Nicola Casati

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The effect of stress triaxiality on the phase transformation in transformation induced plasticity steels: Experimental investigation and modelling the transformation kinetics

Nicola Casati, Efthymios Polatidis, Helena Van Swygenhoven

In situ multiaxial loading during neutron diffraction tests were undertaken on a low-alloyed Quenched and Partitioning (Q&P) Transformation Induced Plasticity (TRIP) Bainitic Ferrite (TBF) steel with dispersed austenite particles. The effect of stress triaxiality on the evolution of the deformation-induced martensite is investigated under uniaxialand equibiaxial-tension as well as tension/compression with a ratio of-1:6. It is shown that transformation is not a monotonic function of stress triaxiality; the amount of deformation-induced martensite is similar under uniaxial and equibiaxial tension but it is significantly smaller under tension/compression. The transformation kinetics are modeled using a recently developed kinetic model that accounts for the stress state and the stability and size of the austenite particles. The larger austenite particles transform first and the mean volume of the austenite particles decreases with increasing strain; the decreasing austenite particle size impedes the phase transformation as the deformation proceeds. It is concluded that stress triaxiality alone cannot account for the differences in the transformation kinetics between different loading states and that the number of potential nucleation sites depends on the stress state.

2021

A miniaturized selective laser melting device for operando X-ray diffraction studies

Nicola Casati, Samy Hocine, Steven Van Petegem, Helena Van Swygenhoven

We report on the development of a miniaturized device for operando X-ray diffraction during laser 3D printing. The printing chamber has a size of only 130 x 135 x 34 mm(3) and is optimized to be installed at synchrotron beamlines. We describe the design considerations, details on the setup and the implementation at two different beamlines of the Swiss Light Source. Its capabilities are demonstrated by ex situ printing of complex shapes and operando X-ray diffraction experiments using Ti-6Al-4V powder. It is shown that the beamline characteristics have an important influence on the X-ray footprints of the microstructural evolution during 3D printing. From the intensity of the diffraction peaks, the evolution of the different phases can be followed during printing. Furthermore, the diffuse scattering signal provides information on the precise location of the laser beam on the sample and the scanning head settling time.

ELSEVIER2020

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Morphology, phase composition, cracks and porosity are investigated in monolithic Ti modified SiOC polymer-derived ceramics pyrolyzed at 1000°C and 1400°C using synchrotron X-ray full field absorption- contrast tomographic microscopy and scanning XRF- and XRD-contrast microscopy. Samples pyrolyzed at 1000°C show a crack-free structure, but pyrolysis at 1400°C results in formation of cracks and at higher Ti content also shows porosity. Tomography revealed the formation of a layered morphology that varies in terms of crystallographic structure and/or Ti stoichiometric concentration. The microstructural observations and electrical conductivity are discussed in terms of pyrolysis temperature and Ti content.

2020