Twinning mediated anisotropic fracture behavior in bioimplant grade hot-rolled pure magnesium

• Anisotropic elastic plastic fracture toughness ( J IC ) was related to initial texture. • Extension, contraction, and double twins (ET, CT, and DT) formed at the crack tip ( a n ) vicinity due to stress triaxiality. • Distinct ET and DT lamellae morphologies and orientations activated for a n ∥, ⊥, and 45° to rolling direction. • Strain incompatibility of matrices with ET domains were higher than the DT domain. • Cracking via matrix-ET Σ15b and matrix-DT Σ23b/Σ15a interfaces led to lower and higher plasticity and J IC , respectively. Bioimplant grade hot-rolled magnesium with equiaxed microstructure and basal texture was examined for fracture toughness (FT) anisotropy using fatigue pre-cracked single-edge notch bending specimens with the notch, a n ∥, ⊥ and 45° to rolling direction (RD). Due to adequate crack-tip plasticity, the size-independent elastic-plastic fracture toughness ( J IC ) were determined. Anisotropic J IC was observed due to different twin lamellae formation w.r.t. notch owing to the initial basal texture with { 10 1 ¯ 0 } and { 11 2 ¯ 0 } poles mostly ∥ and ⊥ to RD. The out-of-plane tensile stresses activated the { 10 1 ¯ 2 } 〈 10 1 ¯ 1 〉 extension twins (ET) as usual with matrix-ET Σ15b coincident site lattice boundary (CSLB) interfaces. While the in-plane tensile stress ⊥ to the crack-tip activated { 10 1 ¯ 1 } 〈 10 1 ¯ 2 〉 contraction twins (CT) that transform into { 10 1 ¯ 1 } - { 10 1 ¯ 2 } double twins (DT) with matrix-DT Σ23b and Σ15a CSLBs. For a n ∥ RD, large DT lamellae fraction formed at ∼30° and few ETs at ∼30° and ∼90° to the notch with crack growth mainly via the Σ23b/Σ15a CSLB interfaces during FT. While, significant DT and ET lamellae developed at ∼0° and ∼60° with cracking via the matrix-DT Σ23b/Σ15a and matrix-ET Σ15b CSLBs for a n ⊥ RD. The DT and ET lamellae activated at ∼15°, and the crack propagated through Σ15b for a n ∼ 45 ∘ to RD. The J IC and the crack-tip plastic zone decreases, while the elastic component of the J-integral ( J el ) and the ET formation increases from a n ∥, ⊥, to ∼ 45 ∘ to RD. The strain incompatibility of matrices was higher with the geometrically hard ETs than DTs. Thus, brittle interlamellar cracking occurred through the Σ15b interfaces. In contrast, almost similar and higher crack-tip plasticity occurred in matrix and DT domains during crack propagation via Σ23b/Σ15a CSLBs. Crack growth through Σ23b/Σ15a led to high J IC , both Σ15b and Σ23b/Σ15a led to moderate J IC , and Σ15b least J IC for a n ∥, ⊥ and 45° to RD, respectively.