Overview

1. 25 µm 25 µm 10 µm 9.45mm 10mm 13mm 0.5 mm Matrix T2 TD {0001} T1 RD 10 µm Double twinning in AZ80 Magnesium alloy J. Jain, J. X. Zou, C. W. Sinclair and W. J. Poole EBSD & TEM Observations Overview T = 77K, e = 0.08 T = 300K, e = 0.08 Compression Direction Compression Direction The prevalence of deformation twinning relative to slip can be modified both through alloying and deformation temperature. We have used temperature in this study to examine the changes in both mechanical properties and microstructure in an Mg-Al-Zn alloy. One focus of this work has been to clearly identify the detailed microstructure development with deformation. Here we report specific observations of uncommon double twinning events observed when deformation occurs at low-temperature. 1 mm Starting Material: AZ80 • Results: • Many double twins are only observed for samples deformed at 77K. • No double twins were observed for samples deformed at 293K. • Confirmation of double twinning by TEM (dark field image shown in inset left) Composition (all in wt%) of AZ80 Details of Observed Double Twins: I: {1012}-{1012} Double Tensile Twin TD Calculated Pole Figure {0001} TD RD Matrix Start from matrix (M) test all possible axis-angle pairs corresponding to known twins. Sequence that best fits is shown in pole figure – this is double tensile twinning T1 M RD T1 50 μm T2 T2 Green: experimental Blue: calculated Optical micrograph and (0001) EBSD pole figure of solution-treated AZ80. The average initial grain size of the material is 32µm. Rotation axis [1120] [1120] ZM = [1 2 1 1] ZT2= [0 0 0 1] [1120] T1 [1103] Matrix primary twin [1210] Primary secondary twin [1120] M [0110] [1010] [1100] [1010] Mechanical Response [0113] [0113] [1010] [1100] [0110] [1103] [1120] [1010] Matrix Secondary Twin Uniaxial compression tests performed on solution treated samples at 77K (liquid nitrogen) and 293K. ZT1 = [1 1 0 0] TEM observations confirm EBSD determined sequence of double tensile twinning observed in samples deformed at 77K Majority of twins exhibit this form of twinning in TEM [1120] T2 [0002] [0002] [1120] Primary Twin II: {1012}-{1011} double tensile-compression twin As above, sequence and type of twinning was confirmed by iteratively computing orientation of twins relative to experiments from axis angle pairs. Note: in tension-compression (TC) case the secondary twin appears to fill the primary twin. In double tensile twinning (TT) secondary twins are fine – related to common rotation axis in TC compared to separate rotations axes in TT Calculated Pole Figure TD T2 M RD T1 Green: experimental Blue: calculated Summary Two double twinning sequences have been found in AZ80 tested in compression at 77K. In contrast only primary twins have been found in room temperature deformed AZ80. The {1012}-{1012} and {1012}-{1011} double twins twin first on {1012} planes followed by re-twinning inside the primary twins on {1012} and {1011} planes respectively. The two double twin types differ considerably in morphology. The methodology used here is able to unambiguously identify the twinning sequence (e.g. tension twin first, compression twin second). This is not possible when only boundary misorientation is used to define the double twins.