Large amount of helium that produced in nuclear reactions gathering in materials can induce problems like helium releasing and materials embrittlement, which let devices failure. And many unique properties of helium in materials also have potential applications in areas such as semiconductor industry. Thus, the behavior of helium in materials has always been in focus. With background of highly helium retention hydrogen (tritium) storage materials, this paper focus on the behavior of helium in Ti and Ti alloy.
Based on the previous work on introduction of helium into materials, the structures of Ti, TiZr alloy and TiMo film are characterized using a transmission electron microscopy. It is found that helium can be in form of bubbles in these materials after annealing. The shapes of helium bubbles can be spheres or polyhedrons while most are shperized polyhedrons. After annealing more time at higher temperature, the bubbles are more like polyhedrons. Bubbles in alloy are more round than those in pure Ti, and bubbles on triple grain boundary are more round than those on grain faces. When two bubbles get closed, the parts nearest each other usually become round, indicating their amalgamation. The positions of helium bubbles can be in grains and on grain junctions (boundaries), the energy is lower for bubbles on junctions of more grains, and the presence of helium bubbles can be greatly changed in nano materials due to high fraction of junctions of four above grains. The growth of helium bubbles in alloys is much inhibited, and TiZr alloy inhibit more than TiMo alloy.
Many release peaks are found when applying thermal desorption on Ti film which contains helium and is annealed 1 hour at 800. It is considered that these release peaks ℃represent the helium releasing in different ways at temperatures. When heating at a constant velocity, the helium nearby the surface starts to release firstly; then the helium on grain boundaries and diclocations which are helium pipeline to surface, and next the
中国科学院金属研究所硕士学位论文 Ti 及 Ti 合金中氦的存在研究
helium of no pipeline to surface.
Ab initio calculations are employed to study the lattice swelling of a Ti atom. It is found the interactions between an octahedral He atom and its first coordination Ti atoms are almost the same when the distances are equal, and are mainly rejection. Due to the short distances between Ti-Ti atoms in HCP Ti, a He atom can swell an octahedral interstice 1/3 in volume, and due to the orientating feature of Ti-Ti interaction, Ti atoms nearest a He atom at the centre octahedral interstice are rejected to different distances.
The interaction potentials of a FCC Ti-He system are attained employing lattice inverse method. The potentials physically reproduce the gathering phenomenon of helium in materials. It was found helium gathering because of Ti-He rejection is stronger than He-He rejection, and when more He atoms coordinate each other, they can get spaces more easily. Another Ti-He two body interaction potential is attained through Møller–Plesset perturbation calculation, which reproduce the helium phenomenon in HCP Ti quite well when employed with an EAM Ti-Ti potential.