EVOLUTION OF MICROSTRUCTURE AND TENSILE-STRENGTH OF RAPIDLY SOLIDIFIED AL 4.7 PCT ZN 2.5 PCT MG 0.2 PCT ZR X WT PCT MN ALLOYS

Analytical transmission electron microscopy and thermal analysis of as -extruded Al-4.7 pct Zn-2.5 pct Mg-0.2 pct Zr-X wt pct Mn alloys, with Mn contents ranging from 0.5 to 2.5 wt pct, were carried out to eluci date the microstructural change and accompanying mechanical properties during subsequent heat treatments. The as-extruded alloy was fabricat ed from rapidly solidified powder and consisted of a fine, metastable manganese dispersoid and the ternary eutectic T phase (Al2Mg3Zn3). Sol ution heat treatment resulted in the formation of the stable Al6Mn pha se and complete dissolution of the T phase. Formation of stable Al6Mn was made by two routes: by phase transition from metastable Mn dispers oids which already existed, and from the supersaturated solid solution by homogeneous nucleation. The density of the Al6Mn phase increased w ith the addition of manganese, while the shape and average size remain ed unchanged. A significant increase in the hardness was observed to c oincide with the formation of the Al6Mn phase. Similarly, the tensile strength increased further after the aging treatment, and the incremen t was constant over the content of Mn in the alloy, which was explaine d by the contribution from the same amount of precipitates, MgZn2. Res ults of thermal analysis indicated that the dissolution of the T phase started near 180 degrees C and that formation of Al6Mn occurred at ab out 400 degrees C, suggesting that further enhancement of strength is possible with the modification of the heat-treatment schedule.