Stress Concentration and Mechanical Strength of Cubic Lattice Architectures

The continuous design of cubic lattice architecture materials provides a wide range of mechanical properties. It makes possible to control the stress magnitude and the local maxima in the structure. This study reveals some architectures specifically designed to reach a good compromise between mass reduction and mechanical strength. Decreased local stress concentration prevents the early occurrence of localized plasticity or damage, and promotes the fatigue resistance. The high performance of cubic architectures is reported extensively, and structures with the best damage resistance are identified. The fatigue resistance and S–N curves (stress magnitude versus lifetime curves) can be estimated successfully, based on the investigation of the stress concentration. The output data are represented in two-dimensional (2D) color maps to help mechanical engineers in selecting the suitable architecture with the desired stress concentration factor, and eventually with the correct fatigue lifetime

Martensite Transformation and Superelasticity at High Temperature of (TiHfZr)74(NbTa)26 High-Entropy Shape Memory Alloy

In this work, a (TiHfZr)(NbTa) 26 (%at) high-entropy quinary alloy has been developed especially for high-temperature superelastic applications and studied over a large range of temperatures. The mechanical properties of this new material were compared with those of other superelastic alloys. The different ingots have been made in a cold crucible from pure metals. Several thermomechanical treatments have been performed on the microstructure of four alloys among them (TiHfZr)(NbTa) 26 alloy. The microstructure of each alloy has been characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and x-ray diffraction technique (XRD) and the mechanical behavior was investigated through three-point bending tests between - 40 and 200C, in quasi-static monotonic and low cycle loading conditions. The effects of the thermomechanical treatments on the static and cyclic thermomechanical mechanical responses have been analyzed in combination with the microstructure investigations of the four studied alloys. It has been shown that the (TiHfZr)(NbTa) 26 alloy presents a martensitic transformation and a superelastic effect over the studied range of temperatures, in the cold-worked state or after solution treatment. Finally, the obtained experimental results have been compared with those of other superelastic alloys demonstrating the features of the developed high-entropy high-temperature superelastic alloy

A continuous crystallographic approach to generate cubic lattices and its effect on relative stiffness of architectured materials

This original work proposes to investigate the transposition of crystallography rules to cubic lattice architectured materials to generate new 3D porous structures. The application of symmetry operations provides a complete and convenient way to configure the lattice architecture with only two parameters. New lattice structures were created by slipping from the conventional Bravais lattice toward non-compact complex structures. The resulting stiffness of the porous materials was thoroughly evaluated for all the combinations of architecture parameters. This exhaustive study revealed attractive structures having high specific stiffness, up to twice as large as the usual octet-truss for a given relative density. It results in a relationship between effective Young modulus and relative density for any lattice structure. It also revealed the opportunity to generate auxetic structures at will, with a controlled Poisson ratio. The collection of the elastic properties for all the cubic structures into 3D maps provides a convenient tool for lattice materials design, for research, and for mechanical engineering. The resulting mechanical properties are highly variable according to architecture, and can be easily tailored for specific applications using the simple yet powerful formalism developed in this work.The authors acknowledge the financial support from French National Research Agency ANR (LabEx DAMAS, Grant no.ANR-11-LABX-0008-01)

Variations of the Elastic Properties of the CoCrFeMnNi High Entropy Alloy Deformed by Groove Cold Rolling

The variations of the mechanical properties of the CoCrFeMnNi high entropy alloy (HEA) during groove cold rolling process were investigated with the aim of understanding their correlation relationships with the crystallographic texture. Our study revealed divergences in the variations of the microhardness and yield strength measured from samples deformed by groove cold rolling and conventional cold rolling processes. The crystallographic texture analyzed by electron back scattered diffraction (EBSD) revealed a hybrid texture between those obtained by conventional rolling and drawing processes. Though the groove cold rolling process induced a marked strengthening effect in the CoCrFeMnNi HEA, the mechanical properties were also characterized by an unusual decrease of the Young’s modulus as the applied groove cold rolled deformation increased up to about 0.5 before reaching a stabilized value. This decrease of the Young’s modulus was attributed to the increased density of mobile dislocations induced by work hardening during groove cold rolling processing

Architectural effect on 3D elastic properties and anisotropy of cubic lattice structures

This article investigates the elastic properties of a large panel of lattice architectures using a continuous description of geometry. The elastic constants of the orthotropic material are determined, and discussed in terms of specific stiffness and of its density dependence. Different kind of topology families are emerging depending on their specific deformation behavior. For some of them, interesting properties in term of traction-compression were measured, while some other families are predominantly adapted to shear loading. Homogenization technique also allows to quantify the anisotropy of the structures and to compare them. Specific structures having quasi-isotropic properties even at low relative densities were detected. Experimental works demonstrated the validity of the numerical models, and highlighted the necessity to consider carefully the effect of defects on the specific strength, which are of the second-order however not negligible. Finally, this article provides user-friendly maps for selection of optimal architectures for a large variety of specific needs, like a target stiffness or anisotropy

Investigation and Composition Characterization of a “NiTi-like” Alloy Combining High Temperature Shape Memory and High Entropy

New high temperature shape memory alloyswith five or more elements are under development and present attractive performances for several functional applications. These active metallic materials are called high entropy and high temperature shape memory alloys (HE-HT-SMAs). This work deals with the characterization of an alloy that combines high temperature shape memory effect and high entropy effect features, a NiCuTiHfZr alloy. The evolution of the phase transformation and the shape memory effect during thermal fatigue was compared with a ternary alloy NiTiZr. Ingots were prepared in a cold crucible and alloys were characterized after thermal cycling at 600 K without a protective gas atmosphere. Optical microscope, X-ray diffraction, and scanning elec- tron microscopy observations showed the presence of martensite in this unpublished alloy at room temperature. The differential scanning calorimetry (DSC) tests showed that martensitic transformation takes place at high temperature. High temperature thermal cycling was performed during a three-point bending tests under constant load without a protective atmosphere. Thermomechanical results showed that high entropy effects increase the operating behavior at high temperature. Hence this new composition of NiCuTiHfZr alloy can be used as an actuator for aerospace and aeronautic application

Matériaux mésostructurés et infiltration métallique avec optimisation du squelette architecturé

The constant search for structural lightening and performance optimization is at the origin of hybrid materials. They are defined as the combination of two (or more) materials, or the structuring of a material in a given volume, according to predetermined organization and scale in order to respond to a specific application. In addition, the recent growth of additive manufacturing offers a new unmatched design freedom. This is at the origin of hybrid materials such as lattices; it is an assembly of micro-beams ordered or not to fill a given volume. This PhD work focuses on the development of a hybrid metallic material with optimized architectured skeleton. At first, we will focus on the optimization of the skeleton. To do this, an original model generation of periodic lattices inspired by crystallography will be proposed. It will allow the generation of a database of structures to which will be added quasiperiodic lattices structures. In a second time, we will focus on the performances of these structures from the point of view of: mechanical behavior and porous network infiltration by a liquid. In both cases, phenomenological relations will be determined in order to relate the rigidity to the relative density, or the rise of an infiltration front according to the geometric characteristics of a skeleton. These relationships, via the parameters that govern them, make it possible to identify the influence of the topology and give rise to selection tools in the form of 2D maps. The X-ray tomography control steps, evaluations of the experimental mechanical properties and infiltration of porous networks make possible the confrontation with the proposed models. These different tools validate the topology and the scale of the optimized skeleton from the mechanical point of view and the infiltration. Finally, composites combining an aluminum alloy and a Ti-6Al-4V skeleton produced by selective laser melting are produced by a cold crucible, Porosity and metallurgical analysis at the Ti / Al interface will reveal the viability of the process, as well as the influence of the processing conditions on the microstructures.La recherche constante d’allégement de structure et d’optimisation des performances est à l’origine des matériaux hybrides. Ils sont définis comme la combinaison de deux matériaux (ou plus), ou la structuration d’un matériau dans un volume donné, selon une organisation et une échelle prédéterminée en vue de répondre à une application spécifique. De plus, l’essor récent de la fabrication additive permet d’entrevoir une liberté de conception inégalée. Ceci est à l’origine de matériaux hybrides tels que les structures lattices ; il s’agit d’un assemblage de micro-poutres ordonnées ou non visant à remplir un volume donné. Ce travail de thèse porte sur l’élaboration d’un matériau hybride métallique à squelette architecturé optimisé. Dans un premier temps, nous nous intéresserons à l’optimisation du squelette. Pour ce faire un modèle original de génération de structures lattices périodiques inspiré de la cristallographie sera proposé. Il permettra la génération d’une base de données de structures à laquelle seront ajoutées des structures lattices quasipériodiques. Dans un second temps, nous nous intéresserons aux performances de ces structures du point de vue du comportement mécanique et de l’infiltration de réseau poreux par un liquide. Dans les deux cas, des relations phénoménologiques seront déterminées afin de relier la rigidité à la densité relative, ou la remonté d’un front d’infiltration en fonction des caractéristique géométrique d’un squelette. Ces relations, via les paramètres qui les gouvernent, permettent de dégager l’influence de la topologie et donne lieu à des outils de sélection sous forme de cartes 2D. Les étapes de contrôle des structures par tomographie X, d’évaluations des propriétés mécaniques expérimentales et d’infiltration de réseaux poreux, rendent possible la confrontation avec les modèles proposés. Ces différents outils valident la topologie et l’échelle du squelette optimisé du point de vue mécanique et de l’infiltration. Pour finir, des composites associant un alliage d’aluminium et un squelette en Ti- 6Al-4V produit par fusion laser sélective, sont élaborées par creuset froid. Une analyse des porosités et de la métallurgie à l’interface Ti/Al révélera la viabilité du procédé, ainsi que l’influence des conditions d’élaborations sur les microstructures

Metal infiltrated mesostructured materials with opmitization of architectured scaffold

La recherche constante d’allégement de structure et d’optimisation des performances est à l’origine des matériaux hybrides. Ils sont définis comme la combinaison de deux matériaux (ou plus), ou la structuration d’un matériau dans un volume donné, selon une organisation et une échelle prédéterminée en vue de répondre à une application spécifique. De plus, l’essor récent de la fabrication additive permet d’entrevoir une liberté de conception inégalée. Ceci est à l’origine de matériaux hybrides tels que les structures lattices ; il s’agit d’un assemblage de micro-poutres ordonnées ou non visant à remplir un volume donné. Ce travail de thèse porte sur l’élaboration d’un matériau hybride métallique à squelette architecturé optimisé. Dans un premier temps, nous nous intéresserons à l’optimisation du squelette. Pour ce faire un modèle original de génération de structures lattices périodiques inspiré de la cristallographie sera proposé. Il permettra la génération d’une base de données de structures à laquelle seront ajoutées des structures lattices quasipériodiques. Dans un second temps, nous nous intéresserons aux performances de ces structures du point de vue du comportement mécanique et de l’infiltration de réseau poreux par un liquide. Dans les deux cas, des relations phénoménologiques seront déterminées afin de relier la rigidité à la densité relative, ou la remonté d’un front d’infiltration en fonction des caractéristique géométrique d’un squelette. Ces relations, via les paramètres qui les gouvernent, permettent de dégager l’influence de la topologie et donne lieu à des outils de sélection sous forme de cartes 2D. Les étapes de contrôle des structures par tomographie X, d’évaluations des propriétés mécaniques expérimentales et d’infiltration de réseaux poreux, rendent possible la confrontation avec les modèles proposés. Ces différents outils valident la topologie et l’échelle du squelette optimisé du point de vue mécanique et de l’infiltration. Pour finir, des composites associant un alliage d’aluminium et un squelette en Ti- 6Al-4V produit par fusion laser sélective, sont élaborées par creuset froid. Une analyse des porosités et de la métallurgie à l’interface Ti/Al révélera la viabilité du procédé, ainsi que l’influence des conditions d’élaborations sur les microstructures.The constant search for structural lightening and performance optimization is at the origin of hybrid materials. They are defined as the combination of two (or more) materials, or the structuring of a material in a given volume, according to predetermined organization and scale in order to respond to a specific application. In addition, the recent growth of additive manufacturing offers a new unmatched design freedom. This is at the origin of hybrid materials such as lattices; it is an assembly of micro-beams ordered or not to fill a given volume. This PhD work focuses on the development of a hybrid metallic material with optimized architectured skeleton. At first, we will focus on the optimization of the skeleton. To do this, an original model generation of periodic lattices inspired by crystallography will be proposed. It will allow the generation of a database of structures to which will be added quasiperiodic lattices structures. In a second time, we will focus on the performances of these structures from the point of view of: mechanical behavior and porous network infiltration by a liquid. In both cases, phenomenological relations will be determined in order to relate the rigidity to the relative density, or the rise of an infiltration front according to the geometric characteristics of a skeleton. These relationships, via the parameters that govern them, make it possible to identify the influence of the topology and give rise to selection tools in the form of 2D maps. The X-ray tomography control steps, evaluations of the experimental mechanical properties and infiltration of porous networks make possible the confrontation with the proposed models. These different tools validate the topology and the scale of the optimized skeleton from the mechanical point of view and the infiltration. Finally, composites combining an aluminum alloy and a Ti-6Al-4V skeleton produced by selective laser melting are produced by a cold crucible, Porosity and metallurgical analysis at the Ti / Al interface will reveal the viability of the process, as well as the influence of the processing conditions on the microstructures

Relationship between Chemical Composition and Ms Temperature in High-Entropy Shape Memory Alloys

Five high entropy alloys with shape memory effect or superelastic effect were prepared by cold crucible melting to understand the effect of their chemical composition on the transformation temperatures. The microstructure and phases of the alloys at room temperature were investigated by optical and scanning electron microscopy. The calorimetric study of these alloys is employed to analyze the reversible transformation from austenite to martensite and to determine the transformation temperatures of the five alloys. The paper presents the results of several published works relating the transformation temperatures of high entropy shape memory alloys (HE-SMA). An experimental database has been built up to understand the influence of the different alloying elements and their related concentrations on the transformation temperature Ms (Martensite Start) of the HE-SMAs. An equation is identified using the database to predict the transformation temperature Ms of a high entropy shape memory alloy as a function of its weighted chemical composition, which exhibits a metallurgical Solutioned Treatment (ST) state and a specific mixing entropy (ΔSmix/R) range

Ultrasonic investigation of the effect of compressive strains on 3D periodic bi-material structures

International audienceDue to the specific elastic properties such as high stiffness to mass ratio, regular microstructure materials are widely used in the industry. The need for nondestructive evaluation is ubiquitous to ensure material quality. As an effective nondestructive testing method, ultrasound has great potential in providing an efficient materials characterization. However, contrary to more convenient ultrasound applications, challenges arise when applying ultrasound to 3D bi-material structures due to the coexistence of different phenomena, including diffraction effects caused by the periodicity. Two linear ultrasound methods, namely the Bragg diffraction and the comb filtering effect, are proposed to address this hurdle. The results show that the comb filtering effect effectively characterizes, respectively, the vertical structural quality of the bi-material. Bragg diffraction can also expose structural variations in the horizontal plane