Yves Bienvenu

Mechanical properties and non-homogeneous deformation of open-cell nickel foams: application of the mechanics of cellular solids and of porous materials

Abstract The mechanical properties of open-cell nickel foams are investigated for the range of densities used in industrial applications for energy storage. The obtained Young’s modulus, compression yield stress and tensile fracture stress are compared to the predictions of models based on periodic, Penrose and Voronoi beam networks. It is found that Gibson and Ashby’s model [L.J. Gibson, M.F. Ashby, Cellular Solids, Cambridge University Press, Cambridge, 1998] provides the proper scaling laws with respect to relative density for almost all investigated properties. The strong anisotropy of the observed overall responses can also be accounted for. The two-dimensional strain field during the tension of a nickel foam strip has been measured using a photomechanical technique. Non-homogeneous deformation patterns are shown to arise. The same technique is used to obtain the strain field around a circular hole in a nickel foam strip. The observed deformation fields are compared to the results of a finite element analysis using anisotropic compressible continuum plasticity.

Dissolution of the γ′ phase in a nickel base superalloy at equilibrium and under rapid heating

Abstract Dissolution of the γ ′ phase in a PM superalloy Astroloy was studied at equilibrium and under rapid heating. The ‘solvus’ temperature of primary and secondary γ ′ precipitates was determined experimentally. The most prominent feature concerns the large departure from equilibrium for the dissolution of the γ ′ precipitates during rapid heating and this departure depends on the initial size of the precipitates. A model for γ ′ dissolution during fast thermal cycling is proposed.

Microstructural and analytical characterization of Al2O3(AlMg) composite interfaces

Abstract Mechanical properties of composite materials depend on the reactions taking place at the ceramic-matrix interface. This study was devoted to the identification of the mechanisms of interaction for a Saffil-(AlMg) composite during squeeze casting and heat treatment. Transmission electron microscopy characterization and microanalysis clearly show migration of magnesium to the interface and reactions during processing of the composite and appropriate heat treatments allowed MgO and MgAl 2 O 4 clusters to be identified along the interface. Additionally, structural hardening of matrix through Mg 2 Si precipitation was made clear. Some evidence of macroheterogeneity in composition is also given.

Influence of thermal treatments on Ag Sn Cu powders in order to reduce mercury contents in dental amalgam.

The mercury content of dental amalgams is a controversial subject with regard to the biological properties of these materials. The object of this study is to optimize the thermal treatments performed on an experimental powder in order to obtain a low mercury ratio (41% by weight) while preserving the desirable clinical qualities of the material. Using atomized powder, two types of thermal treatments are performed: A1, to obtain a partially annealed structure and A2, to obtain a complete homogenization. The kinetics of the amalgamation reaction is mainly evaluated by X-ray diffraction to identify the newly formed phases as a function of setting time. Mechanical properties are evaluated according to the ISO norms at 37 °C. Metallographical examination of the amalgams shows a «Ag-Hg» phase which acts as a matrix incorporating the «Cu-Sn» and «Ag-Sn» compounds. The setting kinetics of the A1 amalgams is linear and slightly more rapid than that of the A2 amalgam. The mechanical properties of the amalgams are significantly improved regarding the higher mercury content commercial amalgams. A specific thermal treatment permits us to slow down the diffusion of mercury between the different intermetallic compounds into the powder. The final amalgam composition, thus, most approaches the stoechiometric ratio calculated from a quaternary diagram.

Durability modelling of a BGA component under random vibration

This study introduces a computation method to assess damage in electronic solder joints under random vibration. It addresses full 3D dynamic behaviour of electronic board. Finite element modelling (FEM) of electronic BGA (ball grid array) and CGA (column grid array) packages assemblies are developed and adjusted with experimental modal identification of the test board. Vibration FEM simulations are performed to calculate stress transfer functions of critical solder joints. Then, solder joint time-stress responses due to an input random excitation of the board are generated. Stress range distributions are established from rainflow counting. Finally, linear damage computation is done and compared with experimental results by using an empirical damage law. The potential and the limitation of this method are discussed.

Mechanical Behavior of Nickel Base Foams for Diesel Particle Filter Applications

An original processing route by powder metallurgy was developped to alloy pure Ni foams so, that the foam becomes refractory for high temperature applications. The modelling of such a foam at high temperature starts from the behavior of the basic constitutive material, then we use micromechanical models to predict the mechanical properties under tension and in compression creep. A 3D finite element analysis of a volume analysed by X-ray tomography is performed to study the foam deformation mechanisms in both conditions.

Architectured bimetallic laminates by roll bonding: bonding mechanisms and applications

Abstract Manufacturing of bimetallic laminates (<2 mm) with an internal architecture by roll bonding allows to obtain a compromise of conflicting properties. Two applications are considered: copper–steel–copper sheets for high power electronic component substrates and lightweight aluminium–steel–aluminium laminates for structural, electromagnetic applications in power generation and for electromagnetic compatibility. They have in common an architecture, with three-dimensional percolating networks of two metals produced with a sufficient precision. The bonding mechanisms are investigated through microstructural and thermomechanical characterisation of the assemblies produced. Roll bonding fills the voids in a central component by plastic deformation and creates the bonds by cold welding. It may be followed or preceded by surface and heat treatments with the objective to improve the cold welding and to relieve the internal stress state. Architectural rules to optimise the properties of the two-phase laminates in view of the applications may be guided by finite element models combining physical and thermomechanical aspects.

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr, Al alloyed ODS steels

Abstract Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y2O3 and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.

Modelling of Sn3.0Ag0.5Cu thermo-mechanical behaviour by a continuum damage approach

A continuum damage model is proposed for Sn3.0Ag0.5Cu solder alloy in pure shear conditions. Experimental tests were performed on specific torsion samples to adjust material parameters of the continuum damage model. The main advantage of this method is to introduce damage process in the mechanical behaviour law of SnAgCu solder. Damage process highly affects the mechanical behaviour of solder alloy during thermal cycle fatigue. Thus, the lifetime of solder can be estimated by using this model without the determination of fatigue laws.

On laser welding of thin steel sheets

Abstract This paper presents a process–structure–property relationship study of laser welds as a continuous consolidation method for joining thin monophased steel foils, thereby providing a more effective, less costly method to construct automotive catalytic converters. A body centred cubic (bcc) iron–chromium–aluminium alloy doped with Mischmetal was utilised in this study. Both pulsed and continuous wave modes were used to establish the limit welding diagrams for lap joint configuration. Actual laser welding parameters were selected using several testing conditions. The laser welds behaved substantially different from the base material under creep and high temperature oxidation. The difference was mainly attributed to the changes in grain morphology, precipitation of aluminium nitrides and carbides, and relocalisation of the reactive elements during liquid metal flow upon keyhole formation, solidification and cooling.