الفهرس 
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Abstract
The aim of the present work is to investigate the influence of formation, growth and dissolution of different phases on the static stress-strain curves without and with superimposed small oscillations using specimens of Al-5wt% Mg alloy in the form of wires of 0.5 mm in diameter.
Following solid solution treatment at 823 K for 2 h, wires and sheets specimens were quenched into cold water at 273 K to get specimens of same initial solid solution state. After quenching both wires and sheets were aged at 373, 403, 433, 473, 523 and 573 K, for different aging times (¼, ½, 1, 2 and 4 h) to produce the desired microstructure of G.P. zones, metastable phase and stable phase.
The effect of aging conditions on the static stress-strain characteristic of Al-5wt% Mg alloy aged at different aging temperatures, Ta, (373-573 K) for various aging times, ta, (¼ - 4 h) has been studied. The stress-strain tests have been performed at different testing temperatures, Tt, of 313, 323, 333 and 343K.
To elucidate the effect of both temperature and time of aging, it was worthy to concentrate on some of the mechanical parameters that can be deduced from the stress-strain curves. Among these parameters yield stress σy, fracture stress σf, Young’s modulus Y, and work hardening coefficient χp - as hardening parameter- , fracture strain f , and slip dislocation distance L - as softening parameters .
The subsequent increase and decrease in the stress-strain parameters with the change in both aging temperature and time can be explained in view of the fact that the strength of age hardening Al-5wt% Mg alloy is governed by the interaction of moving dislocations with the existing precipitates.
The decrease of all hardening parameters (σy, σf, χp and Y) and the corresponding increase in the softening parameters (f and L) with increasing testing temperatures, Tt, might be due to the thermal agitation which facilitates the motion of the mobile dislocations.
It is reported that the nucleation of precipitates in Al-Mg alloy was enhanced by existing of Si atoms in the matrix. Such enhancement was attributed to Mg2Si particles-existed in commercial Al-Mg alloys- which act as precipitation centers for G.P. zones, , and phases, thus stabilizing them.
The values of the activation energy, Q, [ranged from 12.87 to17.56 kJ/mole] were found to be equal that quoted for precipitate-dislocation intersection.
The effect of superimposed torsional oscillations of different frequencies, = 1.53, 1.9, 2.5 and 3.3 Hz under constant torsional strain amplitude, 𝛗 = 5.69 x 10-4 on the static stress-strain behavior of some specimens used in the former section has been investigated.
The frequencies of the applied oscillations should exceed the threshold frequency th. By applying oscillation of different frequencies of certain amplitude to stress-strain test, the strains mainly result from the dislocation movement.
The mean activation energy for stress-strain process under the effect of tortional oscillations is 14.25 kJ/mol. This value of activation energy is in a good agreement with those given in the case of precipitate-dislocation intersection mechanism.