ON THE MECHANICAL PROPRIETIES AND PRECIPITATION KINETICS IN Al – 3 . 3 WT . % Cu ALLOY

The effect of additional Mg on the microstructure, mechanical properties, and transformation kinetics during aging in Al–3.3 wt.% Cu alloy was studied. The compositions and microstructure were examined by X-ray diffraction, Differential scanning calorimetry (DSC) and scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS). The results show that the Mg in the Al–Cu alloy mainly precipitated to the grain boundaries during the process of transformation and formed a ternary Al2CuMg metallic compound and the rate of discontinuous precipitation reaction decreases with increasing concentration of Mg. The activation energy of crystallization was evaluated by applying the Kissinger equation.


Introduction
The aluminum has excellent mechanical properties in the presence of alloying elements, mainly due to the precipitation hardening (e.g., Al-Cu(Mg) alloys).These alloys and materials are widely used in the aeronautics field [1][2][3][4][5].For a concentration of Cu less than 4%, the Al-Cu and Al-Cu-Mg phase diagrams show the formation of a solid solution α.Increasing the concentration leads to precipitation of copper in the form of Guinier Preston zones (GP1 and GP2) and the first compound Al2Cu (θ' metastable, then θ stable), and phases (S' metastable, then S stable) in Al-Cu-Mg alloys [6][7].
The precipitation sequence in the Al-Cu-Mg system can be presented as
The aim of this research is to study the effect of Mg content on the microstructure and mechanical properties of Al-Cu alloy.The possible effect mechanism will also be discussed.

Experimental methods
Two commercials purity Al-Cu-Mg alloys ingots have been studied; the compositions are shown in Table 2. Specimens with the size of 10mm×10mm×12mm were wired-cut from the center of the ingot.The specimens were homogenized at 470 o C for one week and quenched in water.Microstructure, morphologies of fracture surface and compositions of tested alloys were analyzed by Scanning Electron Microscopy (SEM) (JEOL 2000FX) equipped with energy dispersive spectroscopy (EDS).Phase identification of the alloys was further confirmed by analyzing X-ray diffraction patterns generated by PAN alytical X'Pert PRO diffractometer.CuK radiation and a speed of 1°/min were used.The DSC measurements were performed with a NETZSCH 200 PC DSC.Vickers hardness is one of the most testing techniques and scientists.It is a form of microhardness that uses a diamond indenter and is suitable for a wide range of materials.A Yukon 2500 device was used in the microhardness measurements.

Results and discussion
The as-quenched microstructure The as-quenched microstructures are shown in Fig. 1.It is a typical structure that consists of the fine precipitates with a grain boundary.Fig. 2 represents a typical EDS spectrum of the presents alloys in the as-quenched condition.The quantitative analysis of six different EDS spectra indicated that the average chemical compositions were: Al-3.3±0.3 wt.%-Cu-0.96±0.2wt.% Mg and Al-3.28±0.1 wt.% Cu-1.98±0.4wt.% Mg respectively.For studying the heat treatment effect on the discontinuous precipitation in Al-3.3wt.%Cu-1 wt.% Mg and Al-3.3 wt.% Cu-2 wt.% Mg alloys, the samples are homogenized at 470 °C for one week and quenched in water.In this part of the investigation, we present the results of differential scanning calorimetry (DSC) in nonisothermal conditions, previously homogenized and quenched (Fig. 3 and Fig. 4); and age at different heating rates (2, 5, and 10 °C/min).Five main effects may be identified in these thermograms [17][18][19] an exothermic peak; A, between 60 and 130 °C that occurred due to the formation of co-clusters [20,21]; an endothermic effect, B, between 130 and 170 °C, may be attributed to Cu-Mg cocluster dissolution (with possibly some GPB2 dissolution); two exothermic peaks effect, C, and D between about 200 and 300 °C, are attributed to the formation of S' and S phases precipitates respectively; a broad endothermic effect, E, at 300 to 450 °C is identified as progressive dissolution of the S' and S precipitates [22,23].
To determine the activation energy Ea of S' and S phases of our alloys we use the Kissinger method, this method relies on the assumption that, during the temperature increase, the reaction passes through a maximum before decreasing, using the following relation [24]: where C is constant, at the maximum rate of transformation which corresponds to the maximum at the DSC peak T = Tm (d 2 y/dt 2 =0) and R, the perfect gas constant (8.314J/mol•K).
The maximum temperatures of reactions were determined from the slopes of the DSC curves.The activation energy for the formation of the S' and S phases (Al2CuMg) under nonisothermal conditions was calculated from the slope of The value of Ea may be calculated from the slope of each curve it is shown in this following Table 3; these values are in good accordance with the literature [25].The results of the heating rate and Mg concentration effect on precipitation are presented in the relative volume fraction versus temperatures (Fig. 5), this figure shows sigmoidal curves at different temperatures for the discontinuous precipitation (DP).It is clear that as the concentration of Mg increases, the rate of DP reaction decreases (shift curves on basis temperatures).To determinate the Avrami exponent we used the formula, where are: ΔT -The width at half maximum, Ea-activation energy and Rideal gas constant.
The Table 4. shows the values of Avrami exponent (n).Avrami analysis was utilized to study the overall bulk crystallization kinetics after a specific thermal history.The Avrami exponent evaluated empirically, generally between 1 and 4. In the present study, the mean value of Avrami coefficient (n) for S' and S phases is 1.41 and 1.42 respectively; which may correspond to phase transformation mechanism driven by the diffusion.It has been found, that like any other diffusion controlled nucleation and growth process, the reaction front velocity in DP usually records an 'inverse-C' variation with temperature [27].It is suggested that atomic mobility is essential for the time-dependent nucleation.It is noticed that the peak moves towards basis temperatures as much as the concentration of Mg increases.

After DSC treatment
The nonisothermal treatment effect on the transformation of precipitation in Al-3.3wt.%Cu1wt.%Mg and Al-3.3wt.%Cu2wt.%Mg was examined before and after DSC treatments.The initial samples are homogenized at 470°C for one week and quenched in water.The X-ray diffraction spectrum of this quenched alloy which corresponds to supersaturated solid solution αo is shown in Fig. 6 (a).The second nonisothermal treatment applied on quenched alloy is performed by DSC analysis, from room temperature to 470 °C with heating rate 2 °C/min.The DSC curves show an exothermal peak that corresponds to energy dissipation during the discontinuous precipitation.The formation of this new S phase after last treatment is detected by the X-ray diffraction, where the S phase (Al2CuMg) peaks are present in the spectrum, Fig. 6 (b, c).Based on the Al-Cu-Mg phase diagram [28], the solid solubility of Mg is relatively low at room temperature in aluminum alloys.Therefore, the concentration of Mg at the interface of the solid/liquid phases was certain increased correspondingly during the solidification proceeds.
The Vickers hardness value of Al-3.3wt.%Cu-1wt.%Mg and Al-3.3wt.%Cu-2wt.%Mg alloys are presented in Table 5.All samples are aging for various time at 150 o C .Vickers hardness measurements were carried out in order to investigate the effect of mechanical properties with the small addition of magnesium.The Vickers hardness value of sample of Al-3.3wt.%Cu-2wt.%Mg alloy is slightly lower than that of Al-3.3wt.%Cu-1 wt.% Mg alloy.We concluded that the hardening observed is due to the formation of the phase S' which is converted into the hardening phase S in order to obtain the maximum hardening in these alloys.values of the microhardness with the prolongation of aging at 150 °C, is directly attributed to the decrease in the amount of precipitated phases and in particular, the metastable phase S 'and the precipitation of equilibrium phases S. The study of Eskin [29] explained the hardening and precipitation in the Al-Cu-Mg-Si alloying system.The composition and hardening phase in Al-Cu-Mg-Si alloys containing 2.5% -4.5% Cu, are considered with respect to the chemical composition of the supersaturated solid solution.

Conclusion
In this work, the small contents of magnesium and the heating rate of the nonisothermal transformation in the alloy Al-3.3wt.%Cu was studied.Several experimental methods suited to this kind of scientific research, to follow the various structural, to try to understand the kinetics of various phenomena that occur was used Magnesium in the Al-Cu alloy mainly precipitated to the grain boundaries during the process of transformation and formed ternary Al2CuMg phase.The DSC curves show the formation two phases S 'and S, with a mean activation energy 154.57and 154.82 kJ/mol for Al-3.3wt.%Cu-1wt.%Mgand Al-3.3wt.%Cu-2wt.%Mg,respectively.The value of Avrami coefficient (n) for S' and S phases is 1.4; which may correspond to a phase transformation mechanism driven by the diffusion.The rate of discontinuous reaction decreases with the concentration of Mg increases.

Table 3 .
The