Intragranular cross-level twin pairs in AZ31 Mg alloy after sequential biaxial compressions

Sequential double extension twinning in AZ31 Mg alloy happens easily during sequential biaxial compressions. Widespread twin-twin interaction leads to the formation of intragranular cross-level twin pairs, each of which can be defined by a primary twin variant PVi (i = 1–6) and its nearby secondary twin variant PVj-SVk (j, k = 1–6, j ≠ i, k) across the twin-twin boundary between them. Theoretically, 9 possible misorientations exist between the two variants. Misorientation between two crystals can be expressed by a pair of rotation axis (r) and the minimum rotation angle (θ). The crystallography of a cross-level twin pair can be described unanimously by the misorientations between PVi and PVj-SVk (i.e., r1-θ1), PVi and PVj (i.e., r2-θ2), and the host grain and PVj-SVk (i.e., r3-θ3), resulting in 13 possible orientation relationships (ORs) that can be expressed explicitly by the three misorientation angles (θ1, θ2, θ3). A cross-level twin pair is a key element of a crossing or crossed twin structure, which is common in the deformed microstructure. Statistical electron backscatter diffraction analysis reveals that OR2 of (49.7°, 60°, 60°) appears the most frequently. Twinning shear transmission is almost irrelevant to the formation of a cross-level twin pair. Although the majority of the identified twins are high Schmid factor (SF) ones with SF > 0.3 and SF ratio > 0.8, SF analysis fails to explain the highly preference of OR2 and the abnormal absence of OR8 of (44.0°, 60°, 60.4°), probably due to the complexity of the formation of a cross-level twin pair.