Tijmen Vermeij

The interplay between alloy chemistry and nanoscale architecture governs strength and fracture in Al-based metal/ceramic nanolaminates, yet whether these gains are structurally usable remains unresolved. Here, monolithic Al100-zNiz thin films are compared with hybrid Al100-zNiz/AlOxHy nanolaminates (z≤5 at. %), combining nanoindentation, orientation-dependent micropillar compression over varying strain rate and temperature, microtensile testing, microcantilever bending, and post-mortem (scanning) transmission electron microscopy to decouple the roles of Ni alloying and 1 nm atomic layer deposition (ALD)-AlOxHy interlayers on the strength-ductility trade-off.Ni alloying strengthens Al, whereas the architectural (ALD interlayer) strength contribution through refinement diminishes with increasing Ni content, marking a shift from layering- to grain-refinement-driven strengthening that reaches nearly 2 GPa yield stress in Al95Ni5/AlOxHy: the strongest architecture in the series. The applied micromechanical framework combined with S/TEM confirms grain-boundary-mediated shear localisation as the origin of its discrete deformation. This strengthening comes at the expense of toughness: brittle fracture is found in Al95Ni5/AlOxHy versus ductile Al/AlOxHy and Al95Ni5, establishing lateral grain size, and not interlayer chemistry, as the governing parameter linking strength and fracture resistance. This offers a transferable design rule that proposes Al grain size above ~10 nm for pushing strength without sacrificing toughness in this class of architectures.

Deformation twinning is an important plasticity mechanism in hexagonal close-packed (hcp) metals. While the {10-12} twin is extensively studied, the interaction and secondary twinning of the {11-21} extension twin remains less well understood. This study investigates these mechanisms in single-crystal rhenium using in-situ transmission Kikuchi diffraction tensile testing along the [0001] axis. We observe two distinct secondary twinning phenomena. First, native secondary twins form abundantly within primary twins, independent of twin interaction sites. Their nucleation is rationalized by their high Schmid factor and the energetically favorable dissociation of basal dislocations at primary twin boundaries. Second, specific twin-twin interactions trigger a distinct secondary variant selection producing a crossing-like morphology. However, these configurations violate classical geometric criteria for twin crossings (Cahn, Acta Met., 1953). Instead, we demonstrate that this non-classical variant selection is governed by the local stress state and plausibly facilitated by the dissociation of non-basal dislocations. These findings establish a stress- and dislocation-based framework for predicting secondary twinning in {11-21} extension twins, offering new insights into deformation anisotropy of hcp metals.