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Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

Micro-mechanical response of ultrafine grain and nanocrystalline tantalum
Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100-200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20-50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress-strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 µm), which is attributed to the high ratio between pillar diameter and grain size (~10-20). This behavior is quite different from that in monocrystalline specimens where dislocation 'starvation' leads to a significant size dependence of strength. The ultrafine grains exhibit clear 'pancaking' upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20-50 nm) to grain size was in the range 0.2-2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter.

Micro-Mechanical, Nanocrystalline, Response, Tantalum, Ultrafine Grain
2238-7854
1804-1815
Yang, Wen
9635aab5-1d95-42b5-8111-b379ee39464f
Ruestes, Carlos J.
e4a45be3-7f4b-4c92-8f65-ebb0810b500f
Li, Zezhou Li
34a9b0ef-a811-4b01-ad72-aa696eda12fb
Torrents Abad, Oscar
6e331bed-eeca-4571-ad1b-9dd7a3014a47
Langdon, Terence G
86e69b4f-e16d-4830-bf8a-5a9c11f0de86
Heiland, Birgit
3d9d9269-a429-4f28-8cce-3e75c254b526
Koch, Marcus
7bee205f-b24e-4c2f-8b8b-70bfd9a44c43
Arzt, Eduard
ce4bff0a-f984-41b8-9195-2110e95393fe
Meyers, Marc A.
d0a8c07b-3f0f-493b-bef1-3a85246f5e4d
Yang, Wen
9635aab5-1d95-42b5-8111-b379ee39464f
Ruestes, Carlos J.
e4a45be3-7f4b-4c92-8f65-ebb0810b500f
Li, Zezhou Li
34a9b0ef-a811-4b01-ad72-aa696eda12fb
Torrents Abad, Oscar
6e331bed-eeca-4571-ad1b-9dd7a3014a47
Langdon, Terence G
86e69b4f-e16d-4830-bf8a-5a9c11f0de86
Heiland, Birgit
3d9d9269-a429-4f28-8cce-3e75c254b526
Koch, Marcus
7bee205f-b24e-4c2f-8b8b-70bfd9a44c43
Arzt, Eduard
ce4bff0a-f984-41b8-9195-2110e95393fe
Meyers, Marc A.
d0a8c07b-3f0f-493b-bef1-3a85246f5e4d

Yang, Wen, Ruestes, Carlos J., Li, Zezhou Li, Torrents Abad, Oscar, Langdon, Terence G, Heiland, Birgit, Koch, Marcus, Arzt, Eduard and Meyers, Marc A. (2021) Micro-mechanical response of ultrafine grain and nanocrystalline tantalum. Journal of Materials Research and Technology, 12, 1804-1815. (doi:10.1016/j.jmrt.2021.03.080).

Record type: Article

Abstract

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100-200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20-50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress-strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 µm), which is attributed to the high ratio between pillar diameter and grain size (~10-20). This behavior is quite different from that in monocrystalline specimens where dislocation 'starvation' leads to a significant size dependence of strength. The ultrafine grains exhibit clear 'pancaking' upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20-50 nm) to grain size was in the range 0.2-2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter.

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Accepted/In Press date: 19 March 2021
e-pub ahead of print date: 25 March 2021
Published date: 2021
Additional Information: Funding Information: This research is funded by a UC Research Laboratories Grant ( 09-LR-06-118456-MEYM ) and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 . The visit of MAM to Germany was supported by the Humboldt Foundation Award to one of us (MM). Carlos J Ruestes was supported by SiiP- UNCUYO and ANPCyT PICT 2018-0773 . Simulations were conducted at TOKO-FCEN-UNCUYO and Mendieta-CCAD-UNC computing cluster. We thank Professor Peter Hosemann, UC Berkeley, for allowing us to use his nanoindentation machine. Publisher Copyright: © 2021 The Authors.
Keywords: Micro-Mechanical, Nanocrystalline, Response, Tantalum, Ultrafine Grain

Identifiers

Local EPrints ID: 450038
URI: http://eprints.soton.ac.uk/id/eprint/450038
ISSN: 2238-7854
PURE UUID: 058434b0-dca2-4d31-8e60-14d62ceeb32d
ORCID for Terence G Langdon: ORCID iD orcid.org/0000-0003-3541-9250

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Date deposited: 06 Jul 2021 16:31
Last modified: 17 Mar 2024 02:55

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Contributors

Author: Wen Yang
Author: Carlos J. Ruestes
Author: Zezhou Li Li
Author: Oscar Torrents Abad
Author: Birgit Heiland
Author: Marcus Koch
Author: Eduard Arzt
Author: Marc A. Meyers

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