AN EXPERIMENTAL-STUDY OF PHASE-TRANSFORMATIONS IN AN AL-ZN-MG-ZR ALLOY - DSC AND HOT MICROHARDNESS MEASUREMENTS

Isothermal hot microhardness and multi-scan differential scanning calo rimetry (DSC) experiments were carried out on an AlZnMgZr, 7000 series aluminum alloy to characterize the evolution and dissolution of GP zo nes and the formation of the eta' phase. Three stages of hardness evol ution with aging time were observed for aging in the 25-100 degrees C temperature range corresponding to GP zone formation, simultaneous GP zone dissolution and eta' formation, and eta' formation and coarsening . The isothermal activation energy for the GP zone formation from hot microhardness measurements was about 0.35 eV per atom comparable with that from previous hardness measurements reported in the literature. F rom variable heating rate DSC scans, the activation energies for GP zo ne formation and dissolution were 0.75 and 0.92 eV per atom, respectiv ely. The activation energy for eta' phase formation and growth of the as-quenched specimens as well as aged specimens was about 0.59 eV per atom, and the Avrami index for the transformation was inferred to be t he range 2.3-2.8(+/-20%). The activation energies are discussed in ter ms of various diffusional processes in the Al matrix and are compared with values reported in the literature