الفهرس 
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Abstract
The age hardening behavior of 7075 aluminum aloy, solution treated at 470 c, Water quenched and aged at different aging temperatures was studied by hardness measurement .
The effects of different aging temperatures and the minor change in composition on tensile properties of aged alloys were investigated. The effect of pretreatment on aging process was also studied.
The alloys were cast in a metallic mold, homogenized and then extruded.
The heat treatment applied was solution treatment at 470 *c for 24 hours, water quenching and aging at different temperatures. Two types of pretreattment (cold working) and preaing treatment (aging at room temperature before the artificial aging).
The results of the present work indicate that the aging treatment at 120 *c and 150 * c exhibit better tensile properties than those at 170 *c. The peak hardness was reached faster at 150 *c than at 120 *c, where the corresponding aging time was , respectively, 24 and 30 houre.
The compositional effect on the aging behavior of this alloy was pronounced. Increasing the zinc content from 5.33% to 6.39% raises the tensile strength (corresponing to the peak hardness) from 619 Mpa to 655 Mpa in alloys containing the same amount of magnesium (2..16%) after aging at 150 *c.
On the other hand, increasing magnesium content from 1.96% to 2.16% , in alloys of the same zinc content (5.33%) , increases the tensile strength from 585 Mpa to 619 Mpa.
The tensile properties were also improved by increasing the chromimum and manganese contents in similar alloys. Mechanical and thermal pretreatments of the aged alloys control the kinetics of the age hardening process. Cold working before aging increases the initial rate of aging and reduces the time to reach the peak hardness. Upreaging at room temperature retards the initial rate of aging process.
The fracture of tensile speccimens in the under and peak-aged samples was shear failure , but it was cup and cone failure for the over aged samples.
The activation energy of the aging process of this alloy was calclated as 0.82_+0.01 eV.