Non-isothermal precipitation kinetics and its effect on hot working behaviors of an Al–Zn–Mg–Cu alloy

The precipitation kinetics and its effect on hot working behavior of an Al–Zn–Mg–Cu alloy were studied under non-isothermal conditions and over a wide range of temperatures. The characterization techniques employed included in situ electrical resistivity measurements, differential scanning calorimetry, hardness testing and microstructure examination by scanning and transmission electron microscopy. The results showed that the precipitation kinetics during non-isothermal treatments were highly dependent on the thermal paths. Slow cooling processes led to coarse and sparse particle distribution. When heating to high temperatures, very fine precipitates with high density and homogeneous distribution were present at low temperature. Such fine precipitates showed unstable characters and dissolved quickly at higher temperature. Quantification of precipitate evolution during non-isothermal processing was obtained from in situ electrical resistivity measurements. The effects of heating paths on dynamic and static flow behaviors were also demonstrated to be significant. The degree of initial supersaturation controlled the nucleation rates and led to strong effect on dynamic and static precipitation, and on the flow stress softening.