Study on Mechanical Relaxations of 7075 (Al–Zn–Mg) and 2024 (Al–Cu–Mg) Alloys by Application of the Time-Temperature Superposition Principle

Abstract

The viscoelastic response of commercial Al–Zn–Mg and Al–Cu–Mg alloys was measured with a dynamic-mechanical analyzer (DMA) as a function of the temperature (from 30 to 425°C) and the loading frequency (from 0.01 to 150 Hz). The time-temperature superposition (TTS) principle has proven to be useful in studying mechanical relaxations and obtaining master curves for amorphous materials. In this work, the TTS principle is applied to the measured viscoelastic data (i.e., the storage and loss moduli) to obtain the corresponding master curves and to analyze the mechanical relaxations responsible for the viscoelastic behavior of the studied alloys. For the storage modulus it was possible to identify a master curve for a low-temperature region (from room temperature to 150°C) and, for the storage and loss moduli, another master curve for a high-temperature region (from 320 to 375°C). These temperature regions are coincidental with the stable intervals where no phase transformations occur. The different temperature dependencies of the shift factors for the identified master curves, manifested by different values of the activation energy in the Arrhenius expressions for the shift factor, are due to the occurrence of microstructural changes and variations in the relaxation mechanisms between the mentioned temperature regions.

Viscoelastic behavior of a novel aluminum metal matrix composite and comparison with pure aluminum, aluminum alloys, and a composite made of Al–Mg–Si alloy reinforced with SiC particles

“…3 Loss modulus E" vs. temperature T from DMA at frequencies ranging from 0.01 to 100 Hz, for A356 aluminum alloy and a novel composite made of A356 alloy matrix with reinforcement particles from colliery shale (CS). The behavior of ‫"ܧ‬ for most of these materials is again qualitatively similar and shows features that were discussed thoroughly in [4,6,7,13]. Moreover, the mechanical damping/ internal friction exhibits qualitatively the same behavior as ‫.…”

“…1 and Fig. 2 for the various materials are qualitatively similar in most cases and show some common features that were discussed in [4,6,7,13]. The novel AMC shows higher stiffness than the A356 alloy matrix (see Fig.…”

“…For DMA also, rectangular plates with dimensions equal to those mentioned above were machine cut from ingots of A356 alloy and the novel AMC. More details on the studied material and experimental set up can be found for pure aluminum in [4,5], for AA 7075 and 2024 in [13], for AA 6061 reinforced with SiC particles in [8], and for the A356 alloy and the novel AMC in [1,12]. The latter two materials were also analyzed with optical microscope.…”

Dynamic microstructural evolution of an Al–Zn–Mg–Cu alloy (7075) during continuous heating and the influence on the viscoelastic response

“…The experimental data reported in this work, and viscoelasticity measurements from previous dynamic-mechanical analysis [24,28,37], were used to test a model of the viscoelastic response of the Al-Zn-Mg-Cu alloy. In this approach, a reaction rate equation for formation and dissolution of GP zones and a reaction rate equation for η′ phase formation were integrated numerically inside the least-squares nonlinear fitting of a model of the storage modulus, E ′ , i.e., the elastic (real) component of the dynamic tensile modulus, a measure of the deformation energy stored by the material during cyclic loading.…”

“…Table 5 presents the initial values used for the fitting, as well as the best fit kinetic parameters obtained after this process. A thorough discussion of the latter results with respect to values reported in the literature is provided in [24,37]. The initial values for R GPZ (0) and d were guessed from the measurements at the temperature closest to RT, i.e., the measurements for specimen 2 (L = 4.…”