Papers by Keyword: Isothermal Mechanical Spectroscopy

Abstract: An equiatomic CuZr alloy quenched from 1073 K was studied by isothermal mechanical spectroscopy and X-Ray diffraction. Experiments were performed using a very large frequency range (10^-4 Hz – 50 Hz) at different temperatures. For each temperature of measurement, experiment started after complete microstructure stabilization of the sample. At room temperature, the X–Ray diffraction spectrum shows that there are two CuZr monoclinic phases as a consequence of a martensitic transformation. These structures are characterized by the existence of twinning defects for the first one and a high dislocation density for the other. Both monoclinic phases disappear at higher temperatures and first transform into the cubic CuZr phase, then this cubic phase transforms into Cu₁₀Zr₇ and CuZr₂ phases above 763 K. Internal friction spectra exhibit two relaxation peaks (P₁, P₂), at low and high temperatures, respectively. After rapid cooling of the sample from 1273 K, the first peak P₁ appears from room temperature and disappears after annealing above 673 K. The P₂ peak appears at about 800 K and increases for measurements at higher temperature up to 880 K. This temperature range corresponds with the existence of both Cu₁₀Zr₇ and CuZr₂ phases. These two peaks are associated with a relaxation linked to the dislocation microstructure in the two CuZr monoclinic phases for P1 and in the Cu₁₀Zr₇ and CuZr₂ phases for P2.

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: Internal friction peaks observed in single or polycrystals are clearly due to a dislocation relaxation mechanism. Because a sample observed by transmission electron microscopy (TEM) often exhibits in the same time various dislocation microstructures (isolated dislocations, dislocation walls, etc.) it is very difficult to connect the observed relaxation peak with a particular dislocation microstructure. Using isothermal mechanical spectroscopy (IMS), it is easier to compare, for instance, the evolution of a relaxation peak with measurement temperature to the microstructural evolution observed by in-situ TEM at the same temperatures. IMS was used to study a relaxation peak in a 5N aluminium single crystal firstly 1% cold worked and then annealed at various temperatures. TEM experiments performed in the same material at various temperatures equal to the temperatures used for the damping experiments made possible to link this internal friction peak with a relaxation effect occurring inside dislocation walls. In two other experiments in a 4N aluminium polycrystal and in a metal matrix composite with SiC whiskers, it is shown that the observed relaxation peaks are connected to the motion of dislocations inside polygonization boundaries in the first case and in dislocation pile-ups around each whisker in the second one. Theoretical models proposed to explain such relaxation peaks due to a dislocation motion inside a dislocation wall or network are discussed.