| 其他摘要 | As the pure metal containing the highest hydrogen density, titanium (Ti) is widely used to deposit hydrogen (H) and its isotopes (deuterium (D) and tritium (T)) for the high thermodynamic stability of its hydride, its relatively low price and the easy preparation of its film samples. As H and its isotopes are used widely in high technological areas, pure Ti can no longer satisfy the requirement to store H and its isotopes, and the demand for new type storage materials with better mechanical strength and longer application life are increasing. However, due to the difficulty of obtaining large quantity of T, the radioactivity of T and the long age time, both the cycle period and the cost of alloying effect experiments will be dramatically huge.
H-induced embrittlement in superalloys and intermetallics has been studied for many years, and many alloying elements and technical treatments that can suppress H-induced embrittlement have been found, for example, adding boron (B) and zirconium (Zr) in Ni3Al alloys. However, due to the difference in the composition and treatment of experimental samples and the existence of kinds of defects, the understanding of the mechanism of H-induced embrittlement and the suppression mechanism of alloying elements still can not reach to agreement.
The advantage of first-principles study is its ability to investigate the influence of some specific factor on the characteristic of materials by controlling the compositions and structures of materials via computational simulation, which makes it able to find appropriate alloying elements for stabilization of helium (He), to understand the mechanism of H-induced embrittlement and the suppression mechanism of alloying elements on the H-induced embrittlement. Therefore, in this paper, the first-principles discrete variational method within the framework of density functional theory (DFT) is used to study the occupation behaviors of impurities and the alloying effect in two systems.
The first one is about the occupation behavior of He in titanium ditritide and the alloying effect of transition metals on He. Two models are established to study the conditions with low and high He concentration, respectively:
1）In the low He concentration condition, He atom prefers to stay at the original tetrahedral interstice rather than the octahedral interstice of larger space;
2）In the low He concentration condition, the alloying effect of the 3d and 4d transition metals rank in the descending order as: Nb > Y > Zr > Pd > Ru > Tc > Rh > Cr > Mo > Ag > Ti > V > Mn > Sc > Fe > Co > Ni > Cu > Cd > Zn;
3）In the high He concentration condition, the interaction between He atom and metal atom changes from bonding to antibonding, resulting in the moving of He atom from tetrahedral interstices to octahedral interstices;
4）In the high He concentration condition，the alloying effect of the 3d and 4d transition metals rank in the descending order as: Y > Nb > Mo > Zr > Cr > Tc > Ru > Rh > Cu > Sc > V > Ti > Mn > Co > Fe > Ni > Pd > Ag > Cd > Zn;
5）The He-stabilization mechanism of transition element differs with He concentration: in the low He concentration condition, the desired alloying element is to decrease the mobility of He atom by trapping the He atom around it deeply at tetrahedral interstices to prevent the formation of large He bubble; in the high He concentration condition, the ideal alloying elements should have as small repulsive interaction as possible with He atoms to postpone the outbreak of He bubbles.
The second one is on the influence of lattice misfit on the occupation behaviors of H and B in Ni-Ni3Al systems and the effect of B and Zr on the ductility and H-induced embrittlment of Ni-Ni3Al systems:
1）Under small misfit, both H and B prefer to occupy the Ni-rich interstices, but B has priority over H to take such interstices with lower impurity formation energy;
2）When misfit increases, the preferring site of B will change from that in Ni phase region to that in Ni/Ni3Al interface region, while that of H sticks to that in Ni phase region, which exclude the opinion that grain boundary or interface is the trap of H;
3）Based on the Rice-Wang thermodynamic model and theoretical maximum shear stress model, a method using bond order is proposed here to evaluate the influence of impurity on the Griffith work of interfacial cleavage and the maximum shear stress. It is found that H and B have inverse influence on the Griffith work of interfacial cleavage and the maximum shear stress in the Ni/Ni3Al interface region, confirming that H induces embritlement while B contributes to ductility in Ni-Ni3Al alloys;
4）Zr is found to prefer to segregate to Ni-rich region, resulting in its segregation tendency in Ni-Ni3Al alloys ranks in the descending order as: Ni phase region > Ni/Ni3Al interface region > Ni3Al phase region, and making it prefers to substitute for Al atom rather than Ni atom in Ni3Al phase;
5）Bond order analysis indicates that Zr has ductility effect in all the regions in Ni-Ni3Al alloys, respectively, and the effect ranks in ascending order as: Ni phase region < Ni/Ni3Al interface region < Ni3Al phase region;
6）Comparing the influence of B and Zr on the formation energy of H, B is found to increase the formation energy of H no matter where it is, Zr in Ni phase or Ni/Ni3Al interface region has similar but limited effect on the formation energy of H, which is good to suppress H-induced embrittlement; however, Zr in Ni3Al phase decreases the formation energy of H, which is one of the reasons that why Zr can not suppress the environmental embrittlement in Ni3Al alloys as B does. |
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