Nucleation of austenite in mechanically stabilized martensite by localized heating

The nucleation of bcc austenite in a single crystal of a mechanically stabilized 2H-martensite of Cu-Al-Ni shape-memory alloy is studied. The nucleation process is induced by localized heating and observed by optical microscopy. It is observed that nucleation occurs after a time delay and that the nucleation points are always located at one of the corners of the sample (a rectangular bar in the austenite), regardless of where the localized heating is applied. Using a simplified nonlinear elasticity model, we propose an explanation for the location of the nucleation points, by showing that the martensite is a local minimizer of the energy with respect to localized variations in the interior, on faces and edges of the sample, but not at some corners, where a localized microstructure can lower the energy.Comment: Proceedings, ICOMAT 2011, Journal of Alloys and Compounds, in pres

Building hierarchical martensite

Martensitic materials show a complex, hierarchical microstructure containing structural domains separated by various types of twin boundaries. Several concepts exist to describe this microstructure on each length scale, however, there is no comprehensive approach bridging the whole range from the nano- up to the macroscopic scale. Here, we describe for a Ni-Mn-based Heusler alloy how this hierarchical microstructure is built from scratch with just one key parameter: the tetragonal distortion of the basic building block at the atomic level. Based on this initial block, we introduce five successive levels of nested building blocks. At each level, a larger building block is formed by twinning the preceding one to minimise the relevant energy contributions locally. This naturally explains the occurrence of different types of twin boundaries. We compare this scale-bridging approach of nested building blocks with experiments in real and reciprocal space. Our approach of nested building blocks is versatile as it can be applied to the broad class of functional materials exhibiting diffusionless transformations.Comment: 25 pages, including supplementar

3D printing of cubic zirconia lattice supports for hydrogen production

The demand for hydrogen has extraordinarily grown during the last years, being one of the most attractive forms of fuels to produce green energy. Cubic zirconia ceramics are considered promising catalytic supports, and the additive manufacturing of porous 3D structures based on these ceramics could enhance their catalytic performance. Herein, lightweight highly porous (up to 88%) 3D patterned 8 mol% yttria-stabilized cubic zirconia (8YSZ) scaffolds are manufactured by robocasting from pseudoplastic aqueous-based inks to produce catalytic supports for the hydrogen (H2) production. These scaffolds are thermally treated at temperatures ranging between 1000 and 1400 ◦C and, hence, mechanically and electrically characterized. 3D 8YSZ structures sintered at 1200 ◦C, with an appropriate balance between high porosity (86%) and compressive strength (3.7 MPa), are impregnated with palladium (Pd) catalytic nanoparticles and employed in the catalytic dehydrogenation of renewable formic acid (FA) using a fixed-bed reactor. 3D Pd/8YSZ catalyst leads to the continuous production of CO-free H2 with a FA conversion of 32% at T =55 ◦CThis work was supported by the Spanish Government through RTI2018-095052-B-I00, PID2019-105079RB-I00 (MICINN/AEI/FEDER, UE), PID2021-125427OB-I00 (MICINN/AEI/FEDER, UE) and EIN2020- 112153 (MCINN/AEI/10.13039/501100011033) projects, the latter also supported by the European Union through “NextGenerationEU/ PRTR”. M. Koller gratefully acknowledges funding within “Support for International Mobility of Researchers of the Institute of Thermomechanics, Czech Academy of Sciences, part II”, no. CZ.02.2.69/0.0/ 0.0/18_053/0017555 of the Ministry of Education, Youth and Sports of the Czech Republic funded from the European Structure and Investment Funds (ESIF). G. Vega acknowledges the Universidad Aut´onoma de Madrid for the Predoctoral contract. The authors thank J. Mejía for her permanent technical assistance in the catalytic experiment

Thermomechanical properties of single crystals evaluated by impulsive stimulated thermal scattering technique

This paper describes the application of Impulse Stimulated Thermal Scattering (ISTS) for measurement of surface acoustic wave (SAW) velocity and thermal diffusion along a free surface of a strongly anisotropic material. The motivation for this work stems from the study of thermoelastic properties of individual phases of ferroelastics; experimental results were obtained on a single crystal in the austenitic phase a Cu-Al-Ni alloy (bcc single crystal having elastic anisotropy factor of about 12). The measured SAW velocities in specific directions are in a good agreement with the values calculated for the elastic constants obtained by other ultrasonic methods. Similarly, the evaluated thermal diffusivity coefficient of the austenite is consistent with the data in the literature. The proposed approach has also a potential for characterization of thin films grown on anisotropic substrates.status: publishe

Multifunctional 3D-Printed Cellular MAX-Phase Architectures

[EN] The development of porous MAX-phase structures from computer-aided design models and 3D printing strategies is of great interest for the fabricating of cellular parts with geometric and material complexities for advanced technological applications. This is reinforced by the joining of the outstanding properties of MAX phases with the benefits of creating lighter materials with a higher surface area. Here, the additive manufacturing of 3D cellular CrAlC MAX-phase architectures using a direct ink writing technique from concentrated MAX aqueous-based inks is presented. These architectures exhibit multifunctional properties; in particular, high strength and good mechanical cycling behavior, excellent electrical conductivity, tailored heat dissipation, and good thermal cycling resistance, a blend that widens the potential engineering applications of these MAX phases through an innovative approach.This work was supported by Spanish MICINN/FEDER(UE) under project MAT2015-67437-R and by the Czech Science Foundation (Grant No. GA17-01618S). J. J. Moyano acknowledges the financial support of MINECO/FSE (UE) through the FPI contract ref: BES2016-077759 (2015 call). Authors thank Dr. D. Pérez-Coll for his experimental assistance in the electrical measurements

Anisotropic Elasticity of Ceramic Micro-Scaffolds Fabricated by Robocasting

[EN] Anisotropic elastic and acoustic properties of robocast ceramic scaffolds are calculated by finite element method, utilizing real geometries and material parameters obtained from robocast silicon carbide samples. Six types of robocast geometries are studied, showing different material symmetries given by the arrangement of the ceramic rods in the scaffold structures. Due to the macroscopic periodicity of the structures composed of fully sintered ceramic rods, the robocast scaffolds exhibit metamaterial-like elastic and acoustic properties, never observed for natural materials. The effect of the micro-architecture is shown to be crucial: while for tetragonal and orthorhombic structures, strong acoustic focusing along the directions of the rods appears even in the low-frequency limit, hexagonal structures exhibit no energy focusing up to some frequency limit given by the geometry.This work was supported by the Czech Science Foundation (Grant No. GA17-01618S), and by Spanish project MAT2015-67437-R (MINECO, FEDER, UE)

Frequency-dependent acoustic energy focusing in hexagonal ceramic micro-scaffolds

[EN] Acoustic properties of an additive-manufactured SiC scaffold with hexagonal symmetry fabricated by the robocasting method are studied both numerically and experimentally. The numerical analysis is based on the finite element method (FEM) using Bloch boundary conditions. The calculations show both angular and frequency dispersion of the acoustic waves with wavelengths comparable to the spacing between the rods, i.e., on a millimeter scale, indicating interesting acoustic properties in the MHz range. The dispersion character leads to focusing of the energy propagation into the directions of the rods of the hexagonal structure. This is illustrated by modal-based calculations of the propagation of longitudinal and out-of-plane shear wave packets with a dominant wavelength. The experimental analysis consists of two steps, the measurement of the resonant spectrum and shear wave propagation character. The measured resonant spectrum is in good agreement with the one calculated using numerically obtained low-frequency properties of the structure, also showing the quality of the overall manufactured structure. The time-domain measurement shows significant changes in the energy propagation between low and high frequencies, as predicted by FEM calculations.This work was supported by the Czech Science Foundation, Czechia grant no. GA17-01618S and by Spanish projects MAT2015-67437-R (MINECO, Spain, FEDER, Spain, UE) and RTI2018-095052-B-I00 (MCIU/AEI/FEDER, UE).Peer reviewe