Effects of Oxidation on the Nanoscale Mechanisms of Crack Formation in Aluminum

Abstract

Aluminum is an important engineering material used in a variety of applications. Under ambient conditions, a self‐limiting oxide layer forms on the aluminum surface and protects the underlying metal from further oxidation; this oxidation of aluminum affects its mechanical strength. Accordingly, we consider a simple, atomic‐level model of the effect of oxidation on crack formation by examining how cracks form in aluminum and its fully oxidized stable partner α‐Al2O3; the valence electron density plot of the latter is shown in the picture, in which a 1 Å crack has been introduced. We find that the sharp falloff in electron density at Al2O3 surfaces provides a nanoscale explanation for the brittleness of alumina. Structural relaxation diminishes the acceptability of simple interaction models in macroscale simulations and suggests such relaxation should be accounted for in the future.

Discrete dislocation modeling of fatigue crack propagation

“…For example, our phenomenological cohesive relation is not based on a quantitative consideration of oxidation kinetics and the small-strain formulation neglects changes in the surface geometry due to slip steps which can give rise to roughness-induced crack closure. These limitations can be addressed through ab initio calculations such as those recently undertaken by Jarvis et al [46], which could provide the basis for a more accurate cohesive relation, and through a finite strain discrete dislocation formulation which accounts for geometry changes.…”

Scaling of discrete dislocation predictions for near-threshold fatigue crack growth

“…When comparing different materials and presuming that s coh ϰE and d n ϰb, the scaling implies that ⌬K eff th ϰE√b. On the other hand, for a fixed material, but with varying cohesive energy, due for example to chemical effects [36,37], ⌬K eff th ϰ√f n . We also find that varying the flow strength by a factor of two has no effect on the predicted crack growth rate in the lower Paris law regime.…”

Universal binding-energy relation for crystals that accounts for surface relaxation

“…Only when the initial electron density bridges the crack does it heal. 6 Yellow ͑blue͒ signifies high ͑low͒ electron density.…”

“…In order to obtain converged finite element results, the cohesive zone size must be resolved by the mesh; for brittle materials, the cohesive zone size is atomistic, making the calculation prohibitively expensive. 4 Nanometer scale quantum mechanical calculations have provided insight into cracking at the atomic level, [5][6][7][8] but their extrapolation to the macroscopic scale is fraught with difficulty. Indeed, orders-of-magnitude mismatch exist between atomistic predictions of cohesive strengths and critical opening displacements [9][10][11] and measurements of tensile strength in brittle materials obtained from spallation tests, 12 the latter of which are often employed in engineering simulations.…”