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The Fe-1.0Cu (at. %) and Fe-1.2Cu-2.2Mn alloys aged at 450 °C for 0.25 h, 1 h, 2 h, and 16 h after solution treatment at 900 °C for 2 h are investigated to reveal the role of the addition of Mn on the Cu precipitates in Fe-Cu based alloys. Density functional theory (DFT) total energy calculations on point defects and their influence on Cu precipitates are also performed to understand the nucleation and growth of Cu precipitates. Experiments show that addition of Mn can slightly increase the aging peak hardness by 10 HV; by using atom probe tomography (APT) and optical microscopy, we identify that the increase in hardness derives from both grain refinement and the increase of number density of precipitates. DFT calculations show that Mn increases the formation possibility of Frenkel pairs, i.e., atomic vacancy and self-interstitial atoms, and these two types of defects both serve as nucleation sites of Cu precipitates, resulting in the increase of the nucleation centers number density, which is well consistent with our APT experiments on the very initial stage of aging. Moreover, calculated results show that Mn increases the density of atomic vacancy and promotes evolution rate of Cu precipitates, which accounts for our APT experiments that precipitates in Fe-Cu-Mn grow more quickly than that in Fe-Cu does. Finally, we also discuss the relationship between Mn content in reactor pressure vessel steels and its irradiation damage effects.

In this paper the relationship between the surface relaxations and the electron density distributions of surface states of Cu(100), Cu(110), and Cu(111) surfaces is obtained by first-principles calculations. The calculations indicate that relaxations mainly occur in the layers at which the surface states electrons are localized, and the magnitudes of the multilayer relaxations correspond to the difference of electron density of surface states between adjacent layers. The larger the interlayer relaxation is, the larger the difference of electron density of surface states between two layers is.

The strain engineering is an effective method to modulate the optical properties of germanium. The biaxial tensile strain has been extensively studied, most of the investigations focusing on biaxial tensile strain with equal in-plane strain at different crystal orientations, namely symmetric biaxial tensile strain. However, the effect of biaxial tensile strain with unequal in-plane strain at different crystal orientations, namely asymmetric biaxial tensile strain, has not been reported. In this paper, we systematically investigate the effect of asymmetric biaxial tensile strain on the band structure of Ge by using first-principle calculation.#br#We firstly calculate and analyze the dependence of band gap on strain for Ge with asymmetric biaxial tensile strain along three low Miller index planes, i.e., (001), (101) and (111). Then, we present the values of band gap and strain for some typical indirect-to-direct bandgap-transition-points under asymmetric biaxial tensile strain. Finally, we analyze the influence of biaxial tensile strain on the valance band structure. For the asymmetric biaxial tensile strain along the (001) plane, the indirect-to-direct band gap transition only occurs when the strain of one orientation is larger than 2.95%. For asymmetric biaxial tensile strain along the (101) plane, the indirect-to-direct band gap transition only occurs when the strain of one orientation is larger than 3.44%. Asymmetric biaxial tensile strain along the (111) plane cannot transform Ge into direct band gap material.#br#For asymmetric biaxial tensile strains along the (001) and (101) plane, the indirect-to-direct band gap transition points can be adjusted by changing the combination of in-plane strain at different crystal orientations. The value of bandgap of direct-band-gap Ge under biaxial tensile strain is inversely proportional to the area variation induced by application of strain. The asymmetric biaxial tensile strain along the (001) plane is the most effective to transform Ge into direct band gap material among the three types of biaxial strains, which are similar to the symmetric biaxial tensile strains.#br#In addition, the symmetric biaxial tensile strain will remove the three-fold degenerate states of valance band maximum, leading to a removal of the degeneracy between one heavy hole band and the light hole band. For biaxial tensile strain along the (001) and (101) plane, the asymmetric biaxial tensile strain could further remove the degeneracy between another heavy hole band and the light hole band.

It is observed that the addition of Nb or Ge to Zr alloy can improve its corrosion resistance. Because of the extreme importance of the mechanism of oxidation to corrosion properties of Zr alloy, we systematically investigate the O adsorption properties on pure Zr surface and Zr surface with Nb or Ge using first-principle calculations based on density functional theory. Firstly, we present the absorption energies to reveal the influences of Nb and Ge on the O absorption capacity of Zr surfaces, resepctively. According to the calculated absorption energies, we find that Nb and Ge reduce the oxygen absorption capacities of most of surfaces, and the only exception is that Nb enhances the oxygen absorption capacity of Zr(1120) surface. Moreover, the absorption energy of O at favorable site on Zr(0001) surface is much lower than on Zr(1010) or (1120) surface. Therefore, the initial oxidation of polycrystalline Zr should occur at Zr(0001) surface and the absorption capacity of this surface takes a predominant role in the initial oxidation of polycrystalline surface. Secondly, the segregation of Nb or Ge in Zr alloy is anisotropic. We find that the segregation of Ge to surface is exothermic, while the segregation of Nb to surface is endothermic. Nb and Ge migrate to Zr(0001) surface more easily than to Zr(1120) and Zr(1010) surfaces. Therefore, the influence of Nb or Ge on absorption property of Zr(0001) is much larger than that of Zr(1010) or (1120) surface. Based on the adsorption and segregation properties of Nb and Ge on Zr surfaces, both Nb and Ge can reduce the oxygen absorption capacity of Zr surface and inhibit the initial oxidation of Zr alloy surface, which can be used to understand the experimental observation that Nb and Ge can improve the corrosion resistance of Zr alloy. Finally, the electronic structure analysis shows that the influences of Nb and Ge on oxygen adsorption capacity of Zr surface are exerted by changing the d-band distribution. According to Hammer-Norskov d-band center theory, the absorption energy of absorpate on transition metal surface is mainly determined by d-band center. The addition of Nb or Ge to the Zr surface changes the location of d-band of the surface, which results in the variation of absorption energy of O atom on the Zr surface. For absorption at top site on each surface, it is found that the absorption energy of O only depends on the d-band center of the surface atom below the O atoms. However, for absorption at favorable sites on each surface, the absorption energy of O atom is influenced not only by the d-band center of surface atom, but also by atomic structural properties of the surface.