Anke Pyzalla

Evaluation of Residual Stresses in the Bulk of Materials by High Energy Synchrotron Diffraction

High energy synchrotron diffraction is introduced as a new method for residual stress analysis in the bulk of materials. It is shown that energy dispersive measurements are sufficiently precise so that strains as small 10−4 can be determined reliably. Due to the high intensity and the high parallelism of the high energy synchrotron radiation the sample gauge volume can be reduced to approximately 50 μm×1 mm×1 mm compared to gauge volume of one mm3 up to several mm3 achievable by neutron diffraction. The benefits of the high penetration depth and the small gauge volume are demonstrated by the results of stress studies performed on a fiber reinforced ceramic, a functional gradient material and a metal-ceramic compound. Furthermore, it is shown that in case of a cold extruded metal specimen the energy dispersive measurement technique yields simultaneous information about texture and residual stresses and thus allows a detailed investigation of elastic and plastic deformation gradients.

A Comparative Study of Microstructure and Residual Stresses of CMT-, MIG- and Laser-Hybrid Welds

Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) technique. Here microstructure formation and residual stress state in aluminum CMT welds are characterized and compared to those produced by pulsed MIG- and Laser-hybrid techniques. The results show a small heat affected zone (HAZ) in the MIG weld, the HAZ in the CMT and the laser hybrid welds was not visible by optical and scanning electron microscopy. Compared to the MIG welding the CMT process appears to introduce slightly smaller maximum tensile residual stresses into the weld.

Study of Microstructure and Residual Stresses in Dissimilar Al/Steels Welds Produced by Cold Metal Transfer

Recently a new welding technique, the so-called ‘Cold Metal Transfer’ (CMT) technique was introduced, which due to integrated wire feeding leads to lower heat input and higher productivity compared to other gas metal arc (GMA) techniques. Here microstructure formation and residual stress state in dissimilar steel to aluminum CMT welds are investigated. The intermetallic phase seam between the filler and the steel is only a few micrometers thick. Residual stress analyses reveal the formation of the typical residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses exist in the steel plate parent material, tensile residual stresses are present in the heat affected zone of the steel and the aluminum alloy. The area containing tensile residual stresses is larger in the aluminum alloy due to its higher heat conductivity than in the steel. Due to the symmetry in the patented voestalpine welding geometry and the welding from bottom and face side of the weld, the residual stress distributions at the top and at the bottom side of the weld are very similar.

Microstructure and Residual Stresses in Dissimilar Mg-Al-Zn-Alloy Single Overlap Laser Beam Welds

Microstructure, hardness and residual stresses of the laser beam overlap welds between AZ31B sheets and AZ31, AZ61 and AZ80 extruded profiles are investigated using microscopy and X-ray diffraction. The result of the investigations reveal that weld microstructure, the size of the HAZ, precipitate density and the maximum compressive residual stress values depend strongly on the Al content of the weld zone of two Mg-alloys.

Correlation between magnetic properties of CoFe single and CoFe/SiO2 multi-layer thin films and their microstructure, texture and internal stress state

CoFe single and multi-layer systems are deposited by a radio-frequency sputter process. Thickness, roughness, morphology, texture and internal stress state of the layers are determined by X-ray reflectometry, transmission electron microscopy, and diffraction methods. The texture and the internal stress of the layers depend strongly on the parameters of the sputter process. The magnetic properties of the layers are determined from hysteresis curve measurements and magneto-optical Kerr microscopy. A strong correlation between the texture, the internal stress, and the magnetic properties of the CoFe layers is observed.

In-situ investigation of strain relaxation in an Al/Si-MMC using high energy synchrotron radiation

Abstract Thermal and mechanical loading induce phase specific strains/stresses in MMCs. At elevated temperature part of the phase-specific strains/stresses relax. The relaxation of the phase-specific strains/stresses is determined by in-situ experiments using white high-energy synchrotron radiation. The experiments reveal that such time-resolved strain measurements are possible and that short-time phenomena can be accessed using white high-energy synchrotron radiation. The elastic strain relaxation behavior is similar for all lattice planes accessed. The influence of the temperature on the characteristic relaxation time is determined. The characteristic relaxation time appears to be independent of the amount of plastic deformation the sample suffers before strain/stress relaxation.

Growth Stresses and Phase Development in Nanostructured Oxide Scales Formed on Iron Aluminides

The evolution of phase composition and growth stresses in oxide scales growing on the polycrystalline Fe-15at.%Al alloy at 700°C in air was studied by in-situ synchrotron X-ray diffraction and X-ray photoelectron spectroscopy. The oxidation kinetics was determined by thermogravimetry. The results showed that, under these conditions, metastable -Al2O3 appears only during the first minutes of oxidation and the main oxides formed since the early oxidation are -Al2O3 and -Fe2O3. High volume fractions of -Fe2O3 caused accelerated oxidation rates in the first hours. -Al2O3 and -Fe2O3 grow epitaxially, evolving compressive and tensile growth stresses, respectively.

Dependence of Oxidation Behavior and Residual Stresses in Oxide Layers on Armco Iron Substrate Surface Condition

Mass gain during oxidation, texture and residual stresses in oxide layers on polycrystalline Armco iron substrates with different surface conditions are investigated using thermogravimetry microscopy and synchrotron X-rays. The mass gain during oxidation in all samples follows a parabolic law. The parabolic oxidation constant increases with increasing roughness of a mechanically ground respectively polished oxide layer. Electrolytic polishing (grain surface etching) reduces while grain boundary etching increases the parabolic oxidation constant compared to the mechanically polished sample. All oxide layers show columnar growth of the magnetite and a moderate fiber texture. The magnetite contains compressive residual stresses. Under the conditions chosen for the oxidation treatment the magnitude of these compressive residual stresses does not depend on the substrate surface condition.

Dynamic in-situ investigation of the texture and strain state within a plastically deformed solid AlMg3 torsion sample using high energy synchrotron radiation

A solid torsion sample made from non-age hardenable single-phase Al alloy AlMg3 is continuously deformed until failure. The low speed deformation with free ends is carried out at room temperature. For the first time, the dynamic in-situ development of the local texture and strain state within the sample are observed by means of a novel strain and texture scanning technique. The technique is based on the combination of a microfocussed high energy synchrotron beam, a conical slit system and a large area X-ray detector. The experimental results clearly show the deformation dependent evolution of axial forces (the so-called Swift effect). The texture development exhibits a change from the initial 111 / 100 fibre texture to the dominance of ideal torsion texture orientations.

Study of cavity evolution during creep by synchrotron microtomography using a volume correlation method

Introduction Instrument development at third-generation synchrotrons over the last decade has enabled a large variety of in-situ experiments. Among these, X-ray microtomography represents a special case due to the three-dimensional (3D) character of the information acquired [1-3]. The method combines the high photon flux with the nondestructive nature of the investigation, enabling a deeper insight into the dynamics of physical processes. In-situ microtomography is very well suited for damage investigation during high-temperature creep, providing for the first time experimental 3D data about damage evolution. Such findings are of valuable help to understand the dynamics of the underlying physical processes and to promote further developments of adequate damage theories [4-6]. It is the aim of this report to present the qualitative features of cavity evolution during high-temperature creep of a brass alloy.