Vincent Klosek

Modeling of structural hardening in oxide dispersion-strengthened (ODS) ferritic alloys

AbstractBased on a rather simple macroscopic and statistical model, experimentally observed variations of yield stress at room temperature in various ODS alloys were theoretically reproduced. For the first time, yield stress values of ODS steels were calculated by taking into account: (1) two interaction mechanisms between dislocations and nanoprecipitates (shearing or bypassing, simultaneously, depending on the particle size); and (2) the whole, possibly multimodal, nanoparticle distributions experimentally determined by SANS. The relative importances of the various strengthening mechanisms can be easily deduced from these calculations.

Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

In order to study the relationship between residual stress (RS) and the microstructure of friction stir weld (FSW), RS profiles through thickness in the un-welded aluminum alloy 7075 plate and in middle layer of its FSW joint were determined nondestructively by the short-wavelength X-ray diffraction (SWXRD) and neutron diffraction. Microstructure and mechanical properties of the FSW joint were also studied by optical microscopic analysis, and microhardness and tensile strength measurements. RS profiles measured by the two methods had the same distribution trend. The maximum tensile RS tested by SWXRD and neutron diffraction in transverse and longitudinal direction occurred in the weld nugget. Microhardness in the direction perpendicular to the weld line showed a “W” shape distribution. Position of the local maximal extremum of RS in thermo-mechanically affected zone corresponded to that of minimal microhardness. The grain-refined strengthening caused by the recrystallization in the weld nugget kept the joint from fracturing at this region notwithstanding the maximum tensile RS. And the tensile fracture occurred near the boundary of welding zone and thermo-mechanically affected zone where minimum of hardness and maximum of RS appear at the same position.

Variation of Residual Stresses in Drawn Copper Tubes

Seamless tubes are used for many applications, e.g. in heating, transport gases and fluids, evaporators as well as medical use and as intermediate products for hydroforming and various mechanical applications, where the final dimensions normally are given by some cold drawing steps. The first process step – piercing of the billet, for example by extrusion or 3-roll-milling - typically results in ovality and eccentricity in the tube causing non-symmetric material flow during the cold drawing process, i.e. inhomogeneous deformation. Because of this non-axisymmetric deformation and of deviations over tube length caused by moving tools, this process step generates residual stresses. To understand the interconnections between the geometrical changes in the tubes and the residual stresses, the residual strains in a copper tube had been measured by neutron diffraction.

Evaluation of Residual Stresses in Dissimilar Weld Joints

Dissimilar metal joints between pipes of ferritic and austenitic steels are present in primary coolant circuit of pressurized water reactors. Over the last years in particular in USA and Japan, stress corrosion cracks, often associated with weld repairs, have been observed for some dissimilar welds made with an Inconel filler metal. The integrity of this type of components is thus a major safety issue. In this context, the goal of this work is to evaluate the welding residual stresses field for a dissimilar weld joint. A representative bi-metallic tubular weld joint was fabricated and residual stresses profiles in the different weld zones were evaluated by means of the hole drilling and neutron diffraction methods.

In Situ Analysis of Deformation Mechanisms of Cu-Based fcc Materials under Uniaxial Loading

By associating texture determinations and strains measurements by neutron diffraction, the elastoplastic behaviours of families of crystallites with the same crystallographic orientations were characterized in situ in a brass and a bronze alloys under uniaxial loading. The polycrystalline orientation analysis method proposed here allows an intermediate approach between a “local” (intragranular) and a “global” characterization, within the bulk of massive samples.

Experimental Comparison of In-Depth Residual Stresses Measured with Neutron and X-Ray Diffraction with a Numerical Stress Relaxation Correction Method

This study is an experimental comparison of in-depth X-ray diffraction residual stress measurements with neutron diffraction measurements. The goal is to evaluate the relevance of the Savaria-Bridier-Bocher [1] stress relaxation correction method. Neutron diffraction are performed on a bent notched specimen. Destructive X-ray diffraction is performed until 5.25mm below the surface by polishing the material. This polishing induces stress relaxation and X-ray diffraction results have to be corrected. For that purpose, a finite element analysis is realised and show good correlation with neutron measurements results. The application of the stress correction method improves the X-ray measurements especially after 2 mm below the surface. The differences between measured and corrected residual stresses from both diffraction techniques are analyzed and discussed.

Neutron diffraction study of elastoplastic behaviour of Al/SiCp metal matrix composite

The TOF neutron diffraction measurements were done for Al/SiCp metal–matrix composite (17% of SiC) subjected to T6 thermal treatment. Using three separated diffraction peaks of SiC phase and four peaks of Al phase, the lattice strains were measured for both phases independently during in situ tensile test. The experimental results were presented in comparison with elastoplastic model, which allows to find the values of parameters determining plastic deformation of Al matrix (critical resolved shear stress and hardening parameter). Additionally, the results of TOF method were compared with those which were obtained with monochromatic neutron radiation (LLB, Saclay). In the latter experiment Al/SiCp composite containing 25% of SiC was measured. It was shown that after elastoplastic deformation the mismatch stresses determined for both phases relax during tensile deformation.

Lattice Strain Pole Figures Analysis in Titanium during Uniaxial Deformation

A theoretical and experimental study was carry out to investigate deformation mechanisms in a textured titanium alloy. In situ neutron diffraction measurements were performed to analyze different {hk.l} family planes ({10.0}, {10.1}, {11.0} and {00.2}) and determine the corresponding internal strain pole figures. This method was applied to a pure titanium (a-Ti) submitted to a uniaxial tensile load up to 2 %. The experimental data was then used to validate the EPSC model in order to predict the distribution of lattice strains determined by neutron diffraction for various diffraction vector directions. This comparison reveals that the model results were in good agreement with the experimental data and the simulations reproduced the lattice strain development observed on the strain pole figures determined by neutron diffraction.

Study of Micromechanical Behaviour of Two Phase Polycrystalline Materials Using Diffraction and Self Consistent Model

Diffraction methods for lattice strain measurement provide useful information concerning the nature of grains behaviour during elastoplastic deformation. The main advantage of the diffraction methods is the possibility of studying mechanical properties of polycrystalline materials separately in each phase and in groups of grains with a specific orientation. In this work we present application of the neutron and X-ray diffraction to study “in situ” deformation of two phase stainless steels during tensile loading. The experimental results are compared with self-consistent model.

Study of the Influence of Plastic Deformation on Metastable Phases of CuAlBe Shape Memory Alloy by Neutron Diffraction

This work aims to study by neutron diffraction the evolution of metastable phases of CuAlBe shape memory alloy after plastic deformation. Two samples were studied: the first one deformed by cold rolling at a reduction rate of 15% and the second, deformed by cold rolling at 15% followed by hot rolling at 200°C for a reduction rate of 30% respectively. Before plastic deformation, the material is fully austenitic at ambient temperature. Its crystallographic texture is mainly characterized by a <001> partial fibber. After deformation, this partial fibber disappears and the crystallographic texture is composed by isolated orientations. At higher reduction rates, the texture of austenitic phase remaining in the material is characterized by a <111> fibber. The rolling process modifies metastable phase quantities. After deformation at a reduction rate of 15%, the volume fraction of metastable austenite remaining is close to 8%. Plastic deformation also greatly modifies the characteristic transformation temperatures and enlarges the hysteresis. The material plastically deformed after hot rolling presents large variations of intensities of diffraction peaks belonging to martensite phase during a thermal cycle at low temperature. This effect is attributed to a reorganization of variants due to an evolution of crystallographic texture of martensite.