EFFECT OF MAGNESIUM ON THE AGING BEHAVIOR OF AL-ZN-MG-CU AL2O3 METAL-MATRIX COMPOSITES/

The effect of magnesium content on the aging behavior of Al-Zn-Mg-Cu a lloy reinforced with alumina (Al2O3) was studied by using the differen tial scanning calorimetry (DSC) technique and hardness measurement. Th e magnesium contents were studied in the range from 1.23 to 2.97 wt pc t. The addition of magnesium was found to increase the coherent Guinie r-Preston (GP) zones in composites. The apparent formation enthalpy of GP zones of composites (0.1 V-f) was 0.932 cal/g for 1.23 wt pct magn esium content and 1.375 cal/g for 2.97 wt pct magnesium content. The p recipitation time to achieve the maximum hardness in the composites de pends on the magnesium content. The time changed from 12 to 48 hours a s the magnesium content increased from 1.23 to 2.97 wt pct. Both Vicke rs microhardness and Rockwell hardness increased with increasing magne sium content. The maximum hardness occurred in the composites that con tained maximum amounts of GP zones and eta' precipitates. However, the microhardness of the composites was always lower than that of monolit hic alloys due to the alumina fibers which caused the suppression of G P zones and eta' formation in the composites.