Structural characterization of twin boundaries in deformed cobalt
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 , an extension twin like the type usually occurs when a tension strain is applied along the c axis, while a contraction twin, such as 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 twin have been investigated in the experimental and modeling fields at multiple length fields [6], [7], [8]. The actual TBs of twin are not straight, but present a step-terrace interfacial feature, and are not always parallel to the theoretical undistorted twinning planes [6]. Such shocking features can be ascribed to the existence of {0002}|| basal-prismatic (BP or PB) interfaces in the 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 twin, the 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 TBs may be also faceted, similar to the interfacial features of twin. Furthermore, they also proposed that the most expected deformation facets in TBs are || prismatic-third order pyramidal (P3Py) and {0002}|| basal-pyramidal (BPy) interfaces. Here, P3Py interface means that the prismatic plane of the matrix is parallel to the pyramidal plane of the twin and BPy interface means that the {0002} basal plane of the matrix is parallel to the pyramidal plane of the twin. However, until recently, for 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 twin in deformed hcp materials. In this paper, the 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 system. The actual TBs of twin can consist of straight TBs and BPy interfaces. In addition, a high density of basal stacking faults (SFs) was observed within the 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 zone axis is inserted in the left upper corner of Fig. 1, indicating that the twin shown here corresponds to the 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 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 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 twin system has a Burgers vector:
Conclusion
In summary, in this paper, the 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 twinning boundaries and basal-pyramidal (BPy) interfaces. A BPy interface can be stably connected with two parallel twinning boundaries. The existence of the BPy interfaces results in that the actual twining boundaries are not always aligned with the theoretical
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).
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