Fatigue fracture of friction-stir processed Al–Al3Ti–MgO hybrid nanocomposites

• A fine grained in situ Al–Mg matrix nanocomposite was processed via reactive FSP.
• Microstructural features, tensile, and fatigue performance of the prepared nanocomposite were assessed.
• Fatigue strength of the FSPed nanocomposites was improved continuously via incorporation of nanoparticles.
• Ductile fracture behavior with deep-equiaxed dimples on fracture surfaces was observed for Al–Mg alloys.
• A combined ductile-brittle behavior on fatigue loading was noticed for nanocomposites with finer features.

This paper presents experimental results on the fatigue properties of Al-matrix nanocomposites prepared by the friction stir processing (FSP) technique. An Al–Mg alloy (AA5052) with different amounts (∼2 and 3.5 vol%) of pre-placed TiO2 nanoparticles were FSPed up to 6 passes to attain homogenous dispersion of nano-metric inclusions. Microstructural studies by electron microscopic and electron back scattering diffraction (EBSD) techniques showed that nano-metric Al3Ti (50 nm), TiO2 (30 nm), and MgO (50 nm) particles were distributed throughout a fine-grained Al matrix (<2 μm). Consequently, a significant improvement in the tensile strength and hardness was attained. Uniaxial stress-controlled tension–tension fatigue testing (R = 0.1) were utilized to evaluate the fatigue behavior of the prepared nanocomposites. The results were compared with the un-processed (annealed) and FSPed alloy without pre-placing TiO2 particles. It was found that FSP of the aluminum alloy increased the fatigue strength (at 107 cycles) for about 28% and 32% compared with the annealed specimen when the concentration of the reinforcing particles was 2 and 3.5 vol%, respectively. Fractographic analysis determined a ductile fracture behavior with deep-equiaxed dimples for the annealed and FSPed alloy. The facture surface of the nanocomposites revealed a combined ductile–brittle fracture mode with finer dimples. The mechanism of the fatigue fracture and the role of nano-metric inclusions were elaborated.