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

Abstract

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 $\left\{10\overline{1}0\right\}$ and $\left\{11\overline{2}0\right\}$ poles mostly ∥ and ⊥ to RD. The out-of-plane tensile stresses activated the $\left\{10\overline{1}2\right\}〈10\overline{1}1〉$ 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 $\left\{10\overline{1}1\right\}〈10\overline{1}2〉$ contraction twins (CT) that transform into $\left\{10\overline{1}1\right\}$-$\left\{10\overline{1}2\right\}$ 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 $a_n\sim {45}^{\circ }$ 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 $\sim {45}^{\circ }$ 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.