On the Ductility of Magnesium Single Crystals at Ambient Temperature
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DEFORMATION and recrystallization studies of magnesium are usually performed on polycrystalline specimens. In recent years, studies on magnesium single crystals were very rare despite the fact that they can provide a valuable insight into the fundamental mechanisms of deformation and recrystallization, which can be highly ‘‘orientation-dependent’’ according to the strong anisotropic nature of the magnesium single crystal. Pioneer studies on magnesium single crystals were performed nearly 60 years ago.[1–4] Back then, specimens were deformed mostly in compression along different crystallographic directions at ambient and elevated temperatures, and the operating slip and twinning mechanisms were quantified. In the past few years, single-crystal magnesium research has again attracted many researchers to reassess the deformation behavior of Mg single crystals utilizing modern deformation and characterization techniques, such as severe plastic and high-strain-rate deformation, nanoindentation, electron backscatter diffraction (EBSD), highresolution transmission electron microscopy, and crystal plasticity modeling[5–11] in order to obtain more accurate information on the deformation microstructure and the critical resolved shear stresses (CRSSs) of the active slip and twinning systems. In a previous paper[12] we reported on the deformation and recrystallization behavior of Mg single crystals strained in plane-strain compression (PSC) along the cKONSTANTIN D. MOLODOV, Researcher, TALAL AL-SAMMAN and DMITRI A. MOLODOV, Senior Researchers, and GU¨NTER GOTTSTEIN, Senior Researcher (Emeritus), are with the Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, 52056 Aachen, Germany. Contact e-mail: [email protected] Manuscript submitted June 6, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
axis. The results showed that 1011 -compression twinning and some 1011 - 1012 double twinning were important for deformation and dynamic recrystallization (DRX) at elevated temperatures. Significant strain accommodation took place by macroscopic twinning events, whereby the twinned areas were reoriented into soft orientations for basal slip. Basal slip was naturally important for deformation, but it was also important for the development of dynamically recrystallized grains on the basis of extensive dynamic recovery, also known as continuous DRX.[13] The current study investigates the PSC behavior of magnesium single crystals in ‘‘c-axis compression’’ and ‘‘c-axis extension’’ at ambient temperature. These two conditions are well-known to result in a huge asymmetry in the mechanical response due to the relative ease of extension twinning activation during the latter. In terms of ductility c-axis compression always poses a problem since compression twinning and hc + ai slip are usually hard to activate at ambient temperature. c-axis extension, on the other hand, would show better ductility since incipient deformation can be readily accommodated by extension twinning. This twinning mode rotates the bas
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