Al-Zn-Mg合金の応力腐食処理後の時効脆化挙動

Abstract

Change in the tensile strength of an Al-Zn-Mg alloy, which was subjected to the Stress-Corrosion (SC) treatment in 3%NaCl aqueous solution containing K2Cr2O7 was examined in relation to aging temperature, tesile load applied in the SC treatment, and the solution temperature (Ts). The observation of fracture surfaces was also performed by scaning electron microscopy. Results obtained are:(1) The relation between the reciprocal of the solution temperature in the SC treatment and aging time intervals (tb) up to minimum tensile strength is conjectured approximately as: tb∝Nexp(κ⁄Ts), where N and κ are constant.(2) The relation between tensile load applied in the SC treatment and tb is conjectured as: tb∝N′exp(−κ′σ), where N′ and κ′ are constant.(3) The relation between the reciprocal of aging temperature (Ta) and tb is conjectured as: tb∝N″exp(κ″⁄Ta), where N″ and κ″ are constant.(4) It is assumed that grain boundary precipitates formed by aging grow up with prolonged aging time, and in due course, they are decomposed to particles, and are absorbed into three ajacent grain-boundaries.(5) The area of dimples on a fracture surface is small near the surface but large in the center for the specimen just after the SC. With passage of aging time, the area of dimples seems to move gradually from surface side to center, and after prolonged time, the area is very small near the surface. This phenomenon could be explained as the change in the embrittlement region by hydrogen diffusion.