Karim Inal

Experimental approach of a crystallographic cleavage criterion in a cast aged duplex stainless steel

Abstract The stress state distribution in an aged duplex steel is determined in each phase and is then correlated to local damage mechanisms. The measurements are performed at the grain scale because the studied material contains coarse grains up to one millimetre in size. A single-crystal specific X-ray method is required to determine the stress tensor. During in situ loading, the mechanical state determined in the ferritic crystal is associated with both the plastic flow and the damage mechanism observation. The coupling of the techniques makes it possible to establish that for the studied aged material, the stress normal to a {100} plane value of 465 MPa can be considered as a crystallographic criterion for cleavage propagation. The comparison of this value with the Cottrells model seems to be good but the number of dislocations in the pile-up must be verified experimentally.

Inter- and Intragranular Stress Determination with Kossel Microdiffraction in a Scanning Electron Microscope

A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses on the micron scale, using a one cubic micrometer spot. The experimental Kossel line patterns are obtained by way of a CCD camera and are then fully indexed using a home-made simulation program. The so-determined orientation is compared with Electron Back-Scattered Diffraction (EBSD) results, and in-situ tests are performed inside the SEM using a tensile/compressive machine. The aim is to verify a 50MPa stress sensitivity for this technique and to take advantage from this microscope environment to associate microstructure observations (slip lines, particle decohesion, crack initiation) with determined stress analyses.

Multi-scale behaviour modelling of an austeno—ferritic steel

Abstract Polycrystalline approaches are more and more used to characterise the mechanical behaviour of material with the interest to take into account the mechanical properties of the material constituents. In this case, for a better understanding of the polycrystalline material mechanical response, mesoscopic and microscopic parameters must be clearly identified. Our purpose is also to characterise the mechanical behaviour of a cast duplex stainless steel, containing 30% ferrite and 70% austenite, using X-ray diffraction. The mechanical state can be determined using X-ray diffraction technique which enables to measure strains in each phase separately. The measurements are performed at the grain scale because the studied material exhibits coarse grains of up to one millimetre. In this case, only few crystals are irradiated by the incident X-ray beam and the classical sin2 ψ method for stress determination cannot be applied to this material. An adaptation of the single crystal measurement method to large g...

Investigation of local stress around TSVs by micro-Raman spectroscopy and finite element simulation

Stress concentration in through-silicon via (TSV) is studied by a calibrated micro-Raman spectroscopy (μRS) and correlated with numerical simulation. Results show that stress concentration is in transverse direction and it depends on the ratio of via diameter to via spacing. Further investigations on bended TSV lined with copper revealed that the stress level is lower than the one in TSV without copper. Chip-to-wafer 3D-stacking using microinserts thermocompression bonding shows that the 50 μm thick TSVs have negligible mechanical stress effect.

Residual and Internal Stress States in Duplex Steel with TWIP Effect

A new variety of duplex steels with superior mechanical properties has been studied. They exhibit a very interesting combination of strength (tensile strength of 680 MPa) and ductility values (more than 45% total elongation) due to the competition between different plasticity mechanisms. These steels contain two phases: austenite and ferrite and are characterized by low stacking fault energy at room temperature. In this work, four duplex steels with different chemical composition and phase volume fraction are studied. Residual and internal stresses in each phase were determined using the classical X-ray diffraction sin²ψ method. In the as-received state, both longitudinal and transverse residual stresses are in compression (until -350 MPa) for the ferrite and in tension (until +410 MPa) for the austenite. However, residual stresses in the austenitic phase decrease when its volume fraction increases. Moreover, internal stress distribution in one alloy was determined by X-ray diffraction during an in situ tensile test. The austenitic phase stress along the loading direction is higher than the macroscopic applied one, which is higher than the ferritic stress state, verifying a mixture rule and consistent with the initial residual stresses. For an applied macroscopic strain of about 1%, the austenite phase is subjected to a stress of about 600 MPa whereas the stress in the ferritic phase is about 300 MPa. It was also observed that as macroscopic strain increases, stress difference between the austenite and the ferrite decreases.

3D multi-stacking of thin dies based on TSV and micro-inserts interconnections

This paper is dedicated to the full development of several technological modules mandatory for 3-D multiple die stacking. This includes technologies such as thinning, Through Silicon Vias (TSV) and dies interconnection. A via last approach was chosen to be compatible with die integration coming from various foundries where design, die thickness and contact pad metallurgy are predefined. The interconnection between each die is based on chip on wafer bonding and micro-inserts technology. Small spikes of Ni are formed at the interconnection point between the two circuits. The approach allows a very narrow connection (less than 10 μm of interface) and do not necessitate an underfill. A test vehicle which allows characterizing the required technologies has been designed and manufactured. It consists of 50 μm thick dies including TSV and contact lines on both sides. These dies are stacked on silicon interconnection network enabling to extract electrical signal. The electrical continuity through the stacking was tested from the substrate thanks to Daisy Chain. Finally, these technologies were implemented in a SimCard prototype. The test vehicle and final prototype will be fully described in this paper. A technical focus will be done on the most important process steps for the 3D integration which include vias last integration, micro-bumping and thin die stacking. Several electrical Kelvin structures were measured from the substrate to estimate the contact resistance through micro-inserts and TSV and results will be presented and commented.

Numerical Investigations of Smart Card Module: Parametric Analysis and Design Optimization

Ultrathin silicon chips are becoming more and more popular because of market demand for small, light, and high- performance products with noticeable request of reliability and flexibility. In this paper, the flexibility of the Integrated Circuit package is investigated using finite-element (FE) analysis. ANSYS software is used to analyze a single ultrathin die package in a smart card under four-point bending with the aim of developing flexible smart card modules using chips with thickness below 50 mum. Thicknesses of different layers and Youngs modulus of the die adhesive and the encapsulation resin are investigated to find their relative influence on the bending stress field in silicon. The thicknesses of some layers have important influence on bending stress distribution in the module. Decreased copper thickness can reduce considerably the maximal bending stress in silicon die under the same bending condition. As a result, some criteria for the design optimization are given in order to improve the flexibility of the package.

Residual Stress State at Different Scales in Deep Drawn Cup of Unstable Austenitic Steel

In this study, residual stresses state at different scales in the 301LN unstable austenitic steel after deep drawing was determined. The first part of the work deals with the characterization of the martensitic transformation during uniaxial loading. The austenite/martensite content which was determined by X-Ray Diffraction increases until a maximum of 0.6 for 30% strain. Internal stress distribution was determined by coupling in-situ tensile tests with sin²ψ method. As soon as martensite appears, the magnitudes of the internal stresses in this phase were found to be 400 MPa higher than in the austenite. To establish a relation between the complex loading path effect and the phase stress state, deep drawing tests were carried out for different drawing ratios. Both macroscopic tangential residual stresses and residual stresses in the martensite were determined. It appears that the macroscopic tangential residual stresses are positive and increase with increasing drawing ratios and the maximum value is located at middle height of the cup. It is about 850MPa for the Drawing Ratio (DR)=2.00. The tangential residual stresses in the martensite were found to be positive in the external face and have a same evolution as the macroscopic ones.

Inter and Intra Granular Strain Analysis by Microdiffraction Kossel

We have developed a new convenient tool for local stress and strain analysis in the scanning electron microscope. It is based on the Kossel diffraction, physical phenomenon that is known for a long time because of its high accuracy for lattice constant determination in micron regions. The pattern is recorded on a CCD camera allowing a fast and reliable analysis. This technique has been applied to several materials. In-situ tensile tests were performed on a shape memory alloy. During loading, we observe clearly a shift of Kossel lines on the diagram, whose magnitude depends on the (hkl) crystallographic planes. The stress can be deduced from the diffracting plane strain measurement using a single crystal stress analysis.

Stress determination during the mechanically-induced martensite phase transformation in the superelastic alloy CuAlBe by neutron diffraction

This paper focuses on the study of the superelastic behavior associated to the stress induced martensite transformation in a Cu-12.5%Al-0.5%Be [wt. %] shape memory alloy. Neutron diffraction was used to track the evolution of stress in the (β1) austenitic phase during the onset of the stress-induced martensite phase change. A thin flat and a cylindrical specimen was analyzed, allowing us firstly to evaluate the stress evolution in the austenite phase during martensitic transformation with laboratory X-ray and neutron diffraction and secondly to compare differences between methods (sin2ψ, principal stress) for in-situ neutron diffraction experiments.