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Research Papers

Molecular Dynamics Simulations of Orientation Effects During Tension, Compression, and Bending Deformations of Magnesium Nanocrystals

[+] Author and Article Information
Haidong Fan

Department of Mechanical Engineering,
Johns Hopkins University,
Baltimore, MD 21218
Department of Mechanics,
Sichuan University,
Chengdu, Sichuan 610065, China
e-mail: haidongfan8@foxmail.com

Jaafar A. El-Awady

Department of Mechanical Engineering,
Johns Hopkins University,
Baltimore, MD 21218
e-mail: jelawady@jhu.edu

1Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received May 30, 2015; final manuscript received June 25, 2015; published online July 14, 2015. Assoc. Editor: Harold S. Park.

J. Appl. Mech 82(10), 101006 (Oct 01, 2015) (11 pages) Paper No: JAM-15-1279; doi: 10.1115/1.4030930 History: Received May 30, 2015; Revised June 25, 2015; Online July 14, 2015

The deformation modes in magnesium nanocrystals during uniaxial tension, uniaxial compression, and pure bending are investigated using molecular dynamics (MD) simulations at room temperature. For each loading condition, the crystal orientation effects are studied by increasing the crystal c-axis orientation angle θ relative to the loading direction from 0 deg to 90 deg by a 15 deg increment. The simulation results reveal a number of different deformation modes and an obvious tension–compression asymmetry in magnesium nanocrystals. As the c-axis is rotated away from the tension loading direction, the deformation mode at yielding changes from tension twinning (θ ≤ 45 deg) to compression twinning (θ > 45 deg). For compression loading, yielding is dominated by only dislocation slip on the pyramidal (θ < 15 deg), basal (15 deg < θ < 60 deg) and prismatic (θ > 60 deg) planes. The nucleation stress in general decreases with increasing θ for both uniaxial tension and uniaxial compression loadings. For pure bending simulations, the yielding is mostly controlled by the weaker deformation mode between the compressive and tensile sides. The bending nucleation stress also decreases as the c-axis deviates away from the loading direction.

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Figures

Grahic Jump Location
Fig. 1

Schematic of the HCP unit cell showing all possible: (a) slip and (b) twinning systems. The planes are highlighted, and the Burgers vectors are shown by the red arrows (see online version for color).

Grahic Jump Location
Fig. 2

Cross section schematic of the 30 nm × 30 nm × 60 nm simulation cell for the pure bending, uniaxial tension, and uniaxial compression simulations

Grahic Jump Location
Fig. 3

Uniaxial tension deformation for the θ = 0 deg case: (a) deformed cell at 5.48% strain (surface atoms are removed for visualization); (b) closeup view at the {10–12} TT boundary; (c) closeup view of the single layer stacking fault structure in the twin; and (d) nucleation of two stacking faults from the TB at 6.1% strain. All atoms are colored according to their CNA parameter in (a)–(c), where HCP atoms are green, FCC atoms are blue, and other atoms are red, and centrosymmetry parameter in (d) with HCP atoms removed (see online version for color).

Grahic Jump Location
Fig. 4

(a) Cross section through the θ = 15 deg simulation cell deformed to 4.68% strain in uniaxial tension using the Liu et al.' s potential and (b) closeup view of the {11–21} TT. All atoms are colored according to their CNA parameter, where HCP atoms are green, FCC atoms are blue, and other atoms are red (see online version for color).

Grahic Jump Location
Fig. 5

(a) Cross section through the θ = 30 deg simulation cell deformed to 5.08% strain in uniaxial tension. A sequence of snapshots showing an 〈a〉 dislocation intersecting the {11–21} TT is shown in (b) through (d). All atoms are colored according to their CNA parameter, where HCP atoms are green, FCC atoms are blue, and other atoms are red. In (b) through (d), all HCP atoms are removed to facilitate visualization (see online version for color).

Grahic Jump Location
Fig. 6

(a) Cross section through the θ = 45 deg simulation cell deformed to 5.08% strain in uniaxial tension. (b) Closeup view of the intersection of the {11–21} TT with the free surface showing the nucleation of basal dislocations at the intersection. All atoms are colored by their centrosymmetry parameter, and HCP atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 7

Uniaxial tension deformation of: (a) θ = 60 deg simulation cell at 4.28% strain; (b) θ = 75 deg simulation cell at 4.68% strain; and (c) θ = 90 deg simulation cell at 5.48% strain. (d) Closeup view of the {10–11} compression TB in (c). All atoms are colored according to their CNA parameter, where HCP atoms are green, FCC atoms are blue, and other atoms are red (see online version for color). Surface atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 8

Nucleation stress and corresponding first deformation event as a function of the simulation cell orientation angle during uniaxial tension simulations. The following abbreviations are used: TT1 for {10–12} TT; TT2 for {11–21} TT; CT1 for {10–11} CT; Ba for basal slip; Pr for prismatic slip; and PyI for pyramidal I slip.

Grahic Jump Location
Fig. 9

Uniaxial compression deformation for the: (a) θ = 0 deg case at 5.08% strain; (b) θ = 15 deg case at 5.08% strain; (c) θ = 30 deg case at 4.68% strain; and (d) θ = 45 deg case at 4.68% strain. All atoms are colored according to their centrosymmetry parameter. Surface and HCP atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 10

(a) Cross section through the θ = 60 deg simulation cell deformed to 4.68% strain in uniaxial compression. (b) Pre- and (c) post-snapshots of the interaction between a {11–21} TT and a basal dislocation. All the atoms are colored according to their centrosymmetry parameter. Surface and HCP atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 11

(a) Uniaxial compression deformation for the θ = 75 deg simulation cell at 5.08% strain. (b) Cross section through the θ = 90 deg simulation cell at 4.68% uniaxial compressive strain. Prismatic dislocations are apparent in both cases with the inset of (a) showing the formation of a dislocation loop. All the atoms are colored according to their centrosymmetry parameter. Surface and HCP atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 12

Nucleation stress and corresponding first deformation event as a function of the simulation cell orientation angle during uniaxial compression simulations. The following abbreviations are used: TT1 for {10–12} TT; TT2 for {11–21} TT; CT1 for {10–11} CT; Ba for basal slip; Pr for prismatic slip; and PyI for pyramidal I slip.

Grahic Jump Location
Fig. 13

Cross sections showing the deformation modes during pure bending simulations for the: (a) θ = 0 deg case at a bending angle of 17.7 deg; (b) θ = 15 deg case at a bending angle of 16.8 deg; (c) θ = 30 deg case at a bending angle of 37.4 deg; (d) θ = 45 deg case at a bending angle of 28.4 deg; (e) θ = 60 deg case at a bending angle of 16.2 deg; (f) θ = 75 deg case at a bending angle of 32.5 deg; and (g) θ = 90 deg case at a bending angle of 31.7 deg. All the atoms are colored according to their centrosymmetry parameter. HCP atoms are removed to facilitate visualization.

Grahic Jump Location
Fig. 14

Nucleation stress and corresponding first deformation event as a function of the simulation cell orientation angle during pure bending simulations, as well as uniaxial tension and uniaxial compression. The following abbreviations are used: T- for tension side; C- for compression side; TT1 for {10–12} TT; TT2 for {11–21} TT; CT1 for {10-11} CT; Ba for basal slip; Pr for prismatic slip; and PyI for pyramidal I slip.

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