In this work, the influence of cation species on the corrosion behavior of Al–Mg–Si alloy in Cl^- containing solution was studied by immersion test and electrochemical measurement. The results of immersion test showed that severe pitting corrosion occurred at the Al-masking boundary of the sample and more weight loss of samples after immersion test gained in MgCl₂ solution than those in NaCl and CaCl₂ solutions. Anodic polarization curves showed little difference in pitting potential of Al–Mg–Si alloy in Cl^- containing solutions regardless of the cation species. On the other hand, cathodic polarization curves showed the increase of cathodic current density in MgCl₂ solution than those in NaCl and CaCl₂ solutions. This electrochemical behavior suggested the remarkable corrosion of Al–Mg–Si alloy in MgCl₂ solution was caused by the increase of cathodic reaction. The reason why the cathodic reaction increased in MgCl₂ solution was considered that the pH at cathode surface was kept by buffering action of Mg²⁺ because of the less solubility of Mg (OH) ₂.

View full abstract

Precipitates refinement of hypereutectic Al–Si alloy is effective for improvement of its mechanical properties. Our original severe plastic deformation process named Compressive Torsion Process is effective for not only grain refinement but also precipitates refinement. In the present work, Compressive Torsion Process was applied to an extruded hypereutectic Al–Si alloy. Mechanical properties of specimens which were unprocessed and processed with 20 revolutions at 473 K were investigated through the tensile test and upset test. Tensile test was carried out at R. T. to 573 K with 3 initial strain speed conditions. Upset test was carried out at R. T. and 473 K. As a result, tensile strength of processed specimen is increased at R. T. and 373 K. The initial strain speed affects the tensile strength at high temperature; low strain speed decrease the tensile strength compared with unprocessed specimen. Total elongation of processed specimen is increased more than twice as unprocessed specimen. Local elongation is increased at high temperature with low strain speed. Flow stress of processed specimen is higher than unprocessed specimen. Limit upset ratio of processed specimen is higher than unprocessed specimen tested at 473 K.

View full abstract

The localizations of plastic deformation in shear band which initiated at the ahead of crack tip of 7075 aluminum alloys have been investigated via in-situ fracture tests using synchrotron X-ray tomography. Local crack driving forces and local strain distributions were measured from tomographic images. The distributions of crack-tip opening displacement along a crack front line were found to vary randomly across specimen thickness. This was attributed to the interaction of stress shielding/anti-shielding effects. The strain distributions around crack-tip were quite different from the understanding which was derived by the elastic–plastic fracture mechanics. With further loadings, the development of equivalent strains in shear bands increased leading to the localizations of plastic deformation. The widths of shear band were observed to decrease at the same time. The degree of localization was affected not only by well-known factors such as aging conditions but also polycrystalline texture and crystallographic orientation. Beyond more than ten times as large strain region which is defined by the fracture mechanics, particles were damaged with developing shear band. Thus, extensive damage within shear bands has been induced the complex behavior of crack propagation in practical materials.

View full abstract