| Vacancy as the simplest lattice defect is very common in metals. The characters of the vacancy formation and the rules of the atom migration in the bulk,surface and grain boundary are very different. The existence of the vacancy in metals can affects directly the mechanical-physical and chemical property of the metals. The migration of vacancy is the dominant mechanism of atom transport or diffusion in processes like solid phase transformation, void formation and expanding, dislocation and interface migration. Many surface phenomena such as diffusion, surface roughening and segregation, oxidation, corrosion and crystal growth are directly referred to the vacancy at the surfaces. The migration of atom in GB can lead to microstructure development, many phase transformations, creep, and some modes of plastic deformation and fracture, etc. GB diffusion occurs by the diffusion of point defects along the GB. With the advances in photovoltaic technology in recent years, the requirements for the material properties become more sophisticated, and more and more people fling themselves into the thorough study of the point defects in crystal, especially from the micro-level to research the materials defect properties.In this paper, by using the modified analytical embedded-atom method (MAEAM) and a molecular dynamics (MD) method, many characters of the metals have be investigated, which include the formation mechanism of the di-vacancy in FCC metal Pt, the vacancy diffusion in Cu∑= 5 [001] and∑= 9[110] twist grain boundary, the diffusion of single adatom Cu on Cu (001) and (110) surfaces, atomic diffusion in the Fe [001]∑=5 (310) and (210) symmetric tilt grain boundary, the structural and atomic diffusive properties of the ordered Cu3Au (110) surface and the structure and the atomic diffusion properties of the∑=5 [001] twist grain boundary in B2 type intermetallic compound NiAl. The following conclusion can be obtained:(1) For migration of an isolated vacancy in FCC metal Pt, the favorable path is the INN migration. The formation energy of the di-vacancy is the lowest for the INN configuration. From the binding energy point of view, the interaction between the two vacancies is attractive for the INN configuration. The migration of the two vacancies having interaction is always to approach each other along the shortest paths. furthermore, the migration energy for INN-INN migration (real rotation) is the lowest. From the energy minimization point of view, the INN configuration is the most stable configuration of the di-vacancy.(2) For Cu∑=5 [001] twist grain boundary, the vacancy is favorable to be formed on the un-coincident site in the first, second and third layers near the GB plane and this case is enhanced following the third, second and first layers. A single vacancy either on un-coincident site or on coincident site in the forth, third and second layers is favorable to migrate to un-coincident site (its first-nearest-neighbor) in its adjacent layer near the GB. But for the first layer, the favorable migration path of the vacancy on the un-coincident site is between un-coincident sites of the first layer or from'2'or'3'to'cR'or'bR'of the first layer in the rotating grain, which is not the case for the vacancy on the coincident site'1'that is migrated difficultly. So, there are collective tendency of the vacancy in the GB. The effects of the GB on the vacancy formation and migration are only to the forth several layers.(3) For Cu∑=9 [110] twist grain boundary, the formation of the vacancy on sites'2','3','4'and '5'of the 1L are spontaneous especially on sites'2'and'4'. The effects of the GB on the intra-layer migration are mainly for 1L-1L, which result in the favorable migration mechanisms between'2'and'3','3'and'4','5'and'6'. The effects of the GB on the inter-layer migration are mainly for 1L-1L',2L-1L and 3L-1L related to the 1L. In detail, the favorable migration mechanisms are between '2' and 'd'','3' and 'e'','4' and 'b'','5' and 'g'' for 1L-1L', the vacancies on the 2L and 3L can easily migrate to the 1L especially from'a'to '4'or'7','b'to'4'or'9','c'to'9','d'to'2'or'7','e'to'2'or'7'for 2L-1L, and from'2'to '2','3'to'3','4'to'4','5'to'5'for 3L-1L. Combing the formation energy and the diffusive activation energy, The vacancy is favorable to be formed on the first layer, and the vacancy in the first layer of the un-rotating grain is favorable migrated to the first layer of the rotating grain, the vacancy in the second and third layers is favorable migrated to the first layrer in the un-rotating grain. Thus the vacancy is easily collected in the first layer.(4) When adatom migrated on Cu (001) and (110) surfaces, the increment curves of the system energy by hopping mechanism are symmetrical and the saddles are in their midpoints, but the ones by exchange mechanism are dissymmetrical and the saddles are always close to the initial hole positions of the adatom and away from the initial equilibrium positions of the exchanged atom. From the diffusion activation energy and the force acted on the adatom from other atoms, on Cu (001) surface, the diffusion of the adatom proceeds more easily by hopping mechanism than by atomic exchange mechanism, and on Cu (110) surface, the diffusion proceeded by hopping via long bridge is the easiest, and the one by hopping via short bridge is the most difficult.(5) For Fe [001]∑=5 (310) and (210) STGBs, the vacancy formation in the 2L is easier than the ones in the other layers, which indicates the vacancy concentration is the higher in the 2L than the ones in the other layers. For the intra-layer migration, the diffusion activation energies in the 2L are the lowest. For the inter-layer migration, The vacancies in the 1L-8L of (310) STGB as well as 1L-10L of (210) STGB are favorably migrated to the 2L. So the vacancy tends to aggregate near the GB plane.(6) For the mixed CuAu-terminated (110) surface of Cu3Au ordered alloy, the surface layer exhibits rippling that the Au atoms are raised above Cu atoms about 0.117A in the topmost layer. The displacements of the topmost two layers are comparatively larger, while the third layer relaxes slightly and there are no changes in the nether layers. The vacancy is most likely to be formed in the 1L, especially on the Au site. For Cu vacancy originally sited in the 2L and migrated intra-layer and inter-layer, the diffusion without causing the local disorder is the most favorable and the vacancy tends to migrate to the topmost layer. For the vacancy in the topmost layer of the mixed CuAu-terminated (110) surface, the circularity path is preferred over the beeline path due to its smaller diffusion activation energies.(7) For the E=5 [001] twist GB of B2-NiAl ordered alloy, the largest displacement occurs at the first layer parallel to the grain boundary. The coincident site atoms C and 1'are close to the GB and the un-coincident site atoms A and 2'are away from the GB. For the vacancy formation near the GB, the GB effects are maily on the first three layers. The formation energies on the CSs are always higher than the ones on the Un-CSs in the Al-termination grain. This situation is the opposite in the Ni-termination grain except the one in the 1L'. From the minimization of the energy, the vacancy are most easily formed on the A site in the 2L and A'site in the 1L'for the Al-termination and Ni-termination, respectively. For the vacancy originally sited on A site in the 2L, it tends to migrate to the CS-Ni site in the 1L'along the six step jump paths. For the vacancy originally sited on Un-CS-Ni and CS-Ni sites in the 1L', they easily migrated respectively along A'-A'and C'-C' paths. From the energy minimization, this without causing the local disorder migration is the most favorable. |
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