Ideal maximum strengths and defect-induced softening in nanocrystalline-nanotwinned metals

Abstract

Strengthening of metals through nanoscale grain boundaries and coherent twin boundaries is manifested by a maximum strength—a phenomenon known as Hall–Petch breakdown. Different softening mechanisms are considered to occur for nanocrystalline and nanotwinned materials. Here, we report nanocrystalline-nanotwinned Ag materials that exhibit two strength transitions dissimilar from the above mechanisms. Atomistic simulations show three distinct strength regions as twin spacing decreases, delineated by positive Hall–Petch strengthening to grain-boundary-dictated (near-zero Hall–Petch slope) mechanisms and to softening (negative Hall–Petch slope) induced by twin-boundary defects. An ideal maximum strength is reached for a range of twin spacings below 7 nm. We synthesized nanocrystalline-nanotwinned Ag with hardness 3.05 GPa—42% higher than the current record, by segregating trace concentrations of Cu impurity (<1.0 weight (wt)%). The microalloy retains excellent electrical conductivity and remains stable up to 653 K; 215 K better than for pure nanotwinned Ag. This breaks the existing trade-off between strength and electrical conductivity, and demonstrates the potential for creating interface-dominated materials with unprecedented mechanical and physical properties.

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Fig. 1: Hall–Petch strength transition zones in pure and impurity-segregated nanotwinned Ag metals containing either perfect or kinked TBs obtained by large-scale atomistic simulations.
Fig. 2: Twin-boundary defect-motion softening mechanism (negative Hall–Petch slope).
Fig. 3: Transition from positive Hall–Petch strengthening to GB stress-controlled (near-zero Hall–Petch slope) mechanisms.
Fig. 4: Cu-impurity-mixed nanocrystalline-nanotwinned Ag (NNT-Ag) synthesized by magnetron sputtering.
Fig. 5: Electrical conductivity and yield strength of Cu-impurity-mixed NNT-Ag.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper or the Supplementary Materials.

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Acknowledgements

The authors thank A. Engwall for help with synthesis and E. Martinez for the helpful discussion. X.K., Z.P. and F.S. are grateful for the support from the US Department of Energy (DOE) (grant no. DE-SC0016270). The work at the Lawrence Livermore National Laboratory was performed under the auspices of DOE contract no. DE-AC52-07NA27344. J.G., M.F.B. and R.T.O. at the Ames Laboratory were supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract no. DE-AC02-07CH11358. A.C. at the Los Alamos National Laboratory is under DOE contract no. DE-AC52-06NA253. Y.M.W. and D.Q. acknowledge the support of the Laboratory Directed Research and Development (LDRD) programme (grant no. 17-ERD-048) at Lawrence Livermore National Laboratory. J.M. acknowledges support from the National Science Foundation (grant no. DMR-1611342). The simulations in this research used resources of the National Energy Research Scientific Computing Centre, supported by DOE contract no. DE-AC02-05CH11231, and those of the Extreme Science and Engineering Discovery Environment, supported by National Science Foundation (grant no. ACI-1548562).

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Contributions

Y.M.W. and F.S. conceived and guided the research. J.Y., M.F.B., J.G., R.T.O. and Y.M.W. performed the synthesis, mechanical testing and characterizations. D.Q. performed the electrical measurements. A.C. developed the initial MC/MD code. X.K. and F.S. performed the MC/MD simulations and atomistic analysis. Z.P. performed the density-functional theory calculations and MD analysis on Cu metals. J.M. and F.S. developed the continuum softening model. All authors contributed to the data discussion and manuscript preparation.

Corresponding authors

Correspondence to Y. Morris Wang or Frederic Sansoz.

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Supplementary information

Supplementary Information

Supplementary Discussion 1–4, Figs. 1–16, Tables 1–4 and Refs. 1–15.

Supplementary Video 1

Deformation mechanisms of Cu-segregated NC-Ag with no TB. a, Stress versus strain curve of the model during tension up to 10% strain. b, Atomic snapshot of microstructure with atomic strain coloured with scale bar from 0 to 0.5. Atoms in perfect FCC arrangement have been omitted for clarity. c, Dislocation-extraction analysis of the structure. Blue for perfect 1/2<110> dislocations, green for 1/6<112> Shockley dislocations, pink for 1/6<110> stair-rod dislocations, yellow for 1/3<001> Hirth dislocations, cyan for 1/3<111> Frank dislocations and red for other types of dislocation.

Supplementary Video 2

Deformation mechanisms of Cu-segregated nt-Ag containing kinked TBs with λ = 14.8 nm. a, Stress versus strain curve of the model during tension up to 10% strain. b, Atomic snapshot of microstructure with atomic strain coloured with scale bar from 0 to 0.5. Atoms in perfect FCC arrangement have been omitted for clarity. c, Dislocation-extraction analysis of the structure. Blue for perfect 1/2<110> dislocations, green for 1/6<112> Shockley dislocations, pink for 1/6<110> stair-rod dislocations, yellow for 1/3<001> Hirth dislocations, cyan for 1/3<111> Frank dislocations and red for other types of dislocation.

Supplementary Video 3

Close-up view on splitting and migration of twin-boundary kink-step defects inside a single grain of Cu-segregated nt-Ag containing kinked TBs with λ = 1.4 nm.

Supplementary Video 4

Deformation mechanisms of Cu-segregated nt-Ag containing perfect TBs with λ = 3.5 nm. a, Stress–strain curve of the model during tension up to 10% strain. b, Atomic snapshot of microstructure with atomic strain coloured with scale bar from 0 to 0.5. Atoms in perfect FCC arrangement have been omitted for clarity. c, Dislocation-extraction analysis of the structure. Blue for perfect 1/2<110> dislocations, green for 1/6<112> Shockley dislocations, pink for 1/6<110> stair-rod dislocations, yellow for 1/3<001> Hirth dislocations, cyan for 1/3<111> Frank dislocations and red for other types of dislocation.

Supplementary Video 5

Close-up view on grain-boundary sliding and soft slip deformation mechanisms at a grain-boundary interface from Supplementary Video 4. The colours indicate the local von-Mises shear strain.

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Ke, X., Ye, J., Pan, Z. et al. Ideal maximum strengths and defect-induced softening in nanocrystalline-nanotwinned metals. Nat. Mater. 18, 1207–1214 (2019). https://doi.org/10.1038/s41563-019-0484-3

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