Microstructural evolution and aging behavior of cryorolled Al–4Zn–2Mg alloy

The effect of cryorolling on microstructural evolution and aging characteristics of an Al–4Zn–2Mg alloy is investigated using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and hardness measurements. The Al–4Zn–2Mg alloy in solutionized condition was cryorolled to achieve 70–90% deformation. The cryorolled samples were short annealed followed by three different aging treatments viz., natural aging, peak aging and over aging. The EBSD results suggest the presence of ultrafine grains in the cryorolled alloy. DSC thermograms and aging studies revealed that cryorolling enhances the aging kinetics of the alloy. The two-step peak aging treatment indicated that the degree of peak hardness is directly proportional to the extent of deformation and the time to attain it varied inversely with respect to the amount of deformation. TEM studies of cryorolled and peak aged (CRPA) samples showed large number of uniformly distributed, finer and closely spaced η′ strengthening precipitates when compared to bigger precipitates in coarse grain peak aged (CGPA) alloy. Variations in precipitate distribution and morphology between CRPA and CGPA alloys are attributed to the stored energy and high density of dislocations generated during cryorolling..