Orientation dependence of the deformation kink band formation behavior in Zn single crystal
Introduction
Compared with slip and twining, “deformation kinking” is a less common deformation mode, but it is considered important for materials that exhibit strong plastic anisotropy. Deformation kinking in metallic materials was first discovered in Cd (Orowan, 1942), and then in Zn single crystals (Hess and Barrett, 1949) with a hexagonal close-packed (hcp) structure. Currently, the deformation kink band is believed to form in many types of anisotropic materials. These bands form not only in hcp metals but also in materials where only one slip system (shear deformation mode) is predominantly operative, such as: in Ti3SiC2 ceramics (Barcelo et al., 2009, Barsoum and El-Raghy, 1999, Barsoum et al., 1999, Farber et al., 1999, Murugaiah et al., 2004), mica (muscovite) (Basu et al., 2009, Misra and Burg, 2012), graphite (Barsoum et al., 2004), and hexagonal boron nitride (Turan and Knowles, 1995).
Recently, the long-period stacking ordered (LPSO) phase was discovered as a possible strengthening phase of Mg alloys (Kawamura et al., 2001). Since the Mg/LPSO two-phase alloy demonstrates superior mechanical properties when compared with conventional Mg alloys (Hagihara et al., 2010a, Hagihara et al., 2013, Itoi et al., 2008, Kawamura et al., 2001, Kawamura and Yamasaki, 2007, Oñorbe et al., 2012, Oñorbe et al., 2013, Shao et al., 2010, Wang et al., 2012, Yamasaki et al., 2005, Yamasaki et al., 2011, Yoshimoto et al., 2006), there are many possible practical applications for this material. The formation of deformation kink bands is proposed to follow the deformation mechanism of the LPSO phase along the direction parallel to the basal phase (Hagihara et al., 2010b, Hagihara et al., 2010c, Hagihara et al., 2011, Hagihara et al., 2010b, Yamasaki et al., 2013), since only the basal slip is predominantly operative in the LPSO phase, owing to its complicated crystal structure along the c-axis. Thus, understanding the nature of the deformation kink band and its accompanying properties is important to clarify the mechanical properties of these anisotropic materials, including the LPSO phase.
One of the first models to explain the formation of the deformation kink band in hcp crystals was proposed for Zn single crystals by Hess and Barrett (1949), shown in Fig. 1. In this model, cooperative initiation and/or operation followed by arrangements of basal dislocations to align perpendicular to the slip plane are believed to be the basic processes to form the deformation kink boundary. However, experimental observations and analysis of the deformation kink bands, i.e. the examination of the crystallographic natures of the deformation kink band, has not been sufficiently conducted. Actually, the crystallographic nature of the deformation kink band has not yet been sufficiently clarified not only in the anisotropic materials including the LPSO phase but even in the Zn single crystal in which the formation of kink band model was proposed (Barcelo et al., 2009, Hagihara et al., 2015a, Hagihara et al., 2015b, Yamasaki et al., 2013). In the LPSO phase, recently the conflicting opinion that denies the presence of the deformation kink band as the predominant deformation mode was also proposed, that attempts to explain the origin of the deformation bands by the formation of deformation twin (Kishida et al., 2014). Thus, clarifying the validity of the kink band model, shown in Fig. 1, and elucidating the formation mechanism is important for understanding the deformation behavior of the anisotropic materials that accompanies the formation of deformation bands.
In this study, we focused on the deformation behavior of Zn single crystals, in which the formation of deformation kink bands has been widely believed by many researchers (Bell and Cahn, 1957, Gilman, 1954, Gilman and Read, 1953, Hagihara et al., 2015b, Hagihara et al., 2015a, Hess and Barrett, 1949, Jillson, 1950, Mayama et al., 2015, Pieła, 1997, Pieła, 2006, Washburn and Parker, 1952, Wróbel and Pieła, 2010), as a model material. The purpose of this study is to clarify the variations in formation behavior and the crystallographic features of the deformation bands formed in the Zn single crystals with loading orientation, by using scanning electron microscopy with electron backscatter diffraction (SEM-EBSD) pattern analysis and so on. In addition, a computational analysis on the variations in crystal orientation by the formation of the deformation kink bands was conducted using the crystal plasticity finite element method. By combining these experimental and computational results, the general features are examined for deformation kink bands.
Section snippets
Compression tests of Zn single crystals
Zinc single crystals were grown using the Bridgman technique (NEV-DS2, Nisshin giken, Japan) with raw Zn ingots (99.99%) under an Ar-gas atmosphere in a carbon crucible. The crystal growth rate was set at 10.0 mm/h. The crystal orientation was determined for the single crystals by the back Laue X-ray diffraction method, with an accuracy of 1°. The deformation behavior of the single crystal was examined by compression tests. Rectangular specimens, that were approximately 2 × 2 × 5 mm3 in size,
Experimental results
Fig. 3(a–d) shows the typical stress–strain curves of Zn single crystals deformed at RT and 200 °C at the four different loading orientations. In addition, Fig. 3(e) shows the corresponding temperature dependence of the yield stress (defined as a 0.2% offset stress) at those loading orientations. The profiles of the stress–strain curve were largely varied depending on the loading orientation and temperature. At the D-orientation near [5 5 8], the yield stress showed an extremely low value
Discussion
By the SEM-EBSD analysis, the deformation bands formed in the Zn single crystal were found to show three characteristic features on their crystallographic nature. The first is an ambiguous crystal rotation axis that varied on the [0001] zone axis from band to band. The second is an arbitrary crystal rotation angle that was not fixed and exhibited relatively wide distributions. The third is a variation in crystal rotation angle, even within a deformation band boundary itself. These features were
Conclusion
- (1)
In the deformation of a Zn single crystal at the [100] and [8 3 0] loading orientations, where the operation of basal slip was strongly hindered, significant formations of deformation bands were confirmed. At the [110] orientation, the formation frequency of the deformation band was considerably lower than those at the [100] and [8 3 0] orientations, and the plastic deformation was predominantly occurred by the operation of the {112} slip at RT. However, similar deformation
Acknowledgment
This work was supported by a grant-in-aid for Scientific Research on Innovative Areas (Project: “Materials Science on Synchronized LPSO Structure ∼ The Evolution of the Material Science for Innovative Development of the Next-generation Lightweight Structure Materials ∼”) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (grant number: 23109001 and 23109008). This work was also partly supported by the Light Metals Educational Foundation of Japan, and by Council for
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