Impact of Thermal Aging on the Microstructure Evolution and Mechanical Properties of Lanthanum-Doped Tin-Silver-Copper Lead-Free Solders

The authors would like to thank Ste´phanie Blanc (Electrical Engineer at Schlumberger) for her useful contribution to the project, Claude Guyomard and Olivier Naegelen (Arts et Me´tiers ParisTech) for the die design and sample casting, respectively, and Jean-Marc Raulot for his enriching discussions.An extensive study is made to analyze the impact of pure lanthanum on the microstructure and mechanical properties of Sn-Ag-Cu (SAC) alloys at high temperatures. Different compositions are tested; the temperature applied for the isothermal aging is 150 C, and aging times of 10 h, 25 h, 50 h, 100 h, and 200 h are studied. Optical microscopy with cross-polarized light is used to follow the grain size, which is refined from 8 mm to 1 mm for as-cast samples and is maintained during thermal aging. Intermetallic compounds (IMCs) present inside the bulk Sn matrix affect the mechanical properties of the SAC alloys. Due to high-temperature exposure, these IMCs grow and hence their impact on mechanical properties becomes more significant. This growth is followed by scanning electron microscopy, and energy-dispersive spectroscopy is used for elemental mapping of each phase. A significant refinement in the average size of IMCs of up to 40% is identified for the as-cast samples, and the coarsening rate of these IMCs is slowed by up to 70% with no change in the interparticle spacing. Yield stress and tensile strength are determined through tensile testing at 20 C for as-cast samples and after thermal aging at 150 C for 100 h and 200h. Both yield stress and tensile strength are increased by up to 20% by minute lanthanum doping

Estimation of the electron beam-induced specimen heating and the emitted X-rays spatial resolution by Kossel microdiffraction in a scanning electron microscope

Lien vers la version éditeur: http://www.sciencedirect.com/science/article/pii/S0304399112000307A Kossel microdiffraction experimental setup has been developed inside a Scanning Electron Micro-scope for crystallographic orientation, strain and stress determination at a micrometer scale. This paper reports an estimation of copper and germanium specimens heating due to the electron beam bombardment. The temperature rise is calculated from precise lattice parameters measurement considering different currents induced in the specimens. The spatial resolution of the technique is then deduced

Influence of projectile shape on dynamic behavior of steel sheet subjected to impact and perforation

Authors thank Ministry of Science and Higher Education of Poland for financial support under Grants: R00 0097 12. Authors thank also M. Tavian technician in electronics from ENIM for his contribution on the development of the residual velocity measurement sensors.The paper describes a work focused on the process of perforation of steel sheet.Experimental,analytical and numerical investigations have been carried out to analyze in details the perforation process.Based on these approaches,the ballistic properties of the material and the failure modes depending on the projectile nose shape(conical,blunt or hemispherical) have been studied.Different failure modes have been observed,including petaling, plug ejection and circumference necking.The special study about the number of petals has been done for different nose angles using conical shape projectiles.The complete energy balance is also reported and the absorbed energy by the steel sheet has been obtained by measuring initial and residual projectile velocities.A wide range of impact velocities from 35to180m/s has been covered during the tests.All the projectiles are 13mm in diameter and the plates are1mm thick.Moreover,the mass ratio(projectile mass/steel sheet mass) and the ratio between the span of the steel sheet and the diameter of the projectile are constant, equal to 0.38 and 3.85, respectively

Effects Of Thermal Exchange On Material Flow During Steel Thixoextrusion Process

Semi-solid processing is an innovative technology for near net-shape production of components, where the metallic alloys are processed in the semi-solid state. Taking advantage of the thixotropic behavior of alloys in the semi-solid state, significant progress has been made in semi-solid processing. However, the consequences of such behavior on the flow during thixoforming are still not completely understood. To explore and better understand the influence of the different parameters on material flow during thixoextrusion process, thixoextrusion experiments were performed using the low carbon steel C38. The billet was partially melted at high solid fraction. Effects of various process parameters including the initial billet temperature, the temperature of die, the punch speed during process and the presence of a Ceraspray layer at the interface of tool and billet were investigated through experiments and simulation. After analyzing the results thus obtained, it was identified that the aforementioned parameters mainly affect thermal exchanges between die and part. The Ceraspray layer not only plays a lubricant role, but also acts as a thermal barrier at the interface of tool and billet. Furthermore, the thermal effects can affect the material flow which is composed of various distinct zones

Microstructure Evolution and Material Flow of Steel in Semi-solid Forming Process

The present study aims to identify and characterize the development of microstructure and deformation characteristics of steel grades in semi-solid state which is affected by the change in morphologies of microstructure at high temperature. Thixoextrusion tests with different combinations of forming temperature and forming speed were performed. It was identified that several process parameters, such as initial billet and die temperatures or forming speed, affect thermal exchanges thereby influencing the microstructure evolution and material flow. Furthermore, 2D and 3D microstructure characterization was performed on the same sample which was partial remelted and quenched. Reconstructed 3D images were compared with the ones obtained with a Scanning Electron Microscope and an Energy Dispersive Spectrometry system. The good agreement between 2D SEM observations and 3D X-ray microtomography results makes these two techniques efficient to characterize steels in the semi-solid state

Determination of quantity and localization of liquid in the semi-solid state using both 3D X-ray microtomography and 2D techniques for steel thixoforming

The distribution of liquid at the semi solid state is one of the most important parameters for steel thixoforging. It has a great influence on the viscosity of the material, on the flows and finally on the final shape and mechanical properties of the thixoforged parts. Both ex situ and in situ 3D X-ray microtomography characterizations have been carried out to determine the quantity and localization of liquid at high temperature of M2 steel slugs. Microtomography was first performed ex situ at room temperature on samples heated and quenched from semi-solid state. The specimens were also scanned in situ directly at high temperature. The obtained results have been compared to 2D observations using EDS technique in SEM on heated and quenched specimens. They showed a good correlation making both approaches very efficient for the study of the liquid zones at the semi-solid state

Development and mechanical characterization of porous titanium bone substitutes

The authors wish to thank Dr J.-M. Hiver from Institut Jean Lamour, Ecole des Mines de Nancy for his participation in the computed tomography analysis of the porous samplesCommercially Pure Porous Titanium (CPPTi) can be used for surgical implants to avoid the stress shielding effect due to the mismatch between the mechanical properties of titanium and bone. Most researchers in this area deal with randomly distributed pores or simple architectures in titanium alloys. The control of porosity, pore size and distribution is necessary to obtain implants with mechanical properties close to those of bone and to ensure their osseointegration. The aim of the present work was therefore to develop and characterize such a specific porous structure. First of all, the properties of titanium made by Selective Laser Melting (SLM) were characterized through experimental testing on bulk specimens. An elementary pattern of the porous structure was then designed to mimic the orthotropic properties of the human bone following several mechanical and geometrical criteria. Finite Element Analysis (FEA) was used to optimize the pattern. A porosity of 53% and pore sizes in the range of 860 to 1500 μm were finally adopted. Tensile tests on porous samples were then carried out to validate the properties obtained numerically and identif the failure modes of the samples. Finally, FE elastoplastic analyses were performed on the porous samples in order to propose a failure criterion for the design of porous substitutes

Lattice strain measurements using synchrotron diffraction to calibrate a micromechanical modeling in a ferrite–cementite steel

In situ tensile tests were performed at room temperature on a ferrite–cementite steel specifically designed for this study. The evolution of the average stress in ferrite during loading was analyzed by X-ray diffraction.Lattice strain measurements were performed with synchrotron ring diffraction in both ferrite and cementite.These in situ tests were complemented by macroscopic tensile and reversible tensile-compression tests to study the Bauschinger effect. In order to reproduce stresses in ferrite and cementite particles,a recently developed micromechanical Internal Length Mean Field (ILMF) model based on a generalized self-consistent scheme is applied. In this designed ferrite–cementite steel,the third ‘‘phase’’of the model represents finite intermediate‘‘layers’’in ferrite due to large geometrically necessary dislocation (GND) densities around cementite particles. The assumed constant thickness of the layers is calibrated thanks to the obtained experimental data.The ILMF model is validated by realistic estimates of the Bauschinger stress and the large difference between mean stresses in ferrite and in cementite phases.This difference cannot be reproduced by classic two-phase homogenization schemes without intermediate GND layers

Quantification and localization of the liquid zone of partially remelted M2 tool steel using X-ray microtomography and scanning electron microscopy

The authors warmly thank Luc Morhain and Marc Wary (Arts et Métiers ParisTech CER Metz) for their technical support.Thixoforming of steels poses challenges due to the high temperatures involved and the lack of understanding of thermomechanical behavior. The volume fractions of the liquid and solid phases in the semi-solid state are the most important parameters for such a form-ing process, as they affect the viscosity and hence the flow behavior of the material. Two-dimensional observations might not always be sufficient, as the size distribution and the connectivity of phases cannot be obtained from associated measurements, which can only be determined by three-dimensional (3-D) investigation. This paper presents the first application of high-energy X-ray microtomography to the microstructure of steel in the semi-solid state. The microstructure of M2 high-speed tool steel was studied in both as-received and heated-and-quenched states. From the reconstructed images, 3-D information could be obtained and was compared with scanning elec-tron microscopy and energy dispersive spectrometry observations. The volume fraction and the location of liquid phase in the semi-solid state were determined in particular, and the continuous solid skeleton was investigated

Semiautomatic determination of orientations and elastic strain from Kossel microdiffraction

The technique of divergent beam X-ray (Kossel) diffraction is being used for determination of local lattice orientations and lattice distortions (elastic strains). The Kossel patterns are recorded in a scanning electron microscope using a digital camera. The advancement in automation of the of the system relies on computerized interpretation of the Kossel patterns. The paper reports on a package of computer programs facilitating computation of orientations and the strain tensors from the geometry of Kossel conics. The orientation and strain parameters are determined based on conics marked manually on experimental patterns; these marked conics are matched with simulated patterns. The program can simultaneously match multiple patterns originating from the same sample location. The software does not require special orientations, and it is not limited to any particular material. Tests on simulated patterns have been used to confirm the internal consistency of the program. These tests also indicate the achievable accuracy; with the current approach, it is estimated to be about 3×10-4