Stress-assisted atomic diffusion in metastable austenite D03 phase of Cu-Al-Be shape memory alloys

AbstractCu-Al-based shape memory alloys are firm candidates to be used up to 473K. The main limiting aspect is the activation of diffusion processes in the metastable austenite phase, which drive the alloy decomposition. In the present work the study of short-distance diffusion processes has been approached by internal friction. A relaxation peak has been found in the metastable β (D03) phase of a Cu-Al-Be shape memory alloy, around 500K (at 1Hz), with an activation energy of Ea=1.38±0.05eV. An atomic mechanism of elastic dipoles Antisite-Vacancy reorientation, involving stress-assisted short distance Cu-atoms diffusion, has been proposed

Espectros Raman de Rb₂SeO₄

En esta comunicación presentamos resultados preliminares del espectro Raman de Rb₂Se0₄. Este compuesto, que pertenece al grupo espacial Pnam, es isomorfo a temperatura ambiente con K₂Se0₄. Este último es prototipo de la conocida familia de compuestos que tiene por característica las transiciones de fase en secuencia paraeléctrica - inconmensurada - ferroeléctrica. El espectro Raman de los modos internos de Rb₂SeI0₄ tiene, como era de esperar, características similares a las de K₂Se0₄, observándose un corrimiento hacia frecuencias más bajas del orden de 5 cm⁻¹. Los modos de la red muestran que este compuesto también puede considerarse pseudohexagonal ya que la cantidad de modos Big es menor que la calculada por análisis de grupo factor. Más importante, en estas medidas que ahora están siendo llevadas a cabo, no se observa hasta 98 K ningún modo blando que pueda ser reminiscente al comportamiento del modo amplitud característico de la fase inconmensurada en el compuesto prototípico. Nuestros resultados son coincidentes con cálculos teóricos recientes que identifican el incremento del radio catiónico (de K a Rb) como un factor inhibidor de la secuencia de transiciones de fase arriba mencionada.Facultad de Ciencias Exacta

Superelastic damping at nanoscale in ternary and quaternary Cu-based shape memory alloys

Superelasticity is a characteristic thermomechanical property in shape memory alloys (SMA), which is due to a reversible stress-induced martensitic transformation. Nano-compression experiments made possible the study of this property in Cu–Al–Ni SMA micropillars, showing an outstanding ultra-high mechanical damping capacity reproducible for thousands of cycles and reliable over the years. This scenario motivated the present work, where a comparative study of the damping capacity on four copper-based SMA: Cu–Al–Ni, Cu–Al–Be, Cu–Al–Ni–Be and Cu–Al–Ni–Ga is approached. For this purpose, [001] oriented single-crystal micropillars of comparable dimensions (around 1 µm in diameter) were milled by focused ion beam technique. All micropillars were cycled up to two hundred superelastic cycles, exhibiting a remarkable reproducibility. The damping capacity was evaluated through the dimensionless loss factor η, calculated for each superelastic cycle, representing the dissipated energy per cycle and unit of volume. The calculated loss factor was averaged between three micro-pillars of each alloy, obtaining the following results: Cu–Al–Ni η = 0.20 ± 0.01; Cu–Al–Be η = 0.100 ± 0.006; Cu–Al–Ni–Be η = 0.072 ± 0.004 and Cu–Al–Ni–Ga η = 0.042 ± 0.002. These four alloys exhibit an intrinsic superelastic damping capacity and offer a wide loss factor band, which constitutes a reference for engineering, since this kind of micro/nano structures can potentially be integrated not only as sensors and actuators but also as dampers in the design of MEMS to improve their reliability. In addition, the study of the dependence of the superelastic loss factor on the diameter of the pillar was approached in the Cu–Al–Ni–Ga alloy, and here we demonstrate that there is a size effect on damping at the nanoscale.Fil: Gómez Cortés, J.F.. Universidad del País Vasco; EspañaFil: Fuster, Valeria de Los Angeles. Universidad del País Vasco; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Pérez Cerrato, M.. Universidad del País Vasco; EspañaFil: Lorenzo, P.. Universidad del País Vasco; EspañaFil: Ruiz Larrea, I.. Universidad del País Vasco; EspañaFil: Breczewski, T.. Universidad del País Vasco; EspañaFil: Nó, M. L.. Universidad del País Vasco; EspañaFil: San Juan, J. M.. Universidad del País Vasco; Españ

Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds

We consider the evolution of the phase transition from the parent hexagonal phase $P6_{3}/mmc$ to the orthorhombic phase $Pmcn$ that occurs in several compounds of $A'A''BX_{4}$ family as a function of the hcp lattice parameter $c/a$. For compounds of $K_{2}SO_{4}$ type with $c/a$ larger than the threshold value 1.26 the direct first-order transition $Pmcn-P6_{3}/mmc$ is characterized by the large entropy jump $Rln2$. For compounds $Rb_{2}WO_{4}$, $K_{2}MoO_{4}$, $K_{2}WO_{4}$ with $c/a<1.26$ this transition occurs via an intermediate incommensurate $(Inc)$ phase. DSC measurements were performed in $Rb_{2}WO_{4}$ to characterize the thermodynamics of the $Pmcn-Inc-P6_{3}/mmc$ transitions. It was found that both transitions are again of the first order with entropy jumps $0.2Rln2 and$0.3Rln2$. Therefore, at$c/a ~ 1.26$the$A'A''BX_{4}$compounds reveal an unusual Lifshitz point where three first order transition lines meet. We propose the coupling of crystal elasticity with$BX_{4}$ tetrahedra orientation as a possible source of the transitions discontinuity.Comment: 13 pages,1 Postscript figure. Submitted as Brief Report to Phys. Rev. B, this paper reports a new work in Theory and Experiment, directed to Structural Phase Transition

Incommensurate Intermodulation of an Organic Intergrowth Compound Observed by Neutron Scattering

International audienceThe observation of an incommensurate diffraction pattern in urea inclusion compounds by neutron diffraction is reported. It reveals for the first time in these compounds the existence of all kinds of reflections expected for an intermodulated composite. The superspace analysis together with the temperature evolution of some characteristic reflections are presented

Ferroelastic domain structure of (CH 3) 4N CdCl 3 (TMCC) crystal

PACS. 68.35.Rh Phase transitions and critical phenomena – 75.60.Ch Domain walls and domain structure,

Low-frequency dynamics in molecular incommensurate composite: Specific heat of nonadecane/urea inclusion compound

We report the first very low-temperature specific-heat data of the incommensurate n-nonadecane/urea inclusion compound. In addition to the contribution due to normal acoustic phonons, the results show a linear temperature dependence of the specific heat below 1.5 K, assigned to the dynamical disorder of the guest molecules. The excess heat capacity around 4.5 K reveals the low-frequency contribution of nondispersive modes. In contrast to the case of modulated incommensurate compounds, no pinned sliding mode, phason-like mode, could be detected

Light scattering by low-frequency excitations in quasi-periodic

High-resolution light scattering spectroscopy has been performed on a series of n-alkane ${\rm C}_{n}{\rm H}_{2n+2}$/urea inclusion compounds with n = 12, 14, 17\mbox{-}19. The elastic constants of these incommensurate composite crystals have been determined and found to be independent of the chain length of the guest molecules. In addition, no extraneous acoustic-like mode appeared in all spectra. However, quasi-elastic components with different widths are observed for different polarizations. In one of them the characteristics of the scattered light suggest that the quasi-elastic broadening could be generated by a sliding mode, whereas the other detected component is assigned to the reorientational motions of the n-alkane chains about their long axis