Stress corrosion cracking and bio-corrosion behavior of Mg–TiO2 composites

While Mg is an attractive candidate to fabricate biodegradable implants, it suffers from low strength and high corrosion rate. To address these issues, grain refinement, alloying, and composite strengthening have been considered recently. Before using the implants, it is crucial to analyze their mechanical behavior inside a corrosive medium. In the present study, the effects of 2 and 5 vol% submicron (0.3–1 µm) TiO 2 ceramic particle additions on the microstructure, bio-corrosion behavior and mechanical integrity of pure Mg were investigated in both air and simulated body fluid. The composites were fabricated via powder metallurgy followed by extrusion process at 350 o C. Microstructural analysis revealed that the grain size was not significantly changed in the composites and remained 20 ± 3 µm. This can be ascribed to the limited effects of submicron particles on the particle stimulated nucleation, insufficient temperature and strain for recrystallization during extrusion, as well as agglomeration of particles in the matrix. By increasing TiO 2 content, the agglomeration was more pronounced, and porosities of 1.0, 1.15, and 6.85% were measured in pure Mg, Mg–2% TiO 2 , and Mg–5% TiO 2 composites, respectively. The composites showed lower corrosion resistance due to the agglomeration of particles and presence of porosities, both of which contribute to the acceleration of localized corrosion. Results from shear punch test (SPT) indicated that TiO 2 particles enhance the composites’ strength, mainly due to different coefficients of thermal expansion between the matrix and particles, the highest shear strength of 105 MPa being obtained in the Mg–5% TiO 2 composite. SPT showed that the difference in the strength measured in the air and corrosive solution was negligible at the high strain rate of 7.1 × 10 –3 s –1 , while it became significant at slow strain rate of 7.1 × 10 –5 s –1 , because during slow strain rate shear (SSRS) testing that lasted about 10 h, corrosion destroyed the microstructural integrity of the materials. Although the 5% TiO 2 composite showed the lowest corrosion resistance, it exhibited the highest strength in both air (85 MPa) and corrosive solution (77 MPa) during SSRS test, indicating its potential as a suitable candidate for applications demanding high load-bearing capacities.