Papers by Robert O Ritchie
Science Advances, 2024
Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is ce... more Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is central to their application as structural materials. Materials discovery efforts for HEAs are therefore impeded by an incomplete understanding of the chemical factors governing this balance. Through first-principles calculations, this study explores factors governing intrinsic ductility of a crucial subset of HEAs-those with a body-centered cubic (bcc) crystal structure. Analyses of three sets of bcc HEAs comprising nine different compositions reveal that alloy chemistry profoundly influences screw dislocation core structure, dislocation vibrational properties, and intrinsic ductility parameters derived from unstable stacking fault and surface energies. Key features in the electronic structure are identified that correlate with these properties: the fraction of occupied bonding states and bimodality of the d-orbital density of states. The findings enhance the fundamental understanding of the origins of intrinsic ductility and establish an electronic structure-based framework for computationally accelerated materials discovery and design.
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Journal of The Mechanics and Physics of Solids, Aug 1, 2014
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Micron, Oct 1, 2007
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Journal of The Mechanical Behavior of Biomedical Materials, Oct 1, 2020
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Journal of The Mechanical Behavior of Biomedical Materials, Dec 1, 2016
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Research Square (Research Square), May 15, 2023
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Matter, Dec 1, 2020
https://doi.org/10.1016/j.matt.2020.10.029 We thank Huskey, Westneat, and Grubich for the detaile... more https://doi.org/10.1016/j.matt.2020.10.029 We thank Huskey, Westneat, and Grubich for the detailed account on the arapaima and piranha. Their article is not only informational but adds to our understanding of the complex relationship between evolution, environment, and predator-prey dynamics. We are by no means evolutionary biologists, but materials scientists, and the sole goal of our contributions has been to clearly demonstrate that the arapaima scales are a formidable biological structure with great toughness, among the highest among biological materials, with an impressive JIc fracture toughness of ~200 kJm . We unknowingly narrowed Currey’s conclusion about the protection offered by the arapaima scale to piranhas. He states: ‘‘The huge teleost freshwater fish Arapaima gigas, which can exceed 3 m in length, has somewhat similarly layered scales, which probably make it effectively immune to attack (Torres et al., 2008).’’ Torres et al. preceded us in the publication of the results and were the first to analyze the structure of the scale and to suggest their outstanding toughness. Thus, these scales are a superb defense against predators.
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Joule, 2023
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Proceedings of the National Academy of Sciences of the United States of America, Dec 14, 2005
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Social Science Research Network, 2019
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Acta Materialia, Feb 1, 2023
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Micron, Oct 1, 2005
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Freezecasting is considered one of the top candidates to fabricate highly structured porous ceram... more Freezecasting is considered one of the top candidates to fabricate highly structured porous ceramics for numerous applications including bone regeneration implants as well as structural materials. It consists in the unidirectional solidification of ceramic suspensions, thus forming complex microstructures made of ice crystals and ceramic layers, which are then freezedried to remove the ice, and sintered to strengthen the ceramic. It has been shown that the microstructure of the final scaffold can be precisely controlled by varying process parameters such as cooling rate, suspensions density, viscosity, etc. However, precise guidelines are still missing to materials scientists concerning optimal architectures with regards to each specific application. Here, we present our approach to quantitatively master the mechanics of freezecasting scaffolds, from elastic behavior, to onset of cracking and final rupture. Basically, we follow the recent work of Ladeveze & Lubineau for laminated composites. First, from extensive computations on cracked micro-cells, a computational bridge is built between the micro (here, the scaffolds walls and bridges, i.e. 10-100 microns) and macro (the specimen, 10 mm) scales. This bridge allows (i) to characterize the effect of damage on macroscopic stiffness, and (ii) to link macroscopic damage variables and more microscopic ones such as crack densities. Then, instead of using the equivalence between micro and macro energy release rates as in Ladeveze's work, we use the Weibull's theory of rupture probability to derive the damage variables and the mechanical load. At the end, the discrete and homogenized models, together with the computational bridge that link them, provide an extremely valuable insight into the mechanics of ceramic scaffolds made by freezecasting: in a single framework, we derive scaling laws for both stiffness and strength, which are key properties for numerous biomedical and structural applications. Moreover, augmented with a localization controller, the homogenized model will also be used to derive scaling laws for toughness, another property of first interest. These scaling laws will represent precious guidelines for materials scientists to optimize the freezecasting process, and build even stronger materials.
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Materials Today, 2023
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Metallurgical and Materials Transactions, Dec 1, 2005
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MRS Proceedings, 2008
ABSTRACTDynamic friction, wear volumes and wear morphology have been studied for sliding wear in ... more ABSTRACTDynamic friction, wear volumes and wear morphology have been studied for sliding wear in polysilicon in ambient air at μN normal loads using on-chip micron-scale test specimens. With increasing number of wear cycles, the friction coefficients show two distinct types of behavior: (i) an increase by a factor of two and a half to a steady-state regime after peaking at three times the initial value of about 0.10 ± 0.04, with no failure after millions of cycles; (ii) an increase by a factor larger than three followed by failure after ∼105 cycles. Additionally, the average nano-scale wear coefficient sharply increased in the first ∼105 cycles up to about 10−4 and then decayed by an order of magnitude over the course of several million cycles. For both modes of behavior, abrasive wear is the governing mechanism, the difference being attributed to variations in the local surface morphology (and wear debris) between the sliding surfaces. The oxidation of worn polysilicon surfaces only affects the friction coefficient after periods of inactivity (>30 min).
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Journal of The Mechanics and Physics of Solids, Feb 1, 2010
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Papers by Robert O Ritchie