THE MECHANISM OF {101̅2} DEFORMATION TWINNING IN MAGNESIUM
Zhiwei SHAN(),Boyu LIU
Center for Advancing Materials Performance from the Nanoscale, State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China
Cite this article:
Zhiwei SHAN, Boyu LIU. THE MECHANISM OF {101̅2} DEFORMATION TWINNING IN MAGNESIUM. Acta Metall Sin, 2016, 52(10): 1267-1278.
The {101?2} deformation twinning with extremely low activation stress is considered to be one of main reasons for the low strength of magnesium and its alloys at room temperature. In addition, it was found that those generally adopted age-strengthening methods are less effective for magnesium alloys in which postmortem investigation found that {101?2} deformation twinning is still profuse. The formation and propagation mechanism of {101?2} deformation twinning, which are of great importance for designing high strength magnesium alloy, remains elusive or under fervent debate. This paper reviewed the classical definition of deformation twinning, the existing twinning mechanisms, and the recent achievements through in-situ TEM studies on {101?2} deformation twinning. It was found that the {101?2} deformation twinning observed in magnesium are distinct from the classical definition on twinning. It is indeed a brand new room temperature deformation mechanism that can be carried out through unit-cell-reconstruction, without involving twinning dislocations. In addition, the boundaries generated through unit-cell-reconstruction are composed of {0002}/{101?0} interfaces (BP interfaces) and exhibit a terrace-like morphology in 3D space. The unit-cell-reconstruction is essentially different from the traditional dislocation-based twinning mechanism. As a consequence, to develop an effective strengthening strategy based on the nature of this new deformation mechanism would be the key for designing high strength magnesium alloy.
Fig.1 Twinning elements of 111 twin in fcc structure[38] (K1 is the first undistorted (invariant) plane, K2 is the second undistorted (conjugate) plane, η1 is the shear direction, η2 and η′2 are the conjugate shear directions in matrix and twin, respectively. °and ? represent alternative (11?0) planes. A, B and C represent the stacking sequence of (111) planes. a refers to the lattice constant. Dashed lines towards lower right are traces of (111) planes)
Fig.2 SEM images of micro-pillar (a) and ‘dog-bone’ sample (b) of pure magnesium[69]
Fig.3 Measured angle between the {1012?} twin boundary and the loading direction[69] (a and b point to the twin boundary that is approximately parallel to and perpendicular to the loading direction, respectively. c points to the twin boundary generally following the twinning plane)
Fig.4 {101?2} twin boundary is almost parallel (a) or perpendicular (b) to the loading direction[68,69]
Fig.5 The projection of an inclined {101?2} twin boundary [69] (w—width of projection, t—thickness of sample) (a) dark field TEM image showing a band-like twin boundary (b) schematic illustrates that the band-like feature comes from the projection of a inclined twin boundary along the e-beam direction
Fig.6 Snapshots from an in-situ video showing the {101?2} twin boundary migration viewed along [0001] [69] (a) the twin (dark contrast) just formed (b) the twin is expanding with an arch shaped boundary (c) the pillar was under the largest strain (d) the diamond punch was completely retracted
Fig.7 HRTEM images of {101?2} twin boundaries[70] (white dashed lines outline the profile of twin boundary) (a) twin boundary is approximately parallel to the {101?2} plane (1 points to a step parallel to the basal plane in twin) (b) twin boundary is approximately perpendicular to the basal plane of the matrix (1 points to a step parallel to the basal plane in matrix. 2 points to a stacking fault in matrix) (c) twin boundary with zig-zag shape (1 and 3 point to segments of the twin boundary parallel to basal plane in twin. 2 point to a segment approximately along the {101?2} plane. 4 points to a band-like boundary area with its width of about 2 nm) (d, e) serrated twin boundaries exhibit considerable width of about 5~10 nm (f) a band-like twin boundary with its width of about 15 nm
Fig.8 Atomic scale images of BP interfaces (a, c, e) and the corresponding schematics (b, d, f) [39] (a, b) the coexistence of CTB and BP interfaces (c, d) the coexistence of BP and PB interfaces (e, f) a long BP inteface with several steps
Fig.9 One possible route for the unit-cell-reconstruction[68,70] (The matrix hcp cell and its atoms are outlined by dashed lines and circles (light gray) respectively. The new hcp cell and the atoms are outlined by solid lines and circles (dark gray) respectively)
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