SAM

Controllable mesostructure, magnetic properties of soft magnetic Fe-Ni-Si by using selective laser melting from nickel coated high silicon steel powder

KANG, Nan — Mon, 01 Jan 2018 00:00:00 GMT

Controllable mesostructure, magnetic properties of soft magnetic Fe-Ni-Si by using selective laser melting from nickel coated high silicon steel powder KANG, Nan; GUITTONNEAU, Fabrice; LIAO, Hanlin; FU, Y.; AUBRY, E.; EL MANSORI, Mohamed Fe-Ni-Si soft magnetic parts, using Ni coated high silicon steel powder, were manufactured by selective laser melting process. The type of defect changes from porosity to cracks and the relative density increases, from 50% to 99%, with the decreasing laser scanning speed. The microstructural analyses indicate that the low laser scanning speed fully melted the nickel coating and high-silicon steel core. The EBSD study showed that the separated island and lamellar mesostructures appeared on the top and side view respectively. Moreover, no apparent texture were observed. The magnetization saturation of SLM processed sample decreased, as the laser scanning speed was increased. Consequently, the magnetic properties of SLM processed Fe-Ni-Si alloy also showed anisotropic feature in building and scanning directions, which can be attributed to their different mesostructure.

In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110〉 seeded growth Si

TSOUTSOUVA, M.G. — Fri, 01 Jan 2016 00:00:00 GMT

In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110〉 seeded growth Si TSOUTSOUVA, M.G.; RIBERI – BÉRIDOT, T.; REGULA, G.; REINHART, G.; BARUCHEL, José; GUITTONNEAU, Fabrice; MANGELINCK-NOËL, N.; BARRALLIER, Laurent This work is dedicated to the advanced in situ X-ray imaging and complementary ex situ investigations of the growth mechanisms when silicon solidifies on a monocrystalline seed oriented ⟨110⟩ in the solidification direction. It aims at deepening the fundamental understanding of the phenomena that occur throughout silicon crystal growth with a particular focus on mechanisms of formation of defects detrimental for photovoltaic applications. Namely, grain nucleation, grain boundary formation and evolution, grain competition, twining occurrence, dislocation generation and interaction with structural defects are explored and analysed. Nucleation of twin crystals preferentially occurs on {111} facets at the edge of the sample where solid e liquid e vapor triple point lines exist in interaction also with the crucible as well as, at grain boundary grooves at the solid e liquid interface (solid e solid e liquid triple lines), where two grains are in competition, either on the {111} facets of the groove or in the groove. Enhanced undercooling and/or stress accumulation levels are found to act as driving forces for grain nucleation. Additionally, it is demonstrated that twin formation has the property to relax stresses stored in the crystal during the growth process. However, grains formed initially in twin position can undergo severe distortion when they are in direct competition or when they are squeezed in e between grains. Moreover, we show by X-ray Bragg diffraction imaging that on the one hand, coherent S3 ⟨111⟩ grain boundaries efficiently block the propagation of growth dislocations during the solidification process, while on the other hand, dislocations are emitted at the level of incoherent and/or asymmetric S27a ⟨110⟩ at the encounter with either S3 ⟨111⟩ or S9 ⟨110⟩ grain boundaries. Indeed, grain boundaries that deviate from the ideal coincidence orientation act as dislocation sources that spread inside the surrounding crystals.

In-situ synthesis of aluminum/nano-quasicrystalline Al-Fe-Cr composite by using selective laser melting

KANG, Nan — Mon, 01 Jan 2018 00:00:00 GMT

In-situ synthesis of aluminum/nano-quasicrystalline Al-Fe-Cr composite by using selective laser melting KANG, Nan; LIN, X.; GUITTONNEAU, Fabrice; LIAO, H.L.; HUANG, W.D.; CODDET, C.; EL MANSORI, Mohamed In this research, Al-Fe-Cr quasicrystal (QC) reinforced Al-based metal matrix composites were in-situ manufactured by using selective laser melting (SLM) from the powder mixture. The parametrical optimization based on our previous work was performed with focus on laser scanning speed. From the optimized parameters, an almost dense (99.7%) free-crack sample was fabricated with an ultra-fine microstructure. A phase transition from decagonal QC Al65Cu25Fe10Cr5 to icosahedral QC Al91Fe4Cr5 could be observed as laser scanning speed decreases. Differential scanning calorimetry curves show that the QC phase is quiet stable until 500 °C. And then, the effects of annealing temperature on the microstructural and mechanical properties were determined. The results indicate that the recrystallization and growth behavior of α-Al grains could be prevented by QC particle during annealing. Furthermore, the growth of QC particle, which tends to form a porous structure, leads an improvement of Young modulus and decline of ductility.

Microstructural and Mechanical Characterization of the Yb: YAG Laser Welding of High-Pressure Die-Casting Mg-Al-Mn Magnesium Alloy

OUALLAM, Seddik — Wed, 01 Jan 2020 00:00:00 GMT

Microstructural and Mechanical Characterization of the Yb: YAG Laser Welding of High-Pressure Die-Casting Mg-Al-Mn Magnesium Alloy OUALLAM, Seddik; PEYRE, Patrice; DJEGHLAL, Lamine; GUITTONNEAU, Fabrice; BOUTAGHOU, Zoheir; KABA, Liamine; MASSE, Jean-Eric In this work, the Yb:YAG laser beam welding of the magnesium alloy AM60 was studied. A laser power of 2 kW and a welding speed of 3.5 m / min give a different welding quality than that obtained by CO2 laser with the same parameters. The metallurgical characterization, by optical microscopy, showed the formation of four distinct zones : base metal (BM), heat affected zone (HAZ), the partially fusion zone (PFZ) and the fusion zone (FZ), due to the thermal effect produced by the laser welding thermal cycle. Their dimensions are quantified. The microstructural examination using scanning electron microscopy showed the presence of fine dendritic structure in the FZ although the use of electron dispersive spectroscopy analysis confirm that an eutectic Mg17Al12 phase are surrounded by α-Mg solid solution in the HAZ. Electron backscattered diffraction technique revealed an important grain refinement in FZ and considerable twining phenomena in HAZ, but no texture. X-ray diffraction technique has been used, full width at half maximum of diffraction peaks is measured; it also confirmed the grain refinement in FZ in comparison to BM and HAZ. Both microhardness and tensile proprieties of the complete weld joint are similar to those of the BM.

Classification of the acquisition conditions driving the accuracy of strain measurements during in situ DIC with scanning electron microscope

GOULMY, Jean-Patrick — Thu, 13 Jul 2023 00:00:00 GMT

Classification of the acquisition conditions driving the accuracy of strain measurements during in situ DIC with scanning electron microscope GOULMY, Jean-Patrick; GUITTONNEAU, Fabrice; JÉGOU, Sébastien; BARRALLIER, Laurent Performing in situ scanning electron microscope (SEM) tests is an interesting way to visualise strain heterogeneities under mechanical loading. An essential step before performing the tests is to define the acquisition conditions. The aim of this paper is to propose a classification of the acquisition conditions that are most important for the accuracy of strain measurements using digital image correlation (DIC) in in situ SEM tests. More than 200 image pairs were acquired using a field emission gun SEM. The influence of different acquisition conditions was investigated: acceleration voltage, probe current, working distance, magnification, number of integrated images, image resolution, integration and number of integrated images, scan speed, contrast, brightness and exposure time of the sample in a given area. The methodology implemented in this work is an interesting tool for detecting scan line shift, drift distortion, spatial distortion and rastering artefacts. It allows the optimization of SEM acquisition conditions for strain measurements. Finally, optimal acquisition conditions for in situ testing are proposed and used to perform a tensile test on pure copper. The main factors highlighted include the size of the subset used in the DIC, the beam stabilisation time before image acquisition and the size of the images, which play a significant role in the results. It is recommended to apply the methodology to each device to optimise the acquisition conditions.

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

OUADDAH, Hadjer — Wed, 01 Jan 2020 00:00:00 GMT

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation OUADDAH, Hadjer; BECKER, Maike; RIBERI-BÉRIDOT, Thècle; TSOUTSOUVA, Maria; STAMELOU, Vasiliki; REGULA, Gabrielle; REINHART, Guillaume; PÉRICHAUD, Isabelle; GUITTONNEAU, Fabrice; VALADE, Jean-Paul; RACK, Alexander; BOLLER, Elodie; BARUCHEL, José; MANGELINCK-NOËL, Nathalie; BARRALLIER, Laurent To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.

Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries

BECKER, Maike — Wed, 01 Jan 2020 00:00:00 GMT

Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries BECKER, Maike; PIHAN, Etienne; GUITTONNEAU, Fabrice; REGULA, Gabrielle; OUADDAH, Hadjer; REINHART, Guillaume; MANGELINCK-NOËL, Nathalie; BARRALLIER, Laurent Directional solidification of a cast mono silicon seed and of a float-zone (FZ) silicon seed was performed and the grain and defect structures of the seeds as well as of the regrown parts are analyzed. In situ X-ray diffraction imaging enabled the observation of the dislocation arrangements. During the heating process, in the FZ seed, mobile dislocations glide on {111} planes, whereas in the cast mono seed dislocations are arranged in a mainly immobile cellular structure. Ex situ grain orientation mappings reveal the presence of subgrains with misorientations up to 3◦ in the regrown part of the cast mono-seeded sample, which are not observed in the regrown part of the FZ-seeded sample. Subgrain boundaries characterized by misorientations around the [001] growth axis propagate roughly along the growth axis and increase their misorientation by merging with new subgrain boundaries appearing in their vicinity. Although the first inception of subgrain formation cannot be revealed, the comparison of the dislocation arrangements in the two seeds strongly suggests an influence of the latter on subgrain formation. In the regrown part, interactions between subgrain boundaries and twin boundaries show that they can follow Σ3{111} and Σ9{221} grain boundaries or cross Σ3{111} grain boundaries. Whether Σ3 {111} GBs are crossed or not depends among other things on the orientation of the grains on either side of the twin. It demonstrates that the grain orientation relationship and not only the grain boundary character play an important role in the subgrain structure evolution and redistribution in a multicrystalline silicon ingot.

Strain building and correlation with grain nucleation during silicon growth

RIBERI – BÉRIDOT, T. — Tue, 01 Jan 2019 00:00:00 GMT

Strain building and correlation with grain nucleation during silicon growth RIBERI – BÉRIDOT, T.; TSOUTSOUVA, M.G.; REGULA, G.; REINHART, G.; GUITTONNEAU, Fabrice; MANGELINCK-NOËL, N.; BARRALLIER, Laurent This work is dedicated to the grain structure formation in silicon ingots with a particular focus on the crystal structure strain building and its implication in new grain nucleation process. The implied mechanisms are investigated by advanced in situ X-ray imaging techniques during silicon directional solidification. It is shown that the grain structure formation is mainly driven by S3 <111> twin nucleation. Grain competition phenomena occurring during the growth process lead to the creation of higher order twin boundaries, localised strained areas and associated crystal structure deformation. On the one hand, it is demonstrated that local strain building can be directly related to the characteristics of the twin boundaries created during silicon growth due to grain competition. On the other hand, space restriction due to competition during growth can be at the origin of local strain building as well. Finally, the accumulation of all these factors generating strain is responsible for spontaneous new grain nucleation. When occurring, both grain nucleation and subsequent grain structure reorganisation contribute to lower the strain in the growing ingot. It is demonstrated as well that the local distribution of the strained areas created during silicon growth is retrieved after cooling down, from melting temperature to room temperature, on top of an additional larger scale deformation of the sample due to the cooling down only.

Cementite Residual Stress Analysis in Gas-nitrided Low Alloy Steels

GOEKJIAN, S. — Fri, 01 Jan 2016 00:00:00 GMT

Cementite Residual Stress Analysis in Gas-nitrided Low Alloy Steels GOEKJIAN, S.; GUITTONNEAU, Fabrice; BARRALLIER, Laurent; JÉGOU, Sébastien This paper deals with the measurement of residual stresses in cementite after gas-nitriding of a 33CrMoV12-9 steel. During nitriding, precipitation of nanometric alloying elements nitrides and cementite at grain boundaries occurs leading to an increase of superficial hardness and providing compressive residual stresses in the surface layer. The stress state in the ferritic matrix has generally been measured to characterize the mechanical behaviour of the nitrided case while the other phases are not taken into account. In order to better understand the mechanical behaviour (e.g. fatigue life and localization of cracks initiation) of heterogeneous material such as in case of nitrided surfaces, the nature (sign, level) of residual stresses (or pseudo-macro-stresses) of the present phases can be calculated from measurements using X-ray diffraction to select the considered phase. Due to a low volume fraction of cementite through a nitrided case, an approach based on X-ray and electron backscattered diffractions (XRD and EBSD respectively) is proposed to perform stress measurements in cementite. An optimization of the surface preparation (by mechanical and/or chemical polishing techniques) prior to EBSD analysis was performed in order to minimize deformation induced by surface preparation. Pseudo-macro-stresses were calculated in tempered martensite and cementite. Results are compared to local residual stress measurements carried out by a cross-correlation method using EBSD patterns.