Helium Bubble Evolution In Ti Tritides

Tritium is a hydrogen isotope of considerable interest and has many important technology applications,especially in the nuclear engineering.Titanium and zirconium are attractive materials for storage of hydrogen characterized by high storage capacity,stability in air and low storage pressure.However,3He produced by the decay of tritium can lead to drastic changes of macroscopic properties of metal tritides.The behavior of helium in metals is an active research area for decades of years.A lot of researches had been carried out to observe the changes of properties of metal tritides due to helium accumulation.Nevertheless,what is missing from the literature on helium in materials is a systematic experimental study of helium bubbles growth and evolution with time in most metal tritides.This paper gives a systematical study of 3He bubble evolution in Ti tritide by transmission electron microscopy(TEM),X-ray diffraction and helium release from metal tritides.The bubble evolution in Ti tritide was studied by TEM.We show in our studies what we have learned regarding helium bubble nucleation,shape,distribution,growth and interaction,and helium release.The bubble size is increasing with helium content in the Ti tritide until at 0.063 He/Ti the bubble size reach the maximum of 1.3-1.6nm,after that the bubble did not grow up.The density of bubbles is increasing linearly with the helium content in the films.The biggest bubble size is determined by the energy of ejection of Ti in the lattice by 3He and the binding energy of 3He with the bubble.The bubbles began to interconnect with each other at 0.247 He/Ti.Bubbles in Ti tritide prefer to distribute between {111} planes.The shape of bubbles is restricted by the high surface energy of Ti tritide,while the distribution of bubbles is decided by the high strain energy.There are long strip inclusions distributed in films,no bubble is found distributed in the inclusions.Boundaries are the preferential sites for 3He migration than in grains,but only those where the boundary plane is approximately parallel to close-packed planes,which made cracks developing on the grain boundaries.The oxygen distribution on the grain boundary may be the reason for the preferential site of helium migration.The changes of diffraction peaks are studied by XRD during the aging of Ti tritide films.The(111)out-of-plane grains expand out-of-plane less than(311)out-of-plane grains because of bubbles distributed between(111)planes for the full storage of Ti tritide films.The evolution of the lattice parameters and full-width-half-maximum are the results of the generation and migration of helium in the lattice,clustering as bubbles and increasing density of bubbles,interconnection of bubbles,and the ejection of self-interstitial atoms(SIA),formation of dislocation loops,formation of dislocation and dislocation networks from SIA or clusters of SIA.The lattice expansion reach the maximum at 0.063 He/Ti with the bubble size and the concentration of helium in the lattice reaching the maximum.After that,the lattice began contract by the decrease of tritium concentration in the films due to the tritium decay and bubble density increasing until at 0.247 He/Ti the network of bubble interconnection began to produce.Two kinds of Ti tritide films were prepared to study the effect of micro structure on helium migration.The films prepared by electron-beam heating were multi-layered column crystal and the upper layer consist of the stacked flaky grains.The helium release rate of this kind of film change less with storage temperature fluctuate during aging before 0.06 He/Ti.Because of the loosing structure in the films of columnar grains,the distributed stress release easily by grains expansion and the bubble tend to congregate as strips.More cracks were developed on the grain boundaries in the film of column crystal and the crack were widening when near the film surface.The film prepared by resistant-heating consist of compact equiaxed grains with less oxygen contaminate.So there is less grain expansion and higher stress developed,the bubbles tend to interconnect with each other as strings.Cracks were found developed extend to the film surface in all these two kinds of films which may result in the helium accelerate releasing.Depleted zone were found in all these two kinds of films and there are large dislocation loops developed in this zone.The helium distribution was calculated by SRIM program in the TiD2Hex films prepared by helium ion implantation.The helium bubble size,density and distribution were studied in the TiD2Hex films with 60 KeV implanted energy.The bubble tend to congregate as loops and the fcc TiD films transform to hcp Ti film by the heat produced during the ion implantation.The grains in the ZrH2Hex films prepared by hydrogen-helium-Zr codeposition were all nanoparticle.Most of the bubble sizes was 1?1.4nm and almost no stress distributed in this kind of films.The bubble density were increasing linearly with the helium content in this kind of films.This paper provide a research method to study the helium behavior in metal tritides,show the helium bubble evolution during the whole storage of Ti tritide,and indicate that those ways can not simulate the helium behavior in the metal tritide by helium ion implantation and hydrogen-helium codeposition.The influence of the Ti tritide film structure on helium migration and the difference of helium migration between Ti and Zr tritides was also studied in this paper.These findings could greatly accelerated the progress in the search of metal tritides with high helium accommodation.