Julie Bourgon

Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity

ABSTRACT A new bcc Ti-rich high-entropy alloy (HEA) of composition Ti35Zr27.5Hf27.5Nb5Ta5 was designed using the ‘d-electron alloy design’ approach. The tensile behavior displays a marked transformation-induced plasticity effect resulting in a high normalized work-hardening rate of 0.103 without loss of ductility when compared to the reference composition Ti20Zr20Hf20Nb20Ta20. In this paper, a detailed microstructural analysis was performed to understand the deformation process, revealing architectural-type microstructures and a high volume fraction (65%) of internally twinned stress-induced martensite α″ after mechanical testing. This study opens the way to mechanical properties optimization and enhancement of titanium-based HEAs by combining multiple alloying designs. IMPACT STATEMENT For the first time, proof is given that transformation-induced plasticity was triggered in a bcc refractory high-entropy alloy, leading to a twofold increase in the normalized work-hardening rate. GRAPHICAL ABSTRACT

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Hydrogen absorption in 1 nm Pd clusters confined in MIL-101(Cr)

We report here the unprecedented modification of the hydrogen absorption/desorption properties of 1 nm Pd clusters relative to the bulk and nanoparticles down to 2–3 nm. These metal clusters have been synthesized by a facile double solvent impregnation method. They contain on average 33 atoms and are confined/stabilized into a metal-organic-framework with different metal loadings (5–20 wt%). This is the first time, to the best of our knowledge, that 1 nm Pd clusters are effectively confined into a MOF for high metal loadings. Such ultra-small nanoparticles are crystalline with the archetypical fcc structure of the bulk metal, as confirmed by both HR-TEM and in situ EXAFS. Hydrogen absorption/desorption properties of 1 nm Pd clusters have been characterized by both laboratory and synchrotron facilities. Under ambient conditions, 1 nm Pd clusters absorb hydrogen forming solid solutions instead of a hydride phase, as usually encountered for the bulk and Pd nanoparticles down to 2–3 nm. This can be understood by a decrease of the critical temperature of the two-phase region in the Pd–H phase diagram below room temperature. Moreover, the activation energy of hydrogen desorption from Pd clusters strongly decreases relative to bulk Pd. This suggests a change in the rate limiting step from surface recombination or β → α phase transformation usually encountered in bulk Pd to hydrogen diffusion into α and β phases in 1 nm clusters.

Investigation of the local structure of nanosized rhodium hydride

The local structure and the thermal stability of small and well-dispersed RhHx nanoparticles (average size of 1.4 nm) were studied by in situ X-ray Absorption Spectroscopy. The RhHx nanoparticles are stable at room temperature and undergo a structural transition from hydride (fcc) to metal phase (fcc) with a shrinking of the lattice volume due to the desorption of hydrogen. This phase transition occurs in the temperature range of 150-180 °C, in good agreement with the results from thermo-desorption spectroscopy. Above 180 °C, the desorbed nanoparticles undertake important coalescence. In situ transmission electron microscopy performed up to 300 °C proves that this process cannot be only thermal, thus it may be ascribed to a X-ray beam effect.

Influence of high-pressure torsion on the microstructure and the hardness of a Ti-rich high-entropy alloy

High-Pressure Torsion (HPT) is one of the most effective severe plastic deformation techniques in grain refinement. The goal of this study was to investigate the influence of HPT on the microstructure and hardness of a Ti-rich High-Entropy Alloy (HEA). The evolution of the grain size due to 1 turn of HPT was studied by transmission electron microscopy. Besides the refinement of the microstructure, a phase transition also occurred during HPT, as revealed by X-ray diffraction. The initial bcc structure transformed into a martensitic phase throughout the material. The features of this phase transformation were studied on a sample compressed to low strain values. The hardness as a function of the distance from the center in the HPT-processed disk was measured and correlated to the microstructure.

Nanoscale investigation by TEM and STEM-EELS of the laser induced yellowing

Nd-YAG QS laser cleaning of soiled stone at 1064 nm can sometimes result in a more yellow appearance compared to other cleaning techniques. Especially in France, this yellowing effect is still considered as a major aesthetic issue by the architects and conservators. One explanation states that the yellowing is linked to the formation of iron-rich nanophase(s) through the laser beam interaction with black crusts that would re-deposit on the cleaned substrate after irradiation. To characterize these nanophases, a model crust containing hematite was elaborated and laser irradiated using a Nd-YAG QS laser. The color of the sample shifted instantaneously from red to a bright yellow and numerous particles were ablated in a visible smoke. Transmission electron microscopy (TEM) was used to examine the morphology and the crystallinity of the neo-formed compounds, both on the surface of the samples and in the ablated materials. In addition, an investigation of the chemical and structural properties of the nanophases was conducted by X-ray dispersive energy (EDX) and electron energy loss (EELS) spectroscopies. It was found that both the surface of the sample and the ablated materials are covered by crystallized nano-spheres and nano-residues, all containing iron and oxygen, sometimes along with calcium and sulfur. In particular an interfacial area containing the four elements was evidenced between some nanostructures and the substrate. Magnetite Fe3O4 was also identified at the nanoscale. This study demonstrates that the laser yellowing of a model crust is linked to the presence of iron-rich nanophases including CaxFeySzOδ nanostructures and magnetite Fe3O4 at the surface after irradiation.

Laser yellowing effect: study of the nanophases created by laser irradiation of synthetic black crusts using transmission electron microscopy (TEM) and electron paramagnetic resonance (EPR) spectroscopy

Nd:YAG Q-Switched laser cleaning at 1064 nm can sometimes lead to a more yellow appearance of the stone surface in comparison with other cleaning techniques. The yellow hue can originate from different contributions among which the presence of nano-sized residues generated by the laser interaction with the surface materials to be eliminated. In this study, the nature of such residues has been investigated. The analyzed materials are (i) particles collected from a pure gypsum reference plate; (ii) a synthetic crust, composed of 80 wt% natural black crust and 20 wt% synthetic gypsum; (iii) particles ejected from the synthetic crust during laser irradiation. Optical, scanning electron and transmission electron microscopies were used to link color changes at the macro-scale to morphology changes at the submicronand nano-scales. Chemical composition was also obtained at the nano-scale using TEM coupled with energy dispersive X-ray (EDX) spectroscopy. This multi-scale approach was combined with electron paramagnetic resonance spectroscopy (EPR) analysis at low and room temperatures to examine the possible presence of iron-containing species presenting particular magnetic properties in the sample before and after irradiation. Under laser irradiation, both the crust sample and the ejected gypsum particles take a yellow color. This color shift can be linked with morphology changes occurring at the nano-scale: gypsum crystals from the reference plate show a smooth surface, while those coming from the synthetic black crust are, after irradiation, covered by many spherical nanoparticles and a rough nano-layer ranging from less than 20 nm to more than 100

Annealing Effects in Nanograined Al-Cu-Mg Alloy Processed by Equal Channel Angular Pressing

The microstructure and mechanical properties were investigated in an industrial Al-Cu-Mg alloy processed by Equal Channel Angular Pressing ECAP and heating. The die used is formed by two channels intersecting at an angle 90°. Transmission Electron Microscopy (TEM) and orientation (ASTAR) imaging were used in addition to hardness measurements. After heating, a sub-micron grain size is retained. In addition, a further hardening is observed due to secondary precipitation. Differential Scanning Calorimetry (DSC) showed that the activation energy of θ’ precipitation is strongly lowered after ECAP.