Ab initiodetermination of the traction–separation curve for a metal grain boundary: a critical assessment of strategies

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.

H induced decohesion of an Al grain boundary investigated with first principles: General conditions for instant breakage and local delayed fracture

“…In other words, the corrected T-S curve is able to withstand an external stress higher than σ t , but only beyond the transition stage. If the GB cohesive stress keeps rising from here, the strength of the atomic bonds across the GB plane will ultimately decrease to the point of breakage being triggered by thermal activation [36,38]. The GB separation δ c , by construction the point where the critical stress is reached, hence will be the highest value attainable for the intact GB -the ideal breakage point in the present simplified analysis.…”

“…Even for the least affected system examined (θ H = 5×10 -11 ), it seems questionable that a more accurate determination of the separated grain energies will change this conclusion by suggesting breakage by thermal activation at δ < δ c . In addition to such breakage having to be thermodynamically favoured, H would also have to reduce an appreciable energy barrier to bond breakage across the GB plane present at this GB separation [36]. At the larger values of θ H in Fig.…”

“…For a GB decorated with H, adopting the same strategy may produce incorrect answers, however. This is exemplified by the Al Σ5 [100] TGB, which undergoes a structural transformation during the loading stage, linked to the 'bump' on the T-S curve in [36]. This transition implies alterations (Fig.…”

“…For the H-free system (E(δ) from Eq. (7)), both the intact GB and the separated grain configuration energies have been computed [36], in accordance with the scheme of [14]. [36].…”

“…For most GBs, such a site should appear upon sufficient loading, as a consequence of sequential bond breakage and increasing alignment of remaining bonds with the GB normal. For the calculations discussed below, the fcc Al Σ5 [100] 36.87° twist GB (TGB) was used as the model system.…”