Advanced Diffraction Techniques for Microstructure and Stress Characterisation at Multiple Scales

Mario Stefenelli

Research output: ThesisDoctoral Thesis

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Abstract

Mechanical integrity, functional properties and lifetime of engineering components are predetermined by the residual stress state and structural properties of the applied materials. Nowadays, the development of novel materials with tailored functionality requires a detailed knowledge of their structural and mechanical properties at the centimetre down to the nanometre length-scales. For this reason, advanced analytical techniques allowing volume-averaged as well as very local characterisation of phases, residual stresses and microstructure have to be applied to reveal the complex mutli-scale structure-function relationships in technological materials. In this thesis, advanced high-energy X-ray and neutron diffraction techniques operating at various scales are presented and used to analyse three types of structures, nanocrystalline thin films, seamless steel tubes and railway rails, applying always the appropriate spatial resolution. (i) Nanocrystalline thin films are characterised using cross-sectional X-ray nanodiffraction with a spatial resolution as low as 100 nm. In the first study the residual stress gradient in a shot-peened 11.5 µm thick TiN film is characterised and the results are compared to a Laplace space approach characterisation. Additionally the effect of stress relaxation in the thin lamella required as a sample for the nanobeam experiments is addressed. In the second study the residual stress fields across the imprint in a wedge-indented 3 µm CrN-Cr multilayer film are characterised. The results reveal a complex residual stress distribution across the film cross-section with stress peaks ranging from -10 to 2 GPa being in good agreement with a finite element model. It is shown how the ductile Cr sublayers act as a stabilizing component preserving the mechanical integrity of the structure by the encapsulation of the regions with tensile stress peaks in the brittle CrN sublayers. In the third study the fracture properties of 3 µm CrN thin films are examined by coupling four-point bending and energy dispersive X-ray diffraction. Two thin film types, deposited by applying -40 V and -120 V bias voltage, are compared in the as-deposited and annealed state. It is found that the change in the bias voltage modified the observed fracture behaviour in favour for the -120 V film, which however deteriorates upon annealing, whereas the -40 V film is not affected by the heat treatment. (ii) The residual stress distribution inside a roller-straightened railway rail is characterised with neutron diffraction using a gauge volume of 5x5x5 mm3. The longitudinal, vertical and transversal components of the stress tensor are determined and compared with the results from the contour method and finite element modelling. (iii) Three dimensional distributions of triaxial residual stress in seamless steel tubes are studied by high energy synchrotron X-ray diffraction coupled with a conical slit system providing a spatial resolution along the X-ray beam of 0.8 mm. The identified steep residual gradients across the tube walls are correlated with applied cooling conditions and the resulting microstructure.
Translated title of the contributionFortgeschrittene Beugungsverfahren zur Bestimmung von Mikrostruktur und Eigenspannungen auf verschiedenen Größenordungen.
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Keckes, Jozef, Assessor A (internal)
  • Mitterer, Christian, Assessor B (internal)
Publication statusPublished - 2014

Bibliographical note

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Keywords

  • residual stress
  • microstructure
  • texture
  • X-ray
  • neutrons
  • nanodiffraction
  • indent
  • thin film
  • CrN
  • TiN
  • rail
  • seamless tube

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