EFFECT OF GA CONTENT ON STRESS-CORROSION CRACKING CHARACTERISTICS OF AL-9 MASS-PERCENT-MG ALLOYS

The effects of Ga content and aging treatment on stress corrosion crac king characteristics of Al-9 mass%Mg-X mass%Ga alloys are investigated by the measurements of number of precipitates and their occupation ra tios to grain boundary, transmission electron diffraction analysis of precipitates, mechanical properties, time to failure of stress corrosi on cracking and scanning electron micrographs of stress corrosion crac king fracture surfaces for the specimens aged at 453 K in a mixed solu tion of 3 x 10(-2) kmol/m(3) K2Cr2O7 + 1 x 10(-2) kmol/m(3) HCl + 5 x 10(-1) kmol/m(3) NaCl at 303 K under a loaded stress of 0.9 x 0.2% pro of stress. The main results obtained are summarized as follows: (1) Th e number of Al3Mg2 precipitates and its occupation ratios to grain bou ndary of Al-9 mass%Mg-X mass% Ga alloys aged at 453 K increase with in creasing aging time. When the aging time is longer than 40.0 ks, the p recipitates are observed in the transcrystalline, and the precipitate free zone is formed around the grain boundary layer irrespective of st oichiometries of the specimens. The Mg2Ga or MgGa2 compound is detecte d in the precipitates of Ga containing specimens. (2) The effects of G a content and aging treatment on mechanical properties such as U.T.S. and 0.2% proof stress of Al-9 mass%Mg-X mass%Ga alloys are rather smal l. (3) The time to failure of stress corrosion cracking of the specime ns is reduced with increasing aging time irrespective stoichiometries of the specimens used. However, the time to failure of stress corrosio n cracking increases with increasing precipitates in the transcrystall ine of the specimens. The failures from stress corrosion cracking of t he specimens are primarily intercrystalline.