We reveal the microscopic vacancy trapping mechanism for H bubble formation in W based on firstprinciples calculations of the energetics of H-vacancy interaction and the kinetics of H segregation. Vacancy provides an isosurface of optimal charge density that induces collective H binding on its internal surface, a prerequisite for the formation of H 2 molecule and nucleation of H bubble inside the vacancy. The critical H density on the vacancy surface before the H 2 formation is found to be 10 19-10 20 H atoms per m 2. We believe that such mechanism is generally applicable for H bubble formation in metals and metal alloys.
We analyze Ge hut island formation on Si(001), using first-principles calculations of energies, stresses, and their strain dependence of Ge/Si(105) and Ge/Si(001) surfaces combined with continuum modeling. We give a quantitative assessment on strain stabilization of Ge(105) facets, estimate the critical size for hut nucleation or formation, and evaluate the magnitude of surface stress discontinuity at the island's edge and its effect on island stability.
Using a first-principles computational tensile test, we show that the ideal tensile strength of an Al grain boundary ͑GB͒ is reduced with both Na and Ca GB segregation. We demonstrate that the fracture occurs in the GB interface, dominated by the break of the interfacial bonds. Experimentally, we further show that the presence of Na or Ca impurity, which causes intergranular fracture, reduces the ultimate tensile strength when embrittlement occurs. These results suggest that the Na/ Ca-induced intergranular embrittlement of an Al alloy originates mainly from the GB weakening due to the Na/ Ca segregation.
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