Jose San Juan

Abstract Intermetallic γ-TiAl based alloys have found applications in the low-pressure turbine of aircraft engines as well as in the turbocharger unit of automotive engines. However, these light-weight alloys must still be improved, through micro-alloying and tailoring the microstructure, to increase their creep resistance and consequently their maximum working temperature. In this work, a fully nano-lamellar advanced γ-TiAl based alloy doped with small amounts of C and Si is investigated in order to gain a deeper understanding of the atomic mobility mechanisms taking place at high temperature, thus controlling the creep properties. The study was approached through internal friction measurements up to 1223 K. We demonstrate that C has a notable influence on Ti diffusion in α2 phase, leading to an increase of the activation energy for Ti diffusion, which is assessed at ΔETi(α2)=0.32 eV per at% C. An atomic model for the relaxation process is proposed capable to explain this phenomenon. An additional internal friction peak, which, up to now, remained hidden by the high temperature background, was observed in this nano-lamellar TiAl alloy and analyzed through a careful de-convolution of the internal friction spectra. This new relaxation process, with activation energy of 3.70 eV, is attributed to the short distance diffusion of Al atoms in the γ-TiAl lattice. A novel concept of stress-induced cell-lattice reorientation is proposed to explain this relaxation. Finally, a new experimental method to analyze the high temperature internal friction background, which is closely related to the creep behavior, was developed to study the fully nano-lamellar microstructure, whose high temperature background exhibits the highest activation energy ever measured in a γ-TiAl based alloy.

Abstract An experimental ultra-high-vacuum facility capable of measuring the deformation during thermal cycles under load at different temperatures has been designed and developed to characterize the shape memory effect in the environmental and working conditions imposed by the aerospace industry. In the current work the description of the equipment working at 2x10-8 mbar between -180 °C up to 200 °C is presented, as well as several sets of measurements of thermally induced transformation under load in SMAs.

We have developed a new kind of Metal Matrix Composites based on Cu-Al-Ni shape memory alloys, imbibed in a metallic matrix, specifically designed for showing very high damping coefficient, with the characteristic modulus of the metallic materials.
In the present work we present the damping behavior of such metal matrix composites, has been studied by Mechanical Spectroscopy, as a function of temperature, between 150 K and 400 K, and for different frequencies, between 3x10-3 and 3 Hz. These results are very promising, because the internal friction spectra show a very high maximum linked to the thermo elastic martensitic transformation, as well as a high damping background, in a very broad range in temperature. Besides, the temperature of the maximum of damping can be controlled, in order to match the maximum of damping for each particular application.

Shape memory alloys undergo reversible transformations between two distinct phases in response to changes in temperature or applied stress. The creation and motion of the internal interfaces between these phases during such transformations dissipates energy, making these alloys effective mechanical damping materials. Although it has been shown that reversible phase transformations can occur in nanoscale volumes, it is not known whether these transformations have a sample size dependence. Here, we demonstrate that the two phases responsible for shape memory in Cu-Al-Ni alloys are more stable in nanoscale pillars than they are in the bulk. As a result, the pillars show a damping figure of merit that is substantially higher than any previously reported value for a bulk material, making them attractive for damping applications in nanoscale and microscale devices.

ABSTRACT The phase transformation and nanoindentation response as a function of crystallinity of NiTi thin films were assessed. A phase change was detectable in samples with 1% crystallization. Measured mechanical properties indicate that the films soften with crystallization. Films partway through crystallization presented a bimodal response: crystalline regions had moduli similar to fully crystallized films; and amorphous regions had larger moduli (larger than as-deposited amorphous films) attributable to structural relaxation. A modified Voigt model describes the evolution of the modulus in crystallizing films.

Internal friction measurements with a superimposed bias stress have provided evidence for geometrical kink migration on screw dislocations in iron. This intrinsic process causes a relaxation phenomenon in internal friction which has been identified with the occurrence of a subpeak (below 20 K) of the α-peak. The effect of different bias stress has allowed us to evaluate the kink migration

Shape memory alloys undergo reversible transformations between two distinct phases in response to changes in temperature or applied stress. The creation and motion of the internal interfaces between these phases during such transformations dissipates energy, making these alloys effective mechanical damping materials. Although it has been shown that reversible phase transformations can occur in nanoscale volumes, it is not known whether these transformations have a sample size dependence. Here, we demonstrate that the two phases responsible for shape memory in Cu-Al-Ni alloys are more stable in nanoscale pillars than they are in the bulk. As a result, the pillars show a damping figure of merit that is substantially higher than any previously reported value for a bulk material, making them attractive for damping applications in nanoscale and microscale devices.

Advanced intermetallic g-TiAl based alloys, which solidify via the disordered b phase, such as the TNM þ alloy, are considered as most promising candidates for structural applications at high temperatures in aero and automotive industries, where they are applied increasingly. Particularly creep resistant mi-crostructures required for high-temperature application, i.e. fine fully lamellar microstructures, can be attained via two-step heat-treatments. Thereby, an increasing creep resistance is observed with decreasing lamellar interface spacing. Once lamellar structures reach nano-scaled dimensions, deformation mechanisms are altered dramatically. Hence, this study deals with a detailed characterization of the elevated temperature deformation phenomena prevailing in nano-lamellar TiAl alloys by the use of tensile creep experiments and mechanical spectroscopy. Upon creep exposure, microstructural changes occur in the lamellar structure, which are analyzed by the comparative utilization of X-ray diffraction, scanning and transmission electron microscopy as well as atom probe tomography. Creep activation parameters determined by mechanical characterization suggest the dominance of dislocation climb by a jog-pair formation process. The dislocations involved in deformation are, in nano-lamellar TiAl alloys, situated at the lamellar interfaces. During creep exposure the precipitation of b o phase and z-silicide particles is observed emanating from the a 2 phase, which is due to the accumulation of Mo and Si at lamellar interfaces.

Shape-memory alloys capable of a superelastic stress-induced phase transformation and a high displacement actuation have promise for applications in micro-electromechanical systems for wearable healthcare and flexible electronic technologies. However, some of the fundamental aspects of their nanoscale behaviour remain unclear, including the question of whether the critical stress for the stress-induced martensitic transformation exhibits a size effect similar to that observed in confined plasticity. Here we provide evidence of a strong size effect on the critical stress that induces such a transformation with a threefold increase in the trigger stress in pillars milled on [001] L21 single crystals from a Cu-Al-Ni shape-memory alloy from 2 μm to 260 nm in diameter. A power-law size dependence of n = -2 is observed for the nanoscale superelasticity. Our observation is supported by the atomic lattice shearing and an elastic model for homogeneous martensite nucleation.

In the last decades, issues on absorption of vibration energy have attracted much attention in several technological sectors like vibration isolation in high-precision electronics, building protection in civil engineering, etc.. In the past, mainly polymers were used as high-damping materials, but the increasingly demanding working conditions (e.g. high service temperature) have encouraged the scientific community to look for alternative materials. High-damping metallic materials and, in particular, metal matrix composites were proposed as a feasible solution. Among them, shape memory alloys (SMA) intrinsically show very high damping properties, thanks to dissipative movements of the interfaces generated during their characteristic martensitic transformation, which have already found some practical damping applications. Taking advantage of both the high-damping capacity of Cu-Al-Ni SMAs and the flexibility of composite production, very high-damping metal matrix composites were devel...

Damping in structural applications require not only a high damping coefficient η but also a stiff behaviour in order to avoid the deformation of the structure, so the optimization of structural damping require a high merit index η·E. Using a patented technology of ultra-high damping composite materials based on SMA, we have developed some prototypes of dampers which exhibit both stiffness and high damping coefficient. In this work we give a description of such dampers, as well as their characteristics and performances. We analyse some potential applications in several industrial sectors. Besides, these new dampers offer excellent performances at low frequencies, where the dynamic damping systems start to loose their good characteristics, and they also show a good potential to work in series with such devices to extend their performance at low frequencies.

In the last years absorption of vibration energy by mechanical damping have attracted much attention in several fields like vibration reduction in aircraft and machinery industries, nano-scale vibration isolations in electronic industry, vibration damping in civil engineering, etc. For structural applications, materials that combine a high damping capacity and high stiffness at moderate temperatures are required, but unfortunately this combination is not frequent. Usually, the most used high-damping materials are based on polymers due to their viscoelastic behaviour. However, polymeric materials typically show a low elastic modulus and are not stable at relatively low temperatures (≈ 323 K). Therefore, alternate materials for damping applications are needed. Metallic materials, which originally exhibit better mechanical properties (higher modulus and thermal stability) than polymers, with similar damping properties were proposed to replace polymeric ones. In particular, shape memory...

@font-face { "Times"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 0.0001pt; font-size: 12pt; ""; }div.Section1 { page: Section1; } Now a day, high damping materials have attracted the scientific and technological interest, because of their useful applications. Indeed, the use of high damping materials may allow to decrease the acoustic pollution, improve the speed and precision of the tools machines, increase the life of the aeronautical components and to protect buildings against earthquake, for instance. In our laboratory, we have developed a new kind of metal matrix composites, specifically designed to exhibit an ultra-high-damping behavior. These composites are based on a high percentage pre-form of powders of Cu-Al-Ni shape memory alloys (SMA), infiltrated by a soft metallic matrix. The damping properties have been measured by mechanical spectroscopy between 150 K and 400 K as a function of temperature and frequency. Finally, we discuss the o...

Mechanical vibration isolation in several industrial sectors such as electronic, aeronautic, transport, for instance, is necessary in order to improve the quality in new engineering applications and reduce the environment acoustic pollution. Shape memory alloys (SMA) exhibit high damping capacity as well as good mechanical properties in a wide temperature range and can be used as passive dampers to replace the traditionally used polymeric materials, which present poor stiffness and are not stable enough for some structural applications even at relative low temperatures. In this work a new kind of metal matrix composite, based on Cu-Al-Ni SMA powders, has been developed. The damping capacity of these composites has been studied by mechanical spectroscopy as a function of temperature, frequency and strain amplitude. The influence of thermal treatments on the thermo-elastic martensitic transformation as well as on the relative contribution from SMA particles and matrices, have been ana...