Elsevier

Materials Characterization

Volume 116, June 2016, Pages 44-47
Materials Characterization

Structural characterization of 101̅1 twin boundaries in deformed cobalt

https://doi.org/10.1016/j.matchar.2016.04.005Get rights and content

Highlights

  • The interfacial structure of 101̅1 twinning boundaries investigated by HRTEM.

  • The actual 101̅1 TB is not straight, but presents a step-terrace interfacial feature.

  • High density basal stacking faults are observed experimentally within the 101̅1 twin.

Abstract

Deformation twinning is one of the most important strain accommodation mechanisms for deformed hexagonal close-packed materials. During plastic deformation, interfaces such as twinning boundaries usually play a critical role to affect the mechanical properties of many materials with hexagonal structure. As one kind of significant twinning modes, 101̅1 contraction twin usually occurs at the final stage of plastic deformation and serves to relax the stress concentration. Therefore, it is very crucial to understand the interfacial structure of twinning boundaries of 101̅1 twin at the atomic scale if we are to properly tailor twins for microstructural design and applications. In the present work, by means of high-resolution transmission electron microscopy, the 101̅1 deformation twin in deformed cobalt has been investigated. The results show that the twinning boundaries are not straight, but actually consist of 101̅1 TBs and {0002}||1̅011 basal-pyramidal interfaces. In addition, a high density of basal stacking faults is also observed experimentally within the 101̅1 twin. According to these experimental features, the possible mechanism for twinning boundary migration and for the emergence of such abundant basal SFs will be discussed.

Introduction

For materials with hexagonal structure, in addition to dislocation slips, deformation twinning also serves as a significant deformation mechanism during the plastic deformation, in order to satisfy the von Mises criterion for accommodation of the strain at grain boundaries, especially at low temperature or high strain rate [1], [2], [3]. Correspondingly, twinning boundary (TB) usually plays a critical role in plastic deformation and can influence the mechanical properties of many materials with hexagonal structure, such as magnesium (Mg), titanium (Ti) and their alloys [4]. Therefore, it is very crucial to understand the interfacial structure of TBs at the atomic scale if we are to properly tailor twins for microstructural design and applications.

Deformation twins are often categorized as being either extension or contraction. In the hexagonal close-packed (hcp) materials with γ = c/a ratios less than 3, an extension twin like the 101̅2 type usually occurs when a tension strain is applied along the c axis, while a contraction twin, such as 101̅1 twin, often occurs when a compression strain is applied along the c axis [1], [5]. As the most common twinning mode, the interfacial features of 101̅2 twin have been investigated in the experimental and modeling fields at multiple length fields [6], [7], [8]. The actual TBs of 101̅2 twin are not straight, but present a step-terrace interfacial feature, and are not always parallel to the theoretical undistorted 101̅2 twinning planes [6]. Such shocking features can be ascribed to the existence of {0002}||101̅0 basal-prismatic (BP or PB) interfaces in the 101̅2 twin system, as demonstrated in molecular dynamic simulation and experimental observations [7], [9]. Accordingly, Wang et al. [10] firstly proposed that such BP (or PB) interfaces could migrate by glide and climb of twinning dislocations (TDs), combined with atomic shuffling. In a recent paper, the existence of BP or PB interface was further confirmed by Barrett et al. [11] within the framework of topological theory of crystallographic defect. Although occurring less frequently than the 101̅2 twin, the 101̅1 contraction twin is also observed in some deformed materials with hexagonal structure and serves to relax the stress concentration at the final stage of deformation [5], [12], [13]. As predicted by Barrett et al. [11], the 101̅1 TBs may be also faceted, similar to the interfacial features of 101̅2 twin. Furthermore, they also proposed that the most expected deformation facets in 101̅1 TBs are 1̅010||101̅3 prismatic-third order pyramidal (P3Py) and {0002}||1̅011 basal-pyramidal (BPy) interfaces. Here, P3Py interface means that the 1̅010 prismatic plane of the matrix is parallel to the 101̅3 pyramidal plane of the twin and BPy interface means that the {0002} basal plane of the matrix is parallel to the 1̅011 pyramidal plane of the twin. However, until recently, for 101̅1 twin in deformed hcp materials, only P3Py interface has been reported experimentally [13].

The aim of the present work is to shed more light on the interfacial structure of 101̅1 twin in deformed hcp materials. In this paper, the 101̅1 deformation twin in deformed cobalt (Co) was investigated by means of high-resolution transmission electron microscopy (HRTEM). The results show that BPy interfaces indeed exist in the 101̅1 system. The actual TBs of 101̅1 twin can consist of straight 101̅1 TBs and BPy interfaces. In addition, a high density of basal stacking faults (SFs) was observed within the 101̅1 twin. According to these experimental features, the possible mechanism for TBs migration and for the formation of such abundant basal SFs will be discussed below.

Section snippets

Experimental Procedure

The material used in this investigation was high purity (99.99 wt.%) Co. The cylindrical samples were deformed by dynamic compression at room temperature (293 K) with a strain rate of 1 × 102  2 × 103 s 1 to a strain of ε =  0.08. The transmission electron microscopy (TEM) thin foils, cut from cross-section of the compressed bulk sample, were prepared by mechanical grinding, followed by means of double-jet electrolytic polishing in an electrolyte consisting of 10% (volume) perchloric acid and 90% glacial

Results

Fig. 1 presents a cross-sectional bright-field TEM image of deformation twin in deformed Co. The corresponding selected area electron diffraction (SAED) pattern taken from the region containing twin and matrix using a 12̅10 zone axis is inserted in the left upper corner of Fig. 1, indicating that the twin shown here corresponds to the 101̅1 twinning orientation relationship. SFs with the appearance of straight lines marked by the white arrows are not only observed within the twin, but also

Twinning Boundaries Migration

Within the framework of traditional twinning theory, the growth of 101̅1 twin is usually ascribed to the assumed TDs gliding successively along the corresponding TBs [1], [15], [16]. Accompanying TDs gliding, the atoms in the matrix are carried to the correct positions of twin to accomplish 101̅1 twinning process by a combination of shear and local atomic shuffle. According to the crystallographic calculation by Christian et al. [1], the elementary TD for 101̅1 twin system has a Burgers vector:

Conclusion

In summary, in this paper, the 101̅1 deformation twin in deformed Co has been investigated by HRTEM. The results reveal that the twinning boundaries are not coherent, but actually consist of straight 101̅1 twinning boundaries and basal-pyramidal (BPy) interfaces. A BPy interface can be stably connected with two parallel 101̅1 twinning boundaries. The existence of the BPy interfaces results in that the actual 101̅1 twining boundaries are not always aligned with the theoretical 101̅1

Acknowledgements

We gratefully acknowledge Dr. C.D. Barrett (Mississippi State University) and Dr. B. Li (University of Nevada) for valuable advice and discussions. This work was supported by National Natural Science Foundation (NSFC) Nos. 51271208, 51071183, 50890170 and the Basic Research of China (No. 2010CB631004).

References (26)

  • J.W. Christian et al.

    Deformation twinning

    Prog. Mater. Sci.

    (1995)
  • T.A. Sisneros et al.

    Influence of strain rate on mechanical properties and deformation texture of hot-pressed and rolled beryllium

    Mater Sci Eng A

    (2010)
  • G.C. Kaschner et al.

    Role of twinning in the hardening response of zirconium during temperature reloads

    Acta Mater

    (2006)
  • J.F. Nie et al.

    Periodic segregation of solute atoms in fully coherent twin boundaries

    Science

    (2013)
  • J. Koike

    Enhanced deformation mechanisms by anisotropic plasticity in polycrystalline Mg ally at room temperature

    Metall. Mater. Trans. A

    (2005)
  • J. Tu et al.

    Structural characterization of {10-12} twin boundaries in cobalt

    Appl. Phys. Lett.

    (2013)
  • Q. Sun et al.

    Interfacial structure of {10-12} twin tip in deformed magnesium

    Scr. Mater.

    (2014)
  • J. Wang et al.

    (-1012) twinning nucleation mechanism in hexagonal-close-packed crytals

    Acta Mater.

    (2009)
  • C.D. Barrett et al.

    The roles of grain boundary dislocations and disclinations in the nucleation of {10-12} twinning

    Acta Mater.

    (2014)
  • J. Wang et al.

    Twinning and de-twinning via glide and clime of twinning dislocations along serrated coherent twin boundaries in hexagonal-close-packed metals

    Math. Res. Lett.

    (2013)
  • C.D. Barrett et al.

    Impact of deformation faceting on {10-12}, {10-11} and {10-13} embryonic twin nucleation in hexagonal close-packed matals

    Acta Mater

    (2014)
  • X. Wu et al.

    Strain-induced grain refinement of cobalt during surface mechanical attrition treatment

    Acta Mater

    (2005)
  • Y.J. Li et al.

    Faceted interfacial structure of twins in Ti formed during equal channel angular pressing

    Scr. Mater.

    (2010)
  • Cited by (17)

    View all citing articles on Scopus
    View full text