Roland Fortunier

Comparison of heat-affected zones due to nanosecond and femtosecond laser pulses using transmission electronic microscopy

This letter presents a method aimed at quantifying the dimensions of the heat-affected zone (HAZ), produced during nanosecond and femtosecond laser–matter interactions. According to this method, 0.1 μm thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes (i.e., 5 and 2 J/cm2, respectively). The grain size in the samples was observed near the microholes. The main conclusion is that a 40 μm wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 μm, which is our observation limit.

Accuracy assessment of elastic strain measurement by EBSD

A detailed accuracy analysis of electron backscatter diffraction (EBSD) elastic strain measurement has been carried out using both simulated and experimental patterns. Strains are determined by measuring shifts between two EBSD patterns (one being the reference) over regions of interest (ROI) using an iterative cross‐correlation algorithm. An original minimization procedure over 20 regions of interests gives a unique solution for the eight independent components of the deviatoric displacement gradient tensor. It is shown that this method leads to strain measurements on simulated patterns with an accuracy better than 10−4. The influence of the projection parameters is also investigated. The accuracy assessment is illustrated by two worked examples: (i) four‐point bending of a silicon single crystal and (ii) Si1 –xGex layers on a Si substrate. Experimental results are compared with finite‐element simulations.

Factors affecting the accuracy of high resolution electron backscatter diffraction when using simulated patterns

High resolution EBSD directly compares electron backscattering patterns (EBSPs), generated in a scanning electron microscope, to measure relative strain and rotation to a precision of ∼ 10(-4) in strain and 10(-4)rad (0.006 °) in rotation. However the measurement of absolute strain and rotation requires reference EBSPs of known strain and orientation (or a far field region of known strain). Recent suggestions of using simulated EBSPs with known strain show much promise. However precise measurement of the experimental geometry (pattern centre) is required. Common uncertainties of 0.5% in pattern centre result in uncertainty of ∼ 10(-3) in strain state. Aberrations in the compact lenses used for EBSP capture can also result in image shifts that correspond to strains/rotations of ± 10(-3) between experimental and simulated EBSPs. Simulated EBSPs can be generated using dynamical or kinematic models (or a combination of the two). The choice in simulation model has a significant effect on the measured shifts, particularly at zone axis and high structure factor bands, due to large intensity variations, and for simple kinematic simulations can result in the measurement of rogue shifts and thus erroneous strain measurements. Calibrant samples of known strain provide a method of measuring the experimental geometry but imprecise stage movement combined with the high depth of field in the SEM could also result in uncertainties in strain of ∼ 10(-3).

Finite element simulation of heat transfer

Part 1: Steady conduction1. Mathematical formulation.2. The finite element method.3. Isoparametric elements.Part 2: Transient conduction, non-linearities, convective heat transfer4. Transient conduction.5. Non-linearites.6. Convective heat transfer.Part 3: Coupled problems7. Radiative heat transfer in cavities.8. Fluid-structure interaction in a pipe.9. Metallurgical phase change.10. Thermal and electrical phenomena.

Crystal rotations during the rolling of large-grained aluminium sheet

Abstract The formation of a rolling texture has been studied by following the crystal orientations of 19 grains in two sheets of large-grained aluminium rolled to thickness reductions up to 80%. The experimental results for the individual grains are compared with the theoretical lattice rotations calculated for different simple grain deformation modes, namely: fully constrained (FC) plane strain compression, and partially constrained plane strain compression in which one or both in-plane shears are unprescribed. The calculations are performed using the classical Bishop-Hill method for FC deformations and the relaxed constraints (RC) method for partially prescribed grain deformations, together with the recent Renouard-Wintenberger minimum 2nd order work hypothesis. The lattice rotations of almost half the grains are in good quantitative agreement with the RC model. Some grains rotate according to the FC (Taylor) model along the minimum 2nd order work path. The rotations of the remaining grains are qualitatively close to the rather similar predictions of both models. The orientation dependence of the deformation mode of the grains can be correlated with the respective plastic work terms for the FC and RC models.

A 3D Hough transform for indexing EBSD and Kossel patterns

A three‐dimensional Hough transform is designed for the detection of conic curves (hyperbolae and ellipses) formed by the gnomonic projection of diffraction Kossel cones. This new procedure is applied to a high‐angular‐accuracy analysis of electron backscatter diffraction (EBSD) patterns and to a fully automatic indexing of X‐ray Kossel patterns in the SEM. The high‐accuracy analysis of EBSD patterns allows for the determination of local elastic strains, without any reference pattern, and with a spatial resolution of a few tens of nanometres. An accuracy of 2 × 10−4 is achieved on geometrically calculated diagrams. This paper presents also the first fully automatic indexing of Kossel patterns. This automatic indexing procedure can be applied to local texture analysis, as well as to local elastic strain measurements. Although the spatial resolution of Kossel is about 1 μm, the accuracy of strain measurement is in this case much higher than that presently obtained on EBSD.

Impact of the transient formation of molecular hydrogen on the microcrack nucleation and evolution in H-implanted Si (001)

We study the implant-induced hydrogenated defects responsible for the Smart Cut™ layer transfer of Si 001 films. Different experimental methods are used to quantify the time dependence of the defect evolution and interactions during isothermal annealings. An optical characterization technique was developed for the statistical analysis of the formation and growth of micrometer size microcracks in the buried implanted layer. We show that the formation of molecular hydrogen is dominated by a transient phenomenon related to the rapid dissociation of the hydrogenated point defects. The impact of the H 2 formation kinetics on the microcrack evolution is described and the physical mechanisms involved in their growth are identified. A comprehensive picture of the fracture phenomenon in H implanted Si leading to the full layer transfer is proposed and discussed.

A method for accurate localisation of EBSD pattern centres

The moving screen technique for pattern centre localisation is revisited. A cross-correlation based iterative procedure is developed to find both the zoom factor and the zoom centre (which is also the pattern centre) between two EBSD diffraction patterns acquired at two camera positions. The procedure involves two steps: first, a rough estimate of the pattern centre position and zoom factor (the ratio of the two detector distances) is obtained by cross-correlating the entire images. Then, based on this first estimate, cross-correlation of smaller regions of interest (ROIs) gives the displacement field which is interpreted as a zoom factor misfit coupled with a zoom centre position misfit. These misfits are iteratively decreased until the displacement field is reduced to the noise level. The procedure is first applied to simulated patterns and it is shown that the iterative procedure converges very rapidly to the exact solution with an accuracy better than 1/100th of pixel. The potential of this technique for experimental patterns is discussed and recommendations for new EBSD detectors are proposed.

Strain field in silicon on insulator lines using high resolution x-ray diffraction

Symmetric and asymmetric reciprocal space maps (RSMs) of silicon on insulator (SOI) lines are obtained using high resolution x-ray diffraction. RSMs calculated from the displacement field simulated using finite element calculations show a good agreement with the experimental RSMs. These calculations indicate the large influence of the displacement field created by the silicon nitride cap and the sensitivity of the RSMs to the gradients of displacement at the edge of the SOI lines. They further show that the RSMs are influenced by local strains but also by local rotations of the crystal lattice connected with the strain distribution.

Grain reorientations in rolled aluminium sheet: comparison with predictions of continuous constraints model

Abstract The crystal orientations of 24 grains that were, initially, roughly equiaxed have been followed during the rolling of an aluminium sheet for thickness reductions up to 75%. The experimental results for the individual grains are compared with the theoretical lattice rotations calculated for three different simple grain deformation modes. These are 1. (i) the classical full constraints (FC) mode, 2. (ii) a relaxed constraints (RC) mode in which both in-compression plane shears are free, and 3. (iii) a recent continuous constraints (CC) mode in which all the shears in the grain are partially relaxed by using a minimum work hardening rate criterion. The grains were classified in four groups according to their final lattice orientation. For the 19 grains in the first two groups, the CC model gives a good quantitative agreement with the experimental results. In particular, the theoretically predicted continuous transition from the FC to the RC deformation mode of the grains is experimentally confirmed. The remaining 5 grains either rotate towards a Brass orientation or deform inhomogeneously.