Dr. Steven Van Petegem

Steven Van Petegem is senior scientist in the Structure and Mechanics of Advanced Materials. He received both his Physics degree and PhD at the Ghent University, Belgium. During his PhD he focussed on the synergy between experimental techniques and modelling to study the microstructure of nanocrystalline metals. In 2003 he started working at the Paul Scherrer Institut at the Small Angle Neutron Diffraction instrument in the NUM department. In 2005 he moved to the Materials Science and Simulations group, where he was co-responsible for the experimental program. Between 2008-2015, he was also co-responsible for the POLDI instrument. Between 2015 and 2021, he worked in the Photons for Engineering and Manufacturing group.

Within the SMAM-group Steven Van Petegem is responsible for guiding PhD students and the coordination of additive manufacturing experiments at the Swiss Light Source and other large scale facilities. He is the PSD representative in the task force 'Offices and Laboratories' that advises the Steering Committee for Environment and energy. He is a tour guide at the Paul Scherrer Institute. He teaches the master course "Materials Science at Large Scale Facilities" at EPFL.

Van Petegem's scientific research is currently focussed on the application of in situ and operando X-ray diffraction and imaging to unveil microstructural changes in metals during thermo-mechanical processing. During past decade, various devices have been developed that allow performing in situ deformation experiments on samples with sizes ranging from micrometres up to tens of centimetres, subjected to uniaxial, biaxial or shear forces. The experiments go hand-in-hand with advanced simulation techniques, such as molecular dynamics and crystal plasticity modeling. More recenty, a miniaturized selective laser melting device was developed. It allows performing X-ray diffraction or imaging experiments during laser 3D printing. This unique device provides unprecedented insight into microstructural changes during solidification at cooling rates up to 1 million degrees per second.  

For an extensive overview we kindly refer you to our publication repository DORA

Operando X-ray diffraction during laser 3D printing
Hocine S; Van Swygenhoven H; Van Petegem S; Chang C; Maimaitiyili T; Tinti G; Ferreira Sanchez D; Grolimund D; Casati N
Materials Today 34 (2020) 30

Ultra-fast operando X-ray diffraction experiments reveal the temporal evolution of low and high temperature phases and the formation of residual stresses during laser 3D printing of a Ti-6Al-4V alloy. The profound influence of the length of the laser-scanning vector  on the evolving microstructure is revealed and elucidated.

Revealing the role of microstructure architecture on strength and ductility of Ni microwires by in-situ synchrotron X-ray diffraction
Purushottam raj purohit, R.  raj purohit; Arya, A.; Bojjawar, G.; Pelerin, M.; Van Petegem, S.; Proudhon, H.; Mukherjee, S.; Gerard, C.; Signor, L.; Mocuta, C.; Casati, N.; Suwas, S.; Chokshi A.H.; Thilly L.
Nature Scientific Reports 2019, 9 (1), 79.

The objective of this work is to understand strengthening and reduction of ductility in nickel microwires with reduction in diameter via high-energy synchrotron X-ray diffraction (XRD). Tensile tests on Ni microwires are performed in combination with XRD to derive the deformation mechanisms taking place in the different grain families. These mechanisms are discussed in view of the initially observed microstructure (grain size and crystallographic micro-texture) and the effect of diameter change by electropolishing. From these results, guidelines are proposed to tailor the strength and ductility of Ni microwires, these considerations being general enough to be extended to other FCC metals.
 

Laue microdiffraction characterisation of as-cast and tensile deformed Al microwires
Deillon, L.; Verheyden, S.; Ferreira Sanchez, D.; Van Petegem, S.; Van Swygenhoven, H.; Mortensen, A.
Philosophical Magazine 2019, 99 (15), 1866-1880.

As-cast and deformed microwires of pure aluminium are characterised by means of Laue synchrotron X-ray microdiffraction maps gleaned over selected areas of the wires. In the as-cast condition, the wires contain a low density of geometrically necessary dislocations. After deformation, the geometrically necessary dislocation density in the microcast wires has increased, despite the fact that the wires can deform without significant imposed strain gradients. The measured GND density values are well below what would be stored were dislocations prevented from escaping along the surface during the deformation of the single-slip wires. It is higher in the multislip sample oriented along [1 1 1] than in single-slip wires, betraying the presence of mutual dislocation blockage. Those observations are consistent with the proposed mechanism, namely that deformation in these crystals is largely driven by repeatedly rotating, likely single-arm, sources which enable most moving dislocations to escape through the free surface, particularly if the wire is oriented for single slip.
 

A Miniaturized Biaxial Deformation Rig for in Situ Mechanical Testing
Van Petegem, S.; Guitton, A.; Dupraz, M.; Bollhalder, A.; Sofinowski, K.; Upadhyay, M. V.; Van Swygenhoven, H.
Experimental Mechanics 2017, 57 (4), 569-580

A unique miniaturized biaxial deformation rig was developed, that allows applying in-plane biaxial stress states and to perform strain path changes. The rig can be mounted inside conventional scanning electron microscope chambers and at synchrotron beam lines. The cruciform shaped sample geometry has been optimized with the aid of finite element simulations and allows reaching reasonable levels of plasticity. Sample preparation is challenging and requires the use of advanced preparation techniques. A proof-of-principle in situ X-ray diffraction experiment revealed that the developed rig operates successfully. This will allow obtaining crucial microstructural information in real-time while the samples are subjected to complex biaxial strain paths.

In-situ neutron diffraction during biaxial deformation
Van Petegem, S.; Wagner, J.; Panzner, T.; Upadhyay, M. V.; Trang, T. T. T.; Van Swygenhoven, H.
Acta Materialia 2016, 105, 404-416.

A change in strain path may have a significant effect on the mechanical response of metals. In order to understand or even predict the macroscopic behaviour under such conditions a detailed knowledge on the microstructural evolution is crucial. Yet relatively little work has been done to quantify and understand how the inter- and intragranular strains are affected during a change in strain path. In this work we present a new multiaxial deformation rig that allows performing in situ proportional and non-proportional loading under neutron diffraction. We demonstrate the capabilities of this new setup for the case of a 316 L stainless steel. We show that the nature and magnitude of intergranular strain strongly depends on the applied stress state and demonstrate that micro yielding and internal strain recovery are responsible for the observed transient softening during a 90° strain path change. We anticipate that this new characterization method will provide previously inaccessible microstructural data that can serve as input for benchmarking current state-of-the-art crystal plasticitymodels.