An inverse Hall-Petch relation in nanocrystalline MgAl 2 O 4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Sokol, Maxim; Halabi, Mahdi; Mordekovitz, Yuval; Kalabukhov, Sergey; Hayun, Shmuel; Frage, N.

doi:10.1016/j.scriptamat.2017.06.049

Cited by 67 publications

(52 citation statements)

References 21 publications

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“…1 The origin of this phenomenon has been extensively discussed in the literature and is generally accepted to be resulting from the dislocation movement pinning by the presence of the increased grain boundary network, associated with the diminished dislocation density 2 . 6 Ryou et al have reported an increase in hardness with decreasing grain size for the same composition, reaching the maximum at a fairly similar grain size as that reported by Sokol et al,18.4 nm. In ceramics, contradictory data are found, even when addressing the samples with same nominal composition.…”

Section: Introductionsupporting

confidence: 68%

“…Although similar behavior has been observed for both metals and ceramics, in metals the existence of an inverse Hall-Petch relationship, i.e., a low-limiting grain size below which the linear relation is no longer observed, is commonly reported in the literature and attributed to a more dominant role of grain boundary sliding and dislocation interactions when grain sizes are below 20 nm. 6,7 The cause for the inverse relation was linked by Ryou et al to the large fraction of interfacial volume that such small grain size introduces to the system, allowing a large population of triple junctions that can nucleate subsurface cracks during indentation. For instance, MgAl 2 O 4 has been the focus of several works targeting reduction of grain sizes to examine the effect on hardness.…”

Section: Introductionmentioning

confidence: 99%

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Grain boundary strengthening in nanocrystalline zinc aluminate

2019

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Fully dense transparent zinc aluminate ceramics with nanoscaled grain sizes were fabricated by Deformable Punch Spark Plasma Sintering (DP‐SPS). Optical transmission spectra showed high transparency, with up to 70% transmitted light in the visible spectrum. Vickers hardness was measured and grain boundary strengthening observed, showing hardness increase from 18.2 GPa up to 22.5 GPa as the grain sizes decreased from 60.3 to 10.1 nm. The trend followed the Hall‐Petch relationship, with hardness linearly proportional to the inverse of square root of grain size. A low grain size limit reported in previous literature below which hardness decreases, known as inverse Hall‐Petch relationship, was not observed within the studied grain size range. Cross‐sections of the hardness tests' indentations were prepared by focused ion beam and observed by electron microscopy and showed radically different crack patterns underneath the indentation imprint when contrasting samples with dissimilar grain sizes, shedding light on the mechanisms behind the observed grain boundary hardening mechanisms.

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Section: Introductionsupporting

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Grain boundary strengthening in nanocrystalline zinc aluminate

2019

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“…In previous studies, MgAl 2 O 4 hardness values as high as 28.4 GPa were achieved when the grain size was on the nanoscale, in accordance with the Hall–Petch relationship . However, an inverse Hall–Petch relationship was also observed, characterized by a decrease in the hardness with a decrease in the grain size below the 30‐nm, with the mechanical properties following an inverse scaling law. This suggests that different mechanisms may influence the size‐dependent hardness increase and must be understood for the optimized production of nanocrystalline ceramics.…”

Section: Introductionsupporting

confidence: 66%

Li₂O‐doped MgAl₂O₄ nanopowders: Energetics of interface segregation

et al. 2019

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Manufacturing nanoceramics is challenging owing to the instability of the grain size resulting from the high driving force toward growth associated with the interfaces. Nanometric ceramics of some oxides have exceptional mechanical and optical properties, eg, magnesium aluminate spinel (MgAl 2 O 4 ). The production of these fully conformed ceramics requires a precursor powder, which generally contains sintering-promoting additives. Li salts are typically used as sintering promoters for MgAl 2 O 4 , but the interface stability associated with the segregation of the additive is poorly understood. In this study, MgAl 2 O 4 samples containing 0-2.86 mol% Li ions were synthesized via a simultaneous-precipitation method in an ethylic medium and subsequently calcined at 800°C in air. The nanopowders exhibited only the MgAl 2 O 4 phase, and the crystallite size was determined by the Li 2 O concentration. The crystallite size was changed via the chemical modification of the interfaces by the segregation of Li ions. The solubility in the bulk material was very low at the fabrication temperature, and small amounts of Li ions saturated the bulk material and segregated to the grain boundaries (GBs), significantly stabilizing the grain-grain interface compared with the surface. The resulting powder was then aggregated further owing to the initial stage of sintering. The surface excess obtained via the selective lixiviation method confirmed that the segregation to the GBs was greater than that to the surface. Energetics calculations confirmed these results, indicating a high enthalpy of segregation at the GBs (ΔH segr GB = −52.0 kJ∕mol) compared with that at the surfaces (ΔH segr s = −31.5 kJ/mol). The enthalpy of segregation together with theinterface excess allowed us to estimate the reduction in the interface energy with Li + segregation of 0.8% to the surface and 11.2% to the GBs. The Li + segregation to the surfaces started by Al 3+ substitution, and for powders with ≥1.8 mol% Li ions, Mg +2 was preferentially substituted at the surfaces.

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“…Grain size refinement, with the resulting expansion of the grain boundary network, may act as an additional effective barrier for dislocations, further reducing plasticity and increasing hardness—often referred to as Hall‐Petch relation . In ceramics, this behavior is more pronounced when grain sizes are below 100 nm, but as grain sizes are further reduced, several studies also report, instead, the existence of an inverse relation as grains are refined below a critical size …”

Section: Introductionmentioning

confidence: 99%

Size‐induced grain boundary energy increase may cause softening of nanocrystalline yttria‐stabilized zirconia

et al. 2019

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An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size‐induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria‐stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra‐fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.

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An inverse Hall-Petch relation in nanocrystalline MgAl 2 O 4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Cited by 67 publications

References 21 publications

Grain boundary strengthening in nanocrystalline zinc aluminate

Grain boundary strengthening in nanocrystalline zinc aluminate

Li₂O‐doped MgAl₂O₄ nanopowders: Energetics of interface segregation

Size‐induced grain boundary energy increase may cause softening of nanocrystalline yttria‐stabilized zirconia

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An inverse Hall-Petch relation in nanocrystalline MgAl 2 O 4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Cited by 67 publications

References 21 publications

Grain boundary strengthening in nanocrystalline zinc aluminate

Grain boundary strengthening in nanocrystalline zinc aluminate

Li2O‐doped MgAl2O4 nanopowders: Energetics of interface segregation

Size‐induced grain boundary energy increase may cause softening of nanocrystalline yttria‐stabilized zirconia

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Li₂O‐doped MgAl₂O₄ nanopowders: Energetics of interface segregation