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Modern dry‐pressing of ceramic powders using spray‐dried granulates cannot avoid the occurrence of defects related to persisting inter‐ and intra‐granulate interstitial voids. These constitute the parent defect size population limiting the application of polycrystalline ceramics in high‐stress conditions. The mitigation of such defects could widen the range of application in technical and biomedical engineering, reduce safe range for design and extend the lifetime of components. Here we use the Weibull size effect on strength in size‐partitioned YSZ feedstocks to explore the viability of changing the density distribution of granulate sizes as an effective strategy to obtain a denser particle packing that could reduce the size distribution of strength‐limiting pressing‐defects. In a direct assessment of critical defect size using multiscale strength testing with a dataset of ∽1300 values, we demonstrate here the success of such an approach in increasing the strength reliability for small volume components, but its ultimate failure in altering the defect size distribution in sintered YSZ ceramics across several length scales. We ultimately show that granule morphology (spherical or dimpled) fails to affect the defect density and size distribution in YSZ ceramics.This article is protected by copyright. All rights reserved.
Modern dry‐pressing of ceramic powders using spray‐dried granulates cannot avoid the occurrence of defects related to persisting inter‐ and intra‐granulate interstitial voids. These constitute the parent defect size population limiting the application of polycrystalline ceramics in high‐stress conditions. The mitigation of such defects could widen the range of application in technical and biomedical engineering, reduce safe range for design and extend the lifetime of components. Here we use the Weibull size effect on strength in size‐partitioned YSZ feedstocks to explore the viability of changing the density distribution of granulate sizes as an effective strategy to obtain a denser particle packing that could reduce the size distribution of strength‐limiting pressing‐defects. In a direct assessment of critical defect size using multiscale strength testing with a dataset of ∽1300 values, we demonstrate here the success of such an approach in increasing the strength reliability for small volume components, but its ultimate failure in altering the defect size distribution in sintered YSZ ceramics across several length scales. We ultimately show that granule morphology (spherical or dimpled) fails to affect the defect density and size distribution in YSZ ceramics.This article is protected by copyright. All rights reserved.
The low toughness and structural stability of nanostructured materials are strongly related to the numerous grain boundaries and interfaces. Among other design stratgies, the use of low‐energy boundaries has turned out to provide the most comprehensive improvement of the property spectrum targeting on ductility, toughness, as well as thermal and microstructural stability upon mechanical loading. Cyclic high‐pressure torsion (CHPT) is one prosperous technique to synthesize low‐angle boundaries (LAGB) at the nanoscale, enabling the production of high‐strength materials. It is presented here with an in‐depth analysis of the structural evolution focusing on the effect of different strain amplitudes and accumulated strains as well as crystal structure to understand how these parameters need to be adjusted to optimize the fraction of LAGBs. Different than expected from classical fatigue testing, the crystal structure seems to play a minor role for the cell structure evolution at comparably large strain amplitudes. It is, therefore, a strong asset that CHPT is feasible to produce nanostructures LAGB boundaries in both FCC and BCC structures. Furthermore, by optimizing the geometry of the anvils, it enables homogenous structural sizes in the entire sample as in contrast to other techniques the strain gradient impact on LAGB formation can be overcome.
Tungsten-copper composites are used in harsh environments because of their superior material properties. This work addresses a tungsten-copper composite made of 20 wt.% copper, which was subjected to grain refinement by high-pressure torsion, whereby the deformation temperature was varied between room temperature and 400 °C to tailor the grain size. Deformation was performed up to microstructural saturation and verified by hardness measurement and scanning electron microscopy. From the refined nanostructured material, micro-cantilever bending beams with cross-sections spanning from 5 × 5 to 35 × 35 µm2 were cut to examine possible size effects and the grain size influence on the fracture behavior. Fracture experiments were performed in situ inside a scanning electron microscope by applying a quasi-static loading protocol with partial unloading steps. Inspection of the fracture surfaces showed that all cantilevers failed in an inter-crystalline fashion. Nevertheless, remaining coarser tungsten grains impacted the resultant fracture toughness and morphology. Cantilevers fabricated from the 400 °C specimen exhibited a fracture toughness of 220 $$\pm $$ ± 50 $$\frac{{\text{J}}}{{{\text{m}}}^{2}}$$ J m 2 . For the room temperature cantilevers, a fracture toughness of 410 $$\pm $$ ± 50 $$\frac{{\text{J}}}{{{\text{m}}}^{2}}$$ J m 2 was observed, which declined to 340 $$\pm $$ ± 30 $$\frac{{\text{J}}}{{{\text{m}}}^{2}}$$ J m 2 for cantilevers < 10 × 10 µm2, confirming a size effect. The increased fracture toughness is attributed to the delamination-like structures formed in the room temperature sample.
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