An ab initio based atomistic model scheme for an approximate determination of the interfacial and strain energies for the entire interface of a fully coherent precipitate in a host lattice is presented. For each given presumed compositionally abrupt interface, the model incorporates the effect of the strain evolution along the interface by use of a sequence of supercells. Each cell in this sequence has been distorted to describe the local interface region in question with the optimal accuracy allowed by periodic boundary conditions. Together, the cells comprise a shell of nm thickness, enclosing the full interface and its strongly affected near vicinity. The computational demands for the scheme are connected with the number of atoms in a given interface region cell, i.e., no scaling with precipitate size occurs -other than the number of cells required. In practice, this allows performing calculations for essentially all precipitate sizes. The scheme has been tested for the case of the main hardening precipitate β'' in the Al-Mg-Si alloy system and compared quantitatively with presently available alternatives. Implementation in an atomic hybrid model scheme for a full description of the precipitate interface energy should be realistic.

No abstract

We have performed a uniaxial tensile test on the Σ5 [1 0 0] 36.87° twist grain boundary (GB) in face-centred cubic Al within the framework of density functional theory in order to derive an atomistic cohesive traction–separation law. Addressing the importance of kinetics to GB breakage, we accompanied our energy-separation curve calculations by two additional studies. Firstly, using the nudged elastic band method, we determined for a series of GB separations the heights of the zero temperature barriers separating intact and broken GB configurations. Secondly, a representative subset of these transition paths was examined at finite temperature with ab initio molecular dynamics. Contrasting prevalent conclusions on GB breakage behaviour, our results suggest that the GB likely stays intact at room temperature well into the range of separations where a broken GB represents the thermodynamically favourable configuration. Given the non-negligible resulting influence on critical tensile stress and work of separation, our findings may be viewed as stressing the need for a kinetic analysis in a general first principles based uniaxial tensile test.

The Zn-containing β" phase in Al-Mg-Si alloys has been investigated by aberration corrected high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), combined with density functional theory (DFT) calculations. The mean intensity of one Si site of the β" phase is higher than the other Si sites, suggesting partial Zn occupancy. DFT studies support that this Si site is competitive for Zn incorporation. While HAADF-STEM image simulations show an influence of the Zn distribution along the β" main growth direction, total energy calculations predict a weak Zn-Zn interaction. This suggests that Zn atoms are not clustering, but uniformly distributed along the atomic columns. The Zn incorporation has a weak influence on the β" phase where Zn is admitted as a "defect" according to the DFT studies.