Solid State Phenomena Vol. 184

Abstract: The Young’s modulus and internal friction of Be polycrystals (grain sizes 6-60 μm) prepared with a powder metallurgy technique were studied acoustically in both amplitude independent and-dependent damping ranges. The measurements were made by composite oscillator at resonant frequencies of longitudinal vibrations of about 100 kHz in the temperature range of 100-873 K. The data were used to get information about micro-flow properties at vibration stresses of 0,2-30 MPa. It was found that the micro-flow diagrams became non-linear at amplitudes higher than 5 MPa. Mechanical properties (acoustic micro-flow stress σ_y, yield point σ_0.2 and ultimate strength σ_В) as functions of grain size have shown clearly a correlation in the framework of Hall and Petch law at room temperature but this similarity is not complete: stresses σ_0.2 and σ_В are one order of magnitude higher and change more steeply with grain size as compared to σ_y. The similarity is not observed for the temperature dependences. The values of σ_0.2(Т) and σ_В(T) decrease smoothly at higher temperatures but σ_у(Т) demonstrates an unusual minimum at ~400 K. The different behavior of the acoustic and mechanical stresses proves, evidently, that the ultrasonic energy losses and the flow stress level are due to the different nature of obstacles for the vibration (near equilibrium positions) and translation movement of dislocations in beryllium.

Abstract: Composites containing 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) reinforced with multiwalled carbon nanotubes (CNTs) with various amounts of CNTs (3Y-TZP / X wt% CNT, X= 0, 0.5, 1.5, 3 and 5) were processed by spark plasma sintering. Microscopic analysis proves that CNTs were well dispersed and embedded in grain boundaries of the sintered body. High temperature mechanical properties have been investigated using mechanical spectroscopy and low stress (6 MPa) creep. The isothermal spectrum (measured at 1600 K) consists of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at low frequency. The absence of a well-marked peak in monolithic 3Y-TZP is justified considering that restoring force decreases at low frequencies or high temperatures due to the elasticity of neighboring grains. Therefore, strain is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. However, with CNT additions the mechanical loss decreases and a better resolved peak was observed. In parallel, the results have shown that the creep rate drastically decreases with CNT additions. These results can be interpreted by the pinning effect of CNTs which can hinder grain boundary sliding at high temperatures, resulting in a creep resistance improvement.

Abstract: Engineering ceramics are being developed to improve their high-temperature mechanical properties and in particular creep resistance. Recently the production of fine grain ceramics undergoes another step-forward with the development of new technologies to produce nanocrystalline materials. The question is whether the properties depending on the grain size can be extrapolated at nanoscale or, on the contrary, new microscopic mechanisms could appear to be dominant at this nanometer grain size. In the present work we study, by mechanical spectroscopy, the high temperature behavior up to 1350°C of a fine grain Zirconia and a nanocrystalline Zirconia sintered in a conventional way. A new forced torsion pendulum, recently built, has been used for the mechanical spectroscopy measurements. The high temperature background (HTB) of internal friction has been measured as a function of temperature for different frequencies in both materials. The analysis of the HTB shows that the fine grain Zirconia exhibits a single process of defects mobility, with an apparent activation enthalpy similar to the one measured by creep. On the contrary, the HTB of the nanocrystalline sample becomes more complex, showing a much higher energy loss, which will be discussed at the light of the internal friction spectra analysis.

Abstract: A relaxation peak has been observed in the internal friction spectrum of 18-carat AuAgCu yellow gold alloys at about 750K for 0.5Hz. It is related to the presence of grain boundaries, since it is absent in the spectrum of single crystals. For the 14-carat yellow gold alloy (Au38%Ag32%Cu30%), a phase decomposition between silver-rich and copper-rich solid solution occurs in the same temperature range. The effects of the phase decomposition on the internal friction and the dynamic modulus are studied by isochronal and isothermal measurements and correlated with the microstructure evolution. Upon cooling, the phase decomposition starts at grain boundaries at about 840K, producing a fine lamellar structure, and the grain boundary peak amplitude strongly decreases. As the phase decomposition progresses at the interior of the grains upon further cooling, the internal friction background increases. It remains very high in heating until solid solution homogenisation, which occurs above 890K. Such an increase of the internal friction background is observed also in the single crystalline alloy and may be attributed to the interfaces between lamellae of the silver and copper-rich phase.

Abstract: Au₆₀Ag₃₀Cu₁₀ (in at%) gold alloy exhibits a mechanical loss spectrum composed of a Zener peak due to Cu atoms in the solid solution and of a second relaxation peak at higher temperature or lower frequency. It is shown that this second peak is related to the presence of grain boundaries as it is absent in the spectrum of a single crystal. This mechanical loss peak, which is stable and reproducible in heating and cooling cycles, is thermally activated with an activation enthalpy of 2.35 eV and an apparent limit relaxation time of 9.6·10^-17s. As it is hard to imagine that a whole grain should slip at once along a touching grain, the relaxation peak is interpreted by a dislocation model, which may account not only for the activation parameters but also for the stress amplitude dependency of the peak.

Abstract: A detailed study of anelastic effects in submicrocrystalline copper using resonance (~70 kHz, 2 K to 320 K) and sub-resonance (0.05-100 Hz, 300 K to 675 K) techniques was carried out. Several relaxation processes were found in the temperature range of 2 K - 675 K: the relaxation loss peaks (Q^-1) near 35 (P1) and 90K (P2) with the activation energy and the pre-exponential factor (H₁ ≈ 0.02 eV, τ_ο1 ≈ 10^-9 s and H₂ ≈ 0.09 eV, τ_ο2 ≈ 10^-11 s) similar to those of the Bordoni and the Niblett-Wilks peaks in coarse-grained Cu. This suggests that the peaks are due to the thermally activated motion of dislocation kinks in the primary and secondary Peierls relief. The mean values of activation parameters (H₃ ≈1.4-1.6 eV, τ_ο3 ≈10^-17 s) of a third thermally activated peak (P3), which was significantly broadened, can be interpreted as a grain boundary peak with uncoupled activation parameters H₃^*≈0.45 eV and τ_ο3^* ≈10^-14 s. A pseudo peak P_R is associated with irreversible recrystallization processes. The influence of annealing on the observed effects is also discussed.

Abstract: The annealing behaviour of temperature-dependent mechanical spectra (vibrating-reed technique) was studied on electrodeposited ultrafine-grained nickel as well as on Ni nanocomposites with small (7 nm) SiO₂ or larger (25 nm) Al₂O₃ nanoparticles. From the response of the different phenomena involved – Young’s modulus, high-temperature damping background, dislocation-and hydrogen-induced low-temperature loss peaks, and magnetomechanical effects – information is obtained on processes such as recovery, grain growth, hydrogen trapping, and dislocation generation by thermal stresses, which are influenced by both kinds of nanoparticles in different ways.

Abstract: Morphology and mechanical resonse of copper nanoparticles with defects have been simulated by means of molecular dynamics simulation. The embedded atom method potential for copper was used to express the interaction of atoms. Four types of model samples were prepared and about 37,000 atoms were contained in each sample. Two of them are cubic shape with {100} surfaces, in which vacancies or interstitials are introduced. The other two samples are once melted and solidified particles with nearly spherical surfaces. The atomic structure is controlled by cooling rate, and crystalline and amorphous structures are realized. Shear and tetragonal strains are applied to the samples and stress-strain relations for the samples are derived. Mechanical damping and internal friction were evaluated from the free decaying oscillations by releasing static strains.

Abstract: Aluminum-matrix-nanoparticle-composites were produced by ball milling of micro scale aluminum powder in air atmosphere with subsequent consolidation by hot extrusion and also additional hot swaging. They were investigated in this condition after step by step isochronal annealing with successive increasing annealing temperature and quenching into water to room temperature. The material was investigated by amplitude dependent damping, hardness and density measurements, all at room temperature. For all measured amplitude dependent internal friction (ADIF) curves the damping increases with increasing strain amplitude. After some annealing treatments a knee occurs in the medium strain amplitude region of these curves. Moreover between annealing temperatures from 360°C to 480°C the strain dependent damping becomes a maximum, i.e. a peak in the ADIF curves occurs. Other ADIF curves of quenched and fatigued material show characteristic peaks that can be attributed to individual single cracks. It is shown that all these effects are due to the formation, opening and compression of cracks present in the sample or created by thermally exerted stresses.

Abstract: Thermally sprayed hardmetal coatings can be used to improve the wear or fatigue resistance of mechanical parts. Depending on the deposition conditions, their microstructure and phase composition are out of equilibrium at different levels due to the extreme heating/cooling rates. In the present study, the changes that occur with temperature variation are monitored by mechanical spectroscopy. Requirements to specimen of mechanical spectroscopy created the need to prepare WC-17%Co coatings of 1.2 mm thickness by high velocity oxy-fuel (HVOF) spraying. The coatings, separated from the substrate by spark erosion, were tested in a forced torsion pendulum between room temperature and 1570 K at a temperature scanning rate of 1K/min. The mechanical loss spectrum shows different features. At 800 K, a maximum M1 is observed in coincidence with a sudden increase of the elastic modulus. The change of the elastic modulus is due to a densification of the material possibly related to cobalt recrystallization. A relaxation peak located at about 1100 K is typically found in WC-Co hardmetals. It is attributed to the movement of dislocations in the cobalt phase. A sharp peak is observed at 1510 K on heating and at 1410 K on cooling. Such peak is due to the reversible transition from W₃Co₃C at high temperature to W₆Co₆C at low temperature as proven by X-ray diffraction. The reversibility of such transformation was observed for the first time.