Robert O Ritchie
University of California, Berkeley, Materials Sciences, Faculty Member
Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article... more
Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article explores the many nonordering origins of commonly reported features using a combination of experimental electron microscopy and multislice diffraction simulations, which suggest that diffuse intensities largely represent thermal and static displacement scattering. A number of observations may reflect additional contributions from planar defects, surface terminations incommensurate with bulk periodicity, or weaker dynamical effects.
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A low-carbon micro-alloyed (LCMA) steel with a body-centered cubic (bcc) crystal structure suitable for extremely low temperatures was developed by overcoming the intrinsic ductile-to-brittle transition in bcc alloys at cryogenic... more
A low-carbon micro-alloyed (LCMA) steel with a body-centered cubic (bcc) crystal structure suitable for extremely low temperatures was developed by overcoming the intrinsic ductile-to-brittle transition in bcc alloys at cryogenic temperatures. The excellent cryogenic-to-ambient impact toughness in the LCMA rolled plate results from its heterogeneous microstructure, which gradually changes from bamboo-like ultrafine grains (~ 1.1 μm) on the surface to relatively equiaxed coarse grains in the core (~ 3.4 μm), accompanied by a distinct texture gradient variation. The heterostructured LCMA steel displays a cryogenic impact toughness of ~200 J/cm 2 at 77 K, which is 24 times higher than the coarse-grained LCMA steel. Such high impact toughness of heterostructured LCMA arises from the coordinated deformation mechanisms over different length-scales coupled with delamination toughening. At 77 K, the heterostructured steel plate deforms by forming cellular sub-structures at the core to the surface, which refines the microstructure and promotes hetero-deformation induced (HDI) hardening to improve intrinsic toughening. Moreover, the subsequent delamination process induces extrinsic toughening by shielding and blunting the cracks, with the local plane-stress conditions induced by delamination promoting ductile fracture of the coarse grains in the core regions. This low alloy steel with its heterogeneous microstructure exhibits extraordinary impact toughness at cryogenic temperatures highlights the possibility of materials design strategies for sustainable development.
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Abstract Fatigue resistance is invariably critical for structural materials, but is rarely considered in the development of new bioinspired materials. Here the fatigue behavior and damage mechanisms of a nacre-like ceramic... more
Abstract Fatigue resistance is invariably critical for structural materials, but is rarely considered in the development of new bioinspired materials. Here the fatigue behavior and damage mechanisms of a nacre-like ceramic (yttria-stabilized zirconia) - polymer (polymethyl methacrylate) composite, which resembles human tooth enamel in its stiffness and hardness, were investigated under cyclic compression to simulate potential service conditions. The composite has a brick-and-mortar structure which exhibits a staircase-like fracture behavior; it displays a transition in cracking mode from the fracture of the ceramic bricks to separation along the inter-brick polymer phase with increasing stress amplitude. The nacre-like structure functions to induce crack deflection, increase the roughness of the crack surfaces, and promote the mutual sliding between bricks during fracture; this results in high fatigue resistance, which enhances the potential of this composite for dental applications.
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Abstract This work intends to manipulate the internal flow units in Zr55Cu30Ni5Al10 bulk-metallic glasses (BMGs) through plasma-assisted hydrogenation to generate a positive microalloying effect on plasticity. Based on the cooperative... more
Abstract This work intends to manipulate the internal flow units in Zr55Cu30Ni5Al10 bulk-metallic glasses (BMGs) through plasma-assisted hydrogenation to generate a positive microalloying effect on plasticity. Based on the cooperative shear model theory, serration-flow statistics during nanoindentation loading and creep tests during the holding stage were used to analyze the influence of hydrogen on the behavior of flow units in BMGs. Experimental observations showed that the hydrogen in the Zr55Cu30Ni5Al10 BMGs caused mechanical softening, plasticity improvement, and structural relaxation. Analysis also showed that the average volume, size, and activation energy of internal flow units in the BMGs all increased as a result of the increase in the hydrogen content. The hydrogenation in the BMGs was found to lead to a proliferation of shear bands, which promoted plasticity. The aggregation of these internal flow units reduced the stress required for plastic deformation through shear bands, ultimately causing softening and structural relaxation.
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Carbon nanotubes (CNTs) are an economical and multi‐functional nanofiller that can further elevate the versatile performance of fiber‐reinforced polymer (FRP). The past two decades have seen significant progress in the design,... more
Carbon nanotubes (CNTs) are an economical and multi‐functional nanofiller that can further elevate the versatile performance of fiber‐reinforced polymer (FRP). The past two decades have seen significant progress in the design, fabrication, and characterization of CNTs modified FRPs (CNT‐FRPs). The introduction of CNTs has been proven to enhance the key mechanical properties of CNT‐FRPs and endow the composite with additional functional properties. In this review, the fabrication routes of CNT incorporation into FRPs are first discussed, and then the critical effects of CNTs on various mechanical properties of CNT‐FRPs are described. Next, as a complement to the experimental results, modeling studies on CNT‐FRPs are included to reveal the underlying structural effects, followed by a discussion on the reinforcement and toughening mechanisms of CNT‐FRPs. The intent of this review is to provide a comprehensive summary on CNT modified FRP composites, and to shed light on the future resea...
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High-entropy alloys (HEAs), although often presumed to be random solid solutions, have recently been shown to display nanometer-scale variations in the arrangements of their multiple chemical elements. Here, we study the effects of this... more
High-entropy alloys (HEAs), although often presumed to be random solid solutions, have recently been shown to display nanometer-scale variations in the arrangements of their multiple chemical elements. Here, we study the effects of this compositional heterogeneity in HEAs on their mechanical properties using in situ compression testing in the transmission electron microscope (TEM), combined with molecular dynamics simulations. We report an anomalous size effect on the yield strength in HEAs, arising from such compositional heterogeneity. By progressively reducing the sample size, HEAs initially display the classical “smaller-is-stronger” phenomenon, similar to pure metals and conventional alloys. However, as the sample size is decreased below a critical characteristic length (~180 nm), influenced by the size-scale of compositional heterogeneity, a transition from homogeneous deformation to a heterogeneous distribution of planar slip is observed, coupled with an anomalous “smaller-is...
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Unidirectionally oriented architectures demonstrate a notable efficiency in enhancing the properties of macroporous materials, yet are difficult to construct in a time-and cost-effective fashion. Here a facile approach was exploited for... more
Unidirectionally oriented architectures demonstrate a notable efficiency in enhancing the properties of macroporous materials, yet are difficult to construct in a time-and cost-effective fashion. Here a facile approach was exploited for fabricating oriented macro-porous ceramic materials by employing natural graphite flakes as a fugitive material and preferentially aligning the flakes within ceramic matrices using accumulative rolling technique. Flaky to near-ellipsoid shaped pores with a homogeneous distribution were created in macro-porous zirconia ceramics with their porosity and microstructural characteristics adjustable by controlling the additive amounts of graphite flakes. The resulting materials exhibited a good combination of properties with high compressive strength up to over 1.5 GPa, which exceeds those of most other porous zirconia ceramics with similar porosities, along with low thermal conductivity of 0.92-1.85 Wm − 1 ⋅K − 1. This study offers a simple means for developing new oriented macro-porous materials with enhanced properties, and may promote their application by allowing for easy mass production.
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The year 2004 marked the beginning of a new era in the design of metallic materials, as the concept of multiple principal component alloys, commonly known as High-Entropy Alloys (HEAs), was proposed by Cantor and Yeh. The unexpected... more
The year 2004 marked the beginning of a new era in the design of metallic materials, as the concept of multiple principal component alloys, commonly known as High-Entropy Alloys (HEAs), was proposed by Cantor and Yeh. The unexpected single-phase microstructure, instead of the expected brittle intermetallic compounds, was attributed to the large entropy of mixing and immediately caught the attention of the scientific community. Today, HEAs are considered important advanced materials and a broad range of alloys using nominally the same design principle have been investigated. Despite that, the CrMnFeCoNi (Cantor) alloy stands out as the most successful HEA due to its outstanding mechanical properties and microstructure. In this scenario, variants of the Cantor alloy, named medium-entropy alloys (MEAs), are gaining significant interest as they display a better industrial potential than both HEAs and traditional alloys. These variants of the Cantor alloy with only three or four main elements result in 15 possible combinations. The microstructure of these alloys is discussed in terms of advanced characterization as well as thermodynamic parameters and computational simulation. Moreover, their phase stability is addressed over a wide range of temperatures and strain rates. The mechanical properties, especially the fracture toughness, of the CrFeCoNi and CrCoNi alloys have been reported to be even superior to those of the Cantor alloy and most modern engineering alloys. This is associated with the formation of a continuous sequence of strengthening mechanisms, including hierarchical twin networks, which serve to prolong the strain hardening. The present article reviews and critically assesses, for the first time, recent advances in these Cantor-derived MEAs.
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Studies have established associations between environmental and occupational manganese (Mn) exposure and executive and motor function deficits in children, adolescents, and adults. These health risks from elevated Mn exposure underscore... more
Studies have established associations between environmental and occupational manganese (Mn) exposure and executive and motor function deficits in children, adolescents, and adults. These health risks from elevated Mn exposure underscore the need for effective exposure biomarkers to improve exposure classification and help detect/diagnose Mn-related impairments. Here, neonate rats were orally exposed to 0, 25, or 50 mg Mn/kg/day during early life (PND 1-21) or lifelong through ∼ PND 500 to determine the relationship between oral Mn exposure and blood, brain, and bone Mn levels over the lifespan, whether Mn accumulates in bone, and whether elevated bone Mn altered the local atomic and mineral structure of bone, or its biomechanical properties. Additionally, we assessed levels of bone Mn compared to bone lead (Pb) in aged humans (age 41-91) living in regions impacted by historic industrial ferromanganese activity. The animal studies show that blood, brain, and bone Mn levels naturally decrease across the lifespan without elevated Mn exposure. With elevated exposure, bone Mn levels were strongly associated with blood Mn levels, bone Mn was more sensitive to elevated exposures than blood or brain Mn, and Mn did not accumulate with lifelong elevated exposure. Elevated early life Mn exposure caused some changes in bone mineral properties, including altered local atomic structure of hydroxyapatite, along with some biomechanical changes in bone stiffness in weanlings or young adult animals. In aged humans, blood Mn ranged from 5.4 to 23.5 ng/mL; bone Mn was universally low, and decreased with age, but did not vary based on sex or female parity history. Unlike Pb, bone Mn showed no evidence of accumulation over the lifespan, and may not be a biomarker of cumulative long-term exposure. Thus, bone may be a useful biomarker of recent ongoing Mn exposure in humans, and may be a relatively minor target of elevated exposure.
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SiC, as one of the most promising candidate ceramics for high temperature structural applications, offers many intrinsic advantages, including high melting temperatures, low density, and high elastic modulus. However, the use of SiC to... more
SiC, as one of the most promising candidate ceramics for high temperature structural applications, offers many intrinsic advantages, including high melting temperatures, low density, and high elastic modulus. However, the use of SiC to date has been severely limited by its poor fracture toughness (∽ 2-3 MPa√m for commercially available materials) and crack-growth resistance. Our study focuses on the development of silicon carbide as a potentially tough, high-temperature, and damage-tolerant material. The approaches include identifying roles of sintering additives in modifying grain morphology, effects of post-annealing on grain boundary phases, and possibility in introducing nanoscale precipitates in SiC grains. Central in these efforts is structural characterization using state-of-the-art electron microscopy.The first success was in situtoughening the SiC, by hot pressing in the presence of A1, B and C additions. Elongated and interlocked grains were developed surrounded by an Al-c...
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Many bioactive elements, long perceived as non-viable for material development, are now emerging as viable building blocks to encode material lifecycle and to ensure our harmonious existence with nature. Yet, there is a significant... more
Many bioactive elements, long perceived as non-viable for material development, are now emerging as viable building blocks to encode material lifecycle and to ensure our harmonious existence with nature. Yet, there is a significant knowledge gap on how bio-elements interface with synthetic counterparts and function outside of their native environments. Here, we show that when enzymes are dispersed as nanoclusters confined within macromolecular matrices, their reaction kinetics, pathway, and substrate selectivity can be modulated to achieve programmable polymer degradation down to repolymerizable small molecules. Specifically, when enzyme nanoclusters are dispersed in trace amount (~0.02 wt%) in polyesters, i.e. poly(caprolactone) (PCL) and poly(lactic acid) (PLA), chain-end mediated processive depolymerization can be realized, leading to scalable bioactive plastics for efficient sorting, such as recovery of precious metal filler from flexible electronics. Present studies demonstrate...
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Research Interests: Materials Engineering, Mechanical Engineering, Nuclear Engineering, Materials Science, Materials, and 10 moreManufacturing Engineering, Mechanical Properties of Materials, Structural Materials for Nuclear Reactors, Composite Material, Materials Design, Nuclear Fuel, Irradiation, TRISO, X ray Computed Tomography, and Special Nuclear Materials
Research Interests: Biochemistry, Evolutionary Biology, Genetics, Biophysics, Materials Science, and 15 moreHematology, Molecular Biology, Biotechnology, Biomimetics, Ecology, Nanoarchitecture, Mechanical Properties of Materials, Composite, Fabrication, Composite Material, Material, Exoskeleton, Fracture Toughness, Bioinspired Materials, and Combination
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Corrosion is a ubiquitous failure mode of materials. Often, the progression of localized corrosion is accompanied by the evolution of porosity in materials previously reported to be either three-dimensional or two-dimensional. However,... more
Corrosion is a ubiquitous failure mode of materials. Often, the progression of localized corrosion is accompanied by the evolution of porosity in materials previously reported to be either three-dimensional or two-dimensional. However, using new tools and analysis techniques, we have realized that a more localized form of corrosion, which we call 1D wormhole corrosion, has previously been miscategorized in some situations. Using electron tomography, we show multiple examples of this 1D and percolating morphology. To understand the origin of this mechanism in a Ni-Cr alloy corroded by molten salt, we combined energy-filtered four-dimensional scanning transmission electron microscopy and ab initio density functional theory calculations to develop a vacancy mapping method with nanometer-resolution, identifying a remarkably high vacancy concentration in the diffusion-induced grain boundary migration zone, up to 100 times the equilibrium value at the melting point. Deciphering the origin...
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Fine-grained nuclear graphite is one of the key structural materials for high temperature gas-cooled reactors as well as several Generation IV nuclear fission reactor designs. However, its deformation and fracture behaviours at elevated... more
Fine-grained nuclear graphite is one of the key structural materials for high temperature gas-cooled reactors as well as several Generation IV nuclear fission reactor designs. However, its deformation and fracture behaviours at elevated temperatures are not well understood. In light of this, the current study focused on investigating the flexural strength and fracture toughness of two fine-grained graphite (SNG623 and T220) using real-time X-ray computed micro-tomography imaging at room temperature, 750 • C and 1100 • C. Specifically, nonlinear-elastic fracture mechanics-based J R (Δa) R-curves at these temperatures were presented with evolution of damage and failure micro-mechanisms, local strain distributions and J-integral fracture analysis, purveying notable findings. Compared to the coarser-grained Gilsocarbon nuclear graphites used in the current UK Advanced Gas-cooled Reactors (AGRs), these modern fine-grained graphites display deficient fracture resistance in the form of far less stable crack growth prior to catastrophic fracture and reduced failure strain at 1100 • C. Moreover, their elevation in strength and toughness at high temperatures is remarkably lower than that of Gilsocarbon graphite. Based on in situ high-temperature Raman spectroscopy mapping, we believe that one of the major causes of this behaviour can be attributed to the smaller magnitude of 'frozen-in' residual stress relaxed at elevated temperatures compared with Gilsocarbon graphite.
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High or medium- entropy alloys (HEAs/MEAs) are multi-principal element alloys with equal atomic elemental composition, some of which have shown record-breaking mechanical performance. However, the link between shortrange order (SRO) and... more
High or medium- entropy alloys (HEAs/MEAs) are multi-principal element
alloys with equal atomic elemental composition, some of which have shown record-breaking mechanical performance. However, the link between shortrange order (SRO) and the exceptional mechanical properties of these alloys has remained elusive. The local destruction of SRO by dislocation glide has been predicted to lead to a rejuvenated state with increased entropy and free energy, creating softer zones within the matrix and planar fault boundaries that enhance the ductility, but this has not been verified. Here, we integrate in situ nanomechanical testing with energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) and directly observe the rejuvenation during cyclic mechanical loading in single crystal CrCoNi at room temperature. Surprisingly, stacking faults (SFs) and twin boundaries (TBs) are
reversible in initial cycles but become irreversible after a thousand cycles,
indicating SF energy reduction and rejuvenation. Molecular dynamics (MD)
simulation further reveals that the local breakdown of SRO in the MEA
triggers these SF reversibility changes. As a result, the deformation features in HEAs/MEAs remain planar and highly localized to the rejuvenated planes, leading to the superior damage tolerance characteristic in this class of alloys.
alloys with equal atomic elemental composition, some of which have shown record-breaking mechanical performance. However, the link between shortrange order (SRO) and the exceptional mechanical properties of these alloys has remained elusive. The local destruction of SRO by dislocation glide has been predicted to lead to a rejuvenated state with increased entropy and free energy, creating softer zones within the matrix and planar fault boundaries that enhance the ductility, but this has not been verified. Here, we integrate in situ nanomechanical testing with energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) and directly observe the rejuvenation during cyclic mechanical loading in single crystal CrCoNi at room temperature. Surprisingly, stacking faults (SFs) and twin boundaries (TBs) are
reversible in initial cycles but become irreversible after a thousand cycles,
indicating SF energy reduction and rejuvenation. Molecular dynamics (MD)
simulation further reveals that the local breakdown of SRO in the MEA
triggers these SF reversibility changes. As a result, the deformation features in HEAs/MEAs remain planar and highly localized to the rejuvenated planes, leading to the superior damage tolerance characteristic in this class of alloys.
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Bioinspired nacre-like structures are effective in toughening materials, yet are difficult to construct in magnesium-ceramic systems. Here, a set of magnesium-MAX phase composites with nacre-like lamellar and brick-and-mortar... more
Bioinspired nacre-like structures are effective in toughening materials, yet are difficult to construct in magnesium-ceramic systems. Here, a set of magnesium-MAX phase composites with nacre-like lamellar and brick-and-mortar architectures are fabricated by pressureless infiltration of the magnesium melt into ice-templated Ti3AlC2 ceramic scaffolds. The structure and mechanical properties of the composites are elucidated with a special focus on the effects of the types of architectures (lamellar or brick-and-mortar) and matrices (pure magnesium or AZ91D alloy) on the toughening mechanisms. The nacre-like architectures are found to play a role in blunting the cracks via plastic deformation and microcracking, and shielding the cracks from applied stress by promoting crack deflection and uncracked-ligament bridging mechanisms. These composites achieve a good combination of specific strength and fracture toughness, which are superior to many other reported magnesium-ceramic and nacre-li...
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Horns from the Bovidae family represent extremely tough natural composites that resist flexural and impact damage during combat. However, the microstructure characteristics of these horns’ sheaths and how this affects their mechanical... more
Horns from the Bovidae family represent extremely tough natural composites that resist flexural and impact damage during combat. However, the microstructure characteristics of these horns’ sheaths and how this affects their mechanical properties remain to be explored. In this work, we report that the Syncerus caffer horn sheath exhibits the highest tensile strength of 183 MPa and fracture energy of 36.8 MJ m3, superior to other reported bulk natural materials. Comprehensive structure characterizations pin down the key factors of the horn sheath as the corrugated lamellae morphology. Finite-element modeling verifies that the critical characteristics of curved corrugated lamellae have a profound effect on the flexural and impact behavior. Furthermore, the ridge region for the highly curved lamellar morphology corresponds to greater disulfide crosslinking with 2.6% sulfur, suggesting a purposeful evolution of the heterogeneous composition. This work aims to provide insights into the bio-inspired design of superior structural materials.
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Electronic waste carries energetic costs and an environmental burden rivaling that of plastic waste due to the rarity and toxicity of the heavy‐metal components. Recyclable conductive composites are introduced for printed circuits... more
Electronic waste carries energetic costs and an environmental burden rivaling that of plastic waste due to the rarity and toxicity of the heavy‐metal components. Recyclable conductive composites are introduced for printed circuits formulated with polycaprolactone (PCL), conductive fillers, and enzyme/protectant nanoclusters. Circuits can be printed with flexibility (breaking strain ≈80%) and conductivity (≈2.1 × 104 S m−1). These composites are degraded at the end of life by immersion in warm water with programmable latency. Approximately 94% of the functional fillers can be recycled and reused with similar device performance. The printed circuits remain functional and degradable after shelf storage for at least 7 months at room temperature and one month of continuous operation under electrical voltage. The present studies provide composite design toward recyclable and easily disposable printed electronics for applications such as wearable electronics, biosensors, and soft robotics.
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Isotropic and near-isotropic graphites are used as moderating material and major core components of operating nuclear fission reactors, such as the UK Advanced Gas-cooled Reactors (AGRs). As such materials are subjected to various... more
Isotropic and near-isotropic graphites are used as moderating material and major core components of operating nuclear fission reactors, such as the UK Advanced Gas-cooled Reactors (AGRs). As such materials are subjected to various stresses and loads in service, the deformation and fracture behaviour of nuclear graphite at operating temperatures is of critical importance to the integrity of the core. Due to experimental limitations, most mechanical tests performed to date to evaluate such properties in graphites have carried out at ambient temperatures, but these experiments can never fully describe the behaviour of graphite at realistic temperatures, i.e ., ~650°C for AGRs and ~1000°C outlet temperatures for Gen IV reactors. For reliable evaluations of the reactor core integrity, in situ at-temperature characterisation of nuclear graphites must be undertaken. In the current work, we present the first quantitative three-dimensional, under-load characterisation of the evolution of dam...
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Research Interests: Materials Engineering, Mechanical Engineering, Materials Science, Computed Tomography, Sintering, and 12 moreDigital Image Correlation, Ceramic matrix composites, Composite Material, Materials Design, Ceramic, Cracking, Flexural Strength, Oxide, Digital Volume Correlation, Shrinkage, X ray Computed Tomography, and Ceramics Composite
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Nacre’s structure-property relationships have been a source of inspiration for designing advanced functional materials with both high strength and toughness. These outstanding mechanical properties have been mostly attributed to the... more
Nacre’s structure-property relationships have been a source of inspiration for designing advanced functional materials with both high strength and toughness. These outstanding mechanical properties have been mostly attributed to the interplay between aragonite platelets and organic matrices in the typical brick-and-mortar structure. Here, we show that crystallographically co-oriented stacks of aragonite platelets, in both columnar and sheet nacre, define another hierarchical level that contributes to the toughening of nacre. By correlating piezo-Raman and micro-indentation results, we quantify the residual strain energy associated with strain hardening capacity. Our findings suggest that the aragonite stacks, with characteristic dimensions of around 20 µm, effectively store energy through cooperative plastic deformation. The existence of a larger length scale beyond the brick-and-mortar structure offers an opportunity for a more efficient implementation of biomimetic design.