.2017 Dec 8;8(1):2000.

doi: 10.1038/s41467-017-02112-z.

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Deep Choudhuri^{1 2}, Srivilliputhur G Srinivasan³, Mark A Gibson^{4 5 6}, Yufeng Zheng⁷, David L Jaeger⁸, Hamish L Fraser⁷, Rajarshi Banerjee^{9 10 11 12}

Affiliations

¹ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. deep.choudhuri@gmail.com.
² Advanced Materials and Manufacturing Processes Institute, University of North Texas, Denton, TX, 76207, USA. deep.choudhuri@gmail.com.
³ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. srinivasan.srivilliputhur@unt.edu.
⁴ CSIRO Manufacturing, Private Bag 10, Clayton South, Clayton, VIC, 3169, Australia.
⁵ School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Carlton, VIC, 3053, Australia.
⁶ Department of Materials Engineering, Monash University, Clayton, VIC, 3800, Australia.
⁷ Center for Accelerated Maturation of Materials, Department of Materials Science and Engineering, The Ohio State University, Columbus, 43210, OH, USA.
⁸ Materials Research Facility, University of North Texas, Denton, TX, 76201, USA.
⁹ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. raj.banerjee@unt.edu.
¹⁰ Advanced Materials and Manufacturing Processes Institute, University of North Texas, Denton, TX, 76207, USA. raj.banerjee@unt.edu.
¹¹ Materials Research Facility, University of North Texas, Denton, TX, 76201, USA. raj.banerjee@unt.edu.
¹² School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore. raj.banerjee@unt.edu.

PMID:29222427
PMCID: PMC5722870
DOI: 10.1038/s41467-017-02112-z

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Deep Choudhuri et al. Nat Commun.2017.

.2017 Dec 8;8(1):2000.

doi: 10.1038/s41467-017-02112-z.

Authors

Deep Choudhuri^{1 2}, Srivilliputhur G Srinivasan³, Mark A Gibson^{4 5 6}, Yufeng Zheng⁷, David L Jaeger⁸, Hamish L Fraser⁷, Rajarshi Banerjee^{9 10 11 12}

Affiliations

¹ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. deep.choudhuri@gmail.com.
² Advanced Materials and Manufacturing Processes Institute, University of North Texas, Denton, TX, 76207, USA. deep.choudhuri@gmail.com.
³ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. srinivasan.srivilliputhur@unt.edu.
⁴ CSIRO Manufacturing, Private Bag 10, Clayton South, Clayton, VIC, 3169, Australia.
⁵ School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Carlton, VIC, 3053, Australia.
⁶ Department of Materials Engineering, Monash University, Clayton, VIC, 3800, Australia.
⁷ Center for Accelerated Maturation of Materials, Department of Materials Science and Engineering, The Ohio State University, Columbus, 43210, OH, USA.
⁸ Materials Research Facility, University of North Texas, Denton, TX, 76201, USA.
⁹ Department of Materials Science and Engineering, University of North Texas, Denton, TX, 76201, USA. raj.banerjee@unt.edu.
¹⁰ Advanced Materials and Manufacturing Processes Institute, University of North Texas, Denton, TX, 76207, USA. raj.banerjee@unt.edu.
¹¹ Materials Research Facility, University of North Texas, Denton, TX, 76201, USA. raj.banerjee@unt.edu.
¹² School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore. raj.banerjee@unt.edu.

PMID:29222427
PMCID: PMC5722870
DOI: 10.1038/s41467-017-02112-z

Abstract

Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc. This microalloying effect raises fundamental questions regarding creep deformation mechanisms. Here, using atomic-scale characterization and first principles calculations, we attribute the 600% increase in creep life in a prototypical Mg-rare earth (RE)-Zn alloy to multiple mechanisms caused by RE-Zn bonding-stabilization of a large volume fraction of strengthening precipitates on slip planes, increase in vacancy diffusion barrier, reduction in activated cross-slip, and enhancement of covalent character and bond strength around Zn solutes along the c-axis of Mg. We report that increased vacancy diffusion barrier, which correlates with the observed 25% increase in interplanar bond stiffness, primarily enhances the high-temperature creep life. Thus, we demonstrate that an approach of local, randomized tailoring of bond stiffness via microalloying enhances creep performance of alloys.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

**Fig. 1**
Creep response and microstructures.a Strain rate vs. time plots show an order of magnitude improvement in the creep life of Mg–Nd–Zn (red) over Mg–Nd (blue) and both differ significantly from Mg (black). Panelsb–d are TEM and HAADF-STEM observations for Mg–Nd and Mg–Nd–Zn systems taken at pointsb–d marked in panela. BFTEMs in panelsb andc show that β′ precipitates (also inset HAADF-STEM along [11 $\bar{2}$ 0]_α) and fine-scale GP zones (arrows), respectively, in Mg–Nd lie on the prismatic planes ofhcp-Mg. In case of Mg–Nd–Zn, HAADF-STEM in paneld shows γ″ and γ precipitates lying on both basal and prismatic planes, respectively.e Raw ion maps from atom probe tomography confirms the presence of Zn in γ″ along with Nd. Scale bars in panelsb–d are 100 nm, 20 nm, and 20 nm, respectively

**Fig. 2**
γ″ structure after Zn substitution.a Heat of formation of γ″ is plotted as a function of the fraction of Mg and Nd sites substituted by Zn.b Heats of formation for D019 and β′. Panelsc–e show experimental observation and DFT calculations agree well:c DFT-derived structure of (Mg_0.8Zn_0.2)₅Nd γ″ supercell sandwiched in a Mg matrix,d atomic resolution HAADF-STEM image of γ″, ande plot comparing interatomic distances obtained from DFT calculations and measurements performed in the HAADF-STEM image

**Fig. 3**
Formation and migration energies of vacancy at Mg sites. Heat of vacancy formation at Mg sites in:a γ″, D0₁₉, and β′ structures;bhcp-Mg lattice with no solutes, Nd and Zn solutes, and only Nd solute. Energies were calculated for vacancies at different nearest neighbor distances to Nd and/or Zn. Energy vs. reaction coordinates plots showing transition state calculations for out-of-plane (c) and in-plane (d) vacancy migration. The insets inc andd schematically show the migration paths inhcp-Mg. Addition of Zn retards Mg vacancy migration by increasing the peak-barrier and final saddle point energies (marked with arrow inc–d)

**Fig. 4**
Bonding character from DFT calculations.a Mg lattice supercells showing Nd and Zn solutes as 1st and 2nd nearest neighbors (NN), only Nd solute, and pure Mg. The encircled Mg atom was displaced along the [0001]_α to calculate the restoring force.b Plot of DFT-calculated force as function of displacement along [0001]_α. The inset histogram shows the calculated bond stiffness increases with Zn addition. Charge isosurface using Δρ = 0.015 eÅ⁻³ in panelsc–e shows the valence electron delocalization along [0001]_α in solid solution, bulk γ″, and γ″–Mg supercells in Mg–Nd–Zn system

**Fig. 5**
Generalized stacking fault (GSF) and dislocation dissociation energies.a Plot depicting the variation of fault energy with planar displacement along 〈11 $\bar{2}$ 0〉_α in pure Mg, Mg–Nd, and Mg–Nd–Zn. Bottom inset figures show two views of the orthogonal cell used in the calculations. Stacking fault energy corresponds to the first minima of the GSF energy curves. Addition of Zn to Mg–Nd further reduces SFE by ~17%.b Histogram comparing the energy required to dissociate a perfect (1/2) 〈 $11 \bar{2} 0$ 〉_α dislocation into two (1/3) 〈1 $\bar{1}$ 00〉_α partials for pure Mg, Mg–Nd, and Mg–Nd–Zn alloys. Adding Zn to Mg–Nd increases the dissociation energy by ~20%. Out-of-plane vacancy migration energies, also indicated in the same plot, are also significantly larger than the dissociation energies

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Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Affiliations

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Authors

Affiliations

Abstract

Conflict of interest statement

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