Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals

doi:10.1007/s40195-015-0353-2

Abstract

Abstract:

Molecular dynamics simulation is employed to study the tension and compression deformation behaviors of magnesium single crystals with different orientations. The angle between the loading axis and the basal <a> direction ranges from 0° to 90°. The simulation results show that the initial defects usually nucleate at free surfaces, but the initial plastic deformation and the subsequent microstructural evolutions are various due to different loading directions. The tension simulations exhibit the deformation mechanisms of twinning, slip, crystallographic reorientation and basal/prismatic transformation. The twinning, crystallographic reorientation and basal/prismatic transformation can only appear in the crystal model loaded along or near the a-axis or c-axis. For the compression simulations, the basal, prismatic and pyramidal slips are responsible for the initial plasticity, and no twinning is observed. Moreover, the plastic deformation models affect the yield strengths for the samples with different orientations. The maximum yield stresses for the samples loaded along the c-axis or a-axis are much higher than those loaded in other directions.

Key words: Molecular dynamics simulations, Magnesium, Plastic deformation, Orientation

Qun Zu, Ya-Fang Guo, Shuang Xu, Xiao-Zhi Tang, Yue-Sheng Wang. Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(3): 301-312.

Figures/Tables 12

Fig. 1 a Square, b cylindrical simulation models for magnesium nano-pillars. c The unit cell of hcp lattice. The yaxis indicates the loading direction, which is always perpendicular to the [1?010]-axis

Table 1 Initial plastic deformation mechanisms of the samples with different orientation θ under tension and compression

θ	Tension		Compression
θ	Square	Cylinder	Square	Cylinder
0°	T1, BSF, CR	√	PrD<a>	√
10°	T1, BSF, CR	√	SB	√
15°	PrD <a>, T1, BSF, CR	√	SB	√
20°	PrD <a>, T1, BSF, CR	√	SB	√
31.6°	PrD <a>, BD<a>	√	BD<a>	√
45°	PrD <a>, BD<a>	BD<a>, PrD<a>	BD<a>	√
58.4°	SB	SB, BD<a>	BD<a>	√
75°	SB	SB, BD<a>	PyD<+a >, BSF, BD<a>	√
80°	SB	√	PyD<c+a >, BSF, BD<a>	√
90°	PyD<c+a>, B/P, T3, BD<a>, BSF	√	PyD<c+a >, BSF, BD<a>	√
Conclusion	θ < 15°, T1, BSF; θ = 15°-45°, PrD< a>; θ = 45°-80°, SB; θ> 80°, PyD<c+a>	θ ~ 45°, BD<a>	θ = 0°, PrD <a>;θ < 20°, SB; θ = 20°-75°, BD<a>; θ > 75°, PyD<c+a>	√

Fig. 2 Structures of initial plasticity under tension at θ = 0° under ε = 7.2% a, and ε = 7.3% b. Magnified atom configurations of initial defects: c T1 and BSF, d basal/prismatic (B/P) interface between the parent lattice and the rotated lattice, and e grain boundary (GB) between the parent lattice and rotated lattice

Fig. 3 Structures of initial plasticity under tension at θ = 20° under ε = 6.1% a and ε = 6.16% b

Fig. 4 Initial deformation mechanism of the circle column viewed along the loading y direction at θ = 45° with the strains of 5.5% a, 5.59% b, and 5.68% c

Fig. 5 Initial deformation mechanism of the square column under tension at θ = 31.6°. a The structure with prior defect of prismatic <a> dislocation. b The structure with the following defect of basal <a> dislocation

Fig. 6 Initial deformation mechanism of square column under tension at θ = 58.4° with the strains of 5.2% a, b and 5.3% c. d Magnified figure of the shear band

Fig. 7 Initial deformation mechanism of circle column under tension at θ = 58.4°. a The prior defect of basal <a> dislocation. b The following defect of shear band

Fig. 8 Initial deformation mechanism of the square column under tension at θ = 90°. a Pyramidal dislocation nucleates at the free surface at the strain of 8.1%. B/P transformation and BSF formation at strain of 8.2% b, 8.7% c and 9.5% d, respectively. e, f Magnified atom configurations of B/P transformation

Fig. 9 Microstructure evolutions of the [1?21?0]-oriented square pillar under compression with the strains of ε = 5%a, ε = 6.7% b, ε = 6.8% c and ε = 6.9% d

Fig. 10 Microstructure evolutions of the square column under compression at θ = 31.6°

Table 2 Initial defects and maximum yield stresses (σ max) with different orientations under the tension and compression

θ	Tension		Compression
θ	σ _max (GPa)	Initial defects	σ _max (GPa)	Initial defects
0°	1.68	T1, BSF	3.92	PrD <a>
10°	1.61	T1, BSF	2.11	SB
15°	1.64	PrD <a>	1.94	SB
31.6°	1.78	PrD <a>	1.73	BD <a>
45°	2.52	PrD <a>	1.64	BD <a>
58.4°	2.35	SB	1.97	BD <a>
75°	2.68	SB	3.23	PyD <+a>
80°	3.31	SB	3.53	PyD <c+a>
90°	4.01	PyD <c+a>	3.54	PyD <c+a>

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