In order to improve our understanding of the fracture behaviour in nanocrystalline micro-cantilevers, it is necessary to elucidate the multiaxial stress and strain fields throughout their irreversible deformation, especially in the regime where simplified homogeneous linear elastic assumptions are not valid anymore. In this work, cross-sectional X-ray nanodiffraction (CSnanoXRD) with a spatial resolution of 200 nm was coupled with an in situ indentation device to uncover the multi-axial strain fields associated with crack growth. Here, (i) a notched clamped cantilever prepared from a multi-layered thin film composed of four alternating brittle CrN and semi-ductile Cr layers on high-speed steel and (ii) a freestanding cantilever fabricated from a high-pressure torsion processed nanocrystalline FeCrMnNiCo alloy were in situ stepwise loaded. Both cantilevers were manufactured by consecutive femto-second laser ablation and focused ion beam milling.
The Cr/CrN clamped cantilever was loaded stepwise to 150 and 460 mN and multi-axial stress distributions were retrieved in a region of interest of 40 × 30 µm2. An effective negative stress intensity of −5.9±0.4MPa m½ accompanied by a plastic zone extending up to 1.4 µm around the notch tip arose in the notched Cr sublayer as a consequence of residual stress in the thin film. The in situ experiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, crack growth in the notched Cr layer to the adjacent CrN-Cr interface occurred at a critical stress intensity of 2.8±0.5MPa m½.
The freestanding FeCrMnNiCo cantilever was loaded to 22, 45 and 34 mN loads, which corresponds to conditions where elastic loading, crack tip blunting and void formation and coalescence with the crack front are the governing mechanisms, respectively. In that case, CSnanoXRD data were evaluated in a region of 30 × 35 µm2 centered around the crack tip. At a load of 22 mN, a bending stress up to ~±1 GPa was evaluated, while directly in front of the notch the crack opening stress raised to ~4 GPa. In a 200 nm circular zone around the notch the measured stress distributions deviated evidently from the linear-elastic fracture mechanics assumptions. At 45 mN, crack opening stresses increased to ~4.5 GPa and up to 1 µm from the crack tip a distinct plastic zone formed. Further loading lead to a breakdown of the commonly assumed crack tip singularity and a significant decrease of the evaluated stress magnitude.
The quantitative experimental stress results provide unprecedented insights into the gradual stress evolution at the crack tip and across the cantilevers as well as associated fracture processes in nanocrystalline materials.