The heterogeneity of persistent slip band nucleation and evolution in metals at the micrometer scale

Disentangling fatigue Metal fatigues when repeatedly loaded, ultimately failing when cracks form and propagate through the material. Lavenstein et al. studied the origins of this process in nickel. Using high-resolution observations, they tracked how dislocations evolved into microstructural feature...

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Bibliographic Details
Published in:Science (American Association for the Advancement of Science) 2020-10, Vol.370 (6513)
Main Authors: Lavenstein, Steven, Gu, Yejun, Madisetti, Dylan, El-Awady, Jaafar A.
Format: Article
Language:English
Subjects:
Banded structure
Compression
Crack initiation
Crack propagation
Crystallography
Crystals
Cyclic loads
Damage
Deformation
Deformation mechanisms
Dipoles
Dislocation
Dislocation density
Ductile fracture
Ductility
Edge dislocations
Evolution
Extrusion
Failure
Fracture mechanics
Growth stage
Heavy metals
Heterogeneity
Metal fatigue
Metals
Microcracks
Microcrystals
Motion
Nickel
Nucleation
Plastic deformation
Roughness
Scanning electron microscopy
Shear strain
Short cracks
Single crystals
Transmission electron microscopy
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Summary:Disentangling fatigue Metal fatigues when repeatedly loaded, ultimately failing when cracks form and propagate through the material. Lavenstein et al. studied the origins of this process in nickel. Using high-resolution observations, they tracked how dislocations evolved into microstructural features called persistent slip bands that preceded crack formation. The evolution of tangles of dislocations to a more regularly spaced pattern form the basis for the persistent slip bands and provide a road map for understanding fatigue cracking in metals. Science , this issue p. eabb2690 High-resolution observations clarify the mechanisms that create microstructures that grow into cracks during metal fatigue. INTRODUCTION Metals are the material of choice for many structural applications because they provide the best compromise between strength and ductility. For applications in which cyclic loading is imposed, fatigue failure plagues all metals, and mitigating it is of great importance. In ductile metals, fatigue cracks initiate as small, microstructurally short cracks that gradually grow with increasing number of loading cycles. Although many studies have been dedicated to the crack-growth stage, the transition from a crack-free to a cracked metal remains one of the most challenging topics in the study of fatigue of metal. The nucleation of microcracks in ductile metals is a consequence of the to-and-fro motion of dislocations during cyclic loading, which leads to dislocation self-organization into long-range ordered structures. Dislocations result from irregularities in the arrangement of atoms in crystalline materials, and their motion leads to plastic deformation. Ladder dislocation structures, more commonly referred to as persistent slip bands (PSBs), are perhaps the most consequential defect structures with regard to fatigue crack initiation. PSBs take the form of regularly spaced, dislocation-dense walls constructed of edge dislocation dipoles with dislocation-sparse channels separating them in a structure resembling a ladder. RATIONALE Our aim is to provide in situ observations and characterization of the formation of PSBs in micrometer-sized Ni single crystals—a representative, model face-centered cubic metal. To do this, we designed a high-frequency microfatigue experiment that replicates the necessary conditions for PSB formation in a very confined material volume. We conducted all experiments in situ in a scanning electron microscope (SEM) on microcryst
ISSN:0036-8075
1095-9203
DOI:10.1126/science.abb2690