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

Journal of Magnesium and Alloys, vol. 12, pp. 3806–3822

Abstract

• 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.

Authors 2

1. Prakash C. Gautam

   Indian Institute of Technology Kharagpur

   Affiliation as printed

   Light Metals and Alloys Research Lab, Department of Metallurgical and Materials Engineering, Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India

2. Somjeet Biswas corresponding Aachen Institut für Metallkunde und Materialphysik

   RWTH Aachen University · Indian Institute of Technology Kharagpur

   Affiliation as printed

   Institut für Metallkunde und Materialphysik (IMM), Rheinisch- Westfälische Technische Hoschule Aachen, Aachen 52074, Germany

   Light Metals and Alloys Research Lab, Department of Metallurgical and Materials Engineering, Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India