneity in size and shape of bubbles could be attributed to the interaction between bubbles and the periphery sessile dislocation loop through emission and absorption of SIAs from cascades as well as the bubble retention from the anchoring effects of dislocation loops.Moreover,the presence of hydrogen could inhibit the diffusion and enhance helium retention,resulting in the formation of larger bubbles.There were interactions between the preexisting dislocation and the irradiation-induced dislocation loops.A defect-free zone was formed near the dislocation line,which reduced the volume number density of loops.The implantation of H helped to reduce the effect of dislocations and obstacles,and promoted the slip and climb of the dislocation.In-situ observation showed that there was an obvious interaction between dislocation loops and bubbles,indicating that 1/3<111>loop was first formed at the initial stage of irradiation,and when the loop grew to a certain size,obvious helium bubble appeared inside its region.With the increase of irradiation fluence,bubbles began to appear outside the dislocation loops in the Pd matrix.The formation of dislocation networks due to the interactions between largesize bubble-loop complexes could induce irradiation hardening.And the heterogenous segregation of bubbles along the peripheries of wavy-shape perfect loops at a high dose could increase the likelihood of void swelling.These two factors probably resulted in the aging of metal Pd.FDLs can transform into PDLs.The process included three mechanisms:two FDLs unfaulted the stacking fault structure and transformed a PDL,a FDL interacted with a PDL and unfaulted the stacking fault structure to form the large-size PDL,and Shockley partial dislocations nucleated inside the FDL and then spread across the loop removing the stacking fault.The unfaulting of FDLs depends on loop interactions,while loop energy is a necessary but insufficient condition for loop unfaulting.The difference in growth behavior between faulted and perfect loop suggested that there existed a critical transition size～32 nm in diameter.The order of hydrogen and helium trapping ability is:PDLs>FDLs>the region outside the loops.The unfaulting process of FDLs has an important role in the hydrogen isotope absorption performance of Pd.