Influence of the electronic polymorphism of Ni on the classification and design of high entropy alloys

According to a recent Hume-Rothery approach, the electron concentration, e/a, and the average radius can be used to identify the domain of stability of HEAs and to estimate the phases that may occur in the alloy. The present study investigates the influence of the electronic polymorphism of nickel on the efficiency of the classification and on the design of HEAs for magnetic applications. Many different compositions were used, based on 4 to 7 elements out of a total 13 different elements (Co, Cr, Fe, Ni, Al, Cu, Pd, Ti, Mn, V, Nb, Sn, Ru). Phases have been determined by X-ray and neutron diffraction as well as in some cases high energy X-ray diffraction. The e/a for the constituent elements is calculated according to Massalski. The two polymorphic electronic structure of nickel, namely (e/a)Ni = 1 or (e/a)Ni = 2 are considered. The average e/a for the alloy is calculated assuming a solid solution case. The electronic structure [Ar] 3d9 4s1 seems to be more appropriate for the classification of HEAs. Based on a Self-organising Map predictions are made for the average magnetic moment at saturation for this electronic structure of Ni. Non-saturated values and data from the literature are compared with the predictions. The consequences of such results when modelling the structure and properties of Ni containing HEAs are presented, in particular the consideration of the shape and transformation of the Brillouin zone

Hume-Rothery for HEA classification and self-organizing map for phases and properties prediction

The Hume-Rothery approach applied in terms of e/a to classify and design quasicrystals and BMG is revisited for the case of HEAs. The results were compared with other parameters used in the literature, namely VEC and delta. The Self-Organizing Map tool is used to classify the experimental results and the experimental map is used to compare the predictions of phases and properties of compositions reported in the literature. According to the Hume-Rothery approach, e/a and the average radius can give a precise rule of thumb to identify the domain of stability of HEAs and to estimate the phases that may occur in the alloy: i) e/a 2.05: bcc. Moreover, e/a is to be preferred to VEC to classify phases in HEAs. Self-organizing maps can be used to make interpolative predictions for new compositions of HEAs with suitable phases for specific properties. Thus, simple combination of e/a and r gives an accurate first estimation to identify compositions with simple phases giving simple diffraction patterns, thus true HEAs as opposed to other compositional domains where complex phases occur

Multiscale multiphysics model for hydrogen embrittlement in polycrystalline nickel

Référence bibliographique : Rol, 102487Appartient à l’ensemble documentaire : Pho20RolAppartient à l’ensemble documentaire : PACA1Image de press

Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion

The microstructure and mechanical properties of 316 L steel have been examined for parts built by a powder bed laser fusion process, which uses a laser to melt and build parts additively on a layer by layer basis.Relative density and porosity determined using various experimental techniques were correlated against laser energy density. Based on porosity sizes, morphology and distributions, the porosity was seen to transition between an irregular, highly directional porosity at the low laser energy density and a smaller, more rounded and randomly distributed porosity at higher laser energy density, thought to be caused by keyhole melting. In both cases, the porosity was reduced by hot isostatic pressing (HIP).High throughput ultrasound based measurements were used to calculate elasticity properties and show that the lower porosities from builds with higher energy densities have higher elasticity moduli in accordance with empirical relationships, and hot isostatic pressing improves the elasticity properties to levels associated with wrought/rolled 316 L. However, even with hot isostatic pressing the best properties were obtained from samples with the lowest porosity in the as-built condition.A finite element stress analysis based on the porosity microstructures was undertaken, to understand the effect of pore size distributions and morphology on the Young's modulus. Over 1–5% porosity range angular porosity was found to reduce the Young's modulus by 5% more than rounded porosity. Experimentally measured Young's moduli for samples treated by HIP were closer to the rounded trends than the as-built samples, which were closer to angular trends.Tensile tests on specimens produced at optimised machine parameters displayed a high degree of anisotropy in the build direction and test variability for as-built parts, especially between vertical and horizontal build directions. The as-built properties were generally found to have a higher yield stress, but lower upper tensile strength and elongation than published data for wrought/hot-rolled plate 316 L. The hot isostatically pressed parts showed a homogenisation of the properties across build directions and properties much more akin to those of wrought/hot-rolled 316 L, with an increase in elongation and upper tensile strength, and a reduction in yield over the as-built samples

A Coupled Electrical-Thermal-Mechanical Modeling of Gleeble Tensile Tests for Ultra-High-Strength (UHS) Steel at a High Temperature

International audienceA coupled electrical-thermal-mechanical model is proposed aimed at the numerical modeling of Gleeble tension tests at a high temperature. A multidomain, multifield coupling resolution strategy is used for the solution of electrical, energy, and momentum conservation equations by means of the finite element method. Its application to ultra-high-strength steel is considered. After calibration with instrumented experiments, numerical results reveal that significant thermal gradients prevail in Gleeble tensile steel specimen in both axial and radial directions. Such gradients lead to the heterogeneous deformation of the specimen, which is a major difficulty for simple identification techniques of constitutive parameters, based on direct estimations of strain, strain rate, and stress. The proposed direct finite element coupled model can be viewed as an important achievement for subsequent inverse identification methods, which should be used to identify constitutive parameters for steel at a high temperature in the solid state and in the mushy state

An investigation of micro-mechanisms in hydrogen induced cracking in nickel-based superalloy 718

Hydrogen embrittlement of the nickel-iron based superalloy 718 has been investigated using slow strain rate tests for pre-charged material and also in-situ hydrogen charging during testing. Fractography analyses have been carried using scanning electron microscopy, electron back-scattering diffraction and orientation image microscopy concentrating on the influence of microstructural features and associated micro-mechanisms leading to hydrogen induced cracking and embrittlement. It was observed that hydrogen induced transgranular cracking initiates at micro-voids in the crystal lattice. Similar behaviour has been observed in multi-scale finite element chemo-mechanical numerical simulations. In contrast, hydrogen induced localized slip intergranular cracking was associated with the formation of micro-voids in intergranular regions. The effects of grain boundary and triple junction character on intergranular hydrogen embrittlement were also investigated. It was observed that low end high angle misorientations (LHAM), 15° 55°. Finally, the use of grain boundary engineering techniques to increase the resistance of super alloy 718 to hydrogen induced cracking and embrittlement is discussed

Cotton in the new millennium: advances, economics, perceptions and problems

Cotton is the most significant natural fibre and has been a preferred choice of the textile industry and consumers since the industrial revolution began. The share of man-made fibres, both regenerated and synthetic fibres, has grown considerably in recent times but cotton production has also been on the rise and accounts for about half of the fibres used for apparel and textile goods. To cotton’s advantage, the premium attached to the presence of cotton fibre and the general positive consumer perception is well established, however, compared to commodity man-made fibres and high performance fibres, cotton has limitations in terms of its mechanical properties but can help to overcome moisture management issues that arise with performance apparel during active wear. This issue of Textile Progress aims to: i. Report on advances in cotton cultivation and processing as well as improvements to conventional cotton cultivation and ginning. The processing of cotton in the textile industry from fibre to finished fabric, cotton and its blends, and their applications in technical textiles are also covered. ii. Explore the economic impact of cotton in different parts of the world including an overview of global cotton trade. iii. Examine the environmental perception of cotton fibre and efforts in organic and genetically-modified (GM) cotton production. The topic of naturally-coloured cotton, post-consumer waste is covered and the environmental impacts of cotton cultivation and processing are discussed. Hazardous effects of cultivation, such as the extensive use of pesticides, insecticides and irrigation with fresh water, and consequences of the use of GM cotton and cotton fibres in general on the climate are summarised and the effects of cotton processing on workers are addressed. The potential hazards during cotton cultivation, processing and use are also included. iv. Examine how the properties of cotton textiles can be enhanced, for example, by improving wrinkle recovery and reducing the flammability of cotton fibre

Hume-Rothery for HEA classification and self-organizing map for phases and properties prediction

International audienc

Validated Computational Modelling Techniques for Simulating Melt Pool Ejecta In Laser Powder Bed Fusion Processing

Industry currently require faster build rates from laser powder bed fusion processes. As such, higher power lasers and multi-laser systems are being explored. Due to instabilities in the melting process, material is ejected from the melt pool in the form of spatter and vapour. Previous work has shown that these ‘ejecta’ can result in attenuation of the laser and redeposition of lager particles onto the powder bed; which can lead to poor mechanical properties. ANSYS Fluent was used to create a CFD model which was validated against hot wire anemometry results from Renishaw’s RenAM 500Q. This was then coupled with a Discrete Phase Model (DPM) to track the ejection of spatter and vapour from the melt pool through the chamber. This has led to a better understanding of the removal of ‘ejecta’, leading to increased mechanical properties and lower rates of build failure.Mechanical Engineerin