Solid State Phenomena Vol. 184

Abstract: The internal friction and elastic modulus variations caused by the structural rearrangement fcc↔fct in Mn-Fe (Cu) antiferromagnetic alloys were studied in this paper. Antiferromagnetic transition exhibits weak first-order features due to the formation of microtwins by modulus softening mechanism. Antiferromagnetic transition also assists subsequent transformation to form twinned martensite. The small hysteresis between direct and reveres martensitic transformations indicates the thermoelastic feature. Both the martensitic and its reverse transformations also depend on the modulus softening mechanism.

Abstract: X-ray and mechanical spectroscopy on liquid-crystalline elastomers give evidence of rubber elasticity, which depends upon the crosslink concentration. After applied macroscopic deformations, mesoscale non-affine deformations in these systems might lead to long relaxation times. Basing on the example of the crosslink-dependent smectic A − nematic (SmA−N) transition in polysiloxanes, we propose to use the three-dimensional Villain spin glass model and reduce it to the lattice version of the three-dimensional XY spin-glass model. By using the Monte Carlo loop algorithm in this model, we found a percolation threshold depending on the crosslink concentration.

Abstract: The present paper addresses the mechanical behaviour of several bulk metallic glasses (BMG). Both small and large deformations are investigated, using mechanical spectroscopy and compression tests, respectively. In the case of a given BMG, the influence of temperature and strain rate (or frequency) on the mechanical response exhibits an attractive similarity when either small or large deformations are applied. Equivalence between temperature and time is clearly evidenced. The same behaviour is observed in many BMG, whatever their chemical composition, and therefore whatever their glass transition temperature. This behaviour is also very similar to that reported in other amorphous materials: polymers or oxide glasses. The same physical model enables a good description of this behaviour. It is based on atomic mobility and localized deformation in “soft” zones. nanocrystallization hinders strongly the atomic mobility and induces a drastic hardening at high temperature.

Abstract: Fair dispersion of polymer and control of component grain size are key properties to achieve high performances building material (i.e. ultra high strength concrete, self-levelling floor, or exterior insulation composite system). As microstructure analysis in an organic/inorganic hydrated co-matrix material is time consuming, mechanical spectroscopy temperature analysis could characterise both the polymer and the hydrates in the same run. The temperature dependence of the storage modulus G’ and the loss modulus G’’ of some composite building material was therefore measured between 173 and 470 K by mechanical spectroscopy (Dynamic Mechanical Analysis). A model material was then defined to enable DMA latex/cement interaction study. The latex was reinforced by either a microfiller (OMYA limestone) or a microfiller and a nanofiller (hydrated Lafarge cement paste CEM I 52.5 R). The latex evaluated in this study was a 210nm styrene butyl acrylate (SBA). The measurements confirmed that polymer environment was not hindered by micro or nanofiller (i.e. cement). The hydrated cement paste transformation onset was measured at 373K, but occurred at higher temperature as latex content increased. ESEM micrographs performed during heating have proven that within the hydrated cement paste many parallel cracks propagated at once, while within SBA hydrated cement paste no cracks were observed. The hydrated cement microstructure was modified by SBA, and became less sensitive to temperature increase due to SBA latex ability to deform.

Abstract: The preparation method of an elastomeric blend can influence the mechanical properties of the vulcanized compound. In this research elastomeric blends composed by natural rubber and styrene butadiene rubber were mixed using two different methods: by mixing in a roll mill and by dissolution of both elastomers in toluene, mixing of both solutions with the curatives and the evaporation of the solvent. Samples with different Natural Rubber/Styrene Butadiene Rubber relation were prepared by both methods and vulcanized at 433K with a system based on sulphur and accelerator (N-t-butyl-2-benzothiazole sulfenamide) up to the time of optimum cure. The blend composition and the preparation methods have a strong influence in the mechanical dynamic properties. Scanning Electron Microscopy observations indicate that, in the blends prepared by the dissolution method, the samples show better miscibility of the constitutive phases than those prepared by the roll milling method. The temperature dependence of the internal friction was studied for each sample using a subresonant forced pendulum at 1 Hz between 190K and 250K. Depending on the blend composition, one or two glass transition temperatures (T_g) associated to the α-relaxation were measured. In the last case each T_g corresponds to each elastomeric phase of the compound. The loss tangent data for each compound was analyzed using a mixture law of two phases in the frame of the Rouse theory. The adjustment of the data to the proposed model was very good for both preparation method and the whole composition range of the compounds. Then it was possible to obtain the T_g, the main relaxation time and the activation energy values of each compound and, in some samples, the respective values for each elastomeric phase.

Abstract: In order to investigate flip-flop transitions of shear transformation zones (STZs) which are believed to be the elementary deformation sites in metallic glasses, internal friction measurements have been conducted for a commercial bulk metallic glass of a Zr-based alloy (Johnson alloy). Since the STZ is an asymmetrical two-level system, it is necessary to measure the internal friction at a high amplitude or under a bias stress condition to detect the transitions. Measurements have been made at temperatures between 130 K and 573 K at frequencies between 0.1 Hz to 10 Hz. The results showed that quite a broad peak appears between 200 K and 500 K in high amplitude internal friction measurements. The broad peak, observed for the first time in metallic glass, is interpreted to be due to flip-flop transitions of STZs having a broad spectrum, 0.5∼1.2 eV, of the activation enthalpy.

Abstract: For hydrogenated amorphous silicon (a-Si:H) films deposited at temperatures between 423 K and 623 K (a-Si:H_423K and so on), the light-induced changes in the internal friction between 80 K and 400 K were studied. The internal friction is associated with H₂ motion in microvoid networks, and shows the mild temperature dependence between about 80 K and 300 K (Q^-1_80-300K) and the almost linear increase above 300 K (Q^-1_>300K). Both Q^-1_80-300K and Q^-1_>300K decrease with increasing the deposition temperature, and show the mild temperature dependence in a-Si:H_623K. The white light soaking with 100 mW/cm² (WLS₁₀₀ and so on) below 300 K caused a change in Q^-1_80-300K and no changes in Q^-1_>300K, respectively, and the light-induced changes in Q^-1_80-300K recovered after annealing at 423 K. The wide distribution of activation energies for H₂ motions between microvoids indicate that most of neighboring microvoids are connected through windows, i.e., the microvoid networks are existing in a-Si:H, and the spatially loose or solid structures are responsible for the low or high activation energies for the H₂ motion between microvoids, respectively. Furthermore, the light-induced hydrogen evolution (LIHE) was observed for WLS₂₀₀ to WLS₄₀₀ in a vacuum between 400 and 500 K, resulting in the disappearance of the internal friction due to the H₂ motion in the microvoid network.

Abstract: We investigate the jamming transition observed in vibrated granular systems composed of millimeter size glass beads. When a granular system is submitted to vibrations with decreasing intensity, it evolves in a way similar to glass-forming liquids: from a low viscosity, liquid-like state, it evolves into an amorphous jammed state. This evolution is observed by the means of an immersed oscillator acting as a torsion pendulum in forced mode. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency and of the vibration intensity. Focusing on the strongly vibrated states, we observe that there are two different dynamic regions. The first is a high fluidization regime, where the internal friction is found to be proportional to the ratio between the pulsation and the vibration intensity: . In this region, the system shows an apparent viscous friction . In the second, low fluidization, regime, we observe a more complex behavior, and the measured internal friction appears to be well described by a relation of the form: . In this second case, the key role is played by a critical breakaway stress, σ_cr, needed to break the network of chains of forces that form between the grains. Finally, if vibration intensities are still reduced, we also observe that onset of jamming is clearly distinguishable: we observe a sharp increase in the apparent dynamic modulus together with a peak in internal friction. This transition presents important similarities to those observed in glasses, and it leads to the second (low vibrations) regime, where the key role is played by the square root of the vibration intensity.

Abstract: The ferromagnetic amorphous phase in rapidly solidified Nd₆₀Fe₃₀Al₁₀ glass forming alloys is investigated in melt spun ribbons (100 µm thick) and in chill cast cylinders (2 mm diameter). The amorphous resulting for these two different quenching rates were characterized by their room temperature hysteresis loops, magnetization and differential calorimetric measurements in the temperature range 300K-900K. The mechanical damping was explored in the 300-560 K temperature range, by measuring the internal friction and the shear modulus in a forced inverted pendulum operating in the frequency range 0.1-10Hz. Simultaneously, the electrical resistance of the samples was measured. The internal friction spectra of both, ribbons and a cylinder, exhibit a local maximum at about 500K, arising in a relaxation mechanism. After some thermal cycles the peak parameters stabilize reaching an apparent activation enthalpy of 1.5 eV and a limit relaxation time τ₀ ≈ 0.4-2.5 . 10^-17s. In both samples, the electrical resistance largely decreases during the first heating run to 560K and remains unchanged during subsequent thermal cycles. No changes in the elastic modulus or in the damping properties are detected at the Curie temperature of the alloys.

Abstract: Liquid Pb–Bi eutectic alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system. The alloy has been characterized in liquid state from melting (125 °C) to 650 °C by mechanical spectroscopy. Experiments have been carried out using hollow reeds of austenitic stainless steel filled with the Pb-Bi alloy and sealed at the extremities. From 350 °C to 520 °C modulus shows a remarkable drop accompanied by a broad internal friction maximum. In the same temperature range radial distribution functions, determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The anelastic phenomena have been attributed to a structural re-arrangement of liquid metal. For comparison, other alloys of the Pb-Bi system with hypo-eutectic composition have been investigated.