Correlations between stress corrosion cracking, grain boundary precipitates and Zn content of Al-Mg-Zn alloys

The stress corrosion cracking susceptibility of traditional Al-Mg alloys modified by Zn was studied by using slow strain rate test both in the air and in 3.5 wt.% NaCl solution acidified with HCl to pH 3. Traditional Al-Mg alloys showed serious susceptibility to intergranular stress corrosion cracking, while Zn modified alloy showed relatively no susceptibility. It suggested that stress corrosion cracking resistance was significantly improved by Zn additions in acidified NaCl solution coupled with a transition from brittle failure to completely ductile failure. Both the distributions of grain boundary precipitations and grain boundary misorientation were modified by Zn addition to the Al-Mg alloys. Traditional Al-Mg alloys showed a continuous distribution of precipitations along the grain boundary, while Zn modified alloy showed a homogeneous precipitation in the matrix and discontinuous precipitation along the grain boundary. The low-angle grain boundaries had good resistance to intergranular stress corrosion cracking even though continuously precipitated along the grain boundary.