Direct observation of a coincident dislocation- and grain boundary-mediated deformation in nanocrystalline iron

Vetterick, G.; Leff, Asher C.; Marshall, Michael; Baldwin, Jon K.; Misra, Amit; Hattar, Khalid Mikhiel; Taheri, Mitra L.

doi:10.1016/j.msea.2017.09.020

Cited by 11 publications

(4 citation statements)

References 55 publications

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“…PED was introduced by Vincent and Midgley in 1994 as a way of recording diffraction intensity that better suits structure determination (29). It then began to gain momentum in the microscopic exploration of deformation behavior of materials due to its crystal orientation indexability and improved spatial resolution compared with EBSD (30)(31)(32)(33). For example, Vetterick et al directly observed grain boundary-mediated plasticity at a crack tip in nanocrystalline Fe using PED (30).…”

Section: Significancementioning

confidence: 99%

“…It then began to gain momentum in the microscopic exploration of deformation behavior of materials due to its crystal orientation indexability and improved spatial resolution compared with EBSD (30)(31)(32)(33). For example, Vetterick et al directly observed grain boundary-mediated plasticity at a crack tip in nanocrystalline Fe using PED (30). Leff, Weinberger, and Taheri demonstrated the viability and utility of estimating dislocation densities using PED misorientation data and discussed the interpretation of dislocation contents under different acquisition parameters (31).…”

Section: Significancementioning

confidence: 99%

See 1 more Smart Citation

Exploring the origins of the indentation size effect at submicron scales

Higgins

Liang

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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The origin of the indentation size effect has been extensively researched over the last three decades, following the establishment of nanoindentation as a broadly used small-scale mechanical testing technique that enables hardness measurements at submicrometer scales. However, a mechanistic understanding of the indentation size effect based on direct experimental observations at the dislocation level remains limited due to difficulties in observing and quantifying the dislocation structures that form underneath indents using conventional microscopy techniques. Here, we employ precession electron beam diffraction microscopy to “look beneath the surface,” revealing the dislocation characteristics (e.g., distribution and total length) as a function of indentation depth for a single crystal of nickel. At smaller depths, individual dislocation lines can be resolved, and the dislocation distribution is quite diffuse. The indentation size effect deviates from the Nix–Gao model and is controlled by dislocation source starvation, as the dislocations are very mobile and glide away from the indented zone, leaving behind a relatively low dislocation density in the plastically deformed volume. At larger depths, dislocations become highly entangled and self-arrange to form subgrain boundaries. In this depth range, the Nix–Gao model provides a rational description because the entanglements and subgrain boundaries effectively confine dislocation movement to a small hemispherical volume beneath the contact impression, leading to dislocation interaction hardening. The work highlights the critical role of dislocation structural development in the small-scale mechanistic transition in indentation size effect and its importance in understanding the plastic deformation of materials at the submicron scale.

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Section: Significancementioning

confidence: 99%

Section: Significancementioning

confidence: 99%

Exploring the origins of the indentation size effect at submicron scales

Higgins

Liang

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

“…This approach ignores the contribution of pairs/sets of statistically stored dislocations (SSD) that do not lead to a net orientation change within the ~5 nm spatial resolution of the ACOM-STEM maps. We analyzed the Nye dislocation density tensor [ 55 ] using an approach that was implemented in the past for electron backscatter diffraction [ 57 ], as well as ACOM-TEM data [ 41 , 58 ]. The GND scalar density ( ρ GND ) can be expressed as the entrywise norm of the Nye dislocation density tensor ( α ) divided by the length of the Burgers vector ( b ):

where α ij corresponds to the curvature components of the dislocation density tensor.…”

Section: Resultsmentioning

confidence: 99%

In Situ TEM Observation of Cooperative Grain Rotations and the Bauschinger Effect in Nanocrystalline Palladium

2021

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We report on cooperative grain rotation accompanied by a strong Bauschinger effect in nanocrystalline (nc) palladium thin film. A thin film of nc Pd was subjected to cyclic loading–unloading using in situ TEM nanomechanics, and the evolving microstructural characteristics were investigated with ADF-STEM imaging and quantitative ACOM-STEM analysis. ADF-STEM imaging revealed a partially reversible rotation of nanosized grains with a strong out-of-plane component during cyclic loading–unloading experiments. Sets of neighboring grains were shown to rotate cooperatively, one after the other, with increasing/decreasing strain. ACOM-STEM in conjunction with these experiments provided information on the crystallographic orientation of the rotating grains at different strain levels. Local Nye tensor analysis showed significantly different geometrically necessary dislocation (GND) density evolution within grains in close proximity, confirming a locally heterogeneous deformation response. The GND density analysis revealed the formation of dislocation pile-ups at grain boundaries (GBs), indicating the generation of back stresses during unloading. A statistical analysis of the orientation changes of individual grains showed the rotation of most grains without global texture development, which fits to both dislocation- and GB sliding-based mechanisms. Overall, our quantitative in situ experimental approach explores the roles of these different deformation mechanisms operating in nanocrystalline metals during cyclic loading.

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“…This approach ignores the contribution of pairs/sets of statistically stored dislocations (SSD) that do not lead to a net orientation change within the~5 nm spatial resolution of the ACOM-STEM maps. We analyzed the Nye dislocation density tensor [55] using an approach that was implemented in the past for electron backscatter diffraction [57], as well as ACOM-TEM data [41,58]. The GND scalar density (ρ GND ) can be expressed as the entrywise norm of the Nye dislocation density tensor (α) divided by the length of the Burgers vector (b):…”

Section: Structural Observations and Deformation Characteristics Of The Nanocrystalline Palladium Filmmentioning

confidence: 99%

<i>In Situ</i> TEM Observation of Cooperative Grain Rotations and the Bauschinger Effect in Nanocrystalline Palladium

2020

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Direct observation of a coincident dislocation- and grain boundary-mediated deformation in nanocrystalline iron

Cited by 11 publications

References 55 publications

Exploring the origins of the indentation size effect at submicron scales

Exploring the origins of the indentation size effect at submicron scales

In Situ TEM Observation of Cooperative Grain Rotations and the Bauschinger Effect in Nanocrystalline Palladium

<i>In Situ</i> TEM Observation of Cooperative Grain Rotations and the Bauschinger Effect in Nanocrystalline Palladium

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