Research Article

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

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Science  09 Oct 2020:
Vol. 370, Issue 6513, eabb2690
DOI: 10.1126/science.abb2690

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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

Structured Abstract

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 microcrystals having rectangular cross sections with nominal dimensions of 12 μm by 13 μm and gauge lengths of 27 μm. We oriented all microcrystals for single slip, with a fully reversible and symmetric tension-compression cyclic loading imposed along the [3¯52¯] crystallographic direction at a constant shear strain amplitude below 1.5 × 10−2 and a loading frequency of 75 Hz. We generated propagation profiles of PSB surface slip markings from the in situ observations, and the dislocation structure postmortem was characterized with transmission electron microscopy (TEM).

RESULTS

We found that PSBs nucleate locally within the microcrystal volume and then propagate gradually until they span the entire slip region. A relatively large number of cycles (>106) was necessary to nucleate PSBs in microcrystals compared with bulk scale, and correspondingly, extreme fatigue lifetimes were exhibited at the micrometer scale. The PSB surface slip markings also seem to have an inherent roughness immediately on formation; after fully propagating, the roughness of the PSB slip markings remains stable with further cyclic loading. The slip traces formed in the first ~10 cycles are also found to identify the locations where PSBs—and thus cracks—eventually form.

CONCLUSION

This gradual evolution suggests certain refinements to conventional PSB models that idealize extrusion formation as a rapid growth process. This is important for nondestructive damage quantification (i.e., failure prediction) and for the physics-based modeling community, because simulating the evolution of fatigue damage is usually too computationally expensive to perform beyond a few hundred cycles. Although the results presented in this paper focus on pure Ni, the fundamental mechanisms identified are common to many metals. Thus, the current insights provide an avenue to connect micrometer-scale deformation mechanisms with fatigue failures at the bulk scale in metals.

High-frequency microfatigue experiments on Ni single crystals.

Successive micrographs from an SEM video (top left) taken during an in situ cyclic loading experiment show the nucleation and propagation of PSB surface slip markings. The proposed dislocation organization mechanisms that explain this are also illustrated schematically (bottom left). A TEM micrograph (right) confirms the presence of the ladderlike self-organized dislocation structure, which is the hallmark of PSBs.

Abstract

Fatigue damage in metals manifests itself as irreversible dislocation motion followed by crack initiation and propagation. Characterizing the transition from a crack-free to a cracked metal remains one of the most challenging problems in fatigue. Persistent slip bands (PSBs) form in metals during cyclic loading and are one of the most important aspects of this transition. We used in situ microfatigue experiments to investigate PSB formation and evolution mechanisms, and we discovered that PSBs are prevalent at the micrometer scale. Dislocation accumulation rates at this scale are smaller than those in bulk samples, which delays PSB nucleation. Our results suggest the need to refine PSB and crack-initiation models in metals to account for gradual and heterogeneous evolution. These findings also connect micrometer-scale deformation mechanisms with fatigue failure at the bulk scale in metals.

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