Microhardness and corrosion properties of hypoeutectic Al–7Si alloy processed by high-pressure torsion

• Grain refinement on corrosion performance of Al–7Si is reported for the first time.
• HPT significantly enhances corrosion performance of Al–7Si alloy in NaCl solution.
• Refined active sites and homogeneity are crucial to enhanced corrosion performance.
• Microhardness of Al–7Si was significantly enhanced upon high torsional strains.

High-pressure torsion (HPT) was used to produce hypoeutectic Al–7Si alloy samples having a range of microstructures to investigate the effect of the grain refinement on its corrosion behavior in 3.5 wt.% NaCl solution for the first time. Optical microscopy measurements reveal that with the HPT processing increased from 1/4 to 10 revolutions under an applied pressure of 6.0 GPa, brittle coarse silicon particles and intermetallic phases were effectively broken into ultrafine-grained particles and redistributed homogeneously into the Al-rich matrix. Open-circuit potential and polarization curves results exhibit that corrosion resistance of the Al–7Si alloy in NaCl solution was significantly enhanced upon high torsion strains, with corrosion rate reduced from 7.41 μm y−1 for the as-received sample to 1.68 μm y−1 for the 10-turn processed sample. Electrochemical impedance spectroscopy analysis combined with characterization of the corroded samples using scanning electron microscopy and energy dispersive X-ray spectroscopy indicates that the enhancement in corrosion performance of the Al–7Si alloy is due to the breakage of coarse silicon particles and intermetallic phases, the microstructure homogeneity and the increased HPT-induced active sites. It is demonstrated that microstructure refinement through HPT processing can significantly improve both microhardness and corrosion properties of the Al–7Si alloy.

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