Effect Of Sn Microalloying On Microstructure And Properties Of Al-Zn-Mg-Cu-Zr Alloy

The strength, fracture toughness and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys were studied by performing tensile tests、plane strain fracture toughness (KIC) tests and fatigue crack propagation (FCP) tests. The microstructures of the experimental alloys were further analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).(1)When Sn is included, growth of the recrystallization grains of Al-Zn-Mg-Cu-Zr alloy is obstructed, and the grain size was refined; the micro-crack propagation could be blocked when it encounters the grain boundary, finer grain size introduced more grain boundaries, thereby improving the fracture toughness and FCP resistance.(2)After homogenize at450℃×48h, the suitable solution treatment for the Al-Zn-Mg-Cu-Zr-(Sn) alloys is470℃×60min, and reach the peak strength by120℃×24h aged.(3)The formation of η’ could be delayed with Sn addition, which make the strength of Sn-containing alloy is lower during0h～42h; while addition of Sn also can suppress the growth and coarsening of η phase, result in the slower rates of softening and the higher strength after42h.(4)In the overaged Al-Zn-Mg-Cu-Zr-Sn alloy,η phases are finer, PFZ become narrower, and grain boundary precipitates are fewer and more discontinuous, all of which lower the tendency of micro-crack of the Sn-containing alloy, resulting in improvement of fracture toughness and FCP resistance, and the value of KIC can reach to35.6MPa m1/2.(5)Due to the formation of Mg2Sn phase, the Mg content of the Sn-containing alloy is lower, and the Zn content is increased relatively, thus improve the fracture toughness and FCP resistance.32figures,3tables,118references.