Solid State Phenomena Vol. 184

Abstract: By comparing the properties of dislocation-related relaxation processes in internal friction spectra and the tensile data of various iron alloys, we found that the temperature dependence of the flow stress is governed by the total distribution of the activation energies of dislocation motion. The derived model explains the origin of an anomalous temperature dependence of the flow stress in thermally aged Fe-1%Cu alloys, and Fe-x%Cr alloys as a function of Cr concentration, as well as the non-uniform temperature dependence of the flow stress in pure Fe.

Abstract: The purpose of this study is to highlight the influence of the strain hardening on internal friction behaviour of a copper single crystal alloy Cu-Al (9% at.) using Isothermal Mechanical Spectroscopy (IMS) technique. To do this, the sample was cold worked about 1% by torsion and tested at different stabilized levels of temperatures. Thus the specimen have been progressively heated to 1179 K and then cooled down to room temperature. The advantage of IMS experiments is to allow to compare the isothermal internal friction spectra obtained during heating (in this case, the annealing temperature (T_A) is equal to the temperature of measurement (T_M) with the measurements performed at various temperatures during cooling after annealing at 1179 K (T_M < T_A in this case). The results obtained in increasing temperature step by step (T_A = T_M) show the existence of two (02) independent relaxation peaks from 914 K to 1179 K. The first one, called here P₁ (at about 0.7 T_Melt at 1Hz), decreased with increasing the annealing temperature, while a new relaxation peak P₂ (at about 0.9 T_Melt at 1Hz) progressively developed. However for high temperature annealing, IMS tests (T_A > T_M) does reveal only one peak P₂. It appears, therefore, clearly that the strain hardening is responsible for the P₁ existence. To monitor the evolution of P₁ and P₂ with the temperature, a method of peaks decomposition already published was applied. The relaxation parameters, deduced from the Arrhenius plots, were thus accurately determined. The mechanism responsible of the P₁ and P₂ peaks appearance has been attributed to the movement of dislocation segments inside the sample microstructure.

Abstract: Al-12 wt% Mg alloys have been studied by isothermal mechanical spectroscopy. The samples were quenched then annealed at various temperatures. Experiments were performed in a very large frequency range (10^-4 Hz – 50 Hz) between room and solidus temperatures. For each temperature of measurement, experiment started after complete microstructure stabilization of the sample and therefore the transient effects due for instance to β (( and β′ precipitation were not observed. Nevertheless, a new relaxation effect was obtained in the reversion temperature range. This effect is not thermally activated. It is maximal at about 0.1 Hz and increases with the temperature of measurement. It completely disappears after annealing at solid solution temperature and successive slow cooling and therefore is linked to the β precipitates. This effect is interpreted as a phase transformation at the precipitate surface induced by the applied stress.

Abstract: Dynamics of structural defects was investigated in situ during quasistatic deformation of polycrystalline Al-1wt.%Si-0.3wt.%Mg and Al-12wt.%Si-0.3wt.%Mg alloys by means of simultaneous measurements of internal friction and acoustoplastic effect. The alloys were subjected to different heat treatments after quenching: natural ageing at room temperature (T4 treatment) and peak ageing at 433 K for 8 hrs (T6 treatment). This enabled us to study the effect of different microstructure components – solute clusters (for the T4 treatment), GP zones and β (( precipitates (for the T6 treatment), coarse Si particles (for Al-12wt.%Si-0.3wt.%Mg) – on the irreversible and reversible components of dislocation motion under the combined action of oscillatory and quasistatic stress. Based on the data obtained, conclusions have been drawn about microstructural mechanisms of the acoustoplastic effect.

Abstract: Acoustic measurements of Young’s modulus and internal friction at frequencies of longitudinal vibrations of about 100 kHz were made in the (5×10^-7 to 2×10^-4) vibration strain amplitude range for Al-Si alloys. Quasi-static bending stress-strain diagrams were studied as well. The samples of the alloy were prepared using a directional crystallization by Stepanov’s method. The compositions varied from 8 to 25 wt.%Si. The micro-plastic deformation of the materials (acoustic experiments) is explained by dislocation motion in grains of α solid solution. The micro-plastic yield stress has a minimum near the eutectic composition; the flow stress of plastic bending increases smoothly when the Si content increases.

Abstract: sothermal mechanical spectroscopy measurements were performed in an Al-51 at % Zn alloy at various temperatures below and above the eutectoid transition temperature: during a heating the α-β eutectoid mixture changes into α solid solution at 550 K. Damping experiments were performed in a very large frequency range (10^-5– 50 Hz) between room temperature and 673 K. Internal friction spectra performed between 200 K and 540 K, exhibit two thermally activated relaxation peaks (P1 and P2). P1 decreases and disappears with the increase of measurement temperature while P2 appears and increases. P2 totally disappears above the eutectoid transition temperature. Above 550 K, a new peak (P3) is evidenced at very low frequency. The relaxation parameters of P3 (limit relaxation time τ₀ = 9×10^-7 and activation energy H = 105 kJ/mole (1.1 eV)) allow to associate this peak with the motion of sub grain boundaries. P1 and P2 (τ₀ ≈ 10^-7 and H ≈ 70 kJ/mole (0.75 eV) for both peaks) are associated with a thermally induced atom diffusion across the α-β interface.

Abstract: In this study, temperature dependence of internal friction, tand, of ECAPed Al-Ni eutectic alloy was measured using a dynamic mechanical analysis (DMA) to discuss the recrystallization behavior of the alloy. The temperature dependence of internal friction of 8 passed samples shows a large peak at around 200 °C on heating, while this peak disappears on the subsequent cooling stage. Moreover, smaller peak was observed for 2 passed samples. Therefore, it can be concluded that the origin of the peak comes from the recrystallization of a-Al.

Abstract: The aim of this work is to study the Amplitude Dependent Internal Friction (ADIF) of magnesium alloy AZ31 at room temperature at the frequency 20kHz. The internal friction of AZ31 at room temperature is mostly influenced by mechanical cycling at strain amplitudes in the microplastic deformation region. An excited state of the AZ31 alloy, which can be associated with a higher internal friction and lower dynamic modulus than usual state, was found immediately after mechanical cycling. When the strain amplitude drops, the diffusion of solute atoms restores the Zener atmosphere and the internal friction relaxes exponentially with the second root of time. The measurement methodology and obtained results are presented.

Abstract: The amplitude dependent damping of two bending beam samples of magnesium alloy AJ91 (9 wt.% Al, 1 wt.% Sr) was measured at room temperature in as cast condition, after quenching from high temperatures into water of room temperature and after various bending cycles to fatigue. Some measurements were performed successively with about 33 Hz and 100 Hz resonant frequency. The measurements show typical dislocation damping in as cast condition, after heat treatment at temperatures lower than 420°C, and cycle numbers lower then 50.000. For higher quenching temperatures the damping increases over the whole measured strain range with increasing quenching temperature and number of cycles to fatigue. After quenching from temperatures higher than 478°C the crack damping becomes dominant. The effects of damping seem to increase with increasing frequency. In one sample damping of individual cracks could be identified in the amplitude dependent damping curves by their characteristic course very similar to the ones postulated in an earlier publication by a simple rheological model [4]. The extending of crack length leads to a shift of the damping to lower strains.

Abstract: Microstructural changes induced in a carbon fiber – magnesium matrix composite during thermal cycling in the range of 100 - 360 K are detected by an ultrasonic technique. The composite was comprised of Mg-2wt.%Si alloy matrix reinforced with long unidirectional carbon fibers (volume fraction of about 30%). Temperature variations of the elastic modulus of the composite are largely determined by elasticity of the carbon fibers stressed by the thermally expanded/contracted matrix. Anelastic properties of the composite (internal friction and modulus defect) are caused by dislocation motion in the matrix. Temperature spectra of anelasticity of the composite are controlled by a competition between creation of fresh mobile dislocations under the action of thermal stresses and immobilization of the fresh dislocations by atmospheres of mobile point defects.