Fabricación y caracterización a escala micro y nano métrica de la SMA de Cu-Al-Ni para su aplicación en MicroElectroMechanical Systems (MEMS)

220 p.Las aleaciones con memoria de forma SMAs (Shape Memory Alloys) presentan un gran potencial para el desarrollo de nuevos MEMS (MicroElectroMechanical Systems) que puedan cubrir la creciente demanda, de sensores y actuadores sofisticados, en todos los campos de la ciencia y la tecnología. Esta tesis aborda el estudio de fabricación y caracterización a escala micro y nanométrica de la SMA de Cu-Al-Ni para su aplicación en MEMS a partir de dos aproximaciones: La fabricación y caracterización microestructural de películas delgadas, y la caracterización mecánica a escala nanométrica de micro y nanopilares monocristalinos tallados mediante bombardeo iónico focalizado (FIB). En el primer caso se presenta el método de aleado de multicapas elementales para obtener esta SMA en forma de película delgada (1 ¿m, espesor), así como su caracterización mediante In-situ-TEM. En el segundo caso, se presenta la caracterización de micro/nano pilares (2 ¿m-260 nm, diametro) mediante ensayos de nano-compresión, a partir de los cuales se ha puesto en evidencia, por primera vez, un notable efecto de tamaño sobre el comportamiento superelástico, que se ha interpretado según un modelo atomístico-elástico. También se evaluó el efecto superelástico y el amortiguamiento mecánico asociado, en función del ciclado y su reproducibilidad en el tiempo (3.5 años). Finalmente se presentan resultados de nano-compresión In-situ-SEM

Fabricación y caracterización a escala micro y nano métrica de la SMA de Cu-Al-Ni para su aplicación en MicroElectroMechanical Systems (MEMS)

220 p.Las aleaciones con memoria de forma SMAs (Shape Memory Alloys) presentan un gran potencial para el desarrollo de nuevos MEMS (MicroElectroMechanical Systems) que puedan cubrir la creciente demanda, de sensores y actuadores sofisticados, en todos los campos de la ciencia y la tecnología. Esta tesis aborda el estudio de fabricación y caracterización a escala micro y nanométrica de la SMA de Cu-Al-Ni para su aplicación en MEMS a partir de dos aproximaciones: La fabricación y caracterización microestructural de películas delgadas, y la caracterización mecánica a escala nanométrica de micro y nanopilares monocristalinos tallados mediante bombardeo iónico focalizado (FIB). En el primer caso se presenta el método de aleado de multicapas elementales para obtener esta SMA en forma de película delgada (1 ¿m, espesor), así como su caracterización mediante In-situ-TEM. En el segundo caso, se presenta la caracterización de micro/nano pilares (2 ¿m-260 nm, diametro) mediante ensayos de nano-compresión, a partir de los cuales se ha puesto en evidencia, por primera vez, un notable efecto de tamaño sobre el comportamiento superelástico, que se ha interpretado según un modelo atomístico-elástico. También se evaluó el efecto superelástico y el amortiguamiento mecánico asociado, en función del ciclado y su reproducibilidad en el tiempo (3.5 años). Finalmente se presentan resultados de nano-compresión In-situ-SEM

Universal Scaling Law for the Size Effect on Superelasticity at the Nanoscale Promotes the Use of Shape‐Memory Alloys in Stretchable Devices

Shape-memory alloys (SMAs) are the most stretchable metallic materials thanks to their superelastic behavior associated with the stress-induced martensitic transformation. This property makes SMAs of potential interest for flexible and wearable electronic technologies, provided that their properties will be retained at small scale. Nanocompression experiments on Cu-Al-Be SMA single crystals demonstrate that micro- and nanopillars, between 2 mu m and 260 nm in diameter, exhibit a reproducible superelastic behavior fully recoverable up to 8% strain, even at the nanoscale. Additionally, these micro-/nanopillars exhibit a size effect on the critical stress for superelasticity, which dramatically increases for pillars smaller than approximate to 1 mu m in diameter, scaling with a power law of exponent n = -2. The observed size effect agrees with a theoretical model of homogeneous nucleation of martensite at small scale and the universality of this scaling power law for Cu-based SMAs is proposed. These results open new directions for using SMAs as stretchable conductors and actuating devices in flexible and wearable technologies.This work was supported by the Spanish Ministry of Economy and Competitiveness, MINECO, projects MAT2017-84069P and CONSOLIDER-INGENIO 2010 CSD2009-00013, as well as by the ELKARTEK-ACTIMAT project from the Industry Department of the Basque Government, and GIU-17/071 from the University of the Basque Country, UPV/EHU. This work made use of the FIB facilities of the SGIKER from the UPV/EHU. V.F. also acknowledges the Post-Doctoral Mobility Grant from the CONICET of Argentina, and J.F.G.-C. acknowledges the Post-Doctoral Grant (ESPDOC18/37) from the UPV/EHU

Thermal Stability of Cu-Al-Ni Shape Memory Alloy Thin Films Obtained by Nanometer Multilayer Deposition

Cu-Al-Ni is a high-temperature shape memory alloy (HTSMA) with exceptional thermomechanical properties, making it an ideal active material for engineering new technologies able to operate at temperatures up to 200 °C. Recent studies revealed that these alloys exhibit a robust superelastic behavior at the nanometer scale, making them excellent candidates for developing a new generation of micro-/nano-electromechanical systems (MEMS/NEMS). The very large-scale integration (VLSI) technologies used in microelectronics are based on thin films. In the present work, 1 μm thickness thin films of 84.1Cu-12.4 Al-3.5Ni (wt.%) were obtained by solid-state diffusion from a multilayer system deposited on SiNx (200 nm)/Si substrates by e-beam evaporation. With the aim of evaluating the thermal stability of such HTSMA thin films, heating experiments were performed in situ inside the transmission electron microscope to identify the temperature at which the material was decomposed by precipitation. Their microstructure, compositional analysis, and phase identification were characterized by scanning and transmission electron microscopy equipped with energy dispersive X-ray spectrometers. The nucleation and growth of two stable phases, Cu-Al-rich alpha phase and Ni-Al-rich intermetallic, were identified during in situ heating TEM experiments between 280 and 450 °C. These findings show that the used production method produces an HTSMA with high thermal stability and paves the road for developing high-temperature MEMS/NEMS using shape memory and superelastic technologies.This research was funded by the Spanish Ministry of Science and Innovation, project PID2021-122160NB-I00, and the GIU-21/024 from the University of the Basque Country, UPV/EHU. This work made use of the TEM and SEM facilities of the Electron Microscopy and Microanalysis of Materials SGIKER of the UPV/EHU, as well as the FIB facilities of EML at NanoGune

Effect of the mother tree age and acorn weight in the regenerative characteristics of Quercus faginea

[EN] The establishment of oak trees is often a slow and difficult process. Hence, it is necessary to determine the characteristics that can lead to improving their regeneration. In this genus, seed size is highly variable both at the interspecific and intraspecific levels, and the effects of intrapopulation variability are not well understood, being even less so for Quercus faginea. In this study, the effects of the age of the mother tree, seed weight and the interaction between these two factors on seed germination, emergence and growth (biomass) were analysed. For this purpose, 16 trees—8 young and 8 old—were selected with the intent to cover the entire range of acorn weights produced in this population. Among the main results, it should be noted that: (1) in older trees, it is easier to find larger acorns; (2) the percentage and the speed of germination of the acorns of young trees is greater than that of old trees; (3) the percentage and the speed of seedling emergence of young trees is greater than that of old trees; and (4) cotyledon weight is the variable that most influences biomass, quite often in a positive way. Therefore, maintaining intrapopulation variability seems to be an approach that most favours the persistence of these populations

Designing for Shape Memory in Additive Manufacturing of Cu–Al–Ni Shape Memory Alloy Processed by Laser Powder Bed Fusion

Shape memory alloys (SMAs) are functional materials that are being applied in practically all industries, from aerospace to biomedical sectors, and at present the scientific and technologic communities are looking to gain the advantages offered by the new processing technologies of additive manufacturing (AM). However, the use of AM to produce functional materials, like SMAs, constitutes a real challenge due to the particularly well controlled microstructure required to exhibit the functional property of shape memory. In the present work, the design of the complete AM processing route, from powder atomization to laser powder bed fusion for AM and hot isostatic pressing (HIP), is approached for Cu–Al–Ni SMAs. The microstructure of the different processing states is characterized in relationship with the processing parameters. The thermal martensitic transformation, responsible for the functional properties, is analyzed in a comparative way for each one of the different processed samples. The present results demonstrate that a final post–processing thermal treatment to control the microstructure is crucial to obtain the expected functional properties. Finally, it is demonstrated that using the designed processing route of laser powder bed fusion followed by a post–processing HIP and a final specific thermal treatment, a satisfactory shape memory behavior can be obtained in Cu–Al–Ni SMAs, paving the road for further applications.This research was supported by the Industry Department of the Basque Government through the ELKARTEK–CEMAP (KK–2020/00047) project, as well as from the GIU–17/071 from the University of the Basque Country UPV/EHU. Financial support was also received from the Spanish Ministry of Economy and Competitiveness, MINECO, through the project MAT2017-84069P. This work made use of the facilities from the Electronic Microscopy and Material Microanalysis Service of the SGIKER from the UPV/EHU. M.P.-C. acknowledges the pre–doctoral grant (PRE_2019_2_0268) from the Education Department of the Basque Country. J.F.G.-C. thanks the post–doctoral grant (ESPDOC18/37) from the UPV/EHU

Strain Relaxation in Cu-Al-Ni Shape Memory Alloys Studied by in Situ Neutron Diffraction Experiments

In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. The observed effects and the measured strain relaxations are in agreement with the predictions of the model proposed to explain this behavior in previous calorimetric studies. In addition, the thermal expansion coefficients of both martensite and austenite phases were measured. The neutron experiments have allowed a complete description of the strains during martensitic transformation, and the obtained conclusions can be extrapolated to other SMA systems. (c) 2019 Author(s).This work was supported by the Spanish Ministry of Economy and Competitiveness (No. MINECO MAT2017-84069-P), as well as by the Consolidated Research Group (No. IT-1090-16) and the ELKARTEK-ACTIMAT project from the Education and Industry Departments of the Basque Government. The University of the Basque Country (UPV/EHU) also supported this work with the Research Group GIU17/071. This work has benefited from the use of NPDF at the Lujan Center at Los Alamos Neutron Science Center, funded by the Department of Energy (DOE) Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC, under DOE Contract No. DE-AC52-06NA25396. The upgrade of NPDF was funded by the National Science Foundation (NSF) through Grant No. DMR 00-76488

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 mu m 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 eta, 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 eta = 0.20 +/- 0.01; Cu-Al-Be eta = 0.100 +/- 0.006; Cu-Al-Ni-Be eta = 0.072 +/- 0.004 and Cu-Al-Ni-Ga eta = 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.This research was supported by the Spanish Ministry of Economy and Competitiveness, MINECO, projects MAT2017-84069P and CONSOLIDER-INGENIO 2010 CSD2009-00013, as well as by the ELKARTEK-CEMAP project from the Industry Department of the Basque Government, and GIU-17/071 from the University of the Basque Country UPV/EHU, Spain. This work made use of the FIB and ICP facilities of the SGIKER from the UPV/EHU. The author V.F. acknowledges the Post-Doctoral Mobility Grant from the CONICET of Argentina, and J.F.G.-C. also acknowledges the Post-Doctoral Grant (ESPDOC18/37) from the UPV/EHU

Internal friction associated with ε martensite in shape memory steels produced by casting route and through additive manufacturing: Influence of thermal cycling on the martensitic transformation

Among the different families of shape memory alloys (SMA), the Fe-Mn-Si-Cr-Ni alloys have attracted a renewed interest because of its low cost, high corrosion resistance and high recovery strength during the shape memory effect, and the new technologies of additive manufacturing offer unforeseen possibilities for this family of SMA. In the present work, the reversible gamma - epsilon martensitic transformation (MT), responsible for the shape memory effect, is studied in two Fe-Mn-Si-Cr-Ni alloys with high (20.2 wt%) and low (15.8 wt%) Mn content, produced by the conventional route of casting and rolling, in comparison with the MT in another similar alloy, with intermediate Mn content (19.4 wt%), which was produced by gas atomization and additive manufacturing through laser metal deposition. The forward and reverse gamma - epsilon MT is studied by mechanical spectroscopy through the internal friction spectra and the dynamic modulus variation, together with a parallel microstructural characterization including in-situ observation of the gamma - epsilon MT during cooling and heating at the scanning electron microscope. The evolution of the transformed fraction of epsilon martensite, evaluated through the integral area of the internal friction peak, was followed along thermal cycling in all three alloys. Both, the internal friction and the electron microscopy studies show that the epsilon martensite amount increases very fast during the first few cycles, and then decreases with a tendency towards its stabilization for many tens of cycles. The results show that the gamma - epsilon MT is more stable on cycling in the additive manufactured sample than in the conventionally processed samples, opening new avenues for designing shape memory steels to be specifically processed through additive manufacturing.This work was supported by the ELKARTEK-CEMAP (KK-2020/00047) project from the Industry Department of the Basque Government, and the GIU-17/071 from the University of the Basque Country, UPV/EHU. This work made use of the SGIKER facilities at the UPV/EHU