Effects of aging conditions on creep behavior of an Al–Mg–Si alloy

Creep behavior of an Al–0.3%Mg–0.5%Si alloy affected by aging condition was investigated to obtain fundamental knowledge on the thermal stability in the service temperature range for the precipitation-hardened aluminum heat-exchanger. The alloy was aged at 175°C for 3 h (under-aged; the specimen will be termed “UA specimen”) and 24 h (peak-aged; “PA specimen”), after solution treatment. These specimens were subjected to creep testing at temperatures of 150°C and 200°C. When the creep temperature was 150°C, the UA specimen showed a lower creep rate than that of the PA specimen. On the other hand, when the creep temperature was 200°C, both specimens showed almost the same creep rate. In accordance with the higher creep resistance, TEM images demonstrated that fine precipitates were formed in the UA specimen, but not in the PA specimen, during creep testing at 150°C. These results suggest that the higher creep resistance of the UA specimen is derived from the dynamic precipitation, probably transition from clusters to needle-shaped precipitates, during creep deformation that increases the numerical density of the precipitates because the testing temperature is lower than the aging temperature. When the creep temperature is 200°C, higher than the aging temperature, however, the dynamic precipitation does not increase the numerical density of the precipitates, since the UA specimen as well as PA specimen is immediately over-aged at an early stage of the testing.