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The modern understanding of metal plasticity and fracturing began about 100 years ago, with pioneering work; first, on crack-induced fracturing by Griffith and, second, with the invention of dislocation-enhanced crystal plasticity by Taylor, Orowan and Polanyi. The modern counterparts are fracture mechanics, as invented by Irwin, and dislocation mechanics, as initiated in pioneering work by Cottrell. No less important was the breakthrough development of optical characterization of sectioned polycrystalline metal microstructures started by Sorby in the late 19th century and leading eventually to modern optical, x-ray and electron microscopy methods for assessments of crystal fracture surfaces, via fractography, and particularly of x-ray and electron microscopy techniques applied to quantitative characterizations of internal dislocation behaviors. A major current effort is to match computational simulations of metal deformation/fracturing behaviors with experimental measurements made over extended ranges of microstructures and over varying external conditions of stress-state, temperature and loading rate. The relation of such simulations to the development of constitutive equations for a hoped-for predictive description of material deformation/fracturing behaviors is an active topic of research. The present collection of articles provides a broad sampling of research accomplishments on the two subjects.

dislocation mechanics --- yield strength --- grain size --- thermal activation --- strain rate --- impact tests --- brittleness transition --- fracturing --- crack size --- fracture mechanics --- Hall-Petch equation --- Griffith equation --- size effect --- mechanical strength --- pearlitic steels --- suspension bridge cables --- dislocation microstructure --- fractal analysis --- plasticity --- representative volume element --- dislocation structure --- dislocation correlations --- dislocation avalanches --- nanotwin --- nanograin --- Au–Cu alloy --- micro-compression --- Cu-Zr --- ECAP --- deformation --- quasi-stationary --- subgrains --- grains --- coarsening --- Cu–Zr --- ultrafine-grained material --- dynamic recovery --- transient --- load change tests --- Charpy impact test --- GMAW --- additive manufacturing --- secondary cracks --- anisotropy --- linear flow splitting --- crystal plasticity --- DAMASK --- texture --- EBSD --- crack tip dislocations --- TEM --- grain rotation --- fatigue --- dislocation configurations --- residual stress --- indentation --- serration --- temperature --- dislocation --- artificial aging --- solid solution --- loading curvature --- aluminum alloy --- holistic approach --- dislocation group dynamics --- dynamic factor --- dislocation pile-up --- yield stress --- dislocation creep --- fatigue crack growth rate

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The present collection of articles focuses on the mechanical strength properties at micro- and nanoscale dimensions of body-centered cubic, face-centered cubic and hexagonal close-packed crystal structures. The advent of micro-pillar test specimens is shown to provide a new dimensional scale for the investigation of crystal deformation properties. The ultra-small dimensional scale at which these properties are measured is shown to approach the atomic-scale level at which model dislocation mechanics descriptions of crystal slip and deformation twinning behaviors are proposed to be operative, including the achievement of atomic force microscopic measurements of dislocation pile-up interactions with crystal grain boundaries or with hard surface coatings. A special advantage of engineering designs made at such small crystal and polycrystalline dimensions is the achievement of an approximate order-of-magnitude increase in mechanical strength levels. Reasonable extrapolation of macro-scale continuum mechanics descriptions of crystal strength properties at micro- to nano-indentation hardness measurements are demonstrated, in addition to reports on persistent slip band observations and fatigue cracking behaviors. High-entropy alloy, superalloy and energetic crystal properties are reported along with descriptions of deformation rate sensitivities, grain boundary structures, nano-cutting, void nucleation/growth micromechanics and micro-composite electrical properties.

Technology: general issues --- crystal strength --- micro-crystals --- nano-crystals --- nano-polycrystals --- nano-wires --- whiskers --- pillars --- dislocations --- hardness --- crystal size dependencies --- fracture --- strain rate sensitivity --- temperature effect --- indentation size effect --- theoretical model --- nano-indentation --- crack growth --- dislocation models --- pile-ups --- kitagawa-takahashi diagram --- fracture mechanics --- internal stresses --- molecular dynamics simulations --- BCC Fe nanowires --- twin boundaries --- de-twinning --- micromechanical testing --- micro-pillar --- bi-crystal --- discrete dislocation pile-up --- grain boundary --- free surface --- anisotropic elasticity --- crystallographic slip --- molecular dynamics --- nanocutting --- iron --- cutting theory --- ab initio calculations --- hydrogen embrittlement --- cohesive strength --- multiaxial loading --- strain rate --- molecular dynamics simulation --- activation volume --- grain growth --- indentation creep --- size effect --- geometrically necessary dislocations --- FeCrAl --- micropillar --- dislocation --- strain hardening --- crystal plasticity simulations --- persistent slip band --- surface hard coating --- fatigue crack initiation --- fatigue --- cyclic deformation --- internal stress --- copper single crystal --- rafting behavior --- phase-field simulation --- crystal plasticity theory --- mechanical property --- ultrafine-grained materials --- intermetallic compounds --- B2 phase --- strain hardening behavior --- synchrotron radiation X-ray diffraction --- HMX --- elastic properties --- linear complexions --- strength --- lattice distortive transformations --- dislocation emission --- grain boundaries --- nanomaterials --- Hall-Petch relation --- metals and alloys --- interfacial delamination --- nucleation --- void formation --- cracking --- alloys --- nanocrystalline --- thermal stability --- IN718 alloy --- dislocation plasticity --- twinning --- miniaturised testing --- in situ electron microscopy --- magnesium --- anode --- tin sulfide --- lithium ion battery --- conversion reaction --- nanoflower --- rapid solidification --- compression

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The present collection of articles focuses on the mechanical strength properties at micro- and nanoscale dimensions of body-centered cubic, face-centered cubic and hexagonal close-packed crystal structures. The advent of micro-pillar test specimens is shown to provide a new dimensional scale for the investigation of crystal deformation properties. The ultra-small dimensional scale at which these properties are measured is shown to approach the atomic-scale level at which model dislocation mechanics descriptions of crystal slip and deformation twinning behaviors are proposed to be operative, including the achievement of atomic force microscopic measurements of dislocation pile-up interactions with crystal grain boundaries or with hard surface coatings. A special advantage of engineering designs made at such small crystal and polycrystalline dimensions is the achievement of an approximate order-of-magnitude increase in mechanical strength levels. Reasonable extrapolation of macro-scale continuum mechanics descriptions of crystal strength properties at micro- to nano-indentation hardness measurements are demonstrated, in addition to reports on persistent slip band observations and fatigue cracking behaviors. High-entropy alloy, superalloy and energetic crystal properties are reported along with descriptions of deformation rate sensitivities, grain boundary structures, nano-cutting, void nucleation/growth micromechanics and micro-composite electrical properties.

Technology: general issues --- crystal strength --- micro-crystals --- nano-crystals --- nano-polycrystals --- nano-wires --- whiskers --- pillars --- dislocations --- hardness --- crystal size dependencies --- fracture --- strain rate sensitivity --- temperature effect --- indentation size effect --- theoretical model --- nano-indentation --- crack growth --- dislocation models --- pile-ups --- kitagawa-takahashi diagram --- fracture mechanics --- internal stresses --- molecular dynamics simulations --- BCC Fe nanowires --- twin boundaries --- de-twinning --- micromechanical testing --- micro-pillar --- bi-crystal --- discrete dislocation pile-up --- grain boundary --- free surface --- anisotropic elasticity --- crystallographic slip --- molecular dynamics --- nanocutting --- iron --- cutting theory --- ab initio calculations --- hydrogen embrittlement --- cohesive strength --- multiaxial loading --- strain rate --- molecular dynamics simulation --- activation volume --- grain growth --- indentation creep --- size effect --- geometrically necessary dislocations --- FeCrAl --- micropillar --- dislocation --- strain hardening --- crystal plasticity simulations --- persistent slip band --- surface hard coating --- fatigue crack initiation --- fatigue --- cyclic deformation --- internal stress --- copper single crystal --- rafting behavior --- phase-field simulation --- crystal plasticity theory --- mechanical property --- ultrafine-grained materials --- intermetallic compounds --- B2 phase --- strain hardening behavior --- synchrotron radiation X-ray diffraction --- HMX --- elastic properties --- linear complexions --- strength --- lattice distortive transformations --- dislocation emission --- grain boundaries --- nanomaterials --- Hall-Petch relation --- metals and alloys --- interfacial delamination --- nucleation --- void formation --- cracking --- alloys --- nanocrystalline --- thermal stability --- IN718 alloy --- dislocation plasticity --- twinning --- miniaturised testing --- in situ electron microscopy --- magnesium --- anode --- tin sulfide --- lithium ion battery --- conversion reaction --- nanoflower --- rapid solidification --- compression --- crystal strength --- micro-crystals --- nano-crystals --- nano-polycrystals --- nano-wires --- whiskers --- pillars --- dislocations --- hardness --- crystal size dependencies --- fracture --- strain rate sensitivity --- temperature effect --- indentation size effect --- theoretical model --- nano-indentation --- crack growth --- dislocation models --- pile-ups --- kitagawa-takahashi diagram --- fracture mechanics --- internal stresses --- molecular dynamics simulations --- BCC Fe nanowires --- twin boundaries --- de-twinning --- micromechanical testing --- micro-pillar --- bi-crystal --- discrete dislocation pile-up --- grain boundary --- free surface --- anisotropic elasticity --- crystallographic slip --- molecular dynamics --- nanocutting --- iron --- cutting theory --- ab initio calculations --- hydrogen embrittlement --- cohesive strength --- multiaxial loading --- strain rate --- molecular dynamics simulation --- activation volume --- grain growth --- indentation creep --- size effect --- geometrically necessary dislocations --- FeCrAl --- micropillar --- dislocation --- strain hardening --- crystal plasticity simulations --- persistent slip band --- surface hard coating --- fatigue crack initiation --- fatigue --- cyclic deformation --- internal stress --- copper single crystal --- rafting behavior --- phase-field simulation --- crystal plasticity theory --- mechanical property --- ultrafine-grained materials --- intermetallic compounds --- B2 phase --- strain hardening behavior --- synchrotron radiation X-ray diffraction --- HMX --- elastic properties --- linear complexions --- strength --- lattice distortive transformations --- dislocation emission --- grain boundaries --- nanomaterials --- Hall-Petch relation --- metals and alloys --- interfacial delamination --- nucleation --- void formation --- cracking --- alloys --- nanocrystalline --- thermal stability --- IN718 alloy --- dislocation plasticity --- twinning --- miniaturised testing --- in situ electron microscopy --- magnesium --- anode --- tin sulfide --- lithium ion battery --- conversion reaction --- nanoflower --- rapid solidification --- compression

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Determinations of the indentation hardness properties of crystals have expanded to cover the full characterizations of their important elastic, plastic and cracking behaviors, particularly as accomplished with the increased measuring capabilities of nanoindentation hardness testing. No crystal structure of any bonding type is either too soft or too hard to prevent measurement with a suitable probing indenter. The current Special Issue is devoted to surveying the topic with emphasis given in a collection of reports to: (1) the diversity of crystals being tested; (2) the variety of measuring techniques; and (3) the wealth of information being obtained.

Crystals