Elsevier

Computational Materials Science

Volume 111, January 2016, Pages 163-174
Computational Materials Science

Super-plasticity via secondary twinning in magnesium nanowire revealed by molecular dynamics simulations

https://doi.org/10.1016/j.commatsci.2015.09.016Get rights and content

Abstract

We have explored the secondary twinning contribution to the ductility of the magnesium nanowire using molecular dynamics simulations. An ultrahigh 60% elongation is presented in 112¯0-oriented nanowires during tensile deformation as result of primary and sequential secondary twinning processes. Crystallographic and stress field analyses identify the dominant contribution from the formation and procreation of {1¯1¯21} mode secondary twin to the elongation. Our results provide new insight of improving structural alloy ductility through twining-induced plasticity at the nanoscale.

Introduction

Magnesium(Mg)-based alloys are promising light-weight structural materials while the limited plasticity is a great challenge to their application, which is common in Hexagonal Close-Packed (HCP) metals such as Titanium (Ti) and Zirconium (Zr) [1], [2], [3], [4], [5], [6], [7], [8]. Increasing the plasticity of HCP metals by twinning can be an alternative way to overcome this limit when deformation twinning is the prevalent deformation mechanism. Such Twinning Induced Plasticity (TWIP) concept is widely adopted to designing advanced steels [9], [10], and has been extended to Mg [4], [5], [11]. Two important but controversial issues related to deformation twinning in HCP structure should be studied in order that TWIP behavior is effectively applied in Mg: (1) selection of appropriate twinning mode to achieve higher plasticity, and (2) size effect of deformation twinning.

Various twinning modes have been discovered in HCP structures, while their contributions to plasticity are not the same. There are at least five different twinning systems reported in HCP structure [12], [13], [14] (i.e. {11¯01}|11¯02¯,{11¯02}|11¯01¯,{11¯03}|33¯02,{1¯1¯21}|1¯1¯26¯, and {1¯1¯22}|1¯1¯24¯). Among these twinning modes, the {11¯01}|11¯02¯ and {11¯02}|11¯01¯ modes are dominant in Mg. {11¯01} twin is called compression (or contraction) twin since it is usually triggered by the compression along c axis of the HCP lattice, and {11¯02} twin is called tension twin in the same way [4], [5], [11], [15]. TWIP behavior via {11¯02} twin is experimentally observed in both Mg single crystalline nanowires (NWs) [11] and micro-meter grained polycrystal [4]. On the contrary, contribution of {11¯01} twin is negligible to plasticity, since the formation of secondary {11¯02} twin in {11¯01} twin accounts for the generation of voids in the material [5].

It is worth to note that the secondary twin, though supports a further way for accommodating deformation strain after the first-step twinning, usually has negative effect to plasticity [5], [16]. Hence an intriguing question is arisen: could secondary twin be beneficial to plasticity in some special conditions? Furthermore, even ternary twinning can be observed after secondary twinning [17], and could it promote plasticity? So far, most studies on secondary twinning mainly focus on the non-Schmid behavior [6], [16] of twin variant selection, where only two types of secondary twin embryo with relatively small Schmid’s factor are experimentally observed in {11¯01} twin. This phenomenon is well explained by the model based on dislocation reactions and corresponding critical shear stress [16], but the question of relationship between secondary twinning and plasticity remains.

Size effect of deformation twinning in HCP metals is more complex than that in wide-studied FCC metals. The trend also shows great difference between single crystalline [1], [3], [11] and poly-crystalline materials [7], [8], [18], [19], [20], [21]. It is initially observed that deformation twinning is more favored in coarse-grained than fine-grained HCP materials and totally vanishes in nano-crystalline HCP materials [7], [8], [18], [19]. Recently, deformation twin has been successfully discovered in nano-crystalline HCP Mg-based alloys. Nevertheless, it is unclear that if such deformation twin is resulted from the contribution of alloy element or ball-milling preparation process [22], [23]. Due to such uncertainty, the studies of TWIP behavior are limited to micrometer grained Mg and Ti [4], [24], and have not extended to nano-crystalline materials yet. With regard to HCP single crystalline nanowires (NWs) or microwires (MWs), the deformation mechanism goes through several transitions from tens of nanometers to several micrometers scale. Deformation twinning emerges in HCP NWs with a diameter from tens to hundreds nanometer or MWs thicker than several micrometers [2], [11], while only slipping of basal or prismatic dislocation occurs in NWs with intermediate size [2], [3], [20], [25]. Within even smaller diameter regime, deformation mechanism of HCP NWs has not been well studied by experiments.

When the size goes to less than tens of nanometers in diameter, ultra-thin NWs generally show abnormal structural and mechanical properties, and TWIP behavior has been discovered in FCC and BCC NWs both in experiments and computer simulations [26], [27], [28]. Since experimental and computer simulation results usually show consistency, it is meaningful to expand the study to ultra-thin HCP NWs by adopting preliminary simulations. Previous molecular dynamics simulations performed on the HCP metal NWs verified several deformation mechanisms such as {11¯02} twinning [11], [56], [57], {11¯01} twinning [11], [13], [30], [57] and phase transformation [29], [30], [31] observed in experiments. Nonetheless, the formation of the secondary twinning and the corresponding mechanism in ultra-thin HCP NWs has not been revealed to our best knowledge.

In view of the aforementioned issues, we proposed to further study the deformation mechanism in ultra-thin Mg NWs, as well as the feasibility of TWIP via secondary twinning in primary {11¯01} twin. Our hypothesis was that the NW first deformed via {11¯01} twinning, following which secondary twinning occurred as a further way to accommodate strain and achieve super-plasticity. We carried out molecular dynamics (MD) simulation of the deformation in Mg NWs, since MD is a powerful tool in estimation and illustrating deformation mechanism of materials in atomic scale. The uni-axial tension processes of {1¯1¯20} oriented Mg NWs were simulated and TWIP behavior was discovered in this work. Super-plasticity was successfully achieved via secondary twinning, while the twinning mode of secondary twin was the {1¯1¯21} mode, a rarely discovered mode in Mg. In Section 2, we presented the details of the simulation conditions. The mechanical properties, structural evolution, crystallography and atomic motion were described in Section 3. The non-Schmid behavior, size dependence of deformation twinning and contribution of Secondary twinning to plasticity were discussed in Section 4.

Section snippets

Material and methods

The MD simulation here mainly included two parts of work: (1) uni-axial tension of <1¯1¯20> oriented single-crystalline Mg NWs, and (2) shear process along the 1¯1¯26 direction of Mg single-crystal. The simulations were carried out by employing the LAMMPS code [32] and visualized with the Atomeye code [33]. Embedded Atomic Method (EAM) potential fit by Sun et al. [34] was chosen in this work since this potential can accurately describe the slipping of full or partial dislocations and is

Mechanical properties and general morphology evolution during tension

Fig. 2 shows the stress–strain curve as well as snapshots of the 4.41 nm × 4.15 nm and the 3.31 nm × 3.11 nm NWs during uni-axial tension process. These two stress–strain curves show similarity before the second yield points (εxx  0.24%), both including elastic deformation and yield of the initial configuration, a platform for the transformation to the second configuration, and elastic deformation of the second configuration. The first yield points (εxx  0.048) in Fig. 2(a) correspond to the formation of

Twinning mode and Schmid’s law

The key to the super plasticity in NW is the secondary {1¯1¯21} twinning in the primary twin configuration. However, the common secondary twinning mode is {11¯02} mode after {11¯01} twinning [6], [16]. This variation is due to the TB energies calculated by the EAM potential in this work, which are 0.103 and 0.070 J/m2 for {11¯02} and {1¯1¯21} TB [58] respectively. Meanwhile, the TB energies are 0.117 and 0.12 J/m2 for {11¯02} and {1¯1¯21} TB calculated by Density Function Theory (DFT) [59], [60].

Conclusion

We present a molecular dynamics simulation study of uni-axial tension deformation of 1¯1¯20-oriented Mg nanowires with {0 0 0 1} and {1¯100} surfaces. An ultra-high ∼60% elongation is observed by a two-step twinning process: a primary {11¯01} and a following secondary {1¯1¯21} twinning. Large area of {1¯1¯21} twin in Mg is first time observed and the twinning elements, e.g. twinning plane, shear plane and direction, are found to be similar as those in other HCP metals. In contrast, {1¯1¯21} twin

Acknowledgements

The authors are grateful for partial financial support from National Natural Science Foundation of China (Nos. 51571141 and 51201105) and National Youth Science Foundation (No. 51201100). The first author are grateful to the kind support of China Scholarship Council (CSC). The authors also acknowledge professor Mark Asta for his valuable suggestions and the access to the computational facilities of the University of California, Berkeley.

References (61)

  • B.L. Mordike et al.

    Mater. Sci. Eng. A

    (2001)
  • M.R. Barnett

    Mater. Sci. Eng. A

    (2007)
  • M.R. Barnett

    Mater. Sci. Eng. A

    (2007)
  • M.R. Barnett et al.

    Acta Mater.

    (2008)
  • X. Wu et al.

    Acta Mater.

    (2005)
  • Y. Wang et al.

    Scr. Mater.

    (2010)
  • J.W. Christian et al.

    Prog. Mater. Sci.

    (1995)
  • B. Li et al.

    Acta Mater.

    (2009)
  • C.D. Barrett et al.

    J. Mech. Phys. Soli.

    (2012)
  • W. Tirry et al.

    Scr. Mater.

    (2011)
  • Y. Zhu et al.

    Prog. Mater. Sci.

    (2012)
  • J. Sun et al.

    Scr. Mater.

    (2013)
  • D.H. Kim et al.

    Acta Mater.

    (2010)
  • X. Wu et al.

    Scr. Mater.

    (2011)
  • Q. Sun et al.

    Mater. Sci. Eng. A

    (2001)
  • C.M. Byer et al.

    Scr. Mater.

    (2010)
  • H.S. Park et al.

    Acta Mater.

    (2006)
  • C. Ni et al.

    Scr. Mater.

    (2013)
  • S. Plimpton

    J. Comput. Phys.

    (1995)
  • J. Li

    Modell. Simul. Mater. Sci. Eng.

    (2003)
  • C.D. Barrett et al.

    Scr. Mater.

    (2012)
  • J. Wang et al.

    Acta Mater.

    (2009)
  • X. Guo et al.

    Acta Mater.

    (2011)
  • J. Wang et al.

    Int. J. Plast.

    (2014)
  • Y. Li et al.

    Scr. Mater.

    (2010)
  • X. Zhang et al.

    Scr. Mater.

    (2012)
  • A.W. Sleeswyk

    Acta Metall.

    (1962)
  • R. Aghababaei et al.

    Acta Mater.

    (2014)
  • H. Song et al.

    Phys. Lett. A

    (2012)
  • J. Wang et al.

    Acta Mater.

    (2009)
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